Compelling evidence for an abundant, non-baryonic, non-luminous (dark) matter component was collected in the last decades. Yet, the nature of the dark matter (DM) remains totally unknown, and the quest for an answer ranks as one of the main issues of the experimental particle physics, astrophysics and cosmology. Weakly Interacting Massive Particles (WIMPs) are creditable, theoretically appealing DM candidates. If these massive relics of the early universe do exist, they are expected to be gravitationally bound to the baryonic visible matter. A direct search for WIMPs in the mass range from a few GeV/c2 to a few TeV/c2 could be based on the detection of nuclear recoils induced by WIMP elastic scattering.
The motion of the Solar System through the galaxy can create an apparent wind of dark matter particles, blowing opposite to the direction of the Sun’s motion toward the Cygnus constellation. The key points for the design of an experiment searching for DM with a directional approach are the expected event rate and the expected angular and energy distribution of recoiling nuclei. The expected event rate does not exceed 1 event/kg/year. Such extremely low rates require strong background suppression. The WIMPs mean velocity inside our galaxy is a few hundred kilometers per second at the location of our Solar System. For these velocities, WIMPs interact with ordinary matter mainly via elastic scattering on nuclei. With expected WIMP masses in the range from 10 GeV to 10 TeV, typical nuclear recoil energies are of the order of 1 – 100 keV. The expected nuclear recoil energy spectrum decreases almost exponentially with energy.
To exploit directionality with light-medium mass scattered nuclei, the required spatial accuracy is in the sub-mm domain for gaseous detectors and in the submicronic range for solid detectors. In the first case the low event rate sets the requirement of very large volumes while in the second case an extremely high resolution is required in order to cope with the very short range of the recoil nuclei.
Experiments for dark matter searches based on solid or liquid targets are not able to measure the direction of nuclear recoils. They search for a WIMP signal as an excess of events over the expected background with possibly an annual modulation of the event rate, if sensitive enough. The gaseous detectors, on the other hand, are capable of reconstructing the three-dimensional tracks of nuclear recoils, but their mass and the corresponding sensitivity are rather limited.
The Nuclear Emulsions for WIMP Search with directional measurement (NEWSdm) project aims at the direct detection of dark matter candidates by measuring the direction of WIMP-induced nuclear recoils in the nuclear emulsions. For this challenge, the detector exploits new generation nuclear emulsions with nanometric grains. An R&D conducted by the Nagoya University in collaboration with the Fujifilm Company has established the production of films with nanometric grains for an ultra-high spatial resolution.
Dedicated R&D resulted in corresponding development of new fully automated scanning systems capable of detecting extremely short tracks, with improved optical technologies overcoming the diffraction limit of conventional systems. Given that nuclear emulsions are time insensitive, the detector will be placed on a standard equatorial telescope to keep its orientation toward the Cygnus constellation.
Scheme of possible experimental setup.
Sensitivity at 90% C.L, in the zero background hypothesis for an experiment with a mass of 10 kg (green) and 100 kg (blue) for two value of detection threshold: 100 nm (dashed lines) and 50 nm (solid line).
Further details can be found at https://arxiv.org/abs/1604.04199