  Proportional electroluminescence (EL) in noble gases has long been used in
two-phase detectors for dark matter search, to record ionization signals
induced by particle scattering in the noble-gas liquid (S2 signals). Until
recently, it was believed that proportional electroluminescence was fully due
to VUV emission of noble gas excimers produced in atomic collisions with
excited atoms, the latter being in turn produced by drifting electrons. In this
work we consider an additional mechanism of proportional electroluminescence,
namely that of bremsstrahlung of drifting electrons scattered on neutral atoms
(so-called neutral bremsstrahlung); it is systemically studied here both
theoretically and experimentally. In particular, the absolute EL yield has for
the first time been measured in pure gaseous argon in the two-phase mode, using
a dedicated two-phase detector with EL gap optically read out by cryogenic PMTs
and SiPMs. We show that the neutral bremsstrahlung effect can explain two
intriguing observations in EL radiation: that of the substantial contribution
of the non-VUV spectral component, extending from the UV to NIR, and that of
the photon emission at lower electric fields, below the Ar excitation
threshold. Possible applications of neutral bremsstrahlung effect in two-phase
dark matter detectors are discussed.
