From Supersymmetry to Dark Matter

1 December 2008

Expected 5-sigma discovery reach, for an integrated luminosity of 1 fb-1, of the 4-jet plus missing transverse energy analyses with various lepton requirements for mSUGRA as a function of m_{0} and m_{1/2}. The horizontal and curved gray lines indicate gluino and squark mass contours respectively in steps of 500 GeV.

One of the biggest mysteries of cosmology and particle physics is the origin of Dark Matter. Evidence for its existence follows from many observations, the most popular being galaxy clusters with an excess of gravitational force and the rotational speed of spiral galaxies. In the past, many Dark Matter candidates have been discussed, but most were discarded. Black holes and brown dwarfs were examined, as well as neutrinos. However, all of them can – at most – only account for a small fraction of Dark Matter. The most likely scenario is that a new unknown fundamental particle is responsible for Dark Matter. Particles that would have been produced in the Big Bang and would have survived until today, generically called weakly interacting massive particles (WIMPS) are the most favored type.

Many direct searches have been trying to find WIMPS. With many kilograms of the purest crystals located deep underground to be protected from the cosmic rays, they hunt these extremely rare weak interactions. Other experiments take place in space: recent exciting news comes from PAMELA. Hooked on the Russian satellite Resurs-DK1 since June 2006, it studies cosmic rays, in particular positrons. Detecting a positron excess above around 10 GeV, the PAMELA collaboration could have found indirect evidence for Dark Matter annihilation.

Astroparticle physics experiments, however, are not the only ones to search for WIMPS. These mysterious particles might even be produced in our accelerators. Can the LHC then give us some clues to solve the Dark Matter puzzle? Could we even measure the mass of a Dark Matter particle? Several theories for physics beyond the Standard Model can provide a Dark Matter candidate, for instance: the lightest Kaluza-Klein particle in models of universal extra dimensions, or the lightest Supersymmetry (SUSY) particle. Searching for SUSY or extra dimensions could hence provide us with valuable hints to understand the mystery of Dark Matter. Here, only SUSY is discussed.

SUSY is a theoretically attractive scenario for physics beyond the Standard Model which provides a suitable Dark Matter candidate if R-parity is conserved. R-parity is a multiplicative quantum number that is introduced in order to conserve the baryonic and leptonic quantum numbers and thus protect the proton lifetime. It is 1 for Standard Model particles and -1 for SUSY particles (sparticles). The consequences of R-parity conservation (RPC) are that sparticles can only be produced in pairs and that each sparticle will gradually decay to the lightest SUSY particle (LSP) which must be stable. Since no stable exotic strong or EM bound states (isotopes) have been observed, the LSP should carry no electromagnetic or color charge, making it a suitable candidate for WIMPS. The detector signature of such an LSP is similar to that of a heavy neutrino. It would escape direct detection resulting in the characteristic feature expected for SUSY events: an imbalance of the transverse energy measured in the detector.

In the CSC efforts, the ATLAS inclusive SUSY search strategy was developed using a sequential approach. Detailed studies have been conducted to define inclusive search channels using specific SUSY benchmark points, denoted SU1, SU2, ... , SU8. Events corresponding to these benchmark points, and all relevant Standard Model backgrounds, were passed through a detailed simulation of the detector.

The insight gained from these detailed studies has been applied to several scans over subsets of SUSY parameter space. By design these scans consist of a large number of signal points. Hence fast, parameterized simulations have been used. The goal is to verify that the inclusive search channels provide sensitivity to a wide range of SUSY models.

The results indicate that ATLAS should discover signals for RPC SUSY with gluino and squark masses of less than about 1 TeV after having accumulated and understood an integrated luminosity of about one inverse femtobarn (fb-1). SUSY at a higher mass scale could still show up later, but detailed studies would be more difficult.

If a signature consistent with SUSY is established, the experimental focus will be to reconstruct the sparticle mass spectrum and to constrain the model parameters. In RPC models, sparticle decay chains cannot be fully resolved since the LSPs escape detection. As a consequence, edge positions rather than mass peaks in invariant mass distributions are measured and fitted. A Standard Model analogy is the W boson mass measurement, where one neutrino escapes detection.

While the position of such edges can be measured fairly precisely, exploiting such measurements for determining individual sparticle masses will be challenging. As an example, the mass of the LSP is found to be 88±60 GeV for the SU3 benchmark parameters and 62±126 GeV for SU4, for an integrated luminosity of 1 fb-1 and 0.5 fb-1,  respectively. The true (Monte Carlo) masses are 118 GeV (SU3) and 60 GeV (SU4).

Once enough edge positions have been measured, model parameters can be constrained. In the initial phase a limited number of measurements with rather large uncertainties would only allow fitting SUSY models with few parameters. Dark Matter properties, as for instance the neutralino relic density, can then be calculated within these models.

Hence, searching for Supersymmetry with R-parity conservation at the LHC might help us understand the origin of Dark Matter. If RPC SUSY is realized at the TeV energy scale, it seems accessible at the LHC within the first few years of running. Several inclusive search channels are defined to cover a wide range of possible SUSY signatures. Following a SUSY discovery, kinematic endpoints can be exploited to pin down the mass spectrum also revealing the LSP mass. A larger data set could then be used to fit SUSY model parameters from which Dark Matter properties could be calculated.

Till Eifert

Université de Genève