EXKALIBUR: Towards a Kaonic Atoms Periodic Table to test Fundamental Interactions

TL;DR

EXKALIBUR addresses the lack of systematic, high-precision kaonic-atom data across the periodic table to disentangle QED and QCD contributions and to constrain kaon–nucleus interactions. The project proposes a detector-matched X-ray spectroscopy program at DAΦNE using SDDs, CZT, HPGe, and VOXES to cover transitions from $10-600\ \mathrm{keV}$ in light to heavy elements, supported by MCDFGME-based cascade calculations and kaon-mass sensitivity studies. Its key contributions include aiming for a kaon-mass precision below $10\ \mathrm{keV}$, building a comprehensive database of nuclear shifts and widths, and enabling precision BSQED tests in strong fields, along with detector development and heavy-element measurements. The work promises to advance tests of low-energy QCD and bound-state QED, guide cascade modeling, and potentially constrain Beyond-Standard-Model mediators, with potential expansion to other facilities such as J-PARC.

Abstract

Kaonic atoms, formed when a negatively charged kaon replaces an electron, provide a unique laboratory to test fundamental interactions at low energies. EXKALIBUR (EXtensive Kaonic Atoms research: from LIthium and Beryllium to URanium) is a program to perform systematic, high-precision X-ray spectroscopy of selected kaonic atoms across the periodic table at the DA$Φ$NE accelerator at the National Laboratory of Frascati (INFN-LNF). Here, we outline its detector-driven strategy: Silicon Drift Detectors for 10-40 keV transitions in light targets (Li, Be, B, O), CdZnTe detectors for 40-300 keV lines in intermediate-$Z$ systems (Mg, Al, Si, S), and a High-Purity Germanium detector for high-$Z$ atoms (Se, Zr, Ta, Mo, W, Pb), complemented by VOXES, a high-resolution crystal spectrometer for sub-eV studies. EXKALIBUR plans to (i) reduce the charged-kaon mass uncertainty below 10 keV, (ii) produce a database of nuclear shifts and widths to constrain multi-nucleon K$^{-}$-nucleus interaction models, and (iii) provide precision data for testing bound-state QED in strong fields. We summarize the planned measurements and expected sensitivities within DA$Φ$NE luminosities.

Figures (4)

• Figure 1: Cascade level structure showing the main de-excitation mechanisms: Auger emission dominates the initial stages, followed by radiative X-ray transitions, and nuclear absorption occurs at the lowest 1s level of the cascade.
• Figure 2: Proposed kaonic atom measurements by EXKALIBUR compared with those performed by SIDDHARTA (orange). Measurements are grouped by the detector technology best suited to each transition energy range: Silicon Drift Detectors (SDDs) for 10-50 keV (red), Cadmium Zinc Telluride (CZT) for 40-300 keV (green), and High Purity Germanium (HPGe) for 100-600 keV (blue).
• Figure 3: Adapted SIDDHARTA-2 setup for the EXKALIBUR campaign with SDDs, equipped with solid Li, Be, and B targets. The apparatus integrates a cylindrical target cell and the 1 mm SDD X-ray detectors.
• Figure 4: Residuals of kaon mass determinations from past experiments compared with the result obtained by SIDDHARTA-2 in kaonic neon. The statistical (orange) and systematic (blue) uncertainties are shown, along with the total uncertainty (black) and the expected precision (<10 keV) achievable with EXKALIBUR (green).