[Paper Review] Probing the dark sector via searches for invisible decays of the Higgs boson at the ILC

Yuji Katō|arXiv (Cornell University)|Feb 27, 2020

Dark Matter and Cosmic Phenomena5 citations

TL;DR

This paper evaluates the International Linear Collider (ILC) with the International Large Detector (ILD) for probing dark matter via invisible Higgs boson decays. Using full detector simulation at √s = 250 GeV and 500 GeV, it achieves a 95% confidence level upper limit of 0.23% on the branching ratio of Higgs invisible decays, outperforming the HL-LHC prospect by a factor of 10 and demonstrating the ILC's superior sensitivity for low-mass dark matter searches.

ABSTRACT

Although the existence of Dark Matter (DM) has been suggested by various astrophysical observations, it has not yet been discovered today. We can assume a scenario in which the particles that account for the DM can interact with the Standard Model particles only through their couplings with the Higgs sector, the so-called Higgs-portal model. This model can be investigated by collider experiment using the invisible decay of Higgs boson. In this study, we evaluate the search ability of International Linear Collider (ILC) for invisible decay of the Higgs using International Large Detector (ILD) full detector simulation. We estimate 95% C.L. upper limit (UL) on the branching ratio of invisible Higgs decays and compare them between two center-of-mass energy conditions: $\\sqrt{s} = 250$ GeV and 500 GeV. In addition, we describe the complementarity of lepton collider experiment to the direct detection experiment about DM search ability.

Motivation & Objective

• To evaluate the sensitivity of the International Linear Collider (ILC) with the ILD detector to invisible decays of the Higgs boson.
• To compare the discovery potential of ILC at √s = 250 GeV and 500 GeV for invisible Higgs decays.
• To assess the complementarity of lepton collider experiments like ILC with direct detection experiments in probing dark matter.
• To quantify the reach of ILC in constraining the Higgs-portal dark matter model through invisible Higgs decay branching ratios.

Proposed method

• Full detector simulation using Mokka/DD4Hep and Geant4-based ILD detector models for √s = 250 GeV and 500 GeV.
• Event reconstruction with the PandoraPFA algorithm in the Marlin framework to reconstruct particles and jets with high resolution.
• Application of multivariate isolation tagging (MVA) for leptons and selection cuts on particle flow objects (PFOs), track multiplicity, and jet kinematics.
• Recoil mass reconstruction to identify Higgs bosons via missing energy in e+e− → ZH with H → invisible and Z → q̄q.
• Significance estimation via root mean square combination across recoil mass bins, followed by 95% C.L. upper limit calculation using the significance of a 10% branching ratio signal.
• Incorporation of beamstrahlung, initial state radiation, and γγ backgrounds in signal and background simulations.

Experimental results

Research questions

• RQ1 What is the sensitivity of the ILC at √s = 250 GeV and 500 GeV to invisible Higgs decays using the ILD detector?
• RQ2 How does the ILC's performance in detecting invisible Higgs decays compare to the HL-LHC prospect?
• RQ3 What is the role of beam polarization and Z → q̄q vs. Z → l̄l decay modes in enhancing sensitivity to invisible Higgs decays?
• RQ4 How does the ILC's reach for dark matter compare to direct detection experiments, especially for low-mass dark matter?
• RQ5 To what extent can the ILC probe the Higgs-portal model through invisible Higgs decay branching ratios?

Key findings

• The 95% C.L. upper limit on the branching ratio of invisible Higgs decays at √s = 250 GeV is 0.23%, significantly better than the HL-LHC prospect of 1.9%.
• The sensitivity at √s = 250 GeV is approximately 10 times better than the HL-LHC, with the hadronic Z → q̄q channel contributing dominantly to the sensitivity.
• The upper limit improves to 0.78% at √s = 500 GeV when combining both beam polarization configurations, though the sensitivity is lower than at 250 GeV due to reduced event rate.
• The ILC's clean environment and recoil mass technique enable high-precision measurement of missing energy, making it ideal for probing invisible Higgs decays.
• Collider experiments like ILC are complementary to direct detection, with superior sensitivity for low-mass dark matter (below ~10 GeV).
• The ILC's reach for dark matter is comparable to other lepton collider projects, highlighting its unique role in probing the Higgs-portal model.