Higgs Properties and Fourth Generation Leptons

TL;DR

The paper investigates a vector-like fourth generation in which quarks acquire mass without electroweak breaking while leptons gain mass from the Higgs. It analyzes Higgs production and decay, predicting SM-like gluon-fusion production but a significant suppression of h→γγ due to lepton loops, with possible new leptonic Higgs decays altering branching ratios. A renormalization-group treatment of the Yukawa and Higgs sectors yields Higgs-mass bounds that depend on the theory's cutoff, suggesting m_h ≈ 175 GeV for high-scale cutoffs and a broader 120–400 GeV window for low-scale cutoffs. The results point to distinctive collider signatures that could test this scenario while remaining perturbative in the quark sector up to high scales.

Abstract

It is possible that there are additional vector-like generations where the quarks have mass terms that do not originate from weak symmetry breaking, but the leptons only get mass through weak symmetry breaking. We discuss the impact that the new leptons have on Higgs boson decay branching ratios and on the range of allowed Higgs masses in such a model (with a single new vector-like generation). We find that if the fourth generation leptons are too heavy to be produced in Higgs decay, then the new leptons reduce the branching ratio for h -> gamma gamma to about 30% of its standard-model value. The dependence of this branching ratio on the new charged lepton masses is weak. Furthermore the expected Higgs production rate at the LHC is very near its standard-model value if the new quarks are much heavier than the weak scale. If the new quarks have masses near the cutoff for the theory then for cutoffs greater than 10^15 GeV, the new lepton masses cannot be much heavier than about 100 GeV and the Higgs mass must have a value around 175 GeV.

Figures (8)

• Figure 1: Ratio of the cross section for $gg\rightarrow h$ in our model to its standard-model value as a function of the Higgs mass. Here we take $m_U'=2m_U"=60~{\rm GeV}$, $m_D'=2m_D"=60~{\rm GeV}$, and plot for $M_Q =M_U=M_D=300~{\rm GeV}$, $600~{\rm GeV}$ and $1~{\rm TeV}$ from bottom to top.
• Figure 2: Ratio of $\Gamma(h\rightarrow \gamma\gamma)$ to its standard-model value. Here we take $m_E'=m_E"=100~{\rm GeV}$ and $200~{\rm GeV}$ from bottom to top.
• Figure 3: Branching ratios of the Higgs decay. Here we take $m_E'=m_E"=100~{\rm GeV}$ and $m_{N}'=m_{N}"=100~{\rm GeV}$. The modes in which the final state is standard-model fermion pair are drawn in solid line ($t\bar{t}$, $b{\bar{b}}$ and $\tau^+ \tau^-$), and the modes in which the final state is standard-model gauge bosons, i.e., $W^+W^-$, $ZZ$, $\gamma\gamma$, $\gamma Z$ and $gg$, are in dashed lines. Here we omit $c \bar{c}$ line. The branching ratios of new leptons, $e'^+e'^-$, $e"^+e"^-$, ${\nu}'\bar{\nu}'$ and ${\nu}"\bar{\nu}"$, are drawn in dot-dashed lines. In this plot, they all are identical. The line shows the branching ratio for $e'^+e'^-$ plus $e"^+e"^-$ (${\nu}'\bar{\nu}'$ plus ${\nu}"\bar{\nu}"$) with sum denoted just by "$e'^+e'^-$" ("${\nu}'\bar{\nu}'$").
• Figure 4: The same as Fig. \ref{['fig:sm4i']}, except for taking $m_N'=m_N"=70~{\rm GeV}$. The branching ratios of $e'^+e'^-$ and $e"^+e"^-$ ($\nu'\bar{\nu}'$ and $\nu"\bar{\nu}"$) are equal.
• Figure 5: Branching ratios of the Higgs decay in the standard model.