Time-based Selection of Kaonic Atom X-ray Events with Quasi-Hemispherical CZT Detectors at the DAFNE collider

TL;DR

Time-based selection using CZT detectors at the DAΦNE collider enables observation of kaonic aluminum X-ray transitions in a high-background setting. The study combines precise timing from CZT detectors with LUMI time-of-flight discrimination and energy-calibrated spectra to isolate kaonic-atom signals, achieving a background suppression of roughly 95 percent and extracting two transitions with significances exceeding five standard deviations. The results demonstrate the viability of room-temperature CZT detectors for high-resolution X-ray spectroscopy in harsh radiation environments and establish a benchmark for timing-based CZT applications in fundamental physics. The methodology is ready to scale to the full kaonic aluminum dataset and to other intermediate-mass kaonic atoms, expanding precision spectroscopy capabilities at collider facilities.

Abstract

This work presents the results of a time-based event selection for searching X-ray signals from kaonic atom X-ray transition using a single quasi-hemispherical Cadmium-Zinc-Telluride (CZT) detector at the DA$Φ$NE collider. To mitigate the high background level in the measured X-ray spectrum, 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 using a CZT detector: for the 5-4 transition at 50~keV, 362~$\pm$~41~(stat.)~$\pm$~20~(sys.) signal events over 1698~$\pm$~197~(stat.)~$\pm$~25~(sys.) background events in 5$σ$ were observed, with a resolution of 9.2\%~FWHM; for the 4-3 transition at 106~keV, 295~$\pm$~50~(stat.)~$\pm$~20~(sys.) signal events over 2939~$\pm$~500~(stat.)~$\pm$~16~(sys.) background events in 5$σ$ were measured, with a resolution of 6.6 ~FWHM. A strong 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 these 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 $γ$-ray spectrometers in fundamental physics and beyond.

Figures (12)

• Figure 1: Schematic areal view of the main rings of DA$\Phi$NE collider. The SIDDHARTA-2 interaction point, together with the CZT detection system setup, is highlighted. Adapted from Bassetti:DAFNERING.
• Figure 2: Schematic view of the experimental setup of the CZT detection system in DA$\Phi$NE. A description of the materials follows: The DA$\Phi$NE beam pipe (around the IP in the Figure, grey) is made by a mixture of aluminum and carbon fiber; the LUMI detectors (housing and lightguides shown wrapped in aluminized mylar shown in silver); the Target (shown in light gray) is an aluminum plate; the CZT detection system’s box (shown in light grey) is made of aluminum; mechanical supports and mounting structures (brackets yellow, kinematic mounts orange) are made of aluminum alloy. Additional small components visible: an electronics/power module (magenta) and support plates (cyan/turquoise).
• Figure 3: Scheme of the DAQ system of the experiment. Adapted from Scordo:2023per.
• Figure 4: Scheme of the data selection optimization process described in the section.
• Figure 5: 2D histogram of TAC1 vs TAC2. Kaon-related coincidences appear as narrow peaks within the green guidelines.