Speaker
Description
The antikaon-nucleus ($\bar{K}$-nucleus) interaction is fundamental to understanding the low-energy QCD and the behavior of strangeness in dense nuclear matter. While kaonic atom X-ray spectroscopy has provided valuable information about the interaction at the nuclear surface, the strength of the potential in the nuclear interior has remained a subject of long-standing debate, characterized by the "shallow vs. deep" potential ambiguity. In particular, the imaginary part of the potential ($W_0$), which describes the absorption dynamics, has been poorly constrained ever. In this talk, we report the first experimental constraint on the imaginary part of the $\bar{K}$-nucleus optical potential through the exclusive measurement of the $K^-$ emission process. The experiment (J-PARC E42) was performed at the K1.8 beamline. We simultaneously measured the inclusive $^{12}{\rm C}(K^-, p)$ and exclusive $^{12}{\rm C}(K^-, p K^-_{esc})$ reactions at 1.8 GeV/$c$, where $K^-_{esc}$ is denoted as a quasi-free Kaon emitted without absorption in the core nucleus. Utilizing the large-acceptance Hyperon Spectrometer, we successfully detected the recoil $K^-$ in coincidence with the forward proton. By performing a simultaneous likelihood fit to the inclusive and exclusive spectra, we extracted the potential depths at the nuclear center: $V_0 = -72^{+3}_{-5} \text{(stat.)} \:^{+0}_{-8}\:(\text{syst.})\text{ MeV}$ and $W_0 = -100^{+7}_{-1} \text{(stat.)}\:^{+0}_{-16}\:(\text{syst.})\text{ MeV}$. The obtained $W_0$ is significantly stronger than predictions from standard chiral unitary models based on one-nucleon absorption, providing a quantitative indication of the dominance of multi-nucleon absorption processes ($K^-NN \to YN$) in the nuclear interior. We will discuss the details of the analysis, the extracted potential parameters, and their implications for the onset of kaon condensation in neutron star interiors.
| Collaboration | J-PARC E42 |
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