Kaonic helium-4 X-ray measurement in SIDDHARTA

The kaonic helium-4 3 d → 2 p X-ray transition was measured in a gaseous target, where Compton scattering in helium is negligible. The X-rays were detected with large-area Silicon Drift Detectors (SDDs) using the timing information of the K + K − pairs produced by ϕ decays at the DAΦNE e + e − collider. A new value of the strong interaction shift of the kaonic 4He 2p state was determined to be 0 ± 6 (stat) ± 2 (syst) eV , which confirms the recently obtained result by the KEK-PS E570 group.