Vector Leptons in the Higgs Triplet Model

With the discovery of the Standard Model (SM) Higgslike scalar at the LHC [1], the SM particle content seems complete. In particular, the mass and couplings of the neutral Higgs boson seem to disfavor an additional chiral generation of quarks and leptons [2]. However, additional vector-like fermions, in which an SM generation is paired with another one of opposite chirality, and with identical couplings, are less constrained, as there is no quadratic contribution to their masses. These states appear as natural extensions of the SM particle content in theories with warped or universal extra dimensions, as Kaluza-Klein excitations of the bulk field [3], in nonminimal supersymmetric extensions of the SM [4], in composite Higgs models [5], in the little Higgs model [6] and in gauged flavor groups [7]. Vector fermions have identical left- and right-handed couplings and can have masses which are not related to their couplings to the Higgs bosons [8]. Depending on the dominant decay mode, the limits on new vector-like fermions range from � 100–600 GeV [9], rendering them observable at the LHC. Vector quarks can modify both the production and decays of the Higgs boson at the LHC, while vector leptons do not carry SUð3Þc charge and can only modify the decay patterns of the Higgs. The study of the latter would be a sensitive probe for new physics. The lepton states contribute to self-energy diagrams for electroweak gauge boson masses and precision observables, and consistent limits on their masses and mixings have been obtained [10,11]. Vector leptons have been studied in the context of the SM [11–13], but less so for models beyond the SM, where they can also significantly alter the phenomenology of the model. In the SM, introducing heavy fermions provides a contribution of the same magnitude and sign as that of the top quark and interferes destructively with the dominant W contribution, reducing the h ! �� rate with respect to its SM value. Recent studies indicate that cancellations between scalar and fermionic contributions allow a wide range of Yukawa and mass mixings among vector states [14]. An investigation of vector leptons in the two-Higgsdoublet model [14] showed that the presence of additional Higgs bosons alleviates electroweak precision constraints. Introducing vector leptons into supersymmetry [15] can improvevacuum stability and enhance the diphoton rate by as much as 50%, while keeping new particle masses above 100 GeVand preserving vacuum stability conditions. In the present work, we investigate vector leptons in the context of the Higgs triplet model (HTM). We do not deal with the LHC phenomenology (pair production and decay) of the extra leptons, which has been discussed extensively in the literature [16], choosing instead to focus on signature features of this model. We have previously shown that in the Higgs triplet model, an enhancement of the h ! �� rate is possible only for the case where the doublet and triplet neutral Higgs fields mix considerably [17]. We extend our analysis to include additional vector-like leptons in the model and investigate how these affect the Higgs diphoton decay rate, with or without significant mixing in the neutral Higgs sector. Originally, both the CMS and ATLAS experiments at LHC observed an enhancement of the Higgs diphoton rate, while the diboson rates (h ! WW � , ZZ � ) have been roughly consistent with SM expectations. At present CMS observes � ðpp ! h Þ� BRðh ! �� Þ¼ 0:77 � 0:27 times the SM rate, while ATLAS observes � ðpp ! h Þ� BRðh ! �� Þ¼ 1:55 þ0:33 � 0:28