Higgs Couplings and Precision Electroweak Data

TL;DR

This work reevaluates precision electroweak data in light of a 125 GeV Higgs, focusing on the $A_{FB}^b$ and $R_b$ tensions. It demonstrates that non-universal shifts in the $Z\bar{b}b$ vertex provide a compelling NP explanation, and connects these EW effects to altered Higgs couplings through vector-like quarks in a custodial framework. The analysis shows that such models can simultaneously fit EW observables and Higgs signal strengths, notably predicting an enhanced $h\to\gamma\gamma$ rate and accessible TeV-scale mirror quarks. The findings offer concrete collider tests and highlight vacuum-stability considerations that guide UV-completion options, emphasizing the intertwined implications for Higgs phenomenology and EW precision tests.

Abstract

In light of the discovery of a Higgs-like particle at the LHC, we revisit the status of the precision electroweak data, focusing on two discrepant observables: 1) the long-standing 2.4 sigma deviation in the forward-backward asymmetry of the bottom quark A_{FB}^b, and 2) the 2.3 sigma deviation in R_b, the ratio of the Z \rightarrow b \bar b partial width to the inclusive hadronic width, which is now in tension after a recent calculation including new two-loop electroweak corrections. We consider possible resolutions of these discrepancies. Taking the data at face value, the most compelling scenario is that new physics directly affects A_{FB}^b and R_b, bringing the prediction into accord with the measured values. We propose a modified `Beautiful Mirrors' scenario which contains new vector-like quarks that mix with the b quark, modifying the Z b\bar b vertex and thus correcting A_{FB}^b and R_b. We show that this scenario can lead to modifications to the production rates of the Higgs boson in certain channels, and in particular a sizable enhancement in the diphoton channel. We also describe additional collider tests of this scenario.

Figures (4)

• Figure 1: Best-fit region for the $Z \bar{b} b$ coupling shifts $\delta g_{Lb}$, $\delta g_{Rb}$. We show here the region with a moderate positive shift to $\delta g_{Rb}$. The SM corresponds to the green point.
• Figure 2: Best-fit regions of the Higgs signal strength data $(1,2,3\sigma)$ in the $r_g-r_b$ plane. Here we have marginalized over $r_\gamma$, and contours of constant $r_\gamma$ are represented by the red lines. There is a quasi-flat direction in the $\chi^2$ function along $r_b \sim 2\, r_g$ for large value of $r_{b,g}$.
• Figure 3: Preferred regions of parameter space in the $Y_B - Y_X$ plane, with the common vector-like mass fixed to $M=800$ GeV. The dark (light) blue area represents the Higgs signal strength $1\sigma$ ($2\sigma$) preferred region. The gray region predicts oblique parameters $S,T$ outside the $1\sigma$ preferred region. The brown shaded region is excluded by $t^\prime$ searches. We also show in orange the contours of constant signal strength in the diphoton channel $\mu_{\gamma\gamma}$.
• Figure 4: Preferred regions of parameter space in the $M - Y_X$ plane, with the common $B$-sector Yukawa fixed to $Y_B = - 65$ GeV. The dark (light) blue area represents the Higgs signal strength $1\sigma$ ($2\sigma$) preferred region. The gray region predict oblique parameters $S,T$ outside the $1\sigma$ preferred region. The brown shaded region is excluded by $t^\prime$ searches. We also show in orange contours of constant signal strength in the diphoton channel. $\mu_{\gamma\gamma}$