Kaonic atoms measured by DEAR experiment at DAPHINE

The DAPHINE electron-positron collider at the Frascati National Laboratories, Italy has made available a unique 'beam' of negative kaons providing so unprecedented conditions for the study of the low-energy kaon-nucleon interaction, a field still largely unexplored. The DEAR (DAPHINE Exotic Atom Research) experiment at DAPHINE and its successor SIDDHARTA (SIlicon Drift Detector for Hadronic Atom Research by Timing Application) aims at a precise determination of the isospin dependent antikaon-nucleon scattering lengths, through a percent level measurement of the Kα line shift and width in kaonic hydrogen, and a similar, being the first one in the same time, measurement of kaonic deuterium. The principle of the DEAR experiment is: low-momentum negative kaons produced in the decay of the phi-mesons at DAPHINE leave the beam pipe, are degraded in energy to a few MeV, enter a gaseous target and are stopped in the gas. The stopped kaons are captured in an outer orbit of the gaseous atoms, forming the exotic kaonic atoms. The kaons cascade down and some of them will reach the ground state. The energy of the X rays emitted in these transitions is measured with a CCD (Charge-Coupled Device) detector. In the first phase of the experiment, dedicated to the calibration and optimization of the setup, the experiment collected data on gaseous kaonic nitrogen. For the first time a pattern of three transitions was measured, and the corresponding yields obtained. In 2002, DEAR performed the most precise measurement ever obtained on the energy of the X rays emitted in the transitions to the ground state of kaonic hydrogen. The DEAR precision was limited by a signal/background ratio of about 1/70. SIDDHARTA collaboration is developing a new set of large area, triggerable X ray Silicon Drift Detector (SDD), which will improve the background rejection, allowing accomplishing the proposed objectives. Moreover, SIDDHARTA will deepen the scientific program, by measuring kaonic helium (related to the deeply bound kaonic nuclear states). Feasibility studies of other types of exotic atoms (sigmonic ones) are underway. (authors)