Exotic Higgs Decays at a Muon Collider

TL;DR

This paper evaluates a future muon collider's ability to detect exotic Higgs decays into light scalars, using machine learning to enhance sensitivity in specific decay channels, surpassing current collider limits.

Contribution

It introduces a detailed analysis of exotic Higgs decay detection at muon colliders, highlighting improved sensitivities over existing methods and exploring different decay modes and energies.

Findings

The 4b decay mode can detect BR(h→SS→4b) down to 10^{-2} at 3 TeV and 10^{-3} at 10 TeV.

The 2b2μ mode can probe BR(h→SS→2b2μ) down to 10^{-5} at 10 TeV.

Sensitivity to BR(h→SS) remains high when S decays predominantly into b-quarks.

Abstract

We study the sensitivity of a future muon collider to exotic Higgs decays in a minimal scenario of Standard Model (SM) augmented with a light singlet scalar $S$. We consider the decay $h\to SS$ and $S$'s subsequently decay back to SM. In particular, we focus on final states with four bottom quarks ($4b$), or two bottom quarks and two muons ($2b2\mu$). Analyses are performed for two muon collider benchmark configurations: center-of-mass collision energy $\sqrt{s}=3~\mathrm{TeV}$ with $1~\mathrm{ab}^{-1}$ data and $\sqrt{s}=10~\mathrm{TeV}$ with $10~\mathrm{ab}^{-1}$ data. Machine-learning techniques are applied to suppress backgrounds and mitigate jet-combinatorics effects in both channels. We find that the $4b$ mode could be sensitive to the branching ratio, BR$(h \to SS \to 4b)$, of ${\cal O}(10^{-2})$ at 3 TeV and ${\cal O}(10^{-3})$ at 10 TeV, significantly improving upon high-luminosity LHC projections. In the Higgs-portal model with $S$ coupling to SM only through mixing with the Higgs, the sensitivities to BR$(h \to SS)$ remain at the same level given ${\cal O}(1)$ branching fraction of $S$ decaying into $b$-quarks. The $2b2\mu$ mode benefits from a clean dimuon resonance and can probe BR$(h\to SS\to 2b2\mu)$ down to $10^{-5}$ level at a 10 TeV muon collider. But the sensitivity to BR$(h \to SS)$ will be significantly reduced due to the small branching fraction of $S$ decaying into muons in the Higgs portal model.