Abstract
We calculate the effective $$ Zb\overline{b} $$ 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 $$ Zb\overline{b} $$ coupling at two loop level, points to a 1.2σ 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 $$ Zb\overline{b} $$ 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.
Highlights
JHEP03(2015)012 bulk corrections originated from the compactification and boundary corrections due to the orbifolding
Dominant contribution to FnmUED comes from Feynman graphs listed in figure 1; as all of the amplitudes listed in figure 1 contain terms proportional to gyt2
We have estimated one loop contribution to the Zbb vertex in the framework of an Universal Extra Dimensional Model where kinetic and Yukawa terms are added to the fixed points of the extra space like dimension
Summary
We will very briefly review the non-minimal Universal Extra Dimensional Model keeping in mind the necessary masses and couplings which will be used in our calculations of effective Zbb coupling and we will restrict ourselves to boundary-localized kinetic and Yukawa terms only. Before delving into the interactions needed for the calculation, let us spend some time discussing the physical eigenstates which are the outcome of mixing of some of the states originally present in the four dimensional effective theory. Necessary interactions involving the Z-boson, fermions and scalars in the four dimensional effective theory can be derived from the above action by inserting the appropriate y dependent profile for the respective five dimensional fields and integrating over the extra direction, y. Greek indices refer to the kind of fields involved in the coupling while roman indices refer to the KK-level of respective fields At this end, let us pay some attention to a pair of overlap integrals I1mn and I2mn which are relevant for our calculation appearing in the interactions listed in appendix A. The expressions for the integrals (upon integrating over y) are given in appendix A along with the necessary Feynman rules
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