Bounds from tt̄ production on R-parity violating models of supersymmetry

We study tt̄ production in R-parity violating supersymmetry. The annihilation channel qq̄ → tt̄ gets new contributions from t-channel exchange of squarks or sleptons. With the data from Tevatron on tt̄ production, we find that the squarkor slepton-exchange processes constrain the B-violating λ couplings or the L-violating λ couplings, respectively. Our bounds are already comparable to the few existing constraints on third-generation R-parity violating couplings, and will improve when more precise measurements of the tt̄ production cross-section become available. We also discuss the effects of these couplings for top production at the LHC. [email protected] [email protected] [email protected] Supersymmetry is considered to be one of the most promising candidates for physics beyond the Standard Model (SM) and, consequently, a significant amount of effort has been devoted to looking for signals of supersymmetry in experiments. The minimal supersymmetric extension of the Standard Model (MSSM) [1] contains, in addition to the usual particles of the Standard Model, their superpartners and two Higgs doublets. While the gauge structure of the MSSM essentially replicates that of the Standard Model, the Yukawa sector of the MSSM is somewhat more complicated. In addition to the usual Yukawa couplings of the fermions to the Higgs (responsible for the fermion masses), other interactions involving squarks or sleptons are possible. The relevant part of the superpotential containing the Yukawa interactions involving squarks or sleptons in the MSSM is given in terms of the chiral superfields by W = λijkǫαβLi L β jE c k + λ ′ ijkǫαβL α i Q β jD c k + ǫαβμiL α i H β 2 + λ ′′ ijkǫ U c iaD c jbD c kd (1) where the Li and Qi are SU(2)-doublet lepton and quark superfields and the Ei, Ui, Di are singlet superfields, H2 is a Higgs superfield, c denotes conjugation, i, j, k are generation indices, α, β are SU(2) indices and a, b, d are SU(3) indices. The couplings λ, λ and μi violate lepton (L) number, whereas the λ coupling violates baryon (B) number. The Band L-violating couplings cannot be present simultaneously, because that would lead to very rapid proton decay. The term ǫαβμiL α i H β 2 is not usually included, because it can be rotated away from the superpotential by a redefinition of the Higgs H1 and the leptonic Li superfields [2] . We will not consider this term any further. It is possible to forbid the existence of all the interactions in Eq. (1) by imposing a discrete symmetry – R-parity. This discrete symmetry may be represented by R = (−1), where S is the spin of the particle, so that the usual particles of the SM have R = 1, while their superpartners have R = −1. The requirement that the MSSM Lagrangian be invariant under R-parity is sufficient to exclude each of the interactions in Eq. (1). However, R-parity conservation is too strong a requirement to ensure proton stability [4] – the latter can simply be ensured by assuming that either the L-violating or the B-violating couplings in Eq. (1) are present, but not both. Relaxing the requirement of R-parity conservation has important implications for supersymmetric particle searches at colliders : a superparticle can decay into standard particles via the R-parity violating couplings in Eq. (1), and, hence, the lightest supersymmetric particle will no longer be stable or escape detection. Thus bounds on MSSM parameters derived from missing energy and momentum signals are no longer valid in R-parity violating scenarios. In this letter, we will not be concerned with the L-violating λ couplings, since we will be interested in the effects of R-parity violation at hadronic colliders. Consequently, we We remark that this redefinition does not leave the full Lagrangian (i.e. including the soft supersymmetry-breaking terms) invariant and can be achieved only at the expense of generating two additional complex parameters in the soft supersymmetry-breaking sector of the Lagrangian [3]. This complication is of no consequence to our analysis.