An event excess observed in the deeply bound region of 12C(K-, p) missing-mass spectrum

We have measured, for the first time, the inclusive missing-mass spectrum of the |$^{12}$|C|$(K^-, p)$| reaction at an incident kaon momentum of 1.8 GeV/|$c$| at the J-PARC K1.8 beamline. We observed a prominent quasi-elastic peak (⁠|$K^-p \rightarrow K^-p$|⁠) in this spectrum. In the quasi-elastic peak region, the effect of secondary interaction is apparently observed as a peak shift, and the peak exhibits a tail in the bound region. We compared the spectrum with a theoretical calculation based on the Green’s function method by assuming different values of the parameters for the |$\bar{K}$|–nucleus optical potential. We found that the spectrum shape in the binding-energy region |$-300 \, \text{MeV} < B_{K} < 40$| MeV is best reproduced with the potential depths |$V_0 = -80$| MeV (real part) and |$W_0 = -40$| MeV (imaginary part). On the other hand, we observed a significant event excess in the deeply bound region around |$B_{K} \sim 100$| MeV, where the major decay channel of |$K^- NN \to \pi\Sigma N$| is energetically closed, and the non-mesonic decay modes (⁠|$K^- NN \to \Lambda N$| and |$\Sigma N$|⁠) should mainly contribute. The enhancement is fitted well by a Breit–Wigner function with a kaon-binding energy of 90 MeV and width 100 MeV. A possible interpretation is a deeply bound state of a |$Y^{*}$|-nucleus system.