The dark side of the Higgs boson

TL;DR

This work proposes that current LHC Higgs exclusions could reflect a large invisible decay width rather than only a suppressed production rate, motivating a search for invisible Higgs decays and a method to infer the invisible width from the $h\to ZZ\to 4\ell$ lineshape in the heavy-mass regime. It develops a quantitative lineshape analysis that accounts for reduced signal strength due to invisibles and shows that, for $m_h$ above ~200 GeV, the total width can be extracted with notable precision (∼20% with ~30 fb$^{-1}$ at 14 TeV), providing an indirect probe of hidden sectors. The paper also examines Higgs-portal dark matter with singlet scalars or fermions, deriving the invisible width, relic-density predictions, and direct-detection constraints, and finds that minimal models are highly constrained unless the relic density constraint is relaxed. Overall, the study links Higgs phenomenology to dark-sector physics, showing how width measurements, rate information, and DM constraints together shape the viable parameter space for Higgs invisible decays and Higgs-portal DM.

Abstract

Current limits from the Large Hadron Collider exclude a standard model-like Higgs mass above 150 GeV, by placing an upper bound on the Higgs production rate. We emphasize that, alternatively, the limit could be interpreted as a lower bound on the total decay width of the Higgs boson. If the invisible decay width of the Higgs is of the same order as the visible decay width, a heavy Higgs boson could be consistent with null results from current searches. We propose a method to infer the invisible decay of the Higgs by using the width of the measured h to ZZ to 4l lineshape, and study the effect on the width extraction due to a reduced signal strength. Assuming the invisible decay product is the dark matter, we show that minimal models are tightly constrained by limits from Higgs searches at the LHC and direct detection experiments of dark matter, unless the relic density constraint is relaxed.

Figures (7)

• Figure 1: Current Higgs search limits at the LHC ATLAS-CONF-2011-112, presented in terms of upper limits on the production cross section. We also show the equivalent suppression in the cross section from an increase in the total decay width of the Higgs that is 1.5, 2, and 3 times the SM total width.
• Figure 2: The left panel shows measured Higgs width in range of Higgs masses in the $4\mu$ and $2e2\mu$ channels for 30 fb$^{-1}$ of integrated luminosity with $\Gamma_h^{\rm inp} = \Gamma_h^{\rm SM}$ and $2 \times \Gamma_h^{\rm SM}$. The $1\sigma$ range is denoted by the light outer curves. The input width, $\Gamma_h^{\rm inp}$, is denoted by the solid black curve. The right panel, on the other hand, shows $1\sigma$ fractional uncertainty extracted from the fit.
• Figure 3: Assuming a SM input width, the uncertainty in width extraction can be converted to a 95% C.L. reach in the invisible width, as shown in the left panel. The right panel shows comparison in invisible width measurements between direct probe via VBF ATLHinv and indirect probe via total width. The direct and indirect probes are complimentary to each other.
• Figure 4: Exclusion reach of a SM total decay width. Comparison are made between total widths that are 2 times, 2.5 times, and 3 times the SM expectation.
• Figure 5: Comparison of two different reduction mechanisms in the event rate.