Latest News from the LHC
Professor Tara Shears
Reported by Alfie Mussett (13Y) and Pameer Saeed (13O)
Edited by Mrs Hare
The above picture shows the professor behind the famous desk.
The big questions we want the LHC to answer include “What is matter made of“ and “What was the universe like in the first 1xE–12 seconds after the Big Bang”? However there are things it doesn’t tell us.
One of the things that the LHC is looking at is the origin of the universe. As of 2013 the universe is believed to be 13.798 ± 0.037 billion years old.
The very earliest universe was so hot, and energetic, that initially no particles existed or could exist (except perhaps in the most fleeting sense), and the forces we see around us today were believed to be merged into one unified force. There were no atoms but there were the ingredients for atoms. Particle accelerators like the LHC attempt to recreate this environment and understand the early universe.
After the Big Bang the universe started to cool and matter started to clump together.
Between 1xE–43 seconds and 1xE–36 seconds after the Big Bang the Grand unification epoch occurred and some bosons were believed to have emerged.
Between 1xE–36 seconds (or the end of inflation) and 1xE–12 seconds after the Big Bang, particle interactions in this phase were energetic enough to create large numbers of exotic particles, including W and Z bosons and Higgs bosons.
Unknown duration, ending 1xE–32(?) seconds after the Big Bang was the inflationary epoch, repopulating the universe with a dense, hot mixture of quarks, anti-quarks and gluons as it entered the electroweak epoch.
After cosmic inflation ended, the universe filled with a quark–gluon plasma. From this point onwards the physics of the early universe is better understood, and less speculative.
Between 1E–12 seconds and 1E–6 seconds after the Big Bang hot quark-gluon plasma gave way to hadrons because the average energy of particle interactions had fallen below the binding energy of hadrons.
Between 1xE–6 seconds and 1 second after the Big Bang hadrons formed including baryons such as protons and neutrons.
Between 1 second and 10 seconds after the Big Bang: The majority of hadrons and anti-hadrons annihilated each other at the end of the hadron epoch, leaving leptons and anti-leptons dominating the mass of the universe. Approximately 10 seconds after the Big Bang the temperature of the universe fell to the point at which new lepton/anti-lepton pairs were no longer created and most leptons and anti-leptons were eliminated in annihilation reactions, leaving a small residue of leptons.
Between 10 seconds and 380,000 years after the Big Bang most leptons and anti-leptons were annihilated at the end of the lepton epoch the energy of the universe was dominated by photons. These photons were still interacting frequently with charged protons, electrons and (eventually) nuclei, and continued to do so for the next 380,000 years.
During the photon epoch the temperature of the universe fell to the point where atomic nuclei began to form. Protons (hydrogen ions) and neutrons began to combine into atomic nuclei in the process of nuclear fusion. Free neutrons combined with protons to form deuterium. Deuterium rapidly fused into helium-4. Nucleosynthesis only lasted for about seventeen minutes, since the temperature and density of the universe had fallen to the point where nuclear fusion could not continue. By this time, all neutrons had been incorporated into helium nuclei and this left about three times more hydrogen than helium-4 (by mass) and only traces quantities of other nuclei existed.
70,000 years after the Big Bang the densities of non-relativistic matter (atomic nuclei) and relativistic radiation (photons) were equal.
377,000 years after the Big Bang Hydrogen and helium atoms began to form as the density of the universe fell. At the end of this recombination, most of the protons in the universe were bound up in neutral atoms. Therefore, the photons’ mean free path became effectively infinite and the photons could now travel freely: the universe had become transparent. This cosmic event is usually referred to as decoupling. The photons present at the time of decoupling are the same photons that we see in the cosmic microwave background (CMB) radiation, after being greatly cooled by the expansion of the Universe.
9 year WMAP data (2012) shows the cosmic microwave background radiation variations throughout the Universe from our perspective, though the actual variations are much smoother than the diagram suggests.
Before decoupling occured, most of the photons in the universe were interacting with electrons and protons in the photon–baryon fluid. The universe was opaque or “foggy” as a result. There is light but not light we could observe through telescopes. This period is known as the dark ages. Telescopes are useless investigating this era so particle accelerators are used to try and recreate it.
As time continued the matter clumped together and hydrogen and helium formed stars. Stars grouped together and galaxies formed.
Everything in the universe is comprised of the same basic particles. If we understand them we understand us.
Matter is made up of twelve fundamental particles, six quarks and six leptons.
These are far too small to measure. The up and down quarks are the most prevalent and make up the protons and neutrons.
A neutrino is an electrically neutral, weakly interacting elementary subatomic particle and belongs in the lepton group.
Fundamental forces keep the particles together.
The W and Z bosons are the force carrying particles for the weak force.
The photon (g) is the force carrier for the electromagnetic force. The electromagnetic force is the interaction responsible for almost all the phenomena encountered in daily life, with the exception of gravity.
The strong force carried by gluons (g) holds quarks together to form protons, neutrons, and other hadron particles. It allows the electrostatic repulsion to be overcome.
Gravity is a big problem. At the moment no particle has been found that would act as the force carrier (although it is often given the name of graviton). Gravity acts on matter particles but is so weak it is ignored in particle physics. To understand just how weak it is use a tiny magnet to pick up a pin from the floor. The electromagnetic force is much greater than the gravitational force.
Fundamental particles have mass due to an all permeating field known as the Higgs field. The Higgs Field is an invisible energy field that exists throughout the universe. The field is accompanied by a fundamental particle called the Higgs Boson, which it uses to continuously interact with other particles. As particles pass through the field they are endowed with the property of mass, much as an object passing through treacle will become slower.
Photons are not affected by the Higgs field so do not have mass but quarks are.
The Higgs boson is invisible but can be created in a particle accelerator. It does not exist for very long so it is identified by the products of its decay.
The Standard Model Lagrangian equation shows the theory of particle physics. The theory looks good and at the moment no measurements disagree with it. But is the theory correct? It doesn’t include gravity. Trying to include gravity only produces rubbish. Is there a deeper underlying theory to be found?
(0) The Higgs boson must exist for the standard model to work but is the Higgs that has been discovered the expected Higgs or a Higgs that belongs to another model?
(1) The standard model doesn’t explain why very little antimatter exists. At the Big Bang there were equal quantities of matter and antimatter. Why then and not now?
Matter and antimatter will annihilate each other producing enormous energies (0.25g would be equivalent to 5ktonnes of TNT) but this has never been seen. At the start of the universe annihilation should have stopped the universe in a few seconds. There must have been a subtle difference between matter and antimatter. We need to understand this to see how the universe evolved.
(2) The standard model can describe the 4% of matter that is visible to us i.e. the atoms but what about the other 96%?
At the moment there is no explanation for dark energy in particle physics but there is a hypothesis for dark matter – SUSY.
SUSY – super symmetrical particles are a candidate for what dark matter could be and it is experimentally unobservable.
Supersymmetry predicts a partner particle for each particle in the Standard Model, to help explain why particles have mass.
According to supersymmetry, every familiar matter particle has a supersymmetric counterpart. Theory predicts most of these counterparts are unstable, so the ones produced in the Big Bang decayed long ago. But the lightest, the neutralino, should be stable and is an ideal dark matter candidate.
The evidence for dark matter is its gravitational effects. There is not enough visible matter give star rotation.
SUSY particles are unobservable but they do predict the existence of other particles.
There are still many questions that we need answering:
How many dimensions?
What is mass?
What about gravity?
12 matter particles?
Where did all the antimatter go?
Mini black holes?
What about the other 96% of the universe?
At the time of writing the biggest particle accelerator, LHC, in the world can be found at CERN.
CERN was founded in 1954. There are 20 member countries, more than 9000 (from 800 universities) scientists representing over 100 nationalities.
LHC: 2 beams of protons collide 40 million x a second at near light speed 100m underground. The ring is 27km in length and straddles France and Switzerland.
You can just make out the gentle curve.
Inside the blue edged tubes can be found the long chains of strong superconducting magnets required to deflect the particles around the ring. Electric fields are required to produce the acceleration.
Collisions occur at four points on the ring (ATLAS, CMS, ALICE and LHCb). An experiment can produce 40 million collisions per second.
Recreating conditions (hot energies) when the universe was a billionth of a second old …… producing the fundamental particles 10-12 after the big bang. This allows us to view matter at the start of the universe.
The above video is a 3- dimensional electronic picture showing the production of the particles. The different types of particles have different signatures which can be used to identify them and work out what they were doing at the start of the universe.
The below image is quite famous now and shows the size of the ATLAS detector by using one of the engineers to give perspective. 20m tall and 45m long with 100 million channels of information.
The eight torodial magnets can be seen with the calorimeter before it was moved into the middle of the detector. This calorimeter measures the energies of particles produced when protons collide in the centre of the detector. ATLAS works along the CMS experiment to search for new physics at the 14 TeV level.
To trace out the paths of the particles you need precise silicon detectors.
A VELO module for LHCb, constructed at Liverpool. Includes silver semi circles. The equipment locates the positions of charged particles. The sensors are of two different designs, one that measures the radius and the other the azimuthal angle at which an ionizing particle passes through the active area.
In ATLAS muon detectors can be found along the edge of the detector.
Identify particles by characteristic signatures in experiment
Add computers: calculate particle paths and energies
Add theory: infer what fundamental process happened
The above image is a transverse slice through CMS.
So what about the Higgs?
Before the LHC this was an open question.
You can buy your own Higgs Boson.
Where the Higgs wasn’t helped find out where it wasn’t. The discovered particle was found to have a different mass to any other particles.
Both ATLAS and CMS presented results in 2012 that a particle of energy 125.3 +/- 0.6GeV at 4.9 s (4.9 standard deviations) had been found. This means that there is a 1×10-6 chance that this particle is not the Higgs. 5 s (1.7 x 10-6) is needed to say that the particle is definitely the Higgs Boson.
In statistics and probability theory, the standard deviation (represented by the Greek letter sigma, σ) shows how much variation or dispersion from the average exists. A low standard deviation indicates that the data points tend to be very close to the mean (also called expected value); a high standard deviation indicates that the data points are spread out over a large range of values.
The animation shows data and compares it with theory.
David Horsey / Los Angeles Times (July 5, 2012)
Everybody knew a new particle was discovered but was it THE Higgs or a Higgs. The Higgs Bosom was proposed fifty years ago and until 2012 there was not enough energy or data to distinguish it from other particles with the same signature.
There are five different ways the Higgs can behave. It looks like the Higgs but still not sure.
Now the focus is on understanding the Higgs. So far it looks like the standard model Higgs but we still lack the precision to know for sure.
1,946 papers since the start of 2012….. Most of them written by theorists.
Higgs could be associated with SUSY.
The Higgs Boson now has the Nobel seal of approval.
LHCb experiment observes new matter-antimatter difference (24 Apr 2013)
The LHCb collaboration at CERN1 submitted a paper to Physical Review Letters on the first observation of matter-antimatter asymmetry in the decays of the particle known as the B0s. It is only the fourth subatomic particle known to exhibit such behaviour.
LHC collisions produce equal amounts of matter and antimatter. Does this situation remain when the products of the collisions stay the same?
b quarks are collided
Bs K–p+, Bs –> K+p–
A typical B0s →μμ decay candidate event is shown above. The two muon tracks from B0s decay are seen as a pair of purple tracks traversing the whole detector in the left image above. The right image shows the zoom around the proton-proton collision point, origin of many particle tracks. The two muon purple tracks originate from the B0s decay point located 50 mm from the proton-proton collision. Collisions therefore do not occur in the centre of the detector.
Matter is greater than antimatter after the collisions and decays. We don’t know why yet.
It is an experimentally observed fact that matter and antimatter behave differently,
And beyond the standard model? Supersymmetry……dark matter? In the dark …..
Mass reach of ATLAS searches for Supersymmetry. Only a representative selection of the available results is shown.
Status of figure: SUSY 2013
Does this mean that somewhere in the universe there are galaxies made of antimatter? If not where did the antimatter go?
Does SUSY have an explanation for dark matter? At the moment there is no evidence for dark matter in the data.
What’s happening next?
LHC restart (13 TeV) in 2015
Greater energy, more data.
More Higgs bosons
More antimatter investigation
Better New Physics searches
What will we find?
………. wait and see ……….