Kaonic atoms studies at DAFNE by the SIDDHARTA experiment

The system at rest makes a sensitive testing ground for chiral SU(3) symmetry in QCD, especially for the explicit symmetry breaking induced by the relatively large mass of the strange quark.At the DAΦNE electron-positron collider of Laboratori Nazionali di Frascati we study kaonic atoms, taking advantage of the low-energy kaons from Φ-mesons decaying nearly at rest. The low-energy antikaon-nucleon interaction in kaonic hydrogen and kaonic deuterium can be investigated under favorable conditions.The DEAR (DAΦNE Exotic Atom Research) experiment at LNF delivered the most precise data on kaonic hydrogen up to now. DEAR and its follow-up experiment SIDDHARTA (Silicon Drift Detector for Hadronic Atom Research by Timing Application) are using X-ray spectroscopy of kaonic atoms to measure the strong interaction induced shift and width of the ground state. SIDDHARTA is the first experiment on kaonic deuterium ever, and kaonic hydrogen is remeasured with improved precision.From the shift and width in K-p and K-d the isospin-dependent antikaon-nucleon scattering lengths can be determined, quantities essential as constraints for low-energy QCD.