Catch Me if You Can: Searching for Gluinos at the Tevatron
One of the most fascinating proposals for new laws of physics that we might discover at higher energies is supersymmetry, the idea that every particle in nature has a partner with the opposite statistics. For every particle carrying a force—for example, the photon or the gluon—there would be a matter particle, and for every matter particle—for example, the quark and the lepton—there would be a particle carrying a new force. In models of supersymmetry, the lightest new particle is often the partner of the photon, called the "bino." This particle is weakly interacting, is not observed by particle physics detectors, and could well make up the cosmic dark matter.
Experiments at the highest-energy accelerator now operating, the Tevatron proton–antiproton collider at Fermilab, have made extensive searches for supersymmetric particles. However, the theory of supersymmetry has many free parameters, and current searches cover only a small part of this parameter space. We have been working with Johan Alwall and Jay Wacker here at SLAC to extend the Tevatron results to new regions of the parameters and to construct more robust strategies for finding supersymmetric particles.
We have been studying searches for the supersymmetric partner of the gluon, called the "gluino." If a pair of gluinos is produced at the Tevatron, each particle will emit quarks and antiquarks, and perhaps also leptons, and decay eventually to the bino. The bino will not be observed, so the resulting event will contain observable jets of particles from the quarks and unbalanced or missing energy carried away by the binos. The CDF and DZero experiments at Fermilab have published limits on the masses of the gluino based on their searches for these jets plus missing energy events.
It is challenging to interpret the results of these searches as bounds on the gluino mass. Models of supersymmetry have more than 100 independent parameters. The values of particle masses and couplings can vary dramatically depending on the mechanism of supersymmetry breaking. The CDF and DZero results are based on simplifying assumptions known as the CMSSM or mSUGRA, in which the masses of supersymmetric particles are predicted from five parameters.
However, the CMSSM is only a narrow slice of the full parameter space. Some of the omitted parameters are inconsequential, but others allow important differences in the physics. For example, the CMSSM fixes the ratio between the masses of the gluino and bino to a constant (approximately 6:1). There are models of supersymmetry in which this ratio is different, and for those models the limits from searches at the Tevatron are not known.
In our work, we have studied how to perform more model-independent searches for gluinos at the Tevatron. Our goal was to design a search strategy that would allow one to set bounds on all kinematically accessible gluinos and binos, not just those whose masses are in a ratio of 6:1. This is challenging because the number of jets expected as a result of gluino production at the Tevatron depends on the relative mass difference between the gluino and bino. When the mass splitting is much larger than the bino mass, a large amount of energy is released in the gluino decay, and we expect events with many well-separated jets and large missing energy. The situation is very different for light gluinos (with masses below 200 GeV) that are nearly degenerate with the bino. In events with these particles, the jets from the decay have low energy and the recoil from the unobserved binos is difficult to see. We have shown that it is possible to discover a gluino of this type by selecting events in which energetic gluons are produced together with the gluino pair. This boosts the gluinos and makes the missing energy more apparent.
A model-independent gluino search should have sensitivity over a wide range of kinematical parameter space. It should be sensitive to cases where the gluino and bino are nearly degenerate, as well as cases where the gluino is far heavier than the bino. We found that missing energy accompanied by 1 or 2 jets was an effective signal for gluinos that are nearly degenerate with a bino. For heavier mass gluinos, the searches for more complex events fared much better. We found that it is possible to scan for gluinos and binos over the full kinematic range with four mutually exclusive searches, for 1, 2, 3, and 4 jets plus missing energy, with cuts optimized for each hypothesis on the gluino/bino mass ratio.
We are now generalizing our results to more complicated decay chains for the gluino. We are also developing a similar analysis for jets plus missing energy searches at the Large Hadron Collider (LHC). When the LHC turns on later this year, we will be probing a new energy frontier. While theoretical models provide some guidance for what we might expect to see, it is important that our experimental searches remain model-independent. We should be sure that we are approaching the analysis in the best way possible to avoid missing any signal of new physics.
My Phuong Le and Mariangela Lisanti, SLAC Today, March 20, 2008