Two-proton correlations in $p$ + Pb and S + Pb collisions at the CERN Super Proton Synchrotron

Two-proton correlations in $^{32}$S + $^{208}$Pb collisions at 200 GeV/c per nucleon and $p$ + $^{208}$Pb collisions at 450 GeV/c are presented, as measured by the focusing spectrometer of the NA44 experiment at CERN. The technique of intensity interferometry of pairs with low relative momentum has been used. A dependence of the correlation function on the event multiplicity is found. A detailed Monte Carlo calculation has been performed for the purpose of comparison of the data with the RQMD model, taking into account effects relating to the acceptance, momentum resolution and track reconstruction in the apparatus, as well as contamination of the proton sample by $\Lambda$ decays. The centrality of the events studied was estimated using data from a silicon multiplicity detector and a GEANT based simulation of that detector. We find that the RQMD predictions do not agree with the data. If the strange baryon contribution is estimated from the available experimental data rather than RQMD, an additional systematic uncertainty margin is introduced, making the discrepancy between Monte Carlo and the data less significant. The difference could be interpreted as an indication that the measured proton source, in S + Pb reactions, is somewhat larger than predicted by RQMD (this is less certain for $p$ + Pb). An alternative interpretation is that dynamical effects in RQMD are different from those in the actual source.