Will SUSY be found?
If supersymmetry exists it will likely show itself through the presence of missing energy and through some particle signatures being produced more frequently than the normal “background” rate. A thorough understanding of the detector and all possible read-outs is therefore necessary to establish that we can’t explain these results by any known phenomena or experimental error before looking to a new explanation.
CMS is well equipped to discover such elusive particles and if SUSY is a true symmetry of Nature at the energy scales of the LHC, it will almost certainly be discovered in CMS.
This means a doubling of every particle we know to make SUSY partners or “sparticles”: matter partners are given an “s”, so the partners of electrons and quarks are called “selectrons” and “squarks”, and force partners become an “ino”, for example a “gluino” for the gluon and a “zino” for the Z boson.
Given that in the 1930s physicist Paul Dirac already doubled the number of known particles by suggesting that each had an associated “antiparticle”, this idea isn’t so strange. But why haven't we ever seen these SUSY particles?
Physicists think that we’ve yet to see the superpartners because SUSY is a broken symmetry, where sparticles have much heavier masses and are less stable than their counterparts. So though SUSY particles would have featured in the same numbers as standard particles in the early Universe, as it expanded and cooled they would have decayed to now only exist in a lightest “fossil” form that is hard to find and detect. But if SUSY particles exist at energy scales compatible with the LHC, up to 2-3 TeV, we will be able to produce and study them in large quantities right here.
Does a SUSY have a dark side?
Because it is both heavy and undetectable, the LSP (neutralino) is a good candidate for dark matter. This relies on the particle being perfectly stable, since dark matter has to have survived from the Big Bang until now. If the LSP is not stable, we can still detect it in CMS through its decay to other particles, rather than through missing energy, but dark matter will remain a mystery.
How to detect a superparticle
SUSY particles produced in the LHC are likely to be pairs of gluinos and/or squarks, heavy particles that will only live for 10-23 seconds before they decay (turn into other more stable and familiar particles) in multiple stages, forming long “decay chains”. The final products will consist of leptons like electrons and muons (producing clean tracks in the experiment), quarks (producing sprays of particles known as jets), and always the lightest supersymmetric particle (LSP).
The LSP is thought to be the lightest of the four "neutralinos", formed as combinations of a zino, a photino and two higgsinos: the SUSY partners of the Z boson, photon and Higgs boson respectively. The LSP is thought to be stable and so marks the end of the decay chain. But because it is weakly interacting and neutral, like the neutrino, it will pass straight through material and not be detected in CMS.
So how do you detect a particle you can’t see and that doesn’t interact with your detector? CMS will be able to detect LSPs, and so SUSY particles, by instead spotting when energy or momentum from the initial collision goes missing.
Momentum in equals momentum out…
One of the most fundamental laws of physics is that ‘momentum is conserved’. In other words, the total momentum before a collision is equal to the total momentum after; in this instance zero, because the protons travel with equal speeds in opposite directions so their momenta cancel out. If the total momentum of the observed particles that emerge from a proton-proton collision does not equal zero, we can deduce there must be an invisible particle somewhere that carried away that missing momentum.
CMS does exactly this: collecting and adding up the momenta and energies of all the emerging particles from a collision (in the “transverse” direction, i.e. at right angles to the beam line). CMS reconstructs the collision like a giant jigaw, and if the emerging particles seem imbalanced, flying out one side but not the other because an undetected LSP has been produced, we see the missing particle as a hole in the puzzle, identifiable due to its missing momentum or energy.
To spot the missing particles CMS is “hermetic” meaning that, to the extent possible, it catches every detectable particle emerging from a collision. Large detectors have cables and support regions that don't detect any kinds of particle and these channels for escape must be minimised to ensure that standard particles can't slip by undetected. This way if the energy or momentum is ‘missing’ it really is due to an invisible particle.
Finding missing energy is why CMS needs a good hadron calorimeter and why it has detectors in the “forward region”, at very shallow angles to the beam line (in addition to the detectors that surround the collision at larger angles) so that particles flying in all directions will be picked up.
Another important consideration in detecting SUSY particles is the ability to detect the “beauty” or “bottom” quarks. The b quark features in a number of decay chains, its sparticle expected to be one of the lighter squarks, but only lives for a very short amount of time before decaying (1 picosecond, one millionth of a millionth of a second). To spot these detectors must have very fine resolution and be placed very close to the beam line.
Pairs of SUSY particles decay in multiple stages producing a cascade of particles, often including further SUSYs, but resulting in three main “signatures” that CMS can look for.
The first uses the fact that the final decay products will often consist of pairs of leptons of opposite charge (e.g. a muon and an anti-muon), along with the ever present LSPs. Looking for unusually high levels of pairs of these particles should be a clear signal of SUSY production and is why we need a high performance detector system for muons , and a good ECAL to detect electrons.
The second common SUSY decay is for gluino or squark pairs to decay into lots of quarks and anti-quarks, along with the two LSPs; in the experiment seen as many narrow sprays of particles called jets and missing energy.
The third way involves spotting an otherwise rare occurrence, the production of pairs of leptons with the same charge. Usually pairs of leptons are produced from decays of a pair of oppositely charged particles, or from the decay of a single neutral particle; either way, conservation of charge means that the two leptons will have opposite charges. However with SUSY we expect to produce pairs of gluinos, which are neutral particles. Furthermore the decay of each gluino is just as likely to produce a positively charged lepton as a negatively charged one (balanced by negatively or positively charged quarks), so a pair of gluinos will often give a pair of leptons with the same charges. Finding unexpectedly large number of same sign leptons produced in events (e.g. two muons rather than a muon and an anti-muon) would also strongly point towards SUSY production.
Supersymmetry doesn’t only increase the number of familiar particles: within supersymmetric models, there are also at least five Higgs bosons – the light scalar (ho), the heavy scalar (Ho), the pseudo scalar (Ao) and the negative and positive charged Higgs particles (H±). These can decay to photons, leptons (including the heaviest known lepton, the tau) and mesons (particles made of quarks), seen as jets in the detector.
Shown in the decay diagram and event display is how a SUSY Higgs might decay to two taus, giving an end product of an electron, jets (caused by the pions) and neutrinos, which will be detected through their missing energy. If supersymmetric Higgs particles exist, one way they could therefore be seen is as an increased frequency of pairs of taus produced as specific energies, as shown in the graph.