Speaker
Description
We performed missing-mass spectroscopy of the ${}^{12}$C($p$,$d$) reaction near the $\eta^\prime$-meson emission threshold to search for $\eta^\prime$-mesic nuclei — bound states of an $\eta^\prime$ meson and a nucleus — in order to investigate in-medium properties of the $\eta^\prime$ meson, which are closely related to the axial U(1) anomaly and chiral symmetry breaking in QCD. We conducted the experiment using a newly developed system that integrates the WASA central detector with the fragment separator FRS at GSI. The missing mass in the ${}^{12}$C($p$,$d$) reaction was precisely measured using the FRS operated as a high-resolution forward spectrometer. The large-acceptance WASA detector was employed in coincidence to tag particles emitted from the decay of $\eta^\prime$-mesic nuclei, thereby suppressing contributions from other background processes in the ($p$,$d$) reaction.
The obtained excitation spectrum of the $^{12}$C($p$,$d$) reaction exhibits two structures below the $\eta^\prime$-meson emission threshold, suggesting the possible formation of $\eta^\prime$-mesic nuclei. A detailed analysis was performed by fitting the spectrum with theoretically calculated spectra varying the real and imaginary parts of the $\eta^\prime$–$^{11}$C optical potential. The analysis results indicate the possible formation of $\eta^\prime$-mesic nuclei for the real potential depth of $\sim −61$ MeV with a local statistical significance of 3.5$\sigma$ and a global significance of 2.1$\sigma$ after accounting for the look-elsewhere effect.
In this contribution, we will present the details of the experiment and discuss the data analysis, results, and future prospects.