  The influence of the electron spin degree of freedom on nonperturbative
electron-positron pair production by high-energy proton impact on an intense
laser field of circular polarization is analyzed. Predictions from the Dirac
and Klein-Gordon theories are compared and a spin-resolved calculation is
performed. We show that the various spin configurations possess very different
production probabilities and discuss the transfer of helicity in this highly
nonlinear process. Our predictions could be tested by combining the few-TeV
proton beam at CERN-LHC with an intense laser pulse from a table-top petawatt
laser source.
