Searching in the dark

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Dr. Xin Ran Liu

https://www.ph.ed.ac.uk/people/xinran-liu

Email: x.liu@ed.ac.uk

https://twitter.com/xinranliu_1?lang=en

https://www.youtube.com/watch?v=78WmmFG_kbA

https://stfc.ukri.org/news-events-and-publications/features/the-story-of-dark-matter/

Dr Liu started his talk with a video (link below)

https://youtu.be/xcZjpAaZREo

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The video was produced by the dark matter UK collaboration and the screenshot, above, shows all the research groups involved.

A summary of the video

Everything is made up of tiny particles of matter. But a physicist will tell you most of this matter seems to be missing. Over 80% of it seems to be hidden and can’t, at the moment, be detected.

These “dark particles” are a form of matter we are not aware of encountering, but they may provide information to unlock the next frontier of physics. They may provide the missing link that tells us how we came to be and where we are going.

DMUK is an alliance of scientists and universities building new technologies that will detect these particles. These include experimenters, theoreticians and astronomers. They are operating in cutting edge laboratories, operating computer simulations and even working deep underground all over the world.

The work is a global effort providing a new wave of exploration that will push forward the frontiers of science. Whatever they find it’ll just be the start.

The talk

Dr Liu explained that the video was an introduction to what he does.

Dr Liu’s research involves hunting for dark matter.

He is based at the University of Edinburgh, which is very proud of the fact that Peter Higgs is a physics professor there.

https://en.wikipedia.org/wiki/Peter_Higgs

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Peter Ware Higgs CH FRS FRSE FInstP (born 29 May 1929) is a British theoretical physicist, Emeritus Professor in the University of Edinburgh, and Nobel Prize laureate for his work on the mass of subatomic particles.

In the 1960s, Higgs proposed that broken symmetry in electroweak theory could explain the origin of mass of elementary particles in general and of the W and Z bosons in particular. This so-called Higgs mechanism, which was proposed by several physicists besides Higgs at about the same time, predicts the existence of a new particle, the Higgs boson, the detection of which became one of the great goals of physics. On 4 July 2012, CERN announced the discovery of the boson at the Large Hadron Collider. The Higgs mechanism is generally accepted as an important ingredient in the Standard Model of particle physics, without which certain particles would have no mass.

https://www2.slideshare.net/xzxzxz/atlas-higgs-cernseminar2012/44

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http://www.eniscuola.net/en/mediateca/decay-of-the-boson-into-photons/

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Graph showing the events observed by CMS and ATLAS in the two-photon decay channel. The ATLAS and CMS detectors observed a peak in the events (the bump) due to the decay of the Higgs boson into photons, an excess signal above the background made by photons produced mainly by the proton-proton collisions. The peak is around 125 GeV. Source: http://cms.web.cern.ch and http://atlas.web.cern.ch

The fourth of July 2012 is now referred to in particle physics as Higgs dependence day.

https://en.wikipedia.org/wiki/Higgs_boson

The Higgs boson is an elementary particle in the Standard Model of particle physics produced by the quantum excitation of the Higgs field, one of the fields in particle physics theory. It is named after physicist Peter Higgs who in 1964 along with five other scientists proposed the Higgs mechanism to explain why some particles have mass. (Particles acquire mass in several ways, but a full explanation for all particles had been extremely difficult). This mechanism required that a spinless particle known as a boson should exist with properties as described by the Higgs Mechanism theory. This particle was called the Higgs boson. A subatomic particle with the expected properties was discovered in 2012 by the ATLAS and CMS experiments at the Large Hadron Collider (LHC) at CERN near Geneva, Switzerland. The new particle was subsequently confirmed to match the expected properties of a Higgs boson.

The discovery of the Higgs boson came almost 50 years after its first prediction and provides further validation of the standard model of particle physics.

https://www.benbest.com/science/standard.html

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https://en.wikipedia.org/wiki/Standard_Model

The Standard Model of particle physics is the theory describing three of the four known fundamental forces (the electromagnetic, weak, and strong interactions, and not including the gravitational force) in the universe, as well as classifying all known elementary particles. It was developed in stages throughout the latter half of the 20th century, through the work of many scientists around the world, with the current formulation being finalized in the mid-1970s upon experimental confirmation of the existence of quarks. Since then, confirmation of the top quark (1995), the tau neutrino (2000), and the Higgs boson (2012) have added further credence to the Standard Model. In addition, the Standard Model has predicted various properties of weak neutral currents and the W and Z bosons with great accuracy.

The standard model is one of the most rigorously tested models in any branch of science but there is a problem with it. It doesn’t account for gravity or the presence of dark matter. It is incredibly successful at describing just 4% of our Universe. This 4% is termed baryonic matter.

https://www.britannica.com/science/baryon

Baryons are heavy subatomic particles that are made up of three quarks and experience the strong nuclear force. Both protons and neutrons, as well as other particles, are baryons.

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Neutron (above left) and Proton (above right). The squiggly lines represent the strong nuclear force (carried by gluons). u = up and d = down quarks

https://en.wikipedia.org/wiki/Dark_matter

Dark matter is a form of matter thought to account for approximately 85% of the matter in the universe and about a quarter of its total mass–energy density or about 2.241 x 10−27 kg/m3.

The idea of dark matter has a surprisingly long history. In a talk given in 1884, Lord Kelvin (University of Glasgow) estimated the number of dark bodies in the Milky Way from the observed velocity dispersion of the stars orbiting around the centre of the galaxy. By using these measurements, he estimated the mass of the galaxy, which he determined is different from the mass of visible stars. Lord Kelvin thus concluded “many of our stars, perhaps a great majority of them, may be dark bodies”.

https://en.wikipedia.org/wiki/William_Thomson,_1st_Baron_Kelvin

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William Thomson, 1st Baron Kelvin, OM, GCVO, PC, PRS, FRSE (26 June 1824 – 17 December 1907) was a British mathematical physicist and engineer born in Belfast.

The first to suggest the existence of dark matter using stellar velocities was Dutch astronomer Jacobus Kapteyn in 1922. Fellow Dutchman and radio astronomy pioneer Jan Oort also hypothesized the existence of dark matter in 1932. Oort was studying stellar motions in the local galactic neighbourhood and found the mass in the galactic plane must be greater than what was observed, but this measurement was later determined to be erroneous.

https://en.wikipedia.org/wiki/Jacobus_Kapteyn (below left)

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Prof Jacobus Cornelius Kapteyn FRS FRSE LLD (19 January 1851 – 18 June 1922) was a Dutch astronomer.

https://en.wikipedia.org/wiki/Jan_Oort (above right)

Jan Hendrik Oort ForMemRS (28 April 1900 – 5 November 1992) was a Dutch astronomer who made significant contributions to the understanding of the Milky Way and who was a pioneer in the field of radio astronomy.

In 1933, Swiss astrophysicist Fritz Zwicky (Caltech), who studied galaxy clusters while working at the California Institute of Technology, made a similar inference as Kapteyn and Oort. Zwicky applied the virial theorem to the Coma Cluster and obtained evidence of unseen mass he called dunkle Materie (‘dark matter’). Zwicky estimated its mass based on the motions of galaxies near its edge and compared that to an estimate based on its brightness and number of galaxies. He estimated the cluster had about 400 times more mass than was visually observable. The gravity effect of the visible galaxies was far too small for such fast orbits; thus, mass must be hidden from view. Based on these conclusions, Zwicky inferred some unseen matter provided the mass and associated gravitation attraction to hold the cluster together.

https://en.wikipedia.org/wiki/Fritz_Zwicky

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Fritz Zwicky (February 14, 1898 – February 8, 1974) was a Swiss astronomer. He worked most of his life at the California Institute of Technology in the United States of America, where he made many important contributions in theoretical and observational astronomy. In 1933, Zwicky was the first to use the virial theorem to infer the existence of unseen dark matter, describing it as “dunkle (kalt) Materie”.

https://en.wikipedia.org/wiki/Coma_Cluster

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The Coma Cluster (Abell 1656) is a large cluster of galaxies that contains over 1,000 identified galaxies. Along with the Leo Cluster (Abell 1367), it is one of the two major clusters comprising the Coma Supercluster. It is located in and takes its name from the constellation Coma Berenices.

Vera Rubin, Kent Ford, and Ken Freeman’s work in the 1960s and 1970s provided further strong evidence, also using galaxy rotation curves. Rubin and Ford worked with a new spectrograph to measure the velocity curve of edge-on spiral galaxies with greater accuracy. This result was confirmed in 1978. An influential paper presented Rubin and Ford’s results in 1980.They showed most galaxies must contain about six times as much dark as visible mass; thus, by around 1980 the apparent need for dark matter was widely recognized as a major unsolved problem in astronomy.

https://en.wikipedia.org/wiki/Vera_Rubin (below left)

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Vera Florence Cooper Rubin (July 23, 1928 – December 25, 2016) was an American astronomer who pioneered work on galaxy rotation rates.

https://en.wikipedia.org/wiki/Kent_Ford_(astronomer) (above centre)

W. [William] Kent Ford, Jr. (born 1931) is an astronomer involved with the theory of dark matter.

https://en.wikipedia.org/wiki/Ken_Freeman_(astronomer) (above right)

Kenneth Charles Freeman AC FAA FRS (born 27 August 1940) is an Australian astronomer and astrophysicist who is currently Duffield Professor of Astronomy in the Research School of Astronomy and Astrophysics at the Mount Stromlo Observatory of the Australian National University in Canberra.

Rubin, Ford and Freeman were looking at the way the stars moved around galaxies. They were expecting the stars further away from the galactic centre to move slower as predicted from the work of Sir Isaac Newton.

https://en.wikipedia.org/wiki/Isaac_Newton

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Sir Isaac Newton PRS (25 December 1642 – 20 March 1726/27) was an English mathematician, physicist, astronomer, theologian, and author (described in his own day as a “natural philosopher”) who is widely recognised as one of the most influential scientists of all time and as a key figure in the scientific revolution.

https://en.wikipedia.org/wiki/Newton%27s_law_of_universal_gravitation

Newton’s law of universal gravitation is usually stated as that every particle attracts every other particle in the universe with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centres.

The equation for universal gravitation thus takes the form:

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where F is the gravitational force acting between two objects, m1 and m2 are the masses of the objects, r is the distance between the centres of their masses, and G is the gravitational constant.

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The above image showed the expected results.

Basically Rubin, Ford and Freeman were expecting stars far away from the galactic centre to travel more slowly than stars close to the centre instead they found the outer stars travelled just as fast.

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What Rubin, Ford and Freeman found.

This result indicated that there must be something holding the galaxies together differently to what was seen and making the stars rotate at the faster speed.

Whatever it is, it can’t be seen which is why it was called dark matter.

https://upload.wikimedia.org/wikipedia/commons/transcoded/3/33/Galaxy_rotation_under_the_influence_of_dark_matter.ogv/Galaxy_rotation_under_the_influence_of_dark_matter.ogv.360p.vp9.webm

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Left: A simulated galaxy without dark matter. Right: Galaxy with a flat rotation curve that would be expected under the presence of dark matter.

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Rotation curve of a typical spiral galaxy: predicted (A) and observed (B). Dark matter can explain the ‘flat’ appearance of the velocity curve out to a large distance from the galactic centre.

Invisible “dark” matter allows greater rotation speeds of the outer part of galaxies

The term “dark” can also be used because of physicists’ lack of knowledge of what this dark matter substance is,

The flat rotation results were really very surprising and suggested that there was something more than gravity holding things together in galaxies.

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Debates have gone on that dark matter doesn’t exist and that the results are caused by gravity acting differently at such large scales but the data that has been collected, so far, doesn’t fit this idea.

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https://en.wikipedia.org/wiki/Cosmic_microwave_background

The Cosmic Microwave Background (CMB, CMBR), in Big Bang cosmology, is electromagnetic radiation which is a remnant from an early stage of the universe, also known as “relic radiation”[citation needed]. The CMB is faint cosmic background radiation filling all space. It is an important source of data on the early universe because it is the oldest electromagnetic radiation in the universe, dating to the epoch of recombination.

Although both dark matter and ordinary matter are matter, they do not behave in the same way. In particular, in the early universe, ordinary matter was ionized and interacted strongly with radiation via Thomson scattering. Dark matter does not interact directly with radiation, but it does affect the CMB by its gravitational potential (mainly on large scales), and by its effects on the density and velocity of ordinary matter. Ordinary and dark matter perturbations, therefore, evolve differently with time and leave different imprints on the cosmic microwave background (CMB).

https://en.wikipedia.org/wiki/Thomson_scattering

Thomson scattering is the elastic scattering of electromagnetic radiation by a free charged particle,

Since the 1970s there is a lot more evidence from other sources to support the existence of dark matter. It would be more difficult to come up with theories to modify gravity that would explain all of these phenomena away.

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So, while physicists know that there should be more matter in the universe than they can see measurements show part of the matter of the Universe is missing. Unfortunately, they have yet to find it. It’s probably going to be the last place they looked

The image below illustrates there are almost as many theories for what dark matter could be out there as there are theoretical physicists at the present time. There maybe even more as I don’t actually know how many physicists class themselves as theoretical physicists (I’m definitely not one).

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Some of the theories are more strongly supported in the physics community than others and many have been ruled out over the years, as physicists continue to build dark matter experiments to look for it.

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So far, all of the evidence for the existence of dark matter has come from astronomical sources and what physicists really want to do is to be able to detect dark matter directly in a laboratory setting.

There are probably three ways to go about doing this and you can summarise them as: make it; break it; shake it (almost a spice girls’ song).

The LHC at CERN is trying to “make it” by colliding particles together.

https://en.wikipedia.org/wiki/Large_Hadron_Collider

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The Large Hadron Collider (LHC) is the world’s largest and highest-energy particle collider and the largest machine in the world. It was built by the European Organization for Nuclear Research (CERN) between 1998 and 2008 in collaboration with over 10,000 scientists and hundreds of universities and laboratories, as well as more than 100 countries. It lies in a tunnel 27 kilometres in circumference and as deep as 175 metres beneath the France–Switzerland border near Geneva.

Issues explored by LHC collisions include: What is the nature of the dark matter that appears to account for 27% of the mass-energy of the universe?

“Break it” – looking for dark natural annihilation signatures from space using Alpha Magnetic Spectrometer.

https://en.wikipedia.org/wiki/Alpha_Magnetic_Spectrometer

The Alpha Magnetic Spectrometer (AMS-02) is a particle physics experiment module that is mounted on the International Space Station (ISS). The experiment is a recognized CERN experiment (RE1). The module is a detector that measures antimatter in cosmic rays; this information is needed to understand the formation of the Universe and search for evidence of dark matter.

https://en.wikipedia.org/wiki/International_Space_Station

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The International Space Station (ISS) is a modular space station (habitable artificial satellite) in low Earth orbit. It is a multinational collaborative project involving five participating space agencies: NASA (United States), Roscosmos (Russia), JAXA (Japan), ESA (Europe), and CSA (Canada).

Physicists can “shake it” where they make a large target detector and wait for the dark matter to smash into it. This method has really been one of the most successful methods of dark matter searches so far, and it is this method that Dr Liu focused on in his talk because this is his research area.

The major challenge to detecting dark matter with a large target mass is that the expected “signature” of the dark matter process is similar to the background radiation on Earth.

https://en.wikipedia.org/wiki/Background_radiation

Background radiation is a measure of the level of ionizing radiation present in the environment at a particular location which is not due to deliberate introduction of radiation sources.

Background radiation originates from a variety of sources, both natural and artificial. These include both cosmic radiation and environmental radioactivity from naturally occurring radioactive materials (such as radon and radium), as well as man-made medical X-rays, fallout from nuclear weapons testing and nuclear accidents.

https://www.schoolphysics.co.uk/age16-19/Nuclear%20physics/Radioactivity/text/Background_radiation/index.html

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Don’t worry, though, as background radiation levels are minute.

This background radiation is only a nuisance because dark matter interacts with the detectors so rarely

Because dark matter interacts so rarely with ordinary matter it’s really like searching for a needle in a haystack. But for Dr Liu’s situation, the haystack is massive.

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The solution to this can be likened to Russian dolls because it’s all about shielding the detector. Sometimes this shielding is called a castle. Castles are built around the detectors to protect them from background radiation.

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Above is an image of Russian dolls.

https://en.wikipedia.org/wiki/Matryoshka_doll

Matryoshka dolls also known as babushka dolls, stacking dolls, nesting dolls, Russian tea dolls, or Russian dolls are a set of wooden dolls of decreasing size placed one inside another. The name matryoshka, literally “little matron”, is a diminutive form of Russian female first name “Matryona” (Матрёна) or “Matryosha”

https://en.wikipedia.org/wiki/Lead_castle

A lead castle, also called a lead cave or a lead housing, is a structure composed of lead to provide shielding against gamma radiation in a variety of applications in the nuclear industry and other activities which use ionizing radiation.

The ZEPLIN-III apparatus in the Boulby mine laboratory had a lead castle.

https://helenthehare.org.uk/2020/12/31/tour-of-the-boulby-mine/

Physicists don’t want to build their detectors on the surface of Earth because the Earth’s surface is continually bombarded by millions of background events caused by cosmic radiation from space.

https://en.wikipedia.org/wiki/Cosmic_ray

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Cosmic rays are high-energy protons and atomic nuclei which move through space at nearly the speed of light. They originate from the sun, from outside of the solar system, and from distant galaxies. They were discovered by Victor Hess in 1912 in balloon experiments.

Upon impact with the Earth’s atmosphere, cosmic rays can produce showers of secondary particles that sometimes reach the surface.

https://en.wikipedia.org/wiki/Victor_Francis_Hess

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Victor Franz Hess (24 June 1883 – 17 December 1964) was an Austrian-American physicist, and Nobel laureate in physics, who discovered cosmic rays. The image above right shows Hess in the balloon basket.

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The Earth is bombarded by about 1.7 million events every second. So, a detector at the surface of the Earth would be swamped by background radiation. So, to get over this problem the laboratories are built underground as this saves on the expense of having to build huge very expensive shields for any detectors on the surface. And in this case, it’s really a game of location, location, location. because there are only very few deep underground laboratories around the world, maybe just about 10.

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The table above is a list of all the world’s underground labs stating their depth, size, and floor space.

There are more than 10 listed, but not all the labs are available to host a dark matter detector. Some are decommissioned and others are dedicated to a single experiment like ice cube.

https://en.wikipedia.org/wiki/IceCube_Neutrino_Observatory

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The IceCube Neutrino Observatory (or simply IceCube) is a neutrino observatory constructed at the Amundsen–Scott South Pole Station in Antarctica. The project is a recognized CERN experiment (RE10). Its thousands of sensors are located under the Antarctic ice, distributed over a cubic kilometre.

The UK is very fortunate in having the Boulby underground laboratory.

https://www.boulby.stfc.ac.uk/Pages/home.aspx

https://www.boulby.stfc.ac.uk/Pages/Overview-of-the-Laboratory.aspx

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The Boulby Underground Laboratory is located at Boulby Mine, between Saltburn and Whitby on the North East coast of England and on the edge of the North Yorkshire Moors. Boulby is a worki​ng potash, polyhalite and rock-salt mine operated by ICL-UK. Reaching 1,400m at its deepest point, it is the deepest mine in Great Britain.

The mine is a major local employer with about 700 direct and 3000 indirect employees.

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The rock salt, potash and polyhalite are Permian evaporite layers left over from the Zechstein sea.

https://en.wikipedia.org/wiki/Salt

Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of salts; salt in its natural form as a crystalline mineral is known as rock salt or halite.

https://en.wikipedia.org/wiki/Potash

Potash includes various mined and manufactured salts that contain potassium in water-soluble form.

https://en.wikipedia.org/wiki/Polyhalite

Polyhalite is an evaporite mineral, a hydrated sulfate of potassium, calcium and magnesium with formula: K2Ca2Mg(SO4)4·2H2O.

https://en.wikipedia.org/wiki/Permian

The Permian is a geologic period and system which spans 47 million years from the end of the Carboniferous period 298.9 million years ago (Mya), to the beginning of the Triassic period 251.902 Mya

https://en.wikipedia.org/wiki/Zechstein

The Zechstein is a unit of sedimentary rock layers of Middle to Late Permian (Guadalupian to Lopingian) age located in the European Permian Basin which stretches from the east coast of England to northern Poland.

The evaporite rocks of the Zechstein formation were laid down by the Zechstein Sea, an epicontinental or epeiric sea that existed in the Guadalupian and Lopingian epochs of the Permian period. The Zechstein Sea occupied the region of what is now the North Sea, plus lowland areas of Britain and the north European plain through Germany and Poland. At times the Zechstein Sea may have connected with the Paleotethys Ocean through southeastern Poland; the point is disputed by researchers.

About 40km of tunnels are mined each year, the roadways being cut in the lower rocksalt layers.

Mining has gone on in the mine for the last 30 to 40 years.

The Boulby mine hosts the dark matter laboratory and Dr Liu has worked there since 2013. He regards it as one of the best facilities in the world.

The photo below was taken outside the Boulby underground lab and it won a photography award.

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Shining a light on dark matter at STFC’s Boulby Underground Laboratory (Credit: STFC/Simon Wright)

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The above image shows the arrangement of the underground laboratory facilities. It is a very nice environment to build an underground science facility because the rock salt is very stable.

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https://www.boulby.stfc.ac.uk/Pages/Overview-of-the-Laboratory.aspx

Being about 1.1km below ground cosmic radiation levels are reduced by a factor of a million or more.

So, it’s a very good place to do rare event science searches.

Physicists have been building laboratories and searching for dark matter since the 1980s with a range of dark matter experiments such as NaIAD, Drift and the ZEPLIN program.

https://www.hep.shef.ac.uk/research/dm/naiad.php

https://en.wikipedia.org/wiki/UK_Dark_Matter_Collaboration

The UK Dark Matter Collaboration (UKDMC) (1987–2007) was an experiment to search for Weakly interacting massive particles (WIMPs). The consortium consisted of astrophysicists and particle physicists from the United Kingdom, who conducted experiments with the ultimate goal of detecting rare scattering events which would occur if galactic dark matter consists largely of a new heavy neutral particle. Detectors were set up 1,100 m underground in a halite seam at the Boulby Mine in North Yorkshire.

In 1996 they published limits that were obtained using room temperature crystals. NAIAD was an array of NaI(Tl) crystals used in the detector and ran from 2000 to 2003, collecting 44.9 kg x years of exposure, setting spin-independent and spin-dependent limits on WIMPs.

The NaIAD experiment currently holds the world’s best limit on the spin-dependent WIMP-nucleon cross-section.

https://en.wikipedia.org/wiki/Directional_Recoil_Identification_from_Tracks

The Directional Recoil Identification from Tracks (DRIFT) detector is a low-pressure negative ion time projection chamber (NITPC) designed to detect weakly interacting massive particles (WIMPs) – a prime dark matter candidate.

There are currently two DRIFT detectors in operation. DRIFT-IId, which is located 1100m underground in the Boulby Underground Laboratory at the Boulby Mine in North Yorkshire, England, and DRIFT-IIe, which is located on the surface at Occidental College, Los Angeles, CA, USA.

The DRIFT collaboration ultimately aims to develop and operate an underground array of DRIFT detectors for observing and reconstructing WIMP-induced nuclear recoil tracks with enough precision to provide a signature of the dark matter halo.

https://en.wikipedia.org/wiki/ZEPLIN-III

The ZEPLIN-III dark matter experiment attempted to detect galactic WIMPs using a 12 kg liquid xenon target. It operated at the Boulby Underground Laboratory (North-East England, UK) in the period 2006–2011. This was the last in a series of xenon-based experiments in the ZEPLIN programme pursued originally by the UK Dark Matter Collaboration (UKDMC). The ZEPLIN-III project was led by Imperial College London and also included the Rutherford Appleton Laboratory and the University of Edinburgh in the UK, as well as LIP-Coimbra in Portugal and ITEP-Moscow in Russia. It ruled out cross-sections for elastic scattering of WIMPs off nucleons above 3.9 x 10−8 pb (3.9 x 10−44 cm2) from the two science runs conducted at Boulby (83 days in 2008 and 319 days in 2010/11).

As dark matter researchers fail to find it, they build bigger detectors, and bigger detectors need bigger laboratories to house them.

The first dark matter lab at Boulby was literally a garden shed (see below) built inside the rock salt tunnel with the detector inside the tent.

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The facility has increased in size over the years with a series of increasingly larger laboratories being built. The latest lab was completed in 2017.

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It is a really impressive facility, but one aspect of the old lab that Dr Liu misses is the fire axe. If there was a fire in the old lab the fire axe was for anyone in there to hack their way out of the wall to escape.

He never got to use it because there has never been a fire in the Boulby mine. In fact, so he was told, it’s safer than average office.

So, what he lost in an axe fire escape he gained in an incredible new underground luxury space laboratory.

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A class 10,000 clean room means that the air inside the room is incredibly clean and free of dust.

https://en.wikipedia.org/wiki/Cleanroom

A cleanroom or clean room is a facility ordinarily utilized as a part of specialized industrial production or scientific research, including the manufacture of pharmaceutical items, integrated circuits, CRT, LCD, OLED and microLED displays. Cleanrooms are designed to maintain extremely low levels of particulates, such as dust, airborne organisms, or vaporized particles. Cleanrooms typically have a cleanliness level quantified by the number of particles per cubic meter at a predetermined molecule measure. The ambient outdoor air in a typical urban area contains 35,000,000 particles for each cubic meter in the size range 0.5 μm and bigger in measurement, equivalent to an ISO 9 cleanroom, while by comparison an ISO 1 cleanroom permits no particles in that size range and just 12 particles for each cubic meter of 0.3 μm and smaller.

The laboratory has a dedicated low background area for building very clean detector components.

There is a class 1000 clean room which is a factor of 10 cleaner again than the general lab and it consists of 4000 cubic metres of experimental space. It’s just really incredible to have this super clean modern facility inside the salty mine tunnel.

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It’s an incredible place to do science and the researchers like working alongside the mining community. The mine is very helpful in supporting the researchers with their emergency health and safety needs, material transportation and just general facility requirements.

When Dr Liu started his PhD, his supervisor told him that working in the mine was like going to Mars. He thought it was a joke. It wasn’t.

Boulby is a Mars and Moon analogue. The caves are being used as a teaching aid for exploration of the Moon and Mars.

NASA felt it was like Mars which is why they are involved in doing some tests of their Mars rovers inside the salt tunnel, because it’s incredibly salty, dry and hot.

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There is a huge range of science going on at Boulby besides the search for dark matter. One of these is the astrobiology and Mars analogue programs

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During the exploration of the mine researchers discovered living things living in the salt crystals. This incredible considering that the salt has been underground for more than 230 million years.

The rover research program is MINAR.

https://www.astrobiology.ac.uk/news

https://www.astrobiology.ac.uk/research/astrobiological-analogues/minar

The UKCA uses its underground astrobiology laboratory at Boulby to carry out its own analogue programme called MINAR (Mine Analogue Research) which has been running since 2012. The programme focuses on the testing and development of planetary science instrumentation in an active mine with a view to improving links between planetary science and the mining community as well as carrying out planetary science and astrobiology.

https://www.youtube.com/watch?v=TY83GuzDHMw

https://atmospheres.research.ltu.se/pages/news/kore_boulby.php

MINAR holds yearly themed workshops for astrobiologists, geologists, biologists etc.

https://www.astrobiology.ac.uk/research/astrobiological-analogues/minar/minar-2017

https://www.liebertpub.com/doi/pdf/10.1089/ast.2017.1713

In 2017 they held EVA sampling (extravehicular activity), ESA astronaut training (for up to 35 people underground), discovered life in brine and tested the first underground rover

https://en.wikipedia.org/wiki/Extravehicular_activity

Extravehicular activity (EVA) is any activity done by an astronaut or cosmonaut outside a spacecraft beyond the Earth’s appreciable atmosphere.

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MINAR involves large teams (usually around 30 people) from many institutions, including NASA and ESA, from around the world.

https://en.wikipedia.org/wiki/NASA

The National Aeronautics and Space Administration is an independent agency of the U.S. federal government responsible for the civilian space program, as well as aeronautics and space research.

https://en.wikipedia.org/wiki/European_Space_Agency

The European Space Agency is an intergovernmental organisation of 22 member states dedicated to the exploration of space.

The centre for astrobiology in the UK is in Edinburgh.

https://www.astrobiology.ac.uk/

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5820684/

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One of Dr Liu’s favourite things was starting an outreach programme involving the Boulby team

https://scratch.mit.edu/users/Remote3/

https://gtr.ukri.org/projects?ref=ST%2FT005629%2F1

https://ras.ac.uk/sites/default/files/posters/Remote3_RAS%20-%20Xin%20Ran%20Liu.pdf

It’s called the Remote cubed project and its aim is to deliver much needed STEM outreach to some of the most remote areas of Scotland.

https://en.wikipedia.org/wiki/Secondary_education_in_Scotland

The project is aimed at S1 and S2 students (the first and second years of secondary school) who are between the ages of 11 and 14. Ten high schools are participating in the first round of the project. Each High School has a team of four to six students and the schools design, build, and program their miniature Mars rover, which they then send to the staff involved at the Boulby underground laboratory, to explore the STFC Mars yard.

https://stfc.ukri.org/

Mars yard is 1.1km underground and the schools control their rover remotely, which is not unlike what happens for real. It’s a mission to Mars scenario

https://mars.nasa.gov/mars2020/

NASAs latest rover is due to arrive on Mars in February 2021.

https://en.wikipedia.org/wiki/Curiosity_(rover)

Curiosity is a car-sized Mars rover designed to explore the Gale crater on Mars as part of NASA’s Mars Science Laboratory (MSL) mission.

The rover is still operational, and as of January 15, 2021, Curiosity has been on Mars for 3001 sols (3083 Earth days) since landing on August 6, 2012.

https://en.wikipedia.org/wiki/Timeline_of_Mars_Science_Laboratory#Current_status

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As mentioned earlier, Boulby is home to lots of exciting experiments besides dark matter. But Dr Liu returned to the topic of dark matter

Back to dark matter

Boulby has been home to lots of dark matter experiments over the last twenty years and one of the most exciting programs was the ZEPLIN program.

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https://www.sciencedirect.com/science/article/pii/S2212686414000223

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The above image is a summary for spin-independent WIMP–nucleon scattering results. Existing limits from the noble gas dark matter experiments ZEPLIN-III, XENON10, XENON100, and LUX, along with projections for DarkSide-50, LUX, DEAP3600, XENON1T, DarkSide G2, XENONnT (similar sensitivity as the LZ project) and DARWIN are shown. DARWIN is designed to probe the entire parameter region for WIMP masses above ∼6 GeV/, until the neutrino background (yellow region) will start to dominate the recoil spectrum. Experiments based on the mK cryogenic technique such as SuperCDMS and EURECA have access to lower WIMP masses.

Boulby pioneered a technology called 2-phase Xenon dark matter detection which uses xenon in both liquid and gaseous phases. It is a very successful world leading technology.

Bouldby now hosts DRIFT (mentioned above), provides research and development for CYGNUS and provides ULB (ultra-low background) material screening for other studies including LUX-ZEPLIN (LZ)

https://indico.cern.ch/event/699961/contributions/3056787/attachments/1694378/2726925/Spooner-CYGNUS-IDM2018v1.pdf

CYGNUS is a multi-latitude directional WIMP experiment.

https://zenodo.org/record/3906362#.YAGlYej7RaQ

The CYGNUS Directional Dark Matter Experiment and Neutrino-Nucleus Scattering

https://www.saqn.org/wp-content/uploads/2020/01/Boulby_SAQN_2020_short-for-web.pdf

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https://en.wikipedia.org/wiki/LZ_experiment

The LUX-Zeplin (LZ) experiment is a WIMP detector. The international collaboration constructing it formed in 2012 by combining the LUX and ZEPLIN groups.

https://en.wikipedia.org/wiki/Large_Underground_Xenon_experiment

The Large Underground Xenon experiment (LUX) aimed to directly detect weakly interacting massive particle (WIMP) dark matter interactions with ordinary matter on Earth.

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The Large Underground Xenon experiment, at Sanford Underground Laboratory, installed 1,480 m underground inside a 260 m3 water tank shield. The experiment was a 370 kg liquid xenon time projection chamber that aimed to detect the faint interactions between WIMP dark matter and ordinary matter.

The links below are videos showing the journey deep underground to the labs, but sped up

https://www.facebook.com/590198364358499/videos/1549440351872479

https://www.facebook.com/590198364358499/videos/1919306968114292

https://www.youtube.com/watch?v=hinSx59xYpc

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Everybody is wearing hi-vis overalls because it is very dark underground and makes it easier to be seen. There’s no natural light and in parts the only light is what they are carrying. They are also wearing steel toed boots, hard hats with head lights and shin guards.

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Waiting for the lift (left) and in the lift (right).

The journey in the lift takes seven minutes.

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Switching on the headlamps and off down the first tunnel. It is a 600m walk to the laboratories

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Dr Liu in his very fetching orange ensemble

Once the researchers get to the laboratories they need to get changed because the labs need to be kept very clean and the mine is dirty. The “outdoor” overalls are covered in dirt, salt and dust

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Dr Liu wearing the clean room suit, clean helmet and shoe covers so that no dust gets trampled into the labs. The boots are also air sprayed down before the researcher enters the lab.

Brief tour of the lab

https://www.facebook.com/590198364358499/videos/339934223427300

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The tour starts where the little arrow is on the above image. It is the changing rooms. The entrance door from the dark, dusty, salty mine. Clothes have to be changed so that this dirt isn’t taken into the laboratories.

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The above image shows the door from the changing rooms

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A super clean area which happens to be the kitchen. In Dr Liu’s opinion, one of the most important rooms as coffee “fuels the science”

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The UK’s deepest toilet? Before they built the new science facility, they didn’t have a dedicated indoors toilet so if anyone needed the “bathroom” they had to leave the lab and find a dark area in the tunnels.

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Leaving the kitchen. Above shows the main lab space.

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Immediately left of the main lab space is an even cleaner area where the detector components, that actually go into constructing the dark matter detectors, are measured. The components need to be handled in a very ultra-clean environment to measure any radiation they are giving off individually, so that the researchers know how much radiation the detectors are showing is down to the components and how much is being recorded from any “events”.

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The above image shows a nitrogen generation facility. Nitrogen is used to purge detectors and liquid nitrogen is used to keep some of the detectors cold.

https://en.wikipedia.org/wiki/Nitrogen

Nitrogen is the chemical element with the symbol N and atomic number 7.

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As with all good science experiments there are lots of cabling, electronics and tech.

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Above is an area for the Cygnus experiment

The Cygnus Collaboration is made up of several research groups which are working towards directional dark matter detection. It is world-wide

https://indico.cern.ch/event/699961/contributions/3056787/attachments/1694378/2726925/Spooner-CYGNUS-IDM2018v1.pdf

https://www.phys.hawaii.edu/cygnus/

The collaboration was formed through a series of directional workshops (2007 Boulby, UK; 2009 MIT, USA; 2011 Modane, France; 2013 Toyama, Japan; 2015 Los Angeles, USA; 2017 JinPing, China; 2018 l’Aquila, Italy) and is a world wide effort with members from Australia, China, Italy, Japan, Spain, United Kingdom, United States and more.

Apparently, Cygnus was designed by Takaaki Kajita, and it is still operational at Boulby.

https://en.wikipedia.org/wiki/Takaaki_Kajita

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Takaaki Kajita (born 9 March 1959) is a Japanese physicist, known for neutrino experiments at the Kamiokande and its successor, Super-Kamiokande. In 2015, he was awarded the Nobel Prize in Physics jointly with Canadian physicist Arthur B. McDonald.

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The above image shows a pile of lead.

Lead is a very dense material, which is why it’s so heavy, and that’s why it’s such a good material to build shields from. It is very effective at absorbing radiation.

https://en.wikipedia.org/wiki/Lead

Lead is a chemical element with the symbol Pb (from the Latin plumbum) and atomic number 82.

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The above image shows the castle from the ZEPLIN III detector. Inside the stainless-steel bars there are bars of lead.

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Cross-sectional view of ZEPLIN-III experiment (CAD) illustrating WIMP and neutron interactions in the liquid xenon target and neutron capture and detection in the anti-coincidence veto detector

ZEPLIN-III experiment: the WIMP detector, built mainly out of copper, included two chambers within a cryostat vessel: the upper one contained 12 kg of active liquid xenon; an array of 31 photomultipliers operated immersed in the liquid to detect prompt scintillation as well as delayed electroluminescence from a thin gas layer above the liquid. The lower chamber contained liquid nitrogen to provide cooling. The detector was surrounded by Gd-loaded polypropylene to moderate and capture neutrons, a potential source of background. The gamma-rays from neutron capture were detected by 52 modules of plastic scintillator placed around the moderator. The shielding was completed by a 20-cm thick lead castle.

The castle provided a shield as being underground isn’t enough. Underground provides a shield from radiation coming from space but the mine, itself, produces radiation

And you’re, you have to shield yourself from the small trace amounts of radiation that’s in the lab again. So that’s why within that’s why this Russian doll effect you go underground.

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Above shows the journey, so far, from the kitchen.

The Russian doll arrangement again. A lab is built to shield from the mine radiation, a castle is built to shield the detector from lab radiation. But even inside the castle there are cleaner inner shells, each shell cleaner than the previous shell to protect the equipment from any stray radiation. At the centre is the detector which is the coolest part of the entire experiment.

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Above, moving further from the kitchen.

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Part of a previous ZEPLIN experiment no longer used. It reminds the researchers of the great dark matter heritage.

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Tuning left. Equipment being tested for an experiment that will be hosted at Boulby. This area is an advanced instrument station test facility for a future experiment.

https://www.boulby.stfc.ac.uk/Pages/Particle-Physics-and-Low-Background-Science.aspx

AIT-NEO

NEO (Neutrino Experiment One) is a 6-tonne antineutrino detector with the initial​​ focus of reactor monitoring for nuclear non-proliferation goals. Antineutrinos are a natural by-product of fission reactor activity, and owing to their highly penetrative nature, serve as an irreducible signal by which reactors can be detected from great distances. The first of a number of planned detectors, NEO’s initial aim is to detect antineutrinos from the nearby Hartlepool Power Station using methods that are​ scalable to larger fiducial volumes.

http://www.hep.ph.bham.ac.uk/general/seminars/slides/Sean-Paling-2020.pdf

A WATer CHerenkov Monitor of Antineutrinos

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The idea is to be able to monitor remotely and see if people are enriching plutonium when you don’t want them to.

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Above is a right turn from the main hall and the tour is finished.

Since ZEPLIN III finished research has expanded and Boulby is now collaborating with Sandford underground research facility to build larger detectors.

https://en.wikipedia.org/wiki/Sanford_Underground_Research_Facility

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The Sanford Underground Research Facility (SURF), or Sanford Lab, is an underground laboratory in Lead, South Dakota. The deepest underground laboratory in the United States, it houses multiple experiments in areas such as dark matter and neutrino physics research, biology, geology and engineering. There are currently 28 active research projects housed within the facility, 24 of which include professors, undergraduate and graduate students from South Dakota universities.

Boulby and Sandford (and other collaborators) are building a larger xenon dark matter detector and it was called Lux, but that one was a hosted in the Sanford underground

http://lz.ac.uk/lz-experiment/about-us/#

https://en.wikipedia.org/wiki/LZ_experiment

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https://lz.lbl.gov/

The LZ detector will employ a 7-tonne liquid xenon target to search for the rare interactions of these particles with ordinary atoms in the detector medium. The experiment will be located one mile underground at the Sanford Underground Research Facility (Lead, South Dakota, USA).

http://lz.ac.uk/lz-experiment/how-it-works/

Boulby, is of course, an active salt mine whereas the Sanford underground research facility is an abandoned goldmine. It was once the largest goldmine in North America, but all mining has now stopped. The tunnels had been flooded, but the water was removed to build the underground science facility there.

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A photo taken by Dr Liu in November 2019. The temperature was about minus 20 degrees Celsius at this point, but it was a glorious sunny day.

It shows the Black Hills of Dakota, which are not very black in the picture.

From the surface it takes eleven minutes to reach the underground dark matter detector built there.

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The above image shows a cross section of the LUX detector, which uses Xenon.

https://en.wikipedia.org/wiki/Xenon

Xenon is a chemical element with the symbol Xe and atomic number 54. It is a colourless, dense, odourless noble gas found in Earth’s atmosphere in trace a Liquid xenon is used in calorimeters to measure gamma rays, and as a detector of hypothetical weakly interacting massive particles, or WIMPs. When a WIMP collides with a xenon nucleus, theory predicts it will impart enough energy to cause ionization and scintillation. Liquid xenon is useful for these experiments because its density makes dark matter interaction more likely and it permits a quiet detector through self-shielding. mounts.

https://en.wikipedia.org/wiki/Calorimeter_(particle_physics)

In particle physics, a calorimeter is an experimental apparatus that measures the energy of particles. Most particles enter the calorimeter and initiate a particle shower and the particles’ energy is deposited in the calorimeter, collected, and measured. The energy may be measured in its entirety, requiring total containment of the particle shower, or it may be sampled. Typically, calorimeters are segmented transversely to provide information about the direction of the particle or particles, as well as the energy deposited, and longitudinal segmentation can provide information about the identity of the particle based on the shape of the shower as it develops. Calorimetry design is an active area of research in particle physics.

When a dark matter particle comes in and hits a xenon atom a burst of light is produced.

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As ions drift into the gaseous phases there is a secondary burst of much brighter light. The time difference between the two flashes of light gives the position of where the interaction happened.

At the top and bottom of the apparatus are light sensitive detectors called PMT is and what they do is they’re able to detect tiny flashes single photons of light.

https://en.wikipedia.org/wiki/Photomultiplier_tube

Photomultiplier tubes (photomultipliers or PMTs for short), members of the class of vacuum tubes, and more specifically vacuum phototubes, are extremely sensitive detectors of light in the ultraviolet, visible, and near-infrared ranges of the electromagnetic spectrum. These detectors multiply the current produced by incident light by as much as 100 million times or 108 (i.e., 160 dB), in multiple dynode stages, enabling (for example) individual photons to be detected when the incident flux of light is low.

Depending on where the interaction between the xenon atom and dark matter particle happened in the detector, you can reconstruct the x, y and z position of the interaction.

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The detector is inside a water tank because tap water is very good at absorbing radiation coming from the rock caverns.

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Dark matter doesn’t interact with anything very often. It can pass through 1.6km of rock and water and will still get into the detector, where it can be seen with its interaction with xenon atoms.

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https://www.youtube.com/watch?v=bKCsiK4ZZBY

https://www.youtube.com/watch?v=R11Z5n3kfYY

One of the reasons why the detector has to be very big is because interactions happen so rarely and researchers don’t want to have lots of background signals to drown out the desired signal.

What have the physicists seen so far?

Unfortunately, the honest answer is, nothing.

But the researchers are not disheartened by that.

https://www.latimes.com/science/sciencenow/la-sci-sn-lux-dark-matter-final-results-20160721-snap-story.html

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The above is a headline from July 2016

The current landscape

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Dark matter publications/ science journals often show extrusion regions. So, when researchers don’t see any dark matter, they often say the detector excluded some region.

A way of describing this in everyday terms is to imagine a magical forest where you are looking for unicorns.

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Researchers are searching a “forest” for a mythical dark matter particle (the magic unicorn) but they haven’t seen any in that part of the forest.

Maybe it’s still in the area indicated above, but larger detectors are needed to probe further because the easier parts of the forest have already been explored.

One of the axes on the above graph is telling you the target size (y axis) and the other is the size of the unicorn in terms of its mass (x axis).

In order to probe further and look for dark matter in these, as yet, unexplored regions of the “forest” bigger detectors are needed.

This is what has been seen at Boulby, where the research started off in garden shed and is now 4000 cubic metres of beautiful lab space.

The search for dark matter has been an evolutionary process.

Liquid Xenon experiments

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Typical detector improvement:

Increased mass; Detector components; Detection and analysis methods; Improved background removal.

The Lux detector used about 250kg of xenon. There are other experiments around the world also trying to find dark matter using similar technology as Dr Liu’s team. Lux ran in 2013.

https://en.wikipedia.org/wiki/Large_Underground_Xenon_experiment

Then there was the PANDAX experiment in China (completed in 2015) that used double the amount of xenon.

https://en.wikipedia.org/wiki/PandaX

The Particle and Astrophysical Xenon Detector, or PandaX, is a dark matter detection experiment at China Jinping Underground Laboratory (CJPL) in Sichuan, China. The experiment occupies the deepest underground laboratory in the world, and is among the largest of its kind.

Then XENON1T used over a tonne of xenon in their detector and that operated in 2017 so those the last large dogmatic detector to take data.

https://en.wikipedia.org/wiki/XENON

The XENON dark matter research project, operated at the Italian Gran Sasso National Laboratory, is a deep underground research facility featuring increasingly ambitious experiments aiming to detect dark matter particles. The experiments aim to detect particles in the form of weakly interacting massive particles (WIMPs) by looking for rare interactions via nuclear recoils in a liquid xenon target chamber. The current detector consists of a dual phase time projection chamber (TPC).

Construction of XENON1T started in Hall B of the Gran Sasso National Laboratory in 2014. The detector contains 3.2 tonnes of ultra radio-pure liquid xenon, and has a fiducial volume of about 2 tonnes. The detector is housed in a 10 m water tank that serves as a muon veto. The TPC is 1 m in diameter and 1 m in height.

The detector project team, called the XENON Collaboration, is composed of 135 investigators across 22 institutions from Europe, the Middle East, and the United States.

Upper limit for spin-independent WIMP-nucleon cross section according to recent data (published Nov. 2017)

The first results from XENON1T were released by the XENON collaboration on May 18, 2017, based on 34 days of data-taking between November 2016 and January 2017. While no WIMPs or dark matter candidate signals were officially detected, the team did announce a record low reduction in the background radioactivity levels being picked up by XENON1T.

Two larger detectors are in the process of being constructed

And now we’re all in the process of construction. The xenon experiments are being upgraded.

XENONnT is an upgrade of the XENON1T experiment underground at LNGS. Its systems will contain a total xenon mass of more than 8 tonnes. Apart from a larger xenon target in its time projection chamber the upgraded experiment will feature new components to further reduce or tag radiation that otherwise would constitute background to its measurements. It is designed to reach a sensitivity (in a small part of the mass-range probed) where neutrinos become a significant background. As of 2019, the upgrade is on-going and first light is expected in 2020 (although the Covid crisis is likely to put the project back).

As mentioned earlier, LUX has now joined with the UK dark matter group to form LZ. The collaborators are in the process of building a seven-tonne dark matter detector underground. It is due to come online sometime in 2021.

So, the typical method of improving detection sensitivity is to make the detectors bigger, make the components cleaner and improve the analysis detection techniques.

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The above image gives some indication of how the detector size has changed. On the left is the LUX detector that was in the Sanford underground research facility (it’s now in a museum). On the right is the new LZ detector (the upgrade to LUX and ZEPLIN).

The images show how much bigger the next generation of dark matter detectors will be.

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https://darwin.physik.uzh.ch/

DARWIN – a next-generation liquid xenon observatory for dark matter and neutrino physics

https://indico.cern.ch/event/577856/contributions/3420279/attachments/1877626/3092486/DARWIN_EPS-HEP2019_Kevin_Thieme.pdf

https://www.youtube.com/watch?v=W_RtH0FVV20

Further detectors are still planned in case XENONnT and LZ doesn’t see dark matter.

More research groups will join forces and build ever larger detectors.

In the case of DARWIN 50 tonnes of liquid xenon would be needed, which would definitely affect the quantity on the planet.

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LUX-ZEPLIN (LZ) is currently under construction.

As well as sourcing the quantities of xenon needed, methods of keeping away any dust or contamination from the surface of these detectors is a major challenge.

Another challenge is getting the detector underground and into its protective shielding without introducing any contamination.

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Each one of these orange dots on the white circle is a photomultiplier tube. They are the tiny eyes of the detector and they’re able to spot the tiniest flashes of light coming from a possible dark matter interaction.

The architecture is now underground.

https://lz.lbl.gov/

Below shows all the people (in 2017) involved with the LZ collaboration. There are about 270 physicists, engineers and technicians from 36 different institutes around the world

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As these experiments grow larger so do the collaborations because lots of great people need to come together and build these detectors, because it’s no longer something one university or institute can do.

Collaborator meetings usually take place twice a year to discuss what progress has been made and where the research needs to go in the next, say, six months.

There is a visitor centre at Sanford, where you can see the LUX detector

It is a very exciting time to be part of the high energy physics world.

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For a long time there had been great theories, like Peter Higg’s boson theory in the 1960s, but it took until the beginning of the 21st century to produce a detector that could do the searches, test the theories and make the discoveries.

Researchers now think they have technology and the designs to really hunt for dark matter and the space for dark matter to be hiding is ever shrinking with each increasingly large experiment.

Particle physicists know that the standard model only explains about 4.9% of the Universe.

Dark matter which makes up 26.8% of the universe and the rest is composed of dark energy, which is even more mysterious than dark matter.

Dr Liu thinks the next decade is a very exciting time for particle physics.

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Dark matter day is always the 31st of October.

On and around October 31st, 2020 the UK deep underground lab at Boulby teamed up with their American counterpart, SURF

https://en.wikipedia.org/wiki/Sanford_Underground_Research_Facility.

Boulby pioneered the technology that SURF now uses and the two experiments have come together to create LUX-ZEPLIN (LZ).

Questions and answers

1) What is the percentage gender balance of the stem outreach in Scotland. I think it’s about remote cubed. I guess they were asking what percentage of the teams were female, I think, is that right?

Unfortunately, we launched our remote cubed project during this year’s MINAR event. So, we were underground with the NASA scientists when they’re doing their tests. We launched our little mini rover which roamed around bumping into the large rover.

Some schools managed to be involved so we launched it. However, it was done in the middle of March 2020, and, as everybody knows lockdown happened towards the end of March. We were just able to get the Lego Mindstorms kits out to the schools, but not all the schools had formed the teams. I think two or three of the schools had formed a team and one school had an entirely female team, but I don’t know the exact gender balance of the of the other teams because I think many of them unfortunately didn’t have time to get the team setup, logos designed and other stuff before the schools were closed.

2) If you haven’t found dark matter yet how do you know that xenon is the element to find it in and that it will release light when it does collide.

We know xenon releases flashes of light when its atoms collide with other forms of background radiation. Which is part of the problem. If we had it on the surface of the earth it would be flashing all the time and it would be really hard to convince anybody there was dark matter with so much background.

Xenon is chosen because it’s a noble gas and can be made it incredibly clean. It has a very heavy atomic mass which makes it a good candidate for dark matter searching

But it’s by no means the only noble gas dark matter detector out there. Others use argon and they are very successful. Xenon has just been a very popular material simply for the reasons mentioned above.

3) What colour is xenon scintillation?

The scintillation is just a burst of a few photons of light. Just the tiniest flash. I did read somewhere that human eyes can pick up just a few photons of light. So maybe instead of the PMT we can just have an array of PhD students looking at them.

But they are just tiny light flashes. I don’t think there’s any colour associated with them.

4) If LZ finds dark matter will scientists still build more dark matter detectors, or will that one, confirmed, find be enough.

I guess if we did find dark matter it would be very exciting and I’m sure we will still want to build larger detectors, because it’s not enough just to find it. We will also want to study its properties as it is an unknown 26% of the universe. We want to find out what it could be.

After its discovery the next thing we definitely want to find out is what its properties are and how it works.

I’m sure many, many more experiments will be built if we did find dark matter and there’d be a lot of interest in studying its property in nature.

5) We’ve had confirmation that xenon flashes wavelength is 175 nanometres in the far, ultraviolet.

I was just thinking that after I said it. Hey, I seem to recall that, but yes, thank you very much to the person who found that fact out.

6) Do you know of any specific ways for graduates to get into dark matter research.

I think we’re always looking for students to sign up after they have finished their undergraduate degrees.

If you have an interest you should look into doing a summer project. I know that the University of Edinburgh has lots of summer projects and you can work with our group here, or one of the many other dark matter groups around the country.

So, look, look out for these. I know that all the LZ collaborating universities in the UK like Edinburgh, Liverpool, Sheffield, UCL, Imperial, Bristol and other universities will have summer programmes.

I myself did an advanced high energy physics masters in particle physics at University College London because I was originally doing something else. I just wanted to refocus my research.

You could consider doing a PhD in the subject if you’re sure that it’s something that you’re really interested in. There’s no better way than doing a PhD in dark matter to really be on the cutting edge of the research.

7) Given that the Higgs boson gives mass to matter. Are you looking for an anti Higgs Boson for dark matter?

I am not. It seems like theory that’s beyond my work, so I can’t really answer that question.

8) How can you discriminate between cosmic rate impacts and dark matter interactions.

So, first of all, Sanford underground lab is very shielded. Cosmic rays are about 10 million times less there than the surface value, but they do occasionally get through. They do get into a detector and there’s various signatures to distinguish cosmic ray backgrounds, from true dark matter events.

The water tank has photomultiplier tubes in it and on the outer regions of the xenon there is a skin region which is also made of xenon itself.

Xenon forms the cleanest “shell” of the dark matter detector and protects the core xenon.

LZ will have about 7 tons of active liquid xenon in a cryostat surrounded by an additional thin region of xenon (‘skin’), liquid organic scintillator and water, all being viewed by photomultiplier tubes (PMTs). Xenon skin, organic scintillator and water will be used as an anticoincidence system to identify and reject events caused by various particles but WIMPs. Below is the schematic of the LZ detector.

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https://www.hep.shef.ac.uk/research/dm/lz.php

So, xenon is the cleanest part of the detector. As xenon is used in several placed of the detector it explains why the earlier diagrams quoted two value of the xenon required.

Many things could interact multiple times with the xenon. Ordinary matter might interact a couple of times a year in the detector and you can pretty sure that it’s not a dark matter event because dark matter interacts so rarely, we haven’t even seen an interaction once yet.

So, if it happens twice in your detector, then you can be pretty sure to rule it out as a dark matter event. And there’s other signatures similar to that which tells us that what we see is not a dark matter particle

9) How hot is it in in the underground labs and does the new Boulby lab have cooling?

The laboratory itself is air conditioned to a nice 20 degrees Celsius, but outside, I think, in the tunnel near the mine face where the miners work can get up to 42 degrees Celsius. So, it’s an incredibly hard environment to work in.

Recently some people successfully competed in the world’s deepest underground marathon, which is just truly incredible to me, that anybody can run that distance in such hot dry environment.

So outside it’s very hot, but it’s air conditioned inside the lab, Sanford is less hot.

At Boulby, you start almost at sea level on the surface and then you drop 1.1km underground, which is why it’s much hotter there. You’re closer to the core of the earth. But at Sanford the elevation of where you start is already one mile up. The Sanford underground research facility is based in Lead South Dakota and Lead is a-mile-high city. So, you start off a mile above sea level already. So, when you go one mile below ground, you’re still just really at sea level. So, it’s not very hot there and I think the tunnels are actually quite cold. When I was there last, I never felt I was hot when walking around the tunnels.

10) It’s amazing that people are running marathons in that heat.

Yes. I never run a marathon period, and I can’t imagine doing it in that heat. Yeah.

11) What happens to the toilet waste.

This question is a really popular question and comes up all the time. I think the waste is flushed into a storage tank which is then removed from the mine periodically,

Back in the old days when we just used the tunnel. I guess the waste remains in the tunnel.

I think in the old days of Sanford underground research facility in the US they used to have an incinerator toilet. Which is very exciting because I think you use the toilet as normal and when you come to flush it, it’s like the gates of Hell opens up and there’s just a huge fire pit underneath. The waste gets incinerated which is pretty terrifying, but they’ve thankfully replaced that now, with an ordinary toilet that pumps out of the facility. So very interesting challenges which you don’t think about usually on surface. But yeah, there are big problems underground that we all used to have.

Compost toilets in the tunnels did not work so well because I think for composting you need some level of moisture and because the tunnel is so hot and dry there wasn’t enough moisture.

I think the composting was not such an effective toilet method.

12) It’s been confirmed that the tanks do get sent out and then they come back down empty.

13) If dark matter exists. What is its predicted density?

Oh, good question. And I’ll have to double check that one. Since this talk is recorded. I don’t want to be pinned on record. But yeah, that one, I, I’m not sure. So, I will check and perhaps I could get back to whoever asked the question with an answer I know is correct.

14) Why is xenon used for detecting WIMPs?

WIMPS stands for weakly interactive massive particles. If we think we have a chance of detecting them, they have to interact via the weak force. If they don’t, then we are really stuck trying to look for them.

https://en.wikipedia.org/wiki/Weak_interaction

In nuclear physics and particle physics, the weak interaction, which is also often called the weak force or weak nuclear force, is the mechanism of interaction between subatomic particles that is responsible for the radioactive decay of atoms. It also participates in nuclear fission.

WIMPs should interact via the weak force and xenon is a good candidate because it’s a very clean noble gas with a high atomic mass making it a bigger target. It is a nice thing to crash into.

15) If you do detect dark matter, what’s next?

Good question. Well, I guess that is a trip to Sweden to receive a Nobel Prize. Well, not me, I’m sure, but the people who were the brains behind the LZ experiment. Well, a few champagnes corks would be popped.

First, the trip to Sweden for the Nobel Prize and then starting work on the detector to study the properties of dark matter.

I think we physicists are never really satisfied with just one discovery; we always want to find out more.

Just like when the atom was discovered. Research then found it was not the smallest thing. I’m sure physicists are already thinking and planning new experiments to further probe deeper into the makeup of the universe.

16) Someone has just popped on the chat that the expected density in our part of the galaxy is expected to be about three dark matter particles per pint glass.

That is an excellent measurement. I will not forget that anytime soon. Thank you.

17) When did you become interested in dark matter?

I had an interest in particle physics during my undergraduate physics degree at University College London and then I did a master’s project in particle physics. A PhD is a long commitment, three to four years, so I really wanted to make sure that it was something I wanted to do and then during the masters it was really fun working with other like-minded people in collaborations.

I did my masters during 2011/2012 and the collaborative nature of particle physics along with the fact that doesn’t just involve being stuck in a lab on your own but working with a group. Meeting people from all over the world with a shared, common goal of looking for dark matter. You get to go to conferences and look at competing rival experiments, many times.

You know, people are very happy to share ideas and collaboratively share knowledge. It’s just a very, very nice environment to work in. So, I would say, if you’re interested in dark matter, you will need to do a relevant undergraduate degree (although there would be apprenticeships for engineering or technical support) and in your final year do a related project in that area or do a masters and then if you’re if you’re still keen a PhD is a good way to go.

There may be summer programmes where you can help with research but if you want to be doing dark matter research on the cutting edge, then I would highly recommend a PhD in that area.

18) Does antimatter exist and if so, what is it?

Oh, it does exist. I did, I did a senior project on it.

So so anti matter is real. The idea is that matter and antimatter are created in pairs. So, whenever you create matter you create an anti-matter counterpart. But that leads to a huge problem. Why is our whole universe just made of matter? Where has all the anti-matter gone?

Anti-matter exists, but not quite in the same way as Angels and Demons, a book written by Dan Brown.

https://en.wikipedia.org/wiki/Angels_%26_Demons

image

Angels & Demons is a 2000 bestselling mystery-thriller novel written by American author Dan Brown and published by Pocket Books and then by Corgi Books. The novel introduces the character Robert Langdon, who recurs as the protagonist of Brown’s subsequent novels. Angels & Demons shares many stylistic literary elements with its sequels, such as conspiracies of secret societies, a single-day time frame, and the Catholic Church. Ancient history, architecture, and symbology are also heavily referenced throughout the book. A film adaptation was released on May 15, 2009.

If you go to hospital for a PET scan, antimatter is being used. PET stands for Positron Emission Tomography. The positron is positive and is the anti-matter form of the electron.

https://en.wikipedia.org/wiki/Positron

https://en.wikipedia.org/wiki/Positron_emission_tomography

image

Positron emission tomography (PET) is a functional imaging technique that uses radioactive substances known as radiotracers to visualize and measure changes in metabolic processes, and in other physiological activities including blood flow, regional chemical composition, and absorption.

Positrons are emitted by some radioactive materials. When they meet electrons, annihilation occurs producing gamma rays. These are detected by gamma cameras and an image is produced.

image

So, we are already using anti-matter in our everyday medicine.

An electron and a positron are exactly the same except the electron has a negative charge and the positron has a positive charge. When they come in contact with each other they annihilate each other releasing energy equivalent to the sum of their masses.

19) About 27% of the Universe is dark matter.

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