Velocity dependence of the mass modifications of $\rho$ and $\omega$ mesons in 12 GeV $p+A$ reactions

This study measured the invariant mass spectra of $\rho$ and $\omega$ mesons in the $e^{+}{e}^{-}$ decay channel for 12 GeV (12.9 GeV/$c$) $p+\mathrm{C}$ and $p+\mathrm{C}\mathrm{u}$ reactions (${\sqrt{s}}_{NN}=5.1$ GeV) at the KEK 12-GeV Proton Synchrotron. The measured spectra were divided into three $\beta \gamma$ regions to examine their velocity dependence. Across all regions, significant excesses were observed on the low-mass side of the $\omega$ meson peak, beyond the contributions of known hadronic sources, in the data of the C and Cu targets. Model calculations were subsequently performed to evaluate the magnitudes of the mass modifications of $\rho$ and $\omega$ mesons.