Measurement of the kaonic hydrogen x-ray spectrum.

The DEAR (DA$\ensuremath{\Phi}$NE exotic atom research) experiment measured the energy of x rays emitted in the transitions to the ground state of kaonic hydrogen. The measured values for the shift $ϵ$ and the width $\ensuremath{\Gamma}$ of the 1$s$ state due to the ${K}^{\ensuremath{-}}p$ strong interaction are ${ϵ}_{1s}=\ensuremath{-}193\ifmmode\pm\else\textpm\fi{}37$ (stat) $\ifmmode\pm\else\textpm\fi{}6$ (syst) eV and ${\ensuremath{\Gamma}}_{1s}=249\ifmmode\pm\else\textpm\fi{}111$ (stat) $\ifmmode\pm\else\textpm\fi{}30$ (syst) eV, the most precise values yet obtained. The pattern of the kaonic hydrogen $K$-series lines, ${K}_{\ensuremath{\alpha}}$, ${K}_{\ensuremath{\beta}}$, and ${K}_{\ensuremath{\gamma}}$, was disentangled for the first time.