First Measurement of the $K^{-}$ Escape Cross Section in the ${}^{12}\mathrm{C}(K^{-},p)$ Reaction

Abstract

We investigated the $\bar{K}$-nucleus interaction through the simultaneous measurement of the inclusive ${}^{12}\mathrm{C}(K^{-},p)$ and exclusive $K^{-}$-escape ${}^{12}\mathrm{C}(K^{-},p{K}_{esc}^{-})$ reactions at $1.8$ GeV/$c$ at J-PARC. The present measurement explicitly focuses on the $K^{-}$ escape process for the first time, successfully accomplishing a direct experimental determination of the imaginary part of the $K^{-}$ optical potential. The differential cross section for the $K^{-}$-escape reaction was determined to be $436\pm 6(\text{stat.})\pm 44(\text{syst.})\mu \text{b/sr}$. A simultaneous likelihood fit yielded real and imaginary potential strengths of $V_0=-72{}_{-5}^{+3}(\text{stat.}){}_{-8}^{+0}(\text{syst.})\text{MeV}$ and $W_0=-100{}_{-1}^{+7}(\text{stat.}){}_{-16}^{+0}(\text{syst.})\text{MeV}$ at the nuclear center, respectively. The derived $W_0$ is significantly stronger than that predicted by theoretical models based on one-nucleon processes, suggesting possible contribution of multi-nucleon involving processes.