measurement of charged hadron pair correlations in two-dimensional ΔηΔϕ 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 19 A , 30 A , 40 A , 75 A , and 150 A GeV/c . Measurements were carried out for unlike-sign and like-sign charge hadron pairs independently. The C(Δη ,Δϕ ) correlation functions were compared with results from a similar analysis on p + p interactions at similar beam momenta per nucleon. General trends of the back-to-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 (Δη ,Δϕ ) = (0,0) with incident energy is observed in central Be + Be collisions. It is not predicted by both models and almost non-existing in proton–proton collisions at the same momentum per nucleon.
