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CERN | Geneva | Switzerland

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Has anybody seen my supersymmetric particles?

This is the last part of a series of three on supersymmetry, the theory many believe could go beyond the Standard Model. First I explained what is the Standard Model and show its limitations. Then I introduced supersymmetry and explained how it would fix the main flaws of the Standard Model. I now review how experimental physicists are trying to discover “superparticles” at the Large Hadron Collider (LHC) at CERN.

If Supersymmetry (or SUSY for short) is as good as it looks, why has none of the new SUSY particles been found yet? There could be many reasons, the simplest being that this theory is wrong and supersymmetric particles do not exist. If that were the case, one would still need another way to fix the Standard Model.

SUSY can still be the right solution if supersymmetric particles have eluded us for some reasons: we might have been looking in the wrong place, or in the wrong way or they could still be out of the reach of current accelerators.

So how does one go looking for supersymmetric particles? One good place to start is at CERN with the Large Hadron Collider or LHC. The 27-km long accelerator is the most powerful in the world. It brings protons into collisions at nearly the speed of light, generating huge amounts of energy in the tiniest points in space.  Since energy and matter are two forms of the same essence, like water and ice, the released energy materializes in the form of fundamental particles. The hope is to create some of the SUSY particles.

One major problem is that nobody knows the mass of all these new particles. And without the mass, it is very much like looking for someone in a large city without knowing the person’s address. All one can do then is comb the city trying to spot that person. But imagine the task if you don’t even know what the person looks like, how she behaves or even in which city, let alone which country she lives in.

Supersymmetry is in fact a very loosely defined theory with a huge number of free parameters. These free parameters are quantities like the masses of the supersymmetric particles, or their couplings, i.e. quantities defining how often they will decay into other particles. Supersymmetry does not specify which value all these quantities can take.

Hence, theorists have to make educated guesses to reduce the zone where one should search for SUSY particles. This is how various models of supersymmetry have appeared. Each one is an attempt at circumscribing the search zone based on different assumptions.

One common starting point is to assume that a certain property called R-parity is conserved. This leads to a model called Minimal SUSY but this model still has 105 free parameters. But with this simple assumption, one SUSY particle ends up having the characteristics of dark matter. Here is how it works: R-parity conservation states that all supersymmetric particles must decay into other, lighter supersymmetric particles. Therefore, the lightest supersymmetric particle or LSP cannot decay into anything else. It remains stable and lives forever, just like dark matter particles do. Hence the LSP could be the much sought-after dark matter particle.

SUSY-cascade-Fermilab Credit: Fermilab

How can the Large Hadron Collider help? Around the accelerator, large detectors act like giant cameras, recording how the newly created and highly unstable particles break apart like miniature fireworks. By taking a snapshot of it, one can record the origin, direction and energy of each fragment and reconstruct the initial particle.

Heavy and unstable SUSY particles would decay in cascade, producing various Standard Model particles along the way. The LSP would be the last possible step for any decay chain. Generally, the LSP is one of the mixed SUSY states with no electric charge called neutralino. Hence, each of these events contains a particle that is stable but does not interact with our detectors. In the end, there would be a certain amount of energy imbalance in all these events, indicating that a particle has escaped the detector without leaving any signal.

At the LHC, both the CMS and ATLAS experiments have searched billions of events looking for such events but to no avail. Dozens of different approaches have been tested and new possibilities are constantly being explored. Each one corresponds to a different hypothesis, but nothing has been found so far.

dijet-monjet Two events with jets as seen in the ATLAS detector. (Left) A very common event containing two jets of particles. The event is balanced, all fragments were recorded, no energy is missing. (Right) A simulation of a mono-jet event where a single jet recoils against something unrecorded by the detector. The imbalance in energy could be the signature of a dark matter particle like the lightest supersymmetric particle (LSP), something that carries energy away but does not interact with the detector, i.e. something we would not see.

One reason might be that all supersymmetric particles are too heavy to have been produced by the LHC. A particle can be created only if enough energy is available. You cannot buy something that costs more money than you have in your pocket. To create heavy particles, one needs more energy. It is still possible all SUSY particles exist but were out of the current accelerator reach. This point will be settled in 2015 when the LHC resumes operation at higher energy, going from 8 TeV to at least 13 TeV.

If the SUSY particles are light enough to be created at 13 TeV, the chances of producing them will also be decupled, making them even easier to find. And if we still do not find them, new limits will be reached, which will also greatly help focus on the remaining possible models.

SUSY has not said its last word yet. The chances are good supersymmetric particles will show up when the LHC resumes. And that would be like discovering a whole new continent.

Pauline Gagnon

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