Beautiful Mirrors and Precision Electroweak Data

TL;DR

The paper investigates whether precision electroweak data anomalies in the bottom sector can be reconciled by introducing vector-like bottom-like quarks that mix with the SM bottom. It analyzes two viable implementations—Standard Mirror Quark Doublets and Top-less Mirror Quark Doublets—showing that both improve fits to EW observables, with distinct implications for Higgs mass guidance and collider signatures. The study also discusses oblique parameters, potential unification of gauge couplings, and cosmological considerations, outlining clear experimental tests at the Tevatron, LHC, and future linear colliders. Overall, it provides a concrete, testable BSM framework that links EW precision data, Higgs phenomenology, collider physics, and high-energy unification questions through the mechanism of bottom-quark mixing with vector-like exotics.

Abstract

The Standard Model (SM) with a light Higgs boson provides a very good description of the precision electroweak observable data coming from the LEP, SLD and Tevatron experiments. Most of the observables, with the notable exception of the forward-backward asymmetry of the bottom quark, point towards a Higgs mass far below its current experimental bound. The disagreement, within the SM, between the values for the weak mixing angle as obtained from the measurement of the leptonic and hadronic asymmetries at lepton colliders, may be taken to indicate new physics contributions to the precision electroweak observables. In this article we investigate the possibility that the inclusion of additional bottom-like quarks could help resolve this discrepancy. Two inequivalent assignments for these new quarks are analysed. The resultant fits to the electroweak data show a significant improvement when compared to that obtained in the SM. While in one of the examples analyzed, the exotic quarks are predicted to be light, with masses below 300 GeV, and the Higgs tends to be heavy, in the second one the Higgs is predicted to be light, with a mass below 250 GeV, while the quarks tend to be heavy, with masses of about 800 GeV. The collider signatures associated with the new exotic quarks, as well as the question of unification of couplings within these models and a possible cosmological implication of the new physical degrees of freedom at the weak scale are also discussed.

Figures (4)

• Figure 1: The forward-backward asymmetry for the $b$-quark as a function of $\sqrt{s}$ for the four solutions of eq.(\ref{['coupnum']}). The signs in the parentheses refer to those for ($\bar{g}_L^b, \bar{g}_R^b)$ in the same order as in eq.(\ref{['coupnum']}) with $(+, +)$ being SM-like. The experimental data correspond to the measurements reported in Refs.DELPHIALEPHL3OPALinoueshimonakanakanoVENUSPEPPETRA_jadePETRA_tasso.
• Figure 2: The regions in the $Z\bar{b} b$ coupling parameter space that are favoured by the observed values of $A^b_{FB}$ (flatter curves) and $R_b$ (steeper curves). For each set, the innermost curve leads to the experimental central value while the sidebands correspond to the $1 \sigma$ and $2 \sigma$ error bars. The Standard Model point is at the origin.
• Figure 3: Region in the $m_H$--$m_{\chi}$ parameter space (in the model with standard mirror quark doublets) that is consistent with the best fit point (marked) at the 68% C.L. and 99.5% C.L. respectively.
• Figure 4: Region in the $m_H$--$m_{\chi}$ parameter space (in the model with top-less mirror quark doublets) that is consistent with the best fit point (marked) at the 68% C.L. and 99.5% C.L. respectively.