Light Scalars in the Extended Georgi-Machacek Model

Abstract

We perform global fits of the CP-conserving Georgi-Machacek (GM) and extended Georgi-Machacek (eGM) models, incorporating a light CP-even beyond the Standard Model (BSM) scalar within the mass range of $90$ GeV to $100$ GeV. These fits combine the Higgs signal strengths and direct search limits from ATLAS and CMS at $\sqrt{s} = 8$ and $13$ TeV, $B$-physics observables, and theoretical constraints arising from next-to-leading order (NLO) unitarity and BFB constraints. From the global fit, we show that the LHC diphoton and LEP $b\bar{b}$ excesses around $95$ GeV are well compatible with the $125$ GeV Higgs data. Whereas the CMS ditau excess is incompatible with the $125$ GeV Higgs signal strength data in both the CP-conserving GM and eGM models. We present the results from the combined fit, including the $95$ GeV Higgs signal strength data. In the eGM model, the triplet VEV cannot exceed $12$ GeV for additional BSM scalar masses below $160$ GeV and approximately $20$ GeV for additional BSM scalar masses above $160$ GeV. The masses of additional BSM scalars cannot exceed $600$ GeV. The maximum mass splitting is of around $120$ GeV within the members of each custodial multiplet, and up to $250$ GeV between the members of different multiplets. In the GM model, these constraints become more stringent: the triplet VEV is limited to below $15$ GeV, which tightens to $4$ GeV once the BSM scalar masses are below $160$ GeV. Masses of the quintet $m_5$ and the triplet $m_3$ are restricted to be below $530$ GeV and $320$ GeV, respectively. A mass hierarchy, $m_5 > m_3$, is favoured in the high-mass region, with the mass splitting constrained to be less than $210$ GeV.