Characterization of the SIDDHARTA-2 Setup via the Kaonic Helium Measurement

The aim of the SIDDHARTA-2 experiment is to perform the first measurement ever of the width and shift induced by the strong interaction to the 2p -> 1s energy transition of kaonic deuterium. This ambitious goal implies a challenging task due to the very low X-ray yield of kaonic deuterium, which is why an accurate and thorough characterization of the experimental apparatus is mandatory before starting the data-taking campaign. Helium-4 is an excellent candidate for this characterization since it exhibits a high yield in particular for the 3d -> 2p transition, roughly 100 times greater than that of the kaonic deuterium. The ultimate goal of the work reported in this paper is to study the performances of the full experimental setup in view of the kaonic deuterium measurement. This is carried out by measuring the values of the shift and the width for the 3d -> 2p energy transition of kaonic helium-4, induced by the strong interaction. The values obtained for these quantities, for a total integrated luminosity of similar to 31/pb, are epsilon 2p=2.0 +/- 1.2(stat)+/- 1.5(syst)eV and Gamma 2p=1.9 +/- 5.7(stat)+/- 0.7(syst)eV. The results, compared to the value of the shift measured by the SIDDHARTA experiment epsilon 2p=0 +/- 6(stat)+/- 2(syst)eV, show a net enhancement of the resolution of the apparatus, providing strong evidence of the potential to perform the challenging measurement of the kaonic deuterium.