A collaboration at the DAΦNE electron–positron collider has measured, with the best precision so far, how the strong force changes the lowest energy level of kaonic hydrogen. Kaonic hydrogen is an atom where an electron is replaced by a negatively charged kaon. The strong force between the kaon and the proton shifts the 1s energy level and makes that level broader. The SIDDHARTA-2 experiment reports a shift ε1s = −303.0 ± 17.0 (stat.) ± 2.5 (syst.) eV and a width Γ1s = 607 ± 62 (stat.) ± 6 (syst.) eV, based on 237 pb−1 of data. This is roughly a twofold improvement in precision over the previous result from SIDDHARTA.

The reason this matters is that those two numbers give direct experimental access to the low-energy antikaon–nucleon interaction. Physicists translate the measured shift and width into a complex number called the K−p scattering length. That quantity is a key input for theoretical descriptions of the Λ(1405) resonance, a short-lived state that sits just below the antikaon–nucleon threshold and is important for models of systems with strange quarks. The SIDDHARTA-2 result yields aK−p = (−0.715 ± 0.054) + i(0.905 ± 0.091) fm when the standard improved Deser relation is used to convert the X‑ray observables to a scattering length.

The team obtained the result by measuring X rays emitted when kaons captured by hydrogen atoms cascade down to low levels. They used an upgraded array of silicon drift detectors with larger active area, better timing, and lower electronic noise. The experiment ran in two periods (December 2023–February 2024 and April–May 2024) and filled a cryogenic target with hydrogen gas at 1.3 bar and 24 K (1.65% of liquid-hydrogen density). Backgrounds were suppressed by a dedicated kaon trigger that reduced beam-related noise by nearly four orders of magnitude, timing selections from the detectors that gave another factor of two suppression, and improved machine conditions that cut collider background by about three times.