Signal formation in superfluid helium-4 for the DELight experiment

As xenon- and argon-based experiments dominate the search for direct interaction of dark matter particles with nuclei for weakly interacting massive particles (WIMPs) with masses above few GeV/c2, the parameter space for light dark matter (LDM) remains mostly unexplored. Indeed, the low energy threshold needed to be sensitive to sub-GeV/c2 LDM could be achieved so far only by solid state semiconductor detector coupled to low-temperature phonon sensors, with the disadvantage of a limited scalability. The DELight experiment will use a target of superfluid helium-4: this technology recovers the scalability of noble liquid experiments, while reaching low thresholds typical of cryogenic semiconductors thanks to its phonon channel. In addition, the light nuclear mass of helium makes it an ideal candidate for LDM searches, allowing DELight to probe LDM masses down to 100 MeV/c2 already during its first phase which will deploy an helium-4 target of about 1 kg. This talk will present the principle of particle detection in superfluid helium-4, focusing on the signal formation processes and expected final observables for the DELight experiment.