International Conference on Exotic Atoms and Related Topics and conference on Low Energy Antiprotons (EXA/LEAP 2024)

Spectroscopy of η'-mesic nuclei with 12C(p,dp) reaction at GSI/FAIR

27 Aug 2024, 15:15

25m

Johannessaal (Vienna)

Parallel I

Speaker

Yoshiki Tanaka (RIKEN)

Description

The possible existence of ${\eta }^{\prime }$ meson nucleus bound states ( ${\eta }^{\prime }$-mesic nuclei) has been attracting interests both theoretically and experimentally, since in-medium properties of the ${\eta }^{\prime }$ meson are closely related to the axial $U(1)$ anomaly and the chiral symmetry in QCD. The especially large mass of the ${\eta }^{\prime }$ meson ($\sim$958 MeV/c${}^2$) compared with the other light pseudoscalar mesons is theoretically explained by an interplay between the axial $U(1)$ anomaly and spontaneous breaking of chiral symmetry in the QCD vacuum. In the nuclear medium, where chiral symmetry is partially restored, the ${\eta }^{\prime }$ meson mass is expected to be reduced. Such a mass reduction would lead to an attractive ${\eta }^{\prime }$-nucleus potential, suggesting the existence of bound ${\eta }^{\prime }$-mesic nuclei. In two experiments to search for ${\eta }^{\prime }$-mesic nuclei, previously performed by using the ($p$,$d$) reaction and the ($\gamma$, $p$) reaction, no significant signal of the ${\eta }^{\prime }$-mesic nuclei was observed due to the limited experimental sensitivities.

We have recently performed a new spectroscopic experiment of the ${}^{12}$C($p$,$dp$) reaction in order to search for ${\eta }^{\prime }$-mesic nuclei with an increased experimental sensitivity. We have integrated the WASA central detector into the fragment separator (FRS) at GSI. A 2.5 GeV proton beam impinged on a carbon target to produce ${\eta }^{\prime }$-mesic states via the ${}^{12}$C($p$,$d$)${}^{11}$C$\otimes {\eta }^{\prime }$ reaction. The missing mass of the reaction is obtained by measuring the deuteron momenta with FRS used as a forward high-resolution spectrometer. Simultaneously, possible decay particles from the ${\eta }^{\prime }$-mesic nuclei, especially high-momentum protons ($\sim$1 GeV/$c$) emitted in the decay via the two-nucleon absorption process, are identified with the WASA detector system surrounding the reaction target in order to improve the signal-to-background ratio of the missing-mass spectrum. First data taking was successfully accomplished in February 2022. In this contribution, preliminary results of this experiment and future prospects will be discussed.