A Higgs Conundrum with Vector Fermions

TL;DR

The paper investigates how heavy vector-like quarks, in singlet and doublet realizations, affect Higgs production via gluon fusion and Higgs decays, while remaining consistent with precision electroweak data.Using NNLO QCD for Higgs production and EW fits to S, T, U and $Z\to b\bar b$, the authors map the viable parameter space and quantify deviations from the Standard Model predictions.They find that, within the allowed regions, deviations in the Higgs production rate and branching ratios are extremely small due to decoupling, making indirect detection via Higgs observables challenging.Direct detection of the heavy vector-like quarks would be required to distinguish these models; the results emphasize the importance of combining precision EW tests with collider searches in exploring BSM vector-like fermions.

Abstract

Many models of Beyond the Standard Model physics involve heavy colored fermions. We study models where the new fermions have vector interactions and examine the connection between electroweak precision measurements and Higgs production. In particular, for parameters which are allowed by precision measurements, we show that the gluon fusion Higgs cross section and the Higgs decay branching ratios must be close to those predicted by the Standard Model. The models we discuss thus represent scenarios with new physics which will be extremely difficult to distinguish from the minimal Standard Model. We pay particular attention to the decoupling properties of the vector fermions.

Figures (15)

• Figure 1: Allowed $95\%$ confidence level regions from the simultaneous fit to $R_b$ and $A_b$ (red shaded), $R_b$ alone (between solid black lines), and $A_b$ alone (between dashed blue lines).
• Figure 2: Contributions to $\Delta T_F$ , $\Delta S_F$, $\Delta U_F$ from a singlet top partner as a function of $\sin \theta_L$ for fixed $M_T=1$ TeV. The results of Eq. \ref{['singapprox']} in the limit $M_T>>m_t$ are shown as $\Delta T_F$(approx), $\Delta S_F$(approx) and $\Delta U_F$(approx).
• Figure 3: Fermion contributions to $\Delta T_F$ , $\Delta S_F$, $\Delta U_F$ in the singlet top partner model for fixed $\sin\theta_L=0.1$. The dotted lines represent the approximate results from Eq. \ref{['singapprox']} in the limit $M_T>>m_t$.
• Figure 4: Same as Fig. \ref{['fg:dt2']} for a smaller $\sin\theta_L=0.05$.
• Figure 5: $95\%$ confidence level upper bound on the mixing angle $\sin\theta_L$ in the singlet top partner model from experimental restrictions on the $S$, $T$ and $U$ parameters.