Physics case


DEAR (DAFNE Exotic Atoms Research) is an experiment devoted to a precision measurement of the scattering lengths of the kaon-nucleon system, through a percent measurement of the Ka line shifts - due to strong interaction - in kaonic hydrogen and kaonic deuterium.

In this sentence by Dalitz the importance of the measurement is lined out:

     "The most important experiment to be carried out in low energy K-meson physics today is the
      definitive determination of the energy level shifts in K-p and K-d atoms, because of their direct
      connection with the physics of the KN interaction and their complete independence of all other
      kind of measurements which bear on this interaction"

In addition to the precise determination of low-energy parameters, a fundamental non-perturbative QCD quantity, such as the KN sigma term, undetermined up to now, will be measured. The sigma term is a quantity which gives an indication of the chiral symmetry breaking part in the total strong interaction Hamiltonian for a nucleon state. Moreover, the KN sigma term is an extremely sensitive and direct measurement of the strangeness content of the proton. One can eventually extract the KN sigma term from accurate measurements of K- nucleon amplitudes at zero energy. This will be possible only with the DEAR experiment.
DAFNE is a unique source of low momentum, monochromatic (127 MeV/c) K-, coming from the decay of the f(1020) meson produced by the e+e- interactions. In the DEAR experimental set-up the K-, after being slowed down, stop inside a low temperature (25 K), pressurized (3 bar) gaseous hydrogen target and eventually form an exotic K-hydrogen atom in a highly excited atomic level, which subsequently decays to the ground level, emitting characteristic X rays. The selected conditions for the density of the hydrogen target reduce the Stark mixing and hence enhance the yield of the 2p-1s x-ray transition.
The x-ray detectors will be CCDs (Charge Coupled Devices), which offer high energy resolution and detection efficiency (140 eV and 60 %, respectively, at 6.5 keV), and, above all, unprecedented background suppression of ionizing particles, neutrons and gammas.