Higgs properties and fourth generation leptons

It is possible that there are additional vectorlike 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 vectorlike 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 $h\to \gamma \gamma$ 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 ${10}^{15}\mathrm{GeV}$, the new lepton masses cannot be much heavier than about 100 GeV and the Higgs mass must have a value around 175 GeV.

Figure 1

Ratio of the cross section for $gg\to h$ in our model to its standard-model value as a function of the Higgs mass. Here we take $m_U^{\prime }=2m_U^{\prime \prime }=60\mathrm{GeV}$, $m_D^{\prime }=2m_D^{\prime \prime }=60\mathrm{GeV}$, and plot for $M_Q=M_U=M_D=300\mathrm{GeV}$, 600 GeV and 1 TeV from bottom to top.

Figure 2

Ratio of $\Gamma (h\to \gamma \gamma )$ to its standard-model value. Here we take $m_E^{\prime }=m_E^{\prime \prime }=100\mathrm{GeV}$ and 200 GeV from bottom to top.

Figure 3

Branching ratios of the Higgs decay. Here we take $m_E^{\prime }=m_E^{\prime \prime }=100\mathrm{GeV}$ and $m_N^{\prime }=m_N^{\prime \prime }=100\mathrm{GeV}$. The modes in which the final state is standard-model fermion pair are drawn in solid lines ($t\overline{t}$, $b\overline{b}$ 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 $c\overline{c}$ line. The branching ratios of new leptons, $e^{\prime +}{e}^{\prime -}$, $e^{\prime \prime +}{e}^{\prime \prime -}$, ${\nu }^{\prime }{\overline{\nu }}^{\prime }$ and ${\nu }^{\prime \prime }{\overline{\nu }}^{\prime \prime }$, are drawn in dot-dashed lines. In this plot, they all are identical. The line shows the branching ratio for $e^{\prime +}{e}^{\prime -}$ plus $e^{\prime \prime +}{e}^{\prime \prime -}$ (${\nu }^{\prime }{\overline{\nu }}^{\prime }$ plus ${\nu }^{\prime \prime }{\overline{\nu }}^{\prime \prime }$) with sum denoted just by “$e^{\prime +}{e}^{\prime -}$” (“${\nu }^{\prime }{\overline{\nu }}^{\prime }$”).

Figure 4

The same as Fig. 3, except for taking $m_N^{\prime }=m_N^{\prime \prime }=70\mathrm{GeV}$. The branching ratios of $e^{\prime +}{e}^{\prime -}$ and $e^{\prime \prime +}{e}^{\prime \prime -}$ (${\nu }^{\prime }{\overline{\nu }}^{\prime }$ and ${\nu }^{\prime \prime }{\overline{\nu }}^{\prime \prime }$) are equal.

Figure 5

Branching ratios of the Higgs decay in the standard model.

Figure 6

Upper and lower bounds on the Higgs mass as a function of cutoff. The results are shown in solid (dot-dashed) lines for the case of $m_E^{\prime }=m_E^{\prime \prime }=100\mathrm{GeV}$ and $m_N^{\prime }=m_N^{\prime \prime }=100(70)\mathrm{GeV}$. Dotted lines show the results in the standard model.

Figure 7

The same plot as Fig. 6 except for taking $m_E^{\prime }=m_E^{\prime \prime }=m_N^{\prime }=m_N^{\prime \prime }=150\mathrm{GeV}$.

Figure 8

Allowed region on the Higgs mass vs fourth generation masses for ${\Lambda }_c=10\mathrm{TeV}$. Here we take $m_E^{\prime }=m_E^{\prime \prime }=m_N^{\prime }=m_N^{\prime \prime }\equiv m_L$.