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

A measurement of charged hadron pair correlations in two-dimensional $$\Delta \eta \Delta \phi $$ Δ η Δ ϕ 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 $$\text {Ge} \text {V}/c$$ Ge / c . Measurements were carried out for unlike-sign and like-sign charge hadron pairs independently. The $$C(\Delta \eta ,\Delta \phi )$$ C ( Δ η , Δ ϕ ) 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 $$(\Delta \eta ,\Delta \phi ) = (0,0)$$ ( Δ η , Δ ϕ ) = ( 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.