Excitation Spectra of the C 12 ( p , d ) Reaction near the η ′ -Meson Emission Threshold Measured in Coincidence with High-Momentum Protons

The missing mass of the ^{12}C(p,d) reaction has been measured near the η^{'}-meson emission threshold in coincidence with a high-momentum proton to selectively collect η^{'}-^{11}C mesic nucleus formation events at GSI, Germany. A 2.5 GeV proton excites a carbon nucleus in order to form an η^{'}-mesic nucleus emitting a deuteron forward with an energy of ∼1.5 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 η^{'}-mesic nucleus. The measured semiexclusive 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 η^{'}-nucleus interaction. The analysis results indicate possible formation of η^{'}-mesic nuclei for the real potential depth of ∼-61 MeV with a local statistical significance of 3.5σ and, taking into account the look-elsewhere effect, a global significance of 2.1σ.