Following negative results in standard WIMP searches, increasing attention has been devoted to models of dark matter with masses in the sub-MeV range. For such masses, the typical momentum and energy transferred by the dark matter to our detectors is such that the most likely signatures of its passage is given by the emission of collective excitations.
I will describe two possible ways to look for dark matter this way: superfluid He-4 for spin-independent interactions and anti-ferromagnets for spin-dependent ones. In particular, I will argue that a powerful tool to describe the interaction between dark matter and collective excitations is that of effective field theories for spontaneously broken spacetime symmetries. In this framework, the collective modes are nothing but Goldstone bosons, and their interactions with dark matter (and among themselves) are completely dictated by symmetry, together with a handful of effective coefficients.
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