Model-Independent Constraints on Lepton-Flavor-Violating Decays of the Top Quark

Abstract

The imminent start of the Large Hadron Collider, which is expected to produce $\sim 10^8$ $t\bar{t}$ pairs per year, provides an unprecedented opportunity for top physics. As the top quark is widely expected to be rather sensitive to effects of new physics, a detailed study of its properties, including rare decays, is called for. A possible, experimentally distinctive decay is the case where a top decays to a light quark and a flavor-violating lepton-antilepton pair. We use an effective operator analysis to place model-independent bounds on contributions to the decays $t\to u e^{\pm} \mu^{\mp}$ and $t\to c e^{\pm} \mu^{\mp}$. We enumerate the dimension-six operators which contribute to these decays and which are invariant under the Standard Model gauge group. We separate these operators into two classes, one with operators where the top quark belongs to an SU(2) doublet and thus can contribute at tree level to low-energy processes, and one class with operators where the top quark is a right-handed singlet and can only contribute to low-energy processes via loop diagrams. We use $B$ and $K$ decays to place limits on the coefficients of some of these operators, but find that several remain unconstrained and could potentially make observable contributions to top decay.