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

TL;DR

This study measures two-particle correlations in central Be+Be collisions at CERN SPS energies, revealing an energy-dependent enhancement at small angles not predicted by existing models, and compares results with p+p interactions.

Contribution

It provides the first detailed two-dimensional correlation analysis in Be+Be collisions across multiple energies, highlighting an unexplained energy-dependent enhancement.

Findings

Back-to-back correlations are similar in Be+Be and p+p but suppressed in magnitude.

An energy-dependent enhancement at (Δη, Δφ) = (0,0) is observed in Be+Be collisions.

Neither EPOS nor UrQMD models predict the observed enhancement.

Abstract

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 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 $C(\Delta\eta,\Delta\phi)$ correlation functions were compared with results from a similar analysis on p+p interactions at similar beam momenta per nucleon. General trends of the 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 $(\Delta\eta,\Delta\phi) = (0,0)$ 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.