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

TL;DR

This paper investigates how the masses of $ ho$ and $ ext{omega}$ mesons change with their velocity in proton-nucleus collisions at 12 GeV, revealing excess low-mass signals beyond known sources.

Contribution

It provides the first velocity-dependent analysis of in-medium mass modifications of $ ho$ and $ ext{omega}$ mesons in $p+A$ reactions at this energy.

Findings

Significant low-mass excesses observed for $ ext{omega}$ mesons across all velocity regions.

Model calculations suggest notable in-medium mass modifications of $ ho$ and $ ext{omega}$ mesons.

Velocity dependence of mass modifications indicates in-medium effects vary with meson velocity.

Abstract

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 $p+\mathrm{Cu}$ 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.