Extra Families, Higgs Spectrum and Oblique Corrections

TL;DR

The paper investigates whether extra chiral fermion families with weak-scale masses $M_f$ can be reconciled with electroweak precision data, and how their presence interacts with the Higgs sector. Using the oblique parameters $S$, $T$, and $U$, it analyzes contributions from extra fermions and from a two-Higgs-doublet sector, both in general terms and within MN2SSM-inspired spectra. It finds that a single extra chiral family can fit current data without additional new physics, whereas three extra families generally require a two-Higgs-doublet extension to offset positive fermionic $T$ with negative Higgs contributions $T_H$, thereby relaxing the SM-like Higgs mass bound. The results reveal correlated constraints across $S$, $T$, and $U$ that tightly shape the viable spectra and suggest specific Higgs-fermion mass patterns for future collider tests. These findings help delineate the parameter space for extended generations and guide experimental searches for mirror fermions and extended Higgs sectors.

Abstract

The standard model accommodates, but does not explain, three families of leptons and quarks, while various extensions suggest extra matter families. The oblique corrections from extra chiral families with relatively light (weak-scale) masses, $M_{f} \sim <H> $, are analyzed and used to constrain the number of extra families and their spectrum. The analysis is motivated, in part, by recent N = 2 supersymmetry constructions, but is performed in a model-independent way. It is shown that the correlations among the contributions to the three oblique parameters, rather than the contribution to a particular one, provide the most significant bound. Nevertheless, a single extra chiral family with a constrained spectrum is found to be consistent with precision data without requiring any other new physics source. Models with three additional families may also be accommodated but only by invoking additional new physics, most notably, a two-Higgs-doublet extension. The interplay between the spectra of the extra fermions and the Higgs boson(s) is analyzed in the case of either one or two Higgs doublets, and its implications are explored. In particular, the precision bound on the SM-like Higgs boson mass is shown to be significantly relaxed in the presence of an extra relatively light chiral family.

Figures (8)

• Figure 1: The ${95\%\,{\rm C.L.}}$ contours for $S$ and $T$ for fixed values of $U$ (within 2$\sigma$ range) and the reference point ${m_H^{\rm sm}}=100$ GeV. The "$\times$" symbols denote the SM Higgs contributions to $(S,\,T)$ for ${m_H^{\rm sm}}=100,\, 200,\, \dots ,\, 1000$ GeV (from left to right) relative to the reference point (the origin of the $S-T$ plane).
• Figure 2: The contributions to $S$, $T$ and $U$ from one extra family of leptons (solid curves) and quarks (dotted curves).
• Figure 3: $U$ contours in the $S-T$ plane for one generation of (mirror) leptons, which are derived from eqs. (\ref{['eq:Sfermion']})-(\ref{['eq:Ufermion']}) for the mass range $50 \leq m_{\ell} \leq 300$ GeV and with no experimental bounds imposed. The shaded areas cannot be theoretically reached.
• Figure 4: $U$ contours in the $S-T$ plane for one generation of (mirror) quarks, which are derived from eqs. (\ref{['eq:Sfermion']})-(\ref{['eq:Ufermion']}) for the mass range $50 \leq m_{q} \leq 300$ GeV and with no experimental bounds imposed. The shaded areas cannot be theoretically reached.
• Figure 5: Contours for $T_H$ in the 2HDM for $m_A=1000$ GeV and $\beta - \alpha=\pi$ (solid line), $\frac{3\pi}{4}$ (dash-dotted line) and $\frac{\pi}{2}$ (dotted line). Here we consider $\tan\beta>1$ ($\frac{\pi}{4}<\beta<\frac{\pi}{2}$) and $-\frac{\pi}{2}<\alpha<0$, so that $\frac{\pi}{4}<\beta-\alpha<\pi$. The $m_{H^{\pm}}$ value is chosen to minimize $T_H$. These contours are derived from eq. (\ref{['eq:Thiggs']}).