Characterize ionization-quenching fluctuations for nuclear recoils in liquid argon

Determine the statistical fluctuations associated with ionization quenching for nuclear recoils in liquid argon at sub-keV to keV energies relevant to S2-only analyses in DarkSide-20k, and establish whether the no-fluctuation (NQ) or the binomial-fluctuation (QF) model accurately describes the data.

Background

In modeling the DarkSide-20k detector response for low-energy signals, electronic-recoil (ER) fluctuations are treated using a Fano factor measured by DarkSide-50. However, the statistical behavior of ionization quenching for nuclear recoils (NR) at low energies has not been empirically established.

To bracket this uncertainty, two extreme models are used: a no-fluctuation (NQ) scenario and a binomial-fluctuation (QF) scenario that partitions quanta into detectable and undetectable channels. The choice between these models affects predicted S2 spectra and hence sensitivity estimates for light WIMP searches.

A dedicated determination of the NR quenching fluctuations in liquid argon would reduce systematic uncertainty in the detector response and improve the robustness of sensitivity projections and potential signal interpretations.