Anatomy of the real Higgs triplet model

Abstract

In this article, we examine the Standard Model extended by a Y = 0 real Higgs triplet, the triangle SM. It contains a CP-even neutral Higgs (triangle(0)) and two charged Higgs bosons (triangle(+/-)), which are quasi-degenerate in mass. We first study the theoretical constraints from vacuum stability and perturbative unitarity and then calculate the Higgs decays, including the loop-induced modes such as di-photons (gamma gamma) and Z gamma. In the limit of a small mixing between the SM Higgs and triangle(0), the latter decays dominantly to WW and can have a sizable branching ratio to di-photon. The model predicts a positive definite shift in the W mass, which is compatible with the current global electroweak fit. At the Large Hadron Collider, it leads to a (i) stau-like signature from pp -> triangle(+)triangle(-) -> tau(+)tau(-)nu nu, (ii) multi-lepton final states from pp -> gamma* -> triangle(+)triangle(-) -> W(+)W(-)ZZ and pp -> W* -> triangle(+/-)triangle(0) -> W(+/-)ZW(+)W(-) as well as (iii) associated di-photon production from pp -> W* -> triangle(+/-)(triangle(0) -> gamma gamma). Concerning (i), the reinterpretation of the recent supersymmetric tau partner search by ATLAS and CMS excludes m(Delta)+/- < 110 GeV at 95% CL. From (ii), some of the signal regions of multi-lepton searches lead to bounds close to the predicted cross-section, but electroweak scale masses are still allowed. For (iii), the recast of the associated di-photon searches by ATLAS and a combined log-likelihood fit of signal and background to data find that out of the 25 signal regions, 10 provide relevant limits on Br(triangle(0) -> gamma gamma) at the per cent level. Interestingly, 6 signal regions show weaker than expected limits at around 152 GeV, leading to a preference for a non-zero di-photon branching ratio of about (0.7 +/- 0.2)%.