High-precision measurement of the kaonic hydrogen 1s level shift and width with SIDDHARTA-2

Kaonic atoms provide a unique experimental probe of strong interaction in the low-energy regime. In particular, the strong-interaction-induced shift (${\epsilon }_{1\text{s}}$) and width (${\mathrm{\Gamma }}_{1\text{s}}$) of kaonic hydrogen directly constrain the low-energy antikaon-nucleon ($\bar{K}N$) interaction at threshold and the theoretical description of the $\mathrm{\Lambda }$(1405) resonance. We report a new high-precision measurement of kaonic hydrogen X-ray transitions performed by the SIDDHARTA-2 experiment at the DA$\mathrm{\Phi }$NE collider (INFN-LNF), based on an integrated luminosity of 237 pb${}^{-1}$. The extracted values, ${\epsilon }_{1\text{s}}=-303.0\pm 17.0(stat\mathrm{.})\pm 2.5(syst\mathrm{.})$ eV and ${\mathrm{\Gamma }}_{1\text{s}}=607\pm 62(stat\mathrm{.})\pm 6(syst\mathrm{.})$ eV, represent the most precise determination to date, improving the precision by approximately a factor-of-two with respect to the previous SIDDHARTA measurement. These results significantly tighten the experimental constraints on theoretical description of the low-energy $\bar{K}N$ interaction.