$S$-wave kaon-nucleon interactions from lattice QCD at the physical point

We investigate S-wave kaon-nucleon ($KN$) interactions with strangeness $S=+1$ in lattice QCD using the time-dependent HAL QCD method. Employing the $(2+1)$-flavor gauge configuration with $m_{\pi }\approx 137\text{ MeV}$ and $m_K\approx 502\text{ MeV}$, we calculate the $KN$ potentials at the leading order in the derivative expansion. The potentials in both isospin channels ($I=1$ and $I=0$) exhibit repulsion at short distances, while only the $I=0$ potential has a small attractive pocket at intermediate distances. The phase shifts computed from these potentials show no signals corresponding to resonances or bound states in both isospin channels, suggesting the absence of the ${\mathrm{\Theta }}^{+}(1540)$ pentaquark in the S-wave $KN$ systems. The scattering lengths result in $a_0^{I=1}=-0.226(5){(}_{-0}^{+5})\text{ fm}$ and $a_0^{I=0}=+0.028(61){(}_{-26}^{+3})\text{ fm}$. Our results of the S-wave cross sections for $I=1$ are consistent with some of the experimental data within $2-3$ $\sigma$, while they deviate from others. The results for $I=0$, combined with recent studies on chiral perturbation theory, suggest that the scattering amplitudes in this channel are dominated by P-wave components rather than S-wave.