R_b and New Physics: A Comprehensive Analysis
P. Bamert, C. P. Burgess, J. M. Cline, D. London, E. Nardi
TL;DR
The paper addresses the LEP/SLC R_b discrepancy by formulating a general framework that covers both tree-level Zbb mixing and one-loop corrections from new scalars/fermions. It analyzes a wide range of models, identifying viable mechanisms that can raise R_b toward the experimental value, including several that align with SUSY ideas, while showing that loop-only t-t' mixing cannot fully explain the data. The authors also discuss oblique constraints, provide detailed formulae for how new physics modifies Zbb couplings, and propose future tests, notably rare B decays and direct searches, to distinguish among competing explanations. Overall, they conclude that while many NP scenarios can alleviate the R_b tension, a robust, unique explanation requires careful consideration of both precision electroweak constraints and upcoming experimental tests.
Abstract
We survey the implications for new physics of the discrepancy between the LEP measurement of $R_b$ and its Standard Model prediction. Two broad classes of models are considered: ($i$) those in which new $Z\bbar b$ couplings arise at tree level, through $Z$ or $b$-quark mixing with new particles, and ($ii$) those in which new scalars and fermions alter the $Z \bbar b$ vertex at one loop. We keep our analysis as general as possible in order to systematically determine what kinds of features can produce corrections to $R_b$ of the right sign and magnitude. We are able to identify several successful mechanisms, which include most of those which have been recently been proposed in the literature, as well as some earlier proposals (\eg\ supersymmetric models). By seeing how such models appear as special cases of our general treatment we are able to shed light on the reason for, and the robustness of, their ability to explain $R_b$.