  The impact of stochasticity effects (SEs) in photon emissions on the proton
energy spectra during laser-plasma interaction is theoretically investigated in
the quantum radiation-dominated regime, which may facilitate SEs experimental
observation. We calculate the photon emissions quantum mechanically and the
plasma dynamics semiclassically via two-dimensional particle-in-cell
simulations. An ultrarelativistic plasma generated and driven by an
ultraintense laser pulse head-on collides with another strong laser pulse,
which decelerates the electrons due to radiation-reaction effect and results in
a significant compression of the proton energy spectra because of the charge
separation force. In the considered regime the SEs are demonstrated in the
shift of the mean energy of the protons up to hundreds of MeV. This effect is
robust with respect to the laser and target parameters and measurable in soon
available strong laser facilities.
