Light Kaluza–Klein states in Randall–Sundrum models with custodial image Original Research Article

We consider Randall–Sundrum scenarios based on image and a discrete parity exchanging L with R. The custodial and parity symmetries can be used to make the tree level contribution to the T parameter and the anomalous couplings of the bottom quark to the Z very small. We show that the resulting quantum numbers typically induce a negative T parameter at one loop that, together with the positive value of the S parameter, restrict considerably these models. There are nevertheless regions of parameter space that successfully reproduce the fit to electroweak precision observables with light Kaluza–Klein excitations accessible at colliders. We consider models of gauge-Higgs unification that implement the custodial and parity symmetries and find that the electroweak data singles out a very well defined region in parameter space. In this region one typically finds light gauge boson Kaluza–Klein excitations as well as light image singlet, and sometimes also doublet, fermionic states, that mix with the top quark, and that may yield interesting signatures at future colliders.