Search for Cosmic-Ray Produced Dark Meson via the $U(1)_\text{D}$ Portal at JUNO

Abstract

We investigate the atmospheric production and subsequent detection of sub-GeV dark mesons within the framework of a confining dark sector coupled to the Standard Model (SM) via a $U(1)_{\text{D}}$ vector portal. High-energy cosmic ray interactions in the atmosphere produce dark quarks through proton bremsstrahlung, rare decays of Standard Model mesons, and Drell-Yan processes, which subsequently hadronize into dark mesons. We adopt a modified Quark Combination Model to describe the non-perturbative dark hadronization process, allowing for a detailed event-level characterization of the dark meson flux. We simulate the flux and the interactions of these relativistic dark mesons in the Jiangmen Underground Neutrino Observatory (JUNO) using the GENIE generator, considering both elastic scattering off nuclei and deep inelastic scattering channels. Adopting a LES-inspired conservative signal definition with $15~\mathrm{MeV}<E_{\rm vis}<100~\mathrm{MeV}$ and vetoing events containing final-state neutrons, we derive projected $90\%$ C.L. sensitivities to the coupling strength between the dark gauge boson and the SM sector for a $20~\mathrm{kton}\!\cdot\!\mathrm{year}$ exposure. For the representative benchmark masses $m_{Z^\prime}=0.001$, $0.01$, and $0.1~\mathrm{GeV}$, the projected JUNO reach corresponds to $g_{\rm SM}^{90}=6.48\times10^{-5}$, $7.75\times10^{-5}$, and $2.37\times10^{-4}$, respectively. This reach is found to be only mildly sensitive to order-one variations of the hadronization parameter, as illustrated by the choices $κ_β=1/3,\,1,$ and $3$.