First direction sensitive search for dark matter with a nuclear emulsion detector at a surface site

Fine-grained nuclear emulsion films have been developed as a tracking detector with nanometric spatial resolution to be used in direction-sensitive dark matter searches, thanks to novel readout technologies capable of exploiting this unprecedented resolution. Emulsion detectors are time insensitive. Therefore, a directional dark matter search with such detector requires the use of an equatorial telescope to absorb the Earth rotation effect. We have conducted for the first time a directional dark matter search in an unshielded location, at the sea level, by keeping an emulsion detector exposed for 39 days on an equatorial telescope mount. The observed angular distribution of the data collected during an exposure equivalent to 0.59 g days agrees with the background model and an exclusion plot was then derived in the dark matter mass and cross-section plane: cross-sections higher than 1.3 × 10^-28 cm^2 and 1.7 × 10^-31 cm^2 were excluded for a dark matter mass of 10 GeV/c^2 and 100 GeV/c^2, respectively. This is the first direction sensitive search for dark matter with a solid-state, particle tracking detector.