Two-particle correlations in azimuthal angle and pseudorapidity in central $^{7}Be + ^{9}Be$ collisions at the CERN Super Proton Synchrotron

A measurement of charged hadron pair correlations in two-dimensional Delta eta Delta phi space is presented. The analysis is based on total 30 million central Be + Be collisions observed in the NA61/SHINE detector at the CERN SPS for incident beam momenta of 19A, 30A, 40A, 75A, and 150A GeV/c. Measurements were carried out for unlike-sign and like-sign charge hadron pairs independently. The C( Delta eta., Delta phi) correlation functions were compared with results from a similar analysis on p + p interactions at similar beam momenta per nucleon. General trends of the backto-back correlations are similar in central Be + Be collisions and p + p interactions, but are suppressed in magnitude due to the increased combinatorial background. Predictions from the Epos and UrQMD models are compared to the measurements. Evolution of an enhancement around (Delta eta., Delta phi) = (0, 0) with incident energy is observed in central Be + Be collisions. It is not predicted by bothmodels and almost non-existing in proton-proton collisions at the same momentum per nucleon.