Abstract
We study extensions of the standard model by one generation of vector-like leptons with non-standard hypercharges, which allow for a sizable modification of the h -> gamma gamma decay rate for new lepton masses in the 300 GeV - 1 TeV range. We analyze vaccum stability implications for different hypercharges. Effects in h -> Z gamma are typically much smaller than in h -> gamma gamma, but distinct among the considered hypercharge assignments. Non-standard hypercharges constrain or entirely forbid possible mixing operators with standard model leptons. As a consequence, the leading contributions to the experimentally strongly constrained electric dipole moments of standard model fermions are only generated at the two loop level by the new CP violating sources of the considered setups. We derive the bounds from dipole moments, electro-weak precision observables and lepton flavor violating processes, and discuss their implications. Finally, we examine the production and decay channels of the vector-like leptons at the LHC, and find that signatures with multiple light leptons or taus are already probing interesting regions of parameter space.
Highlights
Most interesting ones for the LHC to answer
We study extensions of the standard model by one generation of vector-like leptons with non-standard hypercharges, which allow for a sizable modification of the h → γγ decay rate for new lepton masses in the 300 GeV–1 TeV range
We argue that a different hypercharge assignment to the new vector-like leptons can in principle relax the vacuum instability bound, but simultaneously ensures automatically that the leading contributions to dipole moments only arise at the 2-loop level
Summary
The loop induced coupling of the Higgs to two photons is sensitive to the Yukawa coupling of any internal fermion and to its electric charge, see the diagram on the left hand side of figure 1. That in contrast to the n = 0 scenario, for n = 1 there is only one off-diagonal mixing term between the vector-like and the SM leptons As a consequence, both in the n = 2 and n = 1 scenarios, at leading order, the only non-SM contributions to the h → γγ decay rate are generated by the mass matrix ME. While the expression (3.2) captures the leading contributions in an expansion in the ratio of the electro-weak scale over the vector masses, one can go beyond this approximation working with mass eigenstates of the new leptons. Due to their large charges, the new leptons can lead to sizable effects in h → γγ even for moderate values of the Yukawa couplings. For fixed Yukawa couplings, higher charges allow for heavier vector masses
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