Single-top-squark production via R-parity-violating supersymmetric couplings in hadron collisions

In supersymmetric extensions of the standard model, particles may be assigned a new quantum number called R-parity (Rp) [1]. The particles of the standard model are Rp even, and their corresponding superpartners are Rp odd. The bounds on possible Rp-violating couplings are relatively restrictive for the first two generations of quarks and leptons, but much less so for states of the third generation [2]. If Rp is conserved, as is often assumed, superpartners must be produced in pairs, each of which decays to a final state that includes at least one stable lightest supersymmetric particle (LSP). The production rates for pairs of strongly interacting supersymmetric particles, the squarks and gluinos, benefit from the large color couplings of these superpartners to the incident light quarks and gluons in hadronic scattering subprocesses. However, in many models, the squarks and gluinos are relatively heavy, and therefore their pair production incurs a large phase space suppression. In this Letter, the s-channel production of a single squark through an Rp-violating mechanism [3] is considered. The motivation is that the greater phase space offsets the reduced coupling strength in the production. The focus is on the relatively light top squark t̃1 and its subsequent Rp-conserving decays. Thus, the R-parity violation penalty is paid only once, in the initial production, and is offset by the greater phase space relative to pair production. Beginning with the superpotential for Rp-violating couplings, we write the partonic cross section for the process qq′ → t̃1 and compare with that for pair production. Then discussing observability, we focus on one clean Rpconserving decay, t̃1 → bχ̃1 , with χ̃1 → l + ν + χ̃1. Here, l is an electron or muon, and the χ̃1 and χ̃ 0 1 are the chargino and lowest-mass neutralino states of the minimal supersymmetric standard model (MSSM). For top-squark masses in the range of 180–325 GeV, we simulate both the signal and standard model background processes and thereby show that the top squark can be discovered, or the current bound on the size of the Rpviolating couplings λ′′ 3ij can be reduced by up to one order of magnitude with existing data and by two orders of magnitude at the forthcoming run II of the Fermilab Tevatron. In general it is possible to have Rp-violating contributions to the MSSM superpotential of the baryonor lepton-number violating type. However, limits on the proton decay rate severely restrict their simultaneous presence. We therefore assume the existence of a baryonnumber-violating coupling only of the form [4]