Searching for dark photons in $J/\psi$ decays

TL;DR

This paper investigates dark photon signatures in $J/ar{ ext{psi}}$ decays, analyzing both visible and invisible channels within NRQCD, and provides numerical predictions for event yields at BESIII.

Contribution

It offers the first detailed numerical analysis of dark photon effects in $J/ar{ ext{psi}}$ decays, including both visible and invisible decay modes, with predictions for experimental observability.

Findings

Two-body decay channels yield up to 37 events with very low significance.

Four-body decay channels yield about 94-172 events in low mass regions.

Invisible decay channels can produce up to 129 events, with some having better significance.

Abstract

A dark photon is an Abelian gauge boson from a new $U(1)_D$ gauge symmetry, coupled to the Standard Model via kinetic mixing, with $\varepsilon$ inducing an effective coupling to the electromagnetic current and $g_\chi$ to a stable dark matter particle $\chi$. We study $J/\psi$ two-body and four-body decays via a light-mass dark photon ($m_U < 3.0$ GeV) in the framework of non-relativistic QCD (NRQCD), considering both visible and invisible decays of the dark photon into SM fermions or dark sector particles. Numerical results for the decay ratios $\Gamma/\Gamma_{J/\psi}$ and expected event numbers at BESIII are presented, along with the significance $S/\sqrt{B}$ and $p_T$ distributions where applicable. Our results show that, for $m_U < 2m_\chi$, the two-body final state decay channels of the $J/\psi$ mediated by a dark photon have event yields $0\sim 37$ with significances of $10^{-5}\sim10^{-6}$, while the four-body final state channel yields about $94\sim172$ events in the very low mass region $m_U < 0.2\ \text{GeV}$. For $m_U \ge 2m_\chi$, the invisible two-body final state decay channel yields $0\sim12$ events with better significance $10^{-1}\sim10^{-3}$, and the invisible four-body final state decay channel yields $0\sim129$ events, whereas visible decays are all severely suppressed.