Particle physics – Beyond the standard model

Dark Matter searches

Dr Christopher McCabe


Dr McCabe is an STFC Ernest Rutherford Fellow and Lecturer in the Department of Physics at King’s College London. He obtained his DPhil in Theoretical Physics at the University of Oxford in 2011. He was a postdoctoral researcher at the Institute of Particle Physics Phenomenology (IPPP) at Durham University (2011-2014) and at the GRAPPA Centre of Excellence at the University of Amsterdam (2014-2017). He joined King’s College London in 2017. Dr McCabe works at the intersection of particle physics, cosmology and astrophysics to deepen our understanding of dark matter (DM).


How do you search for something that you can’t see? This is the challenge faced by experiments searching for dark matter, which we now know is the most abundant form of matter in the Universe. In this talk Dr McCabe discussed the different experimental techniques that groups around the world are employing to detect dark matter in labs on Earth and what these experiments can tell us about the particle physics properties of dark matter.

My notes from the lecture (if they don’t make sense then it is entirely my fault)

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 energy density. The majority of dark matter is thought to be non-baryonic in nature, possibly being composed of some as-yet undiscovered subatomic particles. Its presence is implied in a variety of astrophysical observations, including gravitational effects which cannot be explained by accepted theories of gravity unless more matter is present than can be seen. For this reason, most experts think dark matter to be abundant in the universe and to have had a strong influence on its structure and evolution. Dark matter is called dark because it does not appear to interact with observable electromagnetic radiation, such as light, and is thus invisible to the entire electromagnetic spectrum, making it extremely difficult to detect using existing astronomical instruments.

We have detected dark matter

The rotation curve of a disc galaxy (also called a velocity curve) is a plot of the orbital speeds of visible stars or gas in that galaxy versus their radial distance from that galaxy’s centre. It is typically rendered graphically as a plot, and the data observed from each side of a spiral galaxy are generally asymmetric, so that data from each side are averaged to create the curve. A significant discrepancy exists between the experimental curves observed, and a curve derived from theory. The theory of dark matter is currently postulated to account for the variance.


Rotation curve of spiral galaxy Messier 33 (yellow and blue points with error bars), and a predicted one from distribution of the visible matter (grey line). The discrepancy between the two curves can be accounted for by adding a dark matter halo surrounding the galaxy.


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

The Bullet Cluster (1E 0657-558) consists of two colliding clusters of galaxies. Strictly speaking, the name Bullet Cluster refers to the smaller subcluster, moving away from the larger one. It is at a co-moving radial distance of 1.141 Gpc (3.7 billion light-years).

Gravitational lensing studies of the Bullet Cluster are claimed to provide the best evidence to date for the existence of dark matter.


X-ray photo by Chandra X-ray Observatory. Exposure time was 140 hours. The scale is shown in megaparsecs. Redshift (z) = 0.3, meaning its light has wavelengths stretched by a factor of 1.3.

Recent studies have presented evidence for tension between measurements from the cosmic microwave background (CMB) and measurements of the growth of the large-scale structure of the Universe.

One of the tensions surrounds the measured value of Hubble’s constant, H0. Local estimates prefer a relatively high value of 73 ± 2 km s1 Mpc1 (Riess et al. 2016), whereas analysis of the CMB prefer a relatively low value of 67 ± 1 km s1 Mpc1 (Planck Collaboration XIII 2016).

A possible reason for this tension is dark matter.

A new theory of gravity might explain the curious motions of stars in galaxies. Emergent gravity, as the new theory is called, predicts the exact same deviation of motions that is usually explained by invoking dark matter. Prof. Erik Verlinde, renowned expert in string theory at the University of Amsterdam and the Delta Institute for Theoretical Physics, published a new research paper in which he expands his groundbreaking views on the nature of gravity.


Erik Peter Verlinde (born 21 January 1962) is a Dutch theoretical physicist and string theorist.

Collision of galaxy clusters

Collisions between galaxy clusters provide a test of the non-gravitational forces acting on dark matter


The galaxy 3C438 and its cluster of galaxies as seen in the optical (left) and in X-rays by the Chandra X-ray Observatory (right). Astronomers have concluded that the hot gas is the result of a collision between two clusters of galaxies. Credit: X-ray: NASA/CXC/CfA/R.P.Kraft; Optical: Pal.Obs. DSS

Clusters are believed to grow as the result of mergers between smaller galaxy groups and from the accretion of gas and dark matter. The energy released in these mergers is largely dissipated in the hot gas within the cluster, where X-ray observations can spot evidence for shocks and high temperatures. Mergers between two equally massive galaxy clusters provide particularly important diagnostics since these energetic collisions have the most dramatic and long-lasting effects. These major mergers are relatively rare events, however. The Bullet Cluster is one recently analysed example, and because it also happens to act as a gravitational lens for background galaxies, it became famous for showing the distribution of its dark matter.



Bullet cluster

Gas collided and formed a shock front. Dark matter passed straight through

fthumb ~ 107(mproton/mDM)

Dr McCabe explained what the above formula means “The equation is a shorthand way of writing that if the dark matter mass (mDM) is the same as the proton mass (mproton), then the flux of dark matter particles through your thumb is about 107 (ie 10 million) particles per second. You can use the equation to work out the flux for other dark matter masses (since we don’t currently know the dark matter mass). For instance, if the dark matter mass is 10 times larger than the proton mass, so that mproton/mdm = 1/10, then the flux of dark matter particles would be about 106 (i.e. 1 million) particles per second or, if the dark matter mass is 100 times smaller the proton mass, so that mproton/mdm = 100, then the flux of dark matter particles would be about 109 (ie 1 billion) particles per second.”

Visible stars embedded in a much larger dark matter halo

According to modern models of physical cosmology, a dark matter halo is a basic unit of cosmological structure. It is a region that has decoupled from cosmic expansion and contains gravitationally bound matter.


Simulations of galaxies, some involving dark matter

If we want to add a dark matter particle, X0, to the standard model we need to know its: mass; spin; parity; lifetime; scattering cross-section on nucleons; production cross section in hadron colliders; self-annihilation cross-section. Unfortunately at the moment we don’t know these things.

Steven Weinberg quote:

“Up to a point the stories of cosmology and particle physics can be told separately. In the end though, they will come together.”


Steven Weinberg ForMemRS (born May 3, 1933) is an American theoretical physicist and Nobel laureate in Physics for his contributions with Abdus Salam and Sheldon Glashow to the unification of the weak force and electromagnetic interaction between elementary particles.

Cosmology (WDMh2 = 0.120±0.001) + Particle physics (ℒ = ℒSM + mqχχ’qq’/Λ3 + …) = Suggests dark and visible matter interactions are generic

W is the dark matter density parameter and ℒ is the Lagrangian density parameter

There are many dark matter candidates


How do you search for something you can’t see?

You listen and feel

Direct detection – feel for “Massive” dark matter

There is a wind of dark matter χ travelling at 250kms-1. It collides with a nucleus at rest and some visible energy is seen


Evis ≈ 1 – 100keV ~ mdark matter

Event rate: few events / year

World’s most sensitive keV energy detectors are used

UK principal involvement: the LUX-ZEPLIN (LZ) detector

The LUX-Zeplin (LZ) experiment is a WIMP detector. The international collaboration constructing it formed in 2012 by combining the LUX and ZEPLIN groups. It is to be located at the Sanford Underground Research Facility (SURF) in South Dakota, and managed by DOE’s Lawrence Berkeley National Lab (Berkeley Lab). The LZ experiment is a next-generation dark matter direct detection experiment. When completed, the experiment will be the world’s most sensitive experiment for WIMPs (Weakly Interacting Massive Particles) over a large range of WIMP masses. In the spring of 2015, LZ passed the ‘Critical Decision Step 1’ or CD-1 review, and became an official DOE project. As of 2018, the installation of the experiment is expected 2019 and start of data collecting 2020.

LZ is a collaboration of 30 institutes in the US, UK, Portugal and Russia. Henrique Araújo from the Imperial College of London leads the UK team on LZ.


diagram of the former Homestake gold mine and laboratory’s spaces nearly a mile underground

LZ is a next generation dark matter experiment. LZ has been selected by the US Department of Energy (DOE) and the US National Science Foundation as one of the three ‘G2’ (for Generation 2) dark matter experiments.

Deep underground, scientists at Sanford Underground Research Facility search for answers to the most fundamental questions about the universe.


The above image shows access to the Davis Lab on the left and the Yates shaft cage on the right.


The LUX-Zeplin (LZ) experiment is one mile below the Sanford underground research facility. The reason for going underground is that it provides a shield against cosmic rays – they could fake a dark matter signal



View of the water tank – provides even more shielding. LZ is installed inside Image credit Carlos Faham

A fish-eye view of the water tank where the LUX detector was deployed, and where the LZ detector will be deployed. The diameter of the tank is 25 feet, the height 20 feet, and it is filled with highly purified water. It provides even more shielding.


Assistant Professor of Physics Luiz de Viveiros and the LUX detector inside the water tank just before it was filled with 72,000 gallons of ultra-purified water, which helps to shield it from cosmic rays and other radiation that would interfere with a dark matter signal. LUX was removed in 2016, and the tank will be used to house the LUX-ZEPLIN detector. IMAGE: MATT KAPUST, SANFORD UNDERGROUND RESEARCH FACILITY

What is the job of a theoretical physicist?

What these people do is to try to figure out how Nature works. That is, why the stars shine, why water is fluid and the sky is blue, what you are made of and why does “it” weigh that much, why the universe expands, or what energy and matter actually are…

Signal = Flux x cross section x detector response

Detectors used in elementary particle and nuclear physics are based on the principle to transfer radiation energy to detector mass. Charged particles are transferring their energy through collisions to atomic electrons leading to excitation and ionisation. In most cases, neutral particles have to produce charged particles first inside the detector volume which in turn are transferring their energy by excitation or ionisation to the detector. All these interaction processes are random processes.

Related to sensitivity is the detector response (function) to the radiation under study. Usually, the output signal of an electrical detector is a current pulse where the time integral of the signal corresponds to the amount of ionization produced by the particle-detector interaction. If the shape of the signal does not depend on the amount of ionisation the amplitude of the signal is a measure for the radiation energy deposited in the detector. If this relation is linear the response of the detector is called to be linear.

It is possible that a particle of defined energy leads to a spectrum of signal amplitudes. This is called the detector response function. E.g. photons with definite energy may interact with the detector material by Compton scattering resulting into a broad spectrum of deposited energies due to the subsequent interaction of the recoil electrons. In contrast, charged particles with definite energies which are stopped within the detector material will rather lead to a Gaussian distribution of signal amplitudes.

Particle flux, the rate of transfer of particles through a unit area ([number of particles] m−2·s−1)

In everyday speech, “cross section” refers to a slice of an object. A particle physicist might use the word this way, but more often it is used to mean the probability that two particles will collide and react in a certain way. For instance, when CMS physicists measure the “proton-proton to top-antitop” cross section, they are counting how many top-antitop pairs were created when a given number of protons were fired at each other.



1) The nucleus recoil energy is related to the dark matter mass

Evis ≈ 1 – 100keV ~ mdark matter

2. Rate of collisions tells you about the force between dark matter and visible matter

These experiments can tell us about:

1. Dark matter mass

2. Force between dark and visible matter (cross section)


Dr McCabe later explained to me

The above graphs are trying to explain in a cartoon version what I believe was shown on the next slide.

Firstly, they show the parameter space the experiments plots their results on: the dark matter cross-section (basically how likely it is for dark matter to interact with normal matter) vs the dark matter mass.
If an experiment makes a dark matter detection (discovery), they can use the results to narrow down the dark matter mass and the cross-section. If you had a perfect experiment, you may be able to reduce the oval to a single point, since you would exactly know the mass and cross-section. But experiments are generally not perfect so they find a range of masses and cross-sections that are compatible with the measurement.
If an experiment doesn’t make a detection (discovery), all they can say is that they didn’t measure dark matter with a certain cross-section (as a function of mass). So all you can do is build a better experiment to try and test smaller values of the cross-section.”


Simplified plot showing the predicted WIMP-nucleon scattering cross section as a function of WIMP mass. Asymmetric dark matter models (light blue area) predict WIMPs with masses of a few GeV/c2; generic WIMP models (dark blue area) predict larger masses of hundreds of GeV/c2 or more. Parts of these parameter regions have been probed and excluded by current experiments (red area). The signal excess of the CDMS experiment points towards relatively light WIMPs (green area). However, the absence of a signal in the LUX experiment is in tension with this result. Most of the expected parameter space could be probed in the near future, until a background from coherent neutrino-nucleus scattering (yellow area) becomes relevant, interfering with possible dark matter signals.

Dark matter has not been discovered yet


Limits and positive claims for WIMP scattering cross-sections

However dark matter can ionise atoms


When a WIMP – a hypothetical dark matter particle collides with a xenon atom, the xenon atom emits a flash of light (gold) and electrons. The flash of light is detected at the top and bottom of the liquid xenon chamber. An electric field pushes the electrons to the top of the chamber, where they generate a second flash of light (red). Photo: Matthew Kapust/Sanford Underground Research Facility

Directly detecting sub-GeV dark matter with electrons from nuclear scattering


Illustration of electron emission from nuclear recoils. If a DM particle scatters off a nucleus (panel 1), we can assume that immediately after the collision the nucleus moves relative to the surrounding electron cloud (panel 2). The electrons eventually catch up with the nucleus, but individual electrons may be left behind and are emitted, leading to ionisation of the recoiling atom (panel 3).

For ionisation you need the following energy ½mDMv2DM >~ Ebinding (~ 12eV) where the mass of dark matter is mDM >~ 5MeV/c2

Unfortunately no discovery yet

Haloscopes and quantum sensors

“Listen” for “ultra-light” dark matter

The axion is a hypothetical elementary particle originally postulated to solve the strong CP problem. The axion is also an extremely attractive dark matter candidate. The axion is the puzzle piece allowing these two mysteries to fit naturally into our understanding of the universe.

The most common axion detection concept is as follows: a cylindrical metal enclosure (called a cavity) is encased by an electromagnet, which provides a strong and uniform magnetic field throughout the cavity. The dimensions and internal structure of the cavity are tuned to be resonant with a specific microwave frequency. An antenna within the cavity samples the amount of microwave energy within the cavity.

If the resonant frequency of the cavity happens to match the frequency associated with the axion mass, excess power will be detected within the cavity (as a result of axion-photon conversion), as compared with the conditions when the magnet is off, or when the cavity is tuned to a different frequency.

Such a device is known as a “haloscope” (since it investigates the dark matter halo of our galaxy), and was proposed by the physicist Pierre Sikivie.

The Axion Dark Matter Experiment (ADMX, also written as Axion Dark Matter eXperiment in the project’s documentation) uses a resonant microwave cavity within a large superconducting magnet to search for cold dark matter axions in the local galactic dark matter halo. Unusually for a dark matter detector, it is not located deep underground. Sited at the Center for Experimental Nuclear Physics and Astrophysics (CENPA) at the University of Washington, ADMX is a large collaborative effort with researchers from universities and laboratories around the world.

Pierre Sikivie invented the axion haloscope in 1983. After smaller scale experiments at the University of Florida demonstrated the practicality of the axion haloscope, ADMX was constructed at Lawrence Livermore National Laboratory in 1995. In 2010 ADMX moved to the Center for Experimental Physics and Astrophysics (CENPA) at the University of Washington. Led by Dr. Leslie Rosenberg, ADMX is undergoing an upgrade that will allow it to be sensitive to a broad range of plausible dark-matter axion masses and couplings.

An axion enters the cavity within the haloscope, interacting with the magnetic field and converting into a microwave photon.


Scheme of the dark matter haloscope used in the ADMX experiment. To search for axions, the resonant frequency of a cavity (left) is shifted by moving tuning rods placed inside it (or some other structure). If the cavity’s resonant frequency (right) matched the frequency of photons coupled to axions, the rate of axion-to-photon conversion would be significantly enhanced, generating power in the cavity (a peak in the microwave power within the cavity is noted). (Image by C. Boutan/Pacific Northwest National Laboratory; adapted by the American Physical Society/Alan Stonebraker.)

The Haloscope At Yale Sensitive To Axion CDM (HAYSTAC) Experiment is a microwave cavity search for cold dark matter (CDM) axions with masses above 20ueV. Located at Yale’s Wright Laboratory in New Haven, Connecticut, the HAYSTAC collaboration consists of members from Yale University, the University of California Berkeley, Lawrence Berkeley National Laboratory, and the University of Colorado at Boulder. HAYSTAC searches for axion dark matter in the galactic halo by searching for a resonant photon signal produced by axion conversion in a magnetic field. The detection of such a signal would provide important clues to the nature of dark matter and the constitution of the mass content of the universe.

A quantum sensor is a quantum device that responds to a stimulus. Usually this refers to a sensor which has quantized energy levels, uses quantum coherence to measure physical quantity, or uses entanglement to improve measurements beyond what can be done with classical sensors. There are 4 criteria for quantum sensors:

1) The system has to have discrete, resolvable energy levels.

2) You can initialize the sensor and you can perform readout (turn on and get answer).

3) You can coherently manipulate the sensor.

4) The sensor interacts with a physical quantity and has some response to that quantity.

Oscillating dark matter can induce changes in fundamental constants

me(x,t) ≈ [1 + 10-22yDM(x,t)]

Induces tiny changes in atoms: a new field opening up

Groups beginning to search for tiny changes with atomic clocks, magnetometers, accelerometers, interferometers…

Quantum Sensors for Axion Detection with the Haloscope at Yale Sensitive to Axion CDM (HAYSTAC)

The Axion Dark Matter Experiment (ADMX) is the world’s first and only experiment to have achieved the necessary sensitivity to “hear” the telltale signs of dark matter axions.

For scientists to “hear” a dark matter particle, it must hit an atom in one of the crystals at the heart of the Cryogenic Dark Matter Search (CDMS) detectors. The crystals are kept cold—close to absolute zero—to reduce atomic movement, keeping the crystals quiet. The detectors “listen” for vibrations inside the crystal, like ears listening for vibrations in the air.

Ultra-light dark matter flux is huge

fthumb ~ 1022(10-6mneutrino/mDM) particles/s

The equation is a shorthand way of writing that if the dark matter mass (mDM) is the same as the neutrino mass (mneutrino), then the flux of dark matter particles through your thumb is about 1022 particles per second.

Better modelled as a wave rather than individual particles


yDM(x,t) =Ö(2rDM) · cos(wt – k · x)/mDM

ADMX: Axion Dark Matter eXperiment involves the University of Sheffield and 8 USA institutions

Schematic of the ADMX axion detector. The RF cavity, 0.5 m diameter × 1.0 m long, is in the bore of an 8.5-T solenoid magnet. Microwave power is amplified by a low-noise cryogenic amplifier and mixed-down to near audio. The result is digitized and processed with FFT electronics and the power spectrum is searched for axion signals.



Axions in magnetic fields convert to microwave photons


Tuneable cavity: Trying to tune onto the axion mass (a dark matter radio?)


Again no discovery yet

Experiments could tell us: the axion dark matter mass and axion-photon coupling

If the axions have a certain mass then they are believed to decay to photons (one axion decays to two photons)


Photons propagating through a transverse magnetic field, with incident Eg and magnet B parallel, may convert into axions.

A Search for Dark Matter Axions with the Orpheus Experiment

Axions are particles that could make up most or all of the dark matter if they have masses below 100 µeV.

The quantum chromo dynamics (QCD) axion’s coupling to photons is often assumed to lie in a narrow band as a function of the axion mass.



• We know dark matter exists… … but we have yet to measure the particle properties

• Many ideas for what dark matter could be… … and many experiments searching for them

• A truly global effort with the UK at the forefront


I would like to thank Dr McCabe for answering my questions and sharing his presentation with me.

If any of the images I have used have broken copyright then it is entirely my fault. Please let me know and I will remove them.

I did find this talk quite difficult to understand. I do have a physics degree but it did not include very much on particle physics or astrophysics (in fact most of what I have learnt came about from teaching the various topics in A level physics).

However, like my students, I find particle physics and astrophysics incredibly interesting (although when it gets very mathematical I do get very worried).

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