  A recent discovery of additional mechanism of electroluminescence (EL) in
noble gases due to the neutral bremsstrahlung (NBrS) effect led to a prediction
that NBrS EL should be present in noble liquids as well. A theoretical model of
NBrS EL in noble liquids was developed accordingly in the frameworks of
Cohen-Lekner and Atrazhev. In this work, we confirm this prediction: for the
first time, visible-range EL has been observed in liquid argon at electric
fields reaching 90~kV/cm, using Gas Electron Multiplier (GEM) and Thick GEM
(THGEM) structures. Absolute light yields of the EL were measured and found to
be in excellent agreement with the theory, provided that the momentum-transfer
cross section of electron scattering is used for calculation of NBrS cross
section (instead of the energy-transfer one).
