Kaonic hydrogen X-ray measurement in SIDDHARTA

Kaonic hydrogen atoms provide a unique laboratory to probe the kaon–nucleon strong interaction at the energy threshold, allowing an investigation of the interplay between spontaneous and explicit chiral symmetry breaking in low-energy QCD. The SIDDHARTA Collaboration has measured the K-series X rays of kaonic hydrogen atoms at the DAΦNE electron–positron collider of Laboratori Nazionali di Frascati, and has determined the most precise values of the strong-interaction induced shift and width of the 1s atomic energy level. This result provides vital constraints on the theoretical description of the low-energy interaction.