Excitation Spectra of the ${}^{12}\mathrm{C}(p,d)$ Reaction near the ${\eta }^{\prime }$-Meson Emission Threshold Measured in Coincidence with High-Momentum Protons

Abstract

The missing mass of the ${}^{12}\mathrm{C}(p,d)$ reaction has been measured near the ${\eta }^{\prime }$-meson emission threshold in coincidence with a high-momentum proton to selectively collect ${\eta }^{\prime }$-${}^{11}\mathrm{C}$ mesic nucleus formation events at GSI, Germany. A 2.5 GeV proton excites a carbon nucleus to form an ${\eta }^{\prime }$-mesic nucleus emitting a deuteron forward with an energy of $\sim 1.6$ GeV. The deuteron is momentum-analyzed by the Fragment Separator used as a high-resolution spectrometer to deduce the excitation energy of the residual system. The large-acceptance detector WASA surrounding the target identifies high-momentum protons emitted in the decay of the ${\eta }^{\prime }$-mesic nucleus. The measured semi-exclusive spectrum exhibits structures below the threshold though the statistical significance is limited. The spectrum is fitted by theoretically calculated spectra varying optical-potential parameters of the ${\eta }^{\prime }$-nucleus interaction. The analysis results indicate ${\eta }^{\prime }$-mesic nuclei formation for the real potential depth of $\sim -61$ MeV with a local statistical significance of $3.5\sigma$ and, taking into account the look-elsewhere effect, a global significance of $2.1\sigma$.

Comments: 6 pages, 4 figures, and 1 supplemental material