Anatomy of the real Higgs triplet model

In this article, we examine the Standard Model extended by a Y=0 real Higgs triplet, the ΔSM. It contains a CP-even neutral Higgs (Δ0) and two charged Higgs bosons (Δ±), 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 (γγ) and Zγ. In the limit of a small mixing between the SM Higgs and Δ0, 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 agrees with the current global electroweak fit. At the Large Hadron Collider, it leads to a (i) stau-like signature from pp→Δ$^+$Δ$^-$→τ$^+$τ$^-$νν¯, (ii) multi-lepton final states from pp→γ$^*$→Δ+Δ−→W$^+$W$^-$ZZ and pp→W$^+$→Δ$^±$Δ$^0$→W$^±$ZW$^+$W$^-$ as well as (iii) associated di-photon production from pp→W$^∗$→Δ$^±$(Δ$^0$→γγ). Concerning (i), the reinterpretation of the recent supersymmetric tau partner search by ATLAS and CMS excludes mΔ$^±$<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. ... mehrFor (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(Δ$^0$→γγ) at the per cent level. Interestingly, 6 signal regions show excesses at around 152 GeV, leading to a preference for a non-zero di-photon branching ratio of about 0.7% with the corresponding significance amounting to about 4σ.