Anomaly of Zbb coupling revisited in MSSM and NMSSM

TL;DR

This paper investigates the impact of supersymmetric models (MSSM and NMSSM) on the Zbb coupling anomaly, analyzing one-loop effects under current experimental constraints, and finds limited potential for these models to resolve the discrepancy.

Contribution

It provides a comprehensive analysis of supersymmetric effects on Zbb coupling, considering all current experimental constraints, and assesses their ability to explain the observed anomaly.

Findings

Supersymmetric corrections can reach -2% for rho_b and -6% for sin^2 θ_eff^b.

Sizable negative corrections are opposite to what is needed to fix the anomaly.

Supersymmetry only slightly improves the fit to data, indicating the anomaly may require physics beyond MSSM and NMSSM.

Abstract

The Zbb coupling determined from the Z-pole measurements at LEP/SLD shows an about 3-sigma deviation from the SM prediction, which would signal the presence of new physics in association with the Zbb coupling. In this work we give a comprehensive study for the full one-loop supersymmetric effects on the Zbb coupling in both the MSSM and the NMSSM by considering all current constraints which are from the precision electroweak measurements, the direct search for sparticles and Higgs bosons, the stability of Higgs potential, the dark matter relic density, and the muon g-2 measurement. We analyze the characters of each type of the corrections and search for the SUSY parameter regions where the corrections could be sizable. We find that the sizable corrections may come from the Higgs sector with light m_A and large tan-beta, which can reach -2% and -6% for rho_b and sin^2 \theta_{eff}^b,respectively. However, such sizable negative corrections are just opposite to what needed to solve the anomaly. We also scan over the allowed parameter space and investigate to what extent supersymmetry can narrow the discrepancy. We find that under all current constraints, the supersymmetric effects are quite restrained and cannot significantly ameliorate the anomaly of Zbb coupling. Compared with \chi^2/dof = 9.62/2 in the SM, the MSSM and NMSSM can only improve it to \chi^2/dof = 8.77/2 in the allowed parameter space. Therefore, if the anomaly of Zbb coupling is not a statistical or systematic problem, it would suggest new physics beyond the MSSM or NMSSM.