The Quantum Level in the Mssm

We compare the standard top quark decay and the charged Higgs decay of the top quark, t → W b and t → H b, at the quantum level in the MSSM. While the SUSY loop corrections to the standard top quark decay are only of a few percent, it turns out that t → H b is a most promising candidate for carrying large quantum SUSY signatures. As a result, the (tanβ,MH±) exclusion plots presented by the CDF Collaboration should be thoroughly revised in the light of the MSSM. Talk presented at the International Workshop on Quantum Effects in the MSSM , Barcelona, September 9-13, 1997. To appear in the Proceedings. Theoretically, Supersymmetry (SUSY) is perhaps the only known framework beyond the Standard Model (SM) which is capable of extending non-trivially the quantum field theoretical structure of the conventional strong and electroweak interactions while keeping all the necessary ingredients insuring internal consistency, such as gauge invariance and renormalizability. In particular, the Minimal Supersymmetric Standard Model (MSSM) [1, 2] has been able to accommodate all known high precision measurements to a similar degree of significance as the Standard Model [2]. Remarkably, SUSY has been able to survive over the years and it has become a “fact” of live for many physicists. Most likely this situation will remain invariable at least until the Tevatron II and LHC eras have explored in full the experimental feasibility of the MSSM. In this talk I propose to dwell on the supersymmetric phenomenology of top quark decays with an eye on the Tevatron and LHC phenomenological capabilities. Among the relevant MSSM top quark decays potentially carrying a direct or indirect SUSY signature, the following two-body modes stand out: i) t → W b ii) t → H b iii) t → t̃a χ0α, iv) t → b̃a χ+i , v) t → t̃a g̃ . (1) Therein, t̃a, b̃a, χ + i , χ 0 α, g̃r (a, i = 1, 2; α = 1, 2, ..., 4; r = 1, 2, ..., 8) denote stop, sbottom, chargino, neutralino and gluino “sparticles”, respectively. (Also quite a few three-body decays are possible and have been studied [3].) Of course, decay i) is the SM top quark decay, and decay ii) into a charged Higgs need not to be a SUSY decay. However, by studying the possible MSSM quantum effects on these decay modes one may hope to unveil indirect traces of the underlying SUSY dynamics. On the other hand the last three decays in (1) do carry an explicit SUSY signature. In general also these decays may require a higher order treatment, the reason being that some of the final state signatures, after the sparticles have decayed into conventional particles and the LSP (typically the lightest neutralino χ01), they may well mimic the standard top quark decay. For example, decay iii) may lead to a signature similar to the standard top quark decay into the final states b l ν or b + 2 jets; for the stop could decay into χ+i b, and subsequently yield the chain χ+i → χ01W ∗ → χ01 l ν or χ01+2 jets. Therefore, a detailed treatment of these direct SUSY modes is in principle desirable to help disentangling the nature of the complicated final configurations and to enable a reliable determination of the top quark cross-section within the MSSM. Barring a light gluino window, which is nowadays harder and harder to maintain, current limits on squark and gluino masses already rule out decay v) and