Time-based selection of kaonic atom x-ray events with quasi-hemispherical CZT detectors at the DAΦNE collider

This work presents the results of a time-based event selection for the search of x-ray signals from kaonic atom transitions using a quasi-hemispherical Cadmium-Zinc-Telluride (CZT) detector at the DA Phi NE collider. To mitigate the high background level in the measured x-ray spectra, a dedicated event selection strategy was developed, exploiting the precise timing correlation between e+e- collisions and detector signals. This approach enabled, for the first time, the observation of two characteristic x-ray transitions from kaonic aluminum atoms with a CZT detector. For the 5-4 transition at 50 keV, 362 +/- 41 (stat.) +/- 20 (sys.) signal events were observed over 1698 +/- 197 (stat.) +/- 25 (sys.) background events within +/- 5 sigma, with an energy resolution of 9.2% FWHM. For the 4-3 transition at 106 keV, 295 +/- 50 (stat.) +/- 20 (sys.) signal events were measured over 2939 +/- 500 (stat.) +/- 16 (sys.) background events, with an energy resolution of 6.6% FWHM. A background suppression of approximately 95% of the triggered data was achieved through this time-based selection. The demonstrated timing capability of the CZT detector proved highly effective in isolating time-correlated events within an 80 ns window, setting an important benchmark for the application of compound semiconductors in timing-based x-ray spectroscopy. These results highlight the potential of CZT-based detection systems for future precision measurements in high-radiation environments, paving the way for compact, room-temperature x-ray and gamma-ray spectrometers in fundamental physics and related fields.