Probing a light dark sector at future lepton colliders via invisible decays of the SM-like and dark Higgs bosons

TL;DR

This paper investigates the potential to detect light dark sector particles at future lepton colliders through invisible decays of the SM-like and dark Higgs bosons, providing a novel collider search strategy for dark matter models involving dark Higgs fields.

Contribution

It presents the first collider search considering both Higgs bosons for invisible decays in a UV-complete dark sector model, using multivariate analysis at the ILC.

Findings

Large parameter space can be probed at the ILC at 250 GeV.

Dark Higgs invisible decay can be detected even when SM-like Higgs decay is below sensitivity.

The decay of dark Higgs into dark sector particles is a key signature of the light dark sector.

Abstract

A renormalizable UV model for Axion-Like Particles (ALPs) or hidden photons, that may explain the dark matter usually involves a dark Higgs field which is a singlet under the standard model (SM) gauge group. The dark sector can couple to the SM particles via the portal coupling between the SM-like Higgs and dark Higgs fields. Through this coupling, the dark sector particles can be produced in either the early universe or the collider experiments. Interestingly, not only the SM-like Higgs boson can decay into the light dark bosons, but also a light dark Higgs boson may be produced and decay into the dark bosons in a collider. In this paper, we perform the first collider search for invisible decays by taking both the Higgs bosons into account. We use a multivariate technique to best discriminate the signal from the background. We find that a large parameter region can be {probed} at the International Linear Collider (ILC) operating at the center-of-mass energy of 250 GeV. In particular, even when the SM-like Higgs invisible decay is a few orders of magnitude below the planned sensitivity reaches of the ILC and the high luminosity LHC (HL-LHC), the scenario can be probed by the invisible decay of the dark Higgs boson produced via a similar diagram. Measuring the dark Higgs boson decay into the dark sector will be a smoking gun signal of the light dark sector. A similar search of the dark sector would be expected in, e.g., Cool Copper Collider (C$^3$), Circular Electron Positron Collider (CEPC), Compact Linear Collider (CLIC) and {Future Circular electron-positron Collider (FCC-ee).