95 GeV Higgs boson in the Georgi-Machacek model

CMS and ATLAS have reported small excesses in the search for low-mass Higgs bosons in the diphoton decay channel at exactly the same mass, 95.4 GeV. These searches rely on improved analysis techniques, enhancing in particular the discrimination against the $Z\to e^{+}{e}^{-}$ background. In models beyond the Standard Model (SM) that extend the Higgs sector with triplets, doubly charged Higgs bosons are predicted which can contribute substantially to the diphoton decay rate of a light Higgs boson. The Georgi-Machacek (GM) Model is of particular interest in this context, since despite containing Higgs triplets it preserves the electroweak $\rho$-parameter to be 1 at the tree level. We show that within the GM model, a Higgs boson with a mass of $\sim 95\mathrm{GeV}$ with a diphoton decay rate as observed by CMS and ATLAS can be well-described. We discuss the diphoton excess in conjunction with an excess in the $b\overline{b}$ final state observed at LEP and an excess observed by CMS in the ditau final state, which have been found at comparable masses with local significances of about $2\sigma$ and $3\sigma$, respectively. The presence of a Higgs boson at about 95 GeV within the GM model would imply good prospects of the searches for additional light Higgs bosons. In particular, the observed excess in the diphoton channel would be expected to be correlated in the GM model with a light doubly charged Higgs boson in the mass range between 100 and 200 GeV, which motivates dedicated searches in upcoming LHC Runs.

Figure 1

Sample distributions in the ${\mu }_{H_1}^{\gamma \gamma }–{\mu }_{H_1}^{bb}$ plane for (a) $X=\gamma \gamma$ [case (i)], (c) $X=\gamma \gamma +bb$ [case (ii)], and (e) $X=\gamma \gamma +bb+\tau \tau$ [case (iii)], as well as in the ${\mu }_{H_1}^{\gamma \gamma }–{\mu }_{H_1}^{\tau \tau }$ plane for (b) case (i), (d) case (ii), and (f) case (iii). The elliptic contours denote the $1\sigma$ bounds of the corresponding 95 GeV excess measurements.

Figure 2

Sample distributions in the (a) $\alpha –v_{\mathrm{\Delta }}$ and the (b) $m_{H_3}–m_{H_5}$ planes for case (i). In plot (a), the solid (dashed) lines denote the ${\kappa }_{XVV}$ (${\kappa }_{Xff}$) contours for $X=h$ and $X=H_1$ in black and purple, respectively. In plot (b), the dashed curve denotes the contour of the coupling mass relation given by Eq. (16) in the exact limit.

Figure 3

Sample distributions in the (a) $m_{H_5}–{\mu }_{\gamma \gamma }^{H_1}$ and (b) $m_{H_5}–{\tilde{\mu }}_{\gamma \gamma }^{H_1}$ planes for case (i).

Figure 4

Sample distributions in the (a) $m_{H_3}–\sigma (gg\to H_3\to \tau \tau )$, (b) $m_{H_5}–\sigma (WZ\to H_5^{\pm }\to WZ)$, and (c) $m_{H_5}–\sigma (WW\to H_5^{\pm \pm }\to WW)$ planes at the 14 TeV LHC for case (i). In (a) we further show the expected 95% CL HL-LHC exclusion bound [105].

Figure 5

Sample distribution in the ${\kappa }_{hff}–{\kappa }_{hVV}$ plane for case (i) and the prospective precision at the $1\sigma$ level (indicated for the SM value) at the HL-LHC (cyan) [105] and the $\mathrm{HL}\text{−}\mathrm{LHC}+\mathrm{ILC}250$ (magenta) [112].

Figure 6

Sample distribution in the $m_{H_1}–{\kappa }_{hVV}^2\times \mathrm{BR}(H_1\to bb)$ plane for case (i) and the 95% CL LEP observed exclusion bound [6] (cyan), the 95% CL LEP expected exclusion bound [6] (magenta), and a projection for the 95% CL ILC250 expected exclusion bound [113] (orange).

Figure 7

Sample distributions for case (i) of $\mathrm{\Delta }{\kappa }_{H_{1}XX}/{\kappa }_{H_{1}XX}$, $XX=bb,\tau \tau ,gg,WW,ZZ$.