The Formalism of Open Quantum Systems for Superfluid Helium Response to Neutrino Scattering

Abstract Superfluid He-4 can serve as a tool for neutrino and dark matter detection. Low-energy neutrino interactions with superfluid He-4 can take place in the form of coherent elastic neutrino-atom scattering (CEvAS), a process that has not been observed so far. The first experimental study of CEvAS is in preparation in the National Center for Physics and Mathematics in Sarov. Using a high-intensity tritium neutrino source and a superfluid He-4 detector, it will have a rich potential to test the neutrino physics of the Standard Model and beyond it at unprecedentedly low energies. For example, one of the observable effects of the physics beyond the Standard Model in CEvAS can be neutrino millicharge and magnetic moment. In this study, we apply the formalism of open quantum systems to describe the evolution of the recoil helium atom after neutrino scattering on superfluid helium. In particular, we have obtained the Lindblad equation for a helium atom with an initial recoil energy of higher than 2 meV, which suffices to remove the atom from the condensate.