$Z \rightarrow b {\bar b}$ in non-minimal Universal Extra Dimensional Model

Abstract

We calculate the effective $Z b\bar b$ coupling at one loop level, in the framework of non-minimal Universal Extra Dimensional (nmUED) model. Non-minimality in Universal Extra Dimensional (UED) framework is realized by adding kinetic and Yukawa terms with arbitrary coefficients to the action at boundary points of the extra space like dimension. A recent estimation of the Standard Model (SM) contribution to $Zb\bar{b}$ coupling at two loop level, points to a $1.2\sigma$ discrepancy between the experimental data and the SM estimate. We compare our calculation with the difference between the SM prediction and the experimental estimation of the above coupling and constrain the parameter space of nmUED. We also review the limit on compactification radius of UED in view of the new theoretical estimation of SM contribution to $Z b\bar{b}$ coupling. For suitable choice of coefficients of boundary-localized terms, 95\% C.L. lower limit on $R^{-1}$ comes out to be in the ballpark of 800 GeV in the framework of nmUED; while in UED, the lower limit on $R^{-1}$ is 350 GeV which is a marginal improvement over an earlier estimate.