Chargino Mass and $R_b$ Anomaly

Abstract

We re-examine the possible magnitude of the supersymmetric contribution to $R_b$ with imposed all available phenomenological constraints and demanding good quality of the global fit to the precision electroweak data. For low $\tan\beta$ we find a new region of the parameter space, with $M_2\approx |\mu|$ and $\mu<0$ where $R_b$ remains large, $\sim0.2180$ even for the lighter chargino as heavy as $90-100$ GeV. It is an interesting mixture of the up-higgsino and gaugino. The r\^ole of various phenomenological constraints is discussed in analitic form and importance of small but non-negligible left-right mixing in the stop sector is emphasized in this context. The large $\tan\beta$ option for enhancement of $R_b$ is also reviewed. The available data do not rule out this scenario.