Kaonic helium-4 X-ray measurement in SIDDHARTA

The kaonic helium-4 3d→2p 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.