Exotic leptons: Higgs, flavor and collider phenomenology

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 → γγ decay rate for new lepton masses in the 300 GeV-1 TeV range. We analyze vacuum stability implications for different hypercharges. Effects in h → Zγ are typically much smaller than in h → γγ, but distinct among the considered hypercharge assignments. Nonstandard 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.

New vector-like fermions and flavor physics

“…(For some earlier studies of such models, see, e.g., refs. [1][2][3][4][5][6][7][8][9][10].) We focus on vector-like fermions that can have Yukawa couplings involving the Higgs field and the SM fermions.…”

“…For B → πνν, Belle II expects to reach a sensitivity at the 1 × 10 −5 level [28], which will provide weaker bounds than B → π + − in the models considered in this paper. 6 Despite the stated uncertainty, the probability that all 7 observed events were due to background was quoted as 0.001 [41]. 7 At the SM level, B → Kνν is expected to get large backgrounds from B → K * νν [28].…”

Light vectorlike fermions in a minimalSU(5)setup

“…In our work, on the other hand, we assume vector-like fermions to be light enough to be accessible at LHC and study current experimental constraints on the couplings of these states to the matter fields. Recent phenomenological analyses of vector-like fermions in different representations can be found for example in [34][35][36][37][38][39][40][41]. The minimal vector-like fermion extension of the original SU (5) proposal requires very few new couplings, preserves renormalizability and reproduces mass relations among charged leptons and down-type quarks in accordance with experimental data.…”

“…The constraints on the diagonal entries of Y e coming from Z-pole physics, [48], [39], are given in Table II. The µ-e conversion in nuclei sets a very severe bound on Y e 12 , since it occurs at tree level in this setup.…”

Virtual signatures of dark sectors in Higgs couplings

“…Similar scenarios have been discussed for instance in refs. [8,40]. The evolution for λ is obtained by solving the coupled system of equations in eq.…”