Researchers from the SIDDHARTA‑2 collaboration report new measurements of X‑ray emission from kaonic copper and, for the first time, from kaonic fluorine. Kaonic atoms form when a negatively charged kaon (a particle containing a strange quark) slows down and replaces an electron in an atom. As the kaon drops through atomic levels it emits X‑rays. The team measured how often specific X‑ray transitions occur per stopped kaon, called absolute yields.

The experiment ran at the DAΦNE electron‑positron collider, which produces low‑momentum kaons when phi (ϕ) mesons decay. The group used a novel room‑temperature Cadmium Zinc Telluride (CZT) detector array developed for high‑resolution X‑ray spectroscopy in this environment. The system had eight small quasi‑hemispherical CZT crystals (about 13×15×5 mm^3 each) inside a thin aluminum enclosure with a 0.27 mm window. A plastic scintillator called LUMIboost, placed closer to the interaction point, helped select kaons by their time of flight.

To turn detector counts into absolute yields the team simulated the full setup with a Geant4 Monte Carlo model. That simulation estimated the detection efficiencies so the number of observed X‑rays could be converted to yields per stopped kaon. The data‑selection used tight timing criteria: a 1 ns window to pick kaon times and a 170 ns window on the CZT‑luminometer time difference. The CZT system was commissioned in the collider and the number of detectors actually used in the analysis varied across runs.

The measured yields show a clear pattern with the principal quantum number n of the atomic levels. This pattern reflects the competition between three processes: Auger electron emission (which dominates at high n), X‑ray emission (which becomes important at lower n), and nuclear capture driven by the strong interaction (which removes kaons from the cascade at the smallest n). In kaonic fluorine the researchers observed a suppression of the 4→3 X‑ray transition compared with higher‑n transitions. This suppression is interpreted as evidence that strong‑interaction effects already influence the cascade at the n=4 level. From that behaviour the team derived a conservative lower limit on the corresponding strong‑interaction width, meaning they set a minimum value for how quickly that atomic level is broadened by the strong force.