Searching for dark photons from dark-scalar decays at CEPC and FCC-ee

TL;DR

This paper explores the potential of CEPC and FCC-ee colliders to detect long-lived dark photons from dark-scalar decays, proposing search strategies for displaced vertices and demonstrating significant sensitivity improvements over current bounds.

Contribution

It introduces new search strategies for long-lived dark photons at future colliders and quantifies their sensitivity, surpassing existing experimental limits.

Findings

Proposed search strategies can detect dark photons with kinetic-mixing parameters much lower than current bounds.

Monte Carlo simulations show high signal acceptance for the targeted decay channels.

Sensitivity to dark photons extends to masses below half of the dark-scalar mass.

Abstract

We investigate the sensitivity of proposed CEPC and FCC-ee with a center-of-mass energy of 240 GeV to long-lived dark photons heavier than 2 GeV that are pair-produced via the prompt decays of a light scalar mixed with the Standard-Model Higgs boson. We compute the production and decay rates of both the light scalar and the dark photon, and develop two search strategies targeting displaced vertices within the inner tracker of the main detectors. Using Monte Carlo simulations, we evaluate the signal acceptance and projected sensitivity for each strategy. Our results show that, for the scalar-Higgs mixing angle set at $10^{-2}$ just below the current upper limit, the proposed searches at CEPC and FCC-ee can probe dark-photon kinetic-mixing parameter several orders of magnitude below existing bounds, for dark photons lighter than half the dark-scalar mass.