Azimuthal correlations in photoproduction and deep inelastic $\boldsymbol{ep}$ scattering at HERA

TL;DR

This study investigates azimuthal correlations and collective phenomena in high-multiplicity electron-proton scattering at HERA, finding no significant collective behaviour similar to high-energy hadronic collisions, but highlighting multiparton interactions.

Contribution

It provides the first detailed analysis of azimuthal correlations in ep scattering across different regimes, revealing the role of multiparton interactions without observing collective effects.

Findings

No significant collective behaviour observed in high-multiplicity ep scattering.

Clear transition in correlations from photoproduction to deep inelastic scattering.

PYTHIA simulations suggest multiparton interactions from photon fluctuations.

Abstract

Collective behaviour of final-state hadrons, and multiparton interactions are studied in high-multiplicity $ep$ scattering at a centre-of-mass energy $\sqrt{s}=318$ GeV with the ZEUS detector at HERA. Two- and four-particle azimuthal correlations, as well as multiplicity, transverse momentum, and pseudorapidity distributions for charged-particle multiplicities $N_{\textrm ch} \geq 20$ are measured. The dependence of two-particle correlations on the virtuality of the exchanged photon shows a clear transition from photoproduction to neutral current deep inelastic scattering. For the multiplicities studied, neither the measurements in photoproduction processes nor those in neutral current deep inelastic scattering indicate significant collective behaviour of the kind observed in high-multiplicity hadronic collisions at RHIC and the LHC. Comparisons of PYTHIA predictions with the measurements in photoproduction strongly indicate the presence of multiparton interactions from hadronic fluctuations of the exchanged photon.