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.