The detection of the non-baryonic dark matter is one of the most outstanding problems in modern physics today. Cosmological and astrophysical observations converge to a standard cosmological model requiring a new kind of particle, a stable weakly interacting massive particle (WIMP). In the context of direct detection of these new particles an alternative, complementary and in any case needed strategy is the development of detectors providing an unambiguous positive WIMP signal. Indeed, directional detection gives a new degree of freedom to reject neutrons, the particles that produce the same expected signal from the nuclear recoil energy. This can be achieved by searching for a correlation of the WIMP signal with the solar motion around the galactic centre, observed as a direction dependence of the WIMP stream , coming from (l = 90o, b = 0o) in galactic coordinates, which happens to be roughly in the direction of the constellation Cygnus. The background events, coming from gamma rays and neutrons produced in the atmosphere or in the rock should follow the Earth’s rotation isotropic in galactic coordinates and very different with respect to the Cygnus direction.
A dedicated statistical study with simulated data analysis has shown that even a low-exposure, directional detector could allow a high significance discovery of galactic Dark Matter even with a sizeable background contamination or to a robust and competitive exclusion curve , depending on the value of the unknown WIMP-nucleon cross section.
The discovery parameter is a signal pointing toward Cygnus since it cannot be mimicked by neutron background.
The MIMAC (MIcro-tpc MAtrix of Chambers) collaboration, composed by the LPSC-Grenoble, the IRFU-CEA-Saclay, the CPPM-Marseille and the LMDN-IRSN-Cadarache, has developed in the last years an original prototype detector based on the direct coupling of the largest (in number of pixels) micromegas with a special developed fast self-triggered electronics showing the feasibility of a new generation of directional detectors.
The MIMAC-Cygnus detector presents the flexibility to change the target and demonstrate the dependence of the WIMP-nucleus cross section on the mass number and eventually to validate a galactic origin of proposed candidates seen in only direct detection measurements.
Only gas TPCs can aim to achieve measurement of the direction of WIMP-induced nuclear recoils. After a WIMP collision, the nuclei recoils with typical energies of 1-100 keV, travel distances of the order of few 100 Å in solids, while in gases this distance can be up to the mm scale, depending on the type and pressure of the gas.
In one MIMAC chamber the electrons produced by ionization are collected towards the grid in the drift space (25 cm in the bi-chamber module) and are amplified by avalanche in a 256 um space just before the anode thus allowing to get information on the X and Y coordinates, see fig.4. To access the X and Y dimensions with a 350 µm spatial resolution, a bulk micromegas with a 20 cm by 20 cm active area, segmented in pixels with a pitch of 350 µm will be used as 2D read-out. In order to reconstruct the third dimension Z of the recoil, the LPSC has developed a self-triggered electronics able to perform the anode sampling at a frequency of 50 MHz. This included a dedicated 64 channels ASIC chip, in its second version (v2) specially developed by our team.