Recent Development and Prospects in Dark Matter Research

Dear Colleagues,

The presence of dark matter (DM) in the Universe is now an established, yet still mysterious, paradigm: deciphering its essence is one of the most compelling and fascinating tasks for fundamental physics today. The last few decades have seen enormous advances in direct DM search, which has led to many orders of magnitude improvement for high >10 GeV/c ² DM masses down to 10^-46 cm² for spin-independent cross section. Unfortunately, no signal evidence has yet been found, except for the long-standing and extensively debated nuclear recoil yield annual modulation claimed by the DAMA/LIBRA experiment. Given the current lack of experimental evidence from indirect, direct and collider searches, the DM sub-GeV/c² mass region has recently received renewed attention and now represents a new research frontier. In this mass region, the extremely low recoil energy deposited in the target medium can be smaller than detection thresholds, and is additionally suppressed by the quenching of nuclear interactions. In order to circumvent this obstacle, some new promising detection strategies involving new modes of operation (e.g., skipper readout, Neganov–Trofimov–Luke boosted phonon signal) and/or new signatures (e.g., the Migdal effect, DM scattering off bound electrons) have recently arisen. In this context, recent years have seen several experiments (with techniques ranging from noble liquids to scintillating bolometers) reporting observed excesses of unexpected low-energy events not compatible with the DM hypothesis. Experimental direct DM searches have hence reached a level of maturity and complexity for which the detectors’ response to signal and the experimental backgrounds need to be understood and controlled at unprecedented levels, in order to avoid the appearance of unexplained excesses. In addition, experiments will soon reach the infamous neutrino fog, beyond which the return on investment in increasing exposure will no longer be favorable. In this context, the measurement of the DM scattering directionality provides a powerful tool to deal with known and unknown backgrounds, and can offer a unique key for the positive unambiguous identification of a DM interaction, with directional techniques having demonstrated the technological readiness to build experiments with competitive sensitivity. With ton-scale detectors at the verge of closing down the “classical” parameter space between 10 GeV/c² and 1 TeV/c² DM mass down to neutrinos, and in parallel innovative approaches trying to expand the search to yet uncharted parameter spaces (i.e., MeV DM masses and electron recoil signatures) and yet-unexploited complementary information (i.e., directionality), the DM direct search field is still extremely fervid and strongly determined to probe the DM hypothesis to its full extent.

Prof. Dr. Elisabetta Baracchini
Guest Editor

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