Results on investigations on diurnal effect by DAMA/NaI with a partial exposure
A diurnal variation of the low energy rate in Dark Matter particles direct searches could be expected
because of the Earth's daily rotation. In fact, duringthe sidereal day the Earth shields
a given detector with avariable thickness, eclipsing the wind of Dark Matter particles.
The induced effect should be a daily variation of their velocity distribution, therefore
of their flux and, of course, of the signal rate measured deep underground. However, this
effect results very small and would be appreciable only in case of high cross section
candidates (to which small halo fraction would correspond).
In the figure theta (angle formed by the Earth velocity
in the galactic framewith the vector joining the center of the Earth to the position
of the Gran Sasso laboratory) as a function of the sidereal time.
In 1999 an exposure of 14962 kg x day was analysed and it gave no evidence of rate variation in the lowest energy region at that sensitivity level; thus, exclusion plots at 90% C.L. in the plane DM particles halo fraction in 0.3 GeVcm-3 unit vs cross section were evaluated in a given model framework for DM particle masses up to 1 TeV with purely spin-independent coupling.
This result also implied that the signal pointed out at 6.3 σ C.L. by the studies on the Dark Matter particle annual modulation signature should account for a halo fraction not lower than 10-3.