Speaker
Description
We report the first result on $\phi$ meson production in proton-nucleus (copper and carbon) collisions at J-PARC using the 30 GeV proton beam, measured via its dielectron decay channel [1]. This measurement was performed at the newly constructed high-momentum beamline at J-PARC. We employed the dielectron spectrometer developed for the J-PARC E16 experiment [2], which aims to study in medium modifications of the $\phi$ meson mass spectrum, while analyzing 26.8 hours of E16 commissioning data. The invariant mass of the $\phi$ meson was successfully reconstructed through the dielectron decay channel. The obtained $\phi$ yield was converted into the total production cross section by assuming the kinematic distribution derived from the event generator JAM. The results are discussed in comparison with $\phi$ meson production in proton-proton collisions at comparable energies.
A 30 GeV proton beam incident on a fixed nuclear target corresponds to a nucleon-nucleon center of mass energy of $\sqrt{s}=7.7$ GeV. This energy region is currently of great interest in heavy ion collision studies of QCD matter at moderate temperature and high baryon density. In particular, an enhancement of hadrons containing $s$ or $\bar{s}$ quarks has been observed in this regime, prompting various theoretical interpretations. Although the present measurement was performed in proton-nucleus collisions, Ref. [3] points out that the strangeness suppression factor $\gamma_s$, defined as the relative pair creation probability of $s$ quarks to $u$ and $d$ quarks, may increase even at around normal nuclear density. The present results and future data with improved statistics may provide valuable input for discussing the observed strangeness enhancement. To investigate such in medium effects, dielectron measurements, which are free from final state strong interactions, provide a powerful probe.
This study demonstrates that a dielectron measurement system has been successfully established at J-PARC. In this presentation, we introduce the system and its performance, and report the results obtained from the commissioning data.
[1] S. Nakasuga et al., Prog. Theor. Exp. Phys., Advance online publication.
https://doi.org/10.1093/ptep/ptag042
[2] K. Aoki et al., J. Subatomic Part. Cosmol. 3, 100019 (2025).
[3] W. Cassing et al., Phys. Rev. C 93, 014902 (2016).
| Collaboration | J-PARC E16 |
|---|
