Precision Higgs Probe of Type-II Seesaw

TL;DR

This paper investigates the type-II seesaw mechanism with an ${ m SU}(2)_{ m L}$-triplet scalar to identify parameter regions that evade direct collider searches. It focuses on the loop-induced Higgs decay to diphotons, $h\to\gamma\gamma$, where charged Higgs states $H^{\pm}$ and $H^{\pm\pm}$ can modify the rate and thereby constrain the model indirectly. The authors derive the relevant scalar potential, mass relations, and couplings, including the trilinear terms $\lambda_{hH^+H^-}$ and $\lambda_{hH^{++}H^{--}}$, and compute the impact on the effective $h\gamma\gamma$ coupling via loop functions, incorporating EW precision data and collider bounds. By projecting future precision on the diphoton signal strength, $\mu_{h\to\gamma\gamma}$, they show that sub-percent measurements at HL-LHC and future lepton colliders can decisively probe the cascade-dominated, presently unconstrained region, potentially excluding most of it or leaving only a few isolated points depending on the facility (e.g., CEPC/FCC-ee or MuC). Overall, the work demonstrates that precision Higgs measurements offer a powerful, complementary avenue to test the type-II seesaw parameter space beyond direct searches.

Abstract

Despite direct searches at the LHC excluding triplet-like Higgs bosons up to several hundred GeV over much of the type-II seesaw model parameter space, parts of it -- most notably those featuring ``cascade decays'' of the charged Higgs bosons into their neutral partners and off-shell $W$ bosons -- still remain unconstrained. Meanwhile, measurements of the diphoton signal strength of the Standard Model (SM) Higgs boson -- potentially modified by loop contributions from triplet-like Higgs states -- are in good agreement with the SM expectation, with combined experimental uncertainties currently at approximately 8\%. Given the trend in previous measurements, it is expected that future precision Higgs measurements at the HL-LHC and a future lepton collider such as CEPC, FCC-ee, or Muon Collider will be consistent the standard diphoton signal strength, albeit with significantly reduced uncertainties, down to about 0.7\%. Presuming this and considering all relevant constraints, we explore whether such increasingly precise diphoton measurements can indirectly probe the parameter space that currently evades direct searches. We find that sub-percent-level determinations of the diphoton rate will decisively probe a substantial fraction of this otherwise elusive region.

Figures (3)

• Figure 1: Decay phase diagram of the doubly-charged Higgs bosons in the $v_\Delta$--$\Delta m$ parameter space. Dashed (solid) contours in magenta, orange, and light blue represent 95% (90%) branching ratios, while black solid contours indicate 50% branching ratios for the corresponding decay modes. The region shown in blue remains unconstrained by the direct searches thus far. For definiteness, we set $m_{H^{\pm\pm}} = 400$ GeV. The yellow-shaded region above the dashed horizontal line is excluded by the current EWPD at 95% CL. For the other triplet-like Higgs states, the decay phase diagrams are qualitatively similar. For the $\Delta m < 0$ scenario, the region with $\Delta m \lesssim -40$ GeV is excluded by EWPD.
• Figure 2: Estimated 95% CL lower limits on $m_{H^{\pm\pm}}$ in the $v_\Delta$--$\Delta m$ parameter space. Entries marked with superscript '$\dagger 1$', ('$\dagger 2$') ['$\dagger 3$'] and '$' respectively denote region where masses of 150--200 (95--200) [80--200] GeV and 55.6--200 GeV are not excluded yet; those with a superscript '#' ('*') refer to limits derived from LEP monophoton searches ($Z$-pole width bound on $Z\to H^+H^-$).
• Figure 4: Direct-search--inaccessible parameter space allowed by theoretical constraints and EWPD at 95% CL (light gray). Overlaid regions in blue shades indicate the parameter space compatible with future $\mu_{h\to\gamma\gamma}$ measurements of increasing precision (darker blue represents higher precision). The zoomed-in panel on the right highlights the cascade–decay regime, characterized by the sequential decay $H^{\pm\pm} \to H^{\pm} \to H^0/A^0$, which can be almost completely probed by the expected precision to be achieved at a future MuC.