Reproducing the Higgs boson data with vector-like quarks

TL;DR

This work investigates vector-like quarks as a minimal, testable explanation for the Higgs-rate deviations observed around $m_h\simeq125$ GeV. By constructing minimal VL-quark models with exotic charges $Q_{em}=8/3$ or $-7/3$, the authors show that Higgs observables can be driven toward their experimental central values through both mixing-enhanced $h\to bb$ decays and loop-induced $h\to\gamma\gamma$ enhancements, while satisfying direct mass bounds and electroweak precision tests (EWPT). The study identifies three model classes (I, II, III) with distinct field contents and provides explicit Lagrangians and decay patterns, revealing a consistent improvement in the global Higgs fit, albeit with some tension against oblique parameters $S$ and $T$. The results predict TeV-scale exotic VL quarks and distinctive collider signatures, potentially connected to custodial RS/composite Higgs frameworks, and offer concrete guidance for future LHC searches and precision Higgs measurements.

Abstract

Vector-Like (VL) quarks arise in the main alternatives to the supersymmetric extensions of the Standard Model (SM). Given the experimental possibility of a 125 GeV Higgs boson with rates significantly different from the SM expectations, it is motivating to study the effects of VL quarks on the Higgs boson cross sections and branching ratios. We perform a systematic search for the minimal field contents and gauge group representations of VL quarks able to significantly improve the fit of the measured Higgs rates, and simultaneously, to satisfy the direct constraints on VL quark masses as well as the electro-weak precision tests. In particular, large enhancements can be achieved in certain diphoton channels - as pointed out by both the ATLAS and CMS Collaborations - optimizing then the Higgs rate fit. This is a consequence of the introduction of VL quarks, with high electric charges of 8/3 or -7/3, which are exchanged in the Higgs-to-diphoton loop. Interestingly, the field contents and formal Higgs couplings obtained here are similar to those of scenarios in warped/composite frameworks arising from different motivations. The various exotic-charge quarks predicted, possibly below the TeV scale, might lead to a rich phenomenology soon at the LHC.

Figures (2)

• Figure 1: Left: Domain leading to $125$ GeV Higgs boson rates inside the experimental $1\sigma$ intervals for the Model II, with the $(b",q_{-4/3})$ doublet ( c.f. Eq.(\ref{['Eq:LagModII']})), in the plan $m_{8/3}$ versus $m'_{8/3}$ (in GeV). The values of the other parameters are fixed at $Y=1.01$, $Y'=1$, $Y_{8/3}=2.5$, $Y_{5/3}=-0.5$, $Y_b =-0.053$, $Y'_b =1$, $Y"_b=1$, $Y_{-1/3}=1$, $m'=1200$ GeV, $m_{-4/3}=900$ GeV, $m_{5/3}=1000$ GeV. Contour-level curves for the physical masses $m_{q^1_{8/3}}$ and $m_{q^{1,2}_{5/3}}$ are also shown. The other mass eigenvalues are almost constant over the shown plan like, $m_{b_1}\approx 4$ GeV, $m_{b_2}\approx 840$ GeV, $m_{b_3}\approx 1290$ GeV, $m_{t_1}\approx 173$ GeV, $m_{q^1_{-4/3}}\approx 900$ GeV, or mainly depending on $m_{8/3}$ like, $m_{t_2}\approx 900 - 1010$ GeV in the domain shown, or around $2$ TeV in this domain: $m_{t_3}\approx 1250 - 3000$ GeV, $m_{q^3_{5/3}}\approx 1500 - 3000$ GeV, $m_{q^2_{8/3}}\approx 1600 - 3000$ GeV. Right: Central values and $1\sigma$ error bars for the strength modifiers $\mu_{h\gamma}$, $\mu_{hV}$, $\mu_{h\tau}$, $\mu_{Vb}$, $\mu_{qW}$, $\mu_{q\gamma}$ and $\mu_{X\gamma}$ (defined in Section \ref{['hdata']}) measured by the experiments indicated in front, for $m_h=125$ GeV. The various strength modifiers are indicated by the associated cross sections and branching ratios. The plus symbols mean that the experimental results are combined. In each case the SM prediction corresponds to $\mu = 1$ leading to the global $\chi_{\rm SM}^2$ value written in the figure. The small black circles correspond to the theoretical predictions of the strength modifiers for the point of parameter space also indicated as a circle on the left-side plot. This parameter set leads to the oblique $S,T$ and Higgs fit $\chi^2$ values indicated near the circle on the top-left part of the figure. The little black squares are associated to a second parameter set where only one of the parameter values is changed: $Y_{8/3}=2.2$.
• Figure 2: Left: Same as in Fig.(\ref{['Fig:Model.II']}) but for the Model I with the $(b",q_{-4/3})$ doublet [see Eq.(\ref{['Eq:model.I.2']})] in the plan $m_{-7/3}$ versus $m'_{-7/3}$ (in GeV). The fixed parameters read as, $Y=1$, $Y_{-7/3}=3$, $Y_b =-0.053$, $Y'_b =1$, $Y_{-1/3} =1$, $m'=1200$ GeV, $m_{-4/3}=900$ GeV. Three contour-level curves for $m_{q^1_{-7/3}}$ are shown. Other masses are almost constant over the plan, $m_{b_1}\approx 4$ GeV, $m_{b_2}\approx 840$ GeV, $m_{b_3}\approx1290$ GeV, $m_{t_1}\approx 173$ GeV, and the remaining ones are, $m_{q^2_{-7/3}}\approx 1750 - 2750$ GeV [in the presented domain], $m_{q^1_{-4/3}}= min(m_{-4/3},m_{-7/3})$, $m_{q^2_{-4/3}}= max(m_{-4/3},m_{-7/3})$ [as there is no mixing term]. Right: Same as in Fig.(\ref{['Fig:Model.II']}) for the experimental data but with theoretical predictions from the Model I [with $(b",q_{-4/3})$]. The black squares are associated to the same input parameter values as for the black circles except that $Y_{-7/3}=2.2$.