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

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 $N_{\text{c}h}\ge 20$ 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.

27 pages, 9 figures. Additional material included as an ancillary file for this arXiv entry

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• An Introduction to PYTHIA 8.2
• Glauber Modeling in High Energy Nuclear Collisions
• The eccentricity in heavy-ion collisions from Color Glass Condensate initial conditions
• Long-range azimuthal correlations in proton-proton and proton-nucleus collisions from the incoherent scattering of partons
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• Perfect QCD -- a new Universal approach to soft QCD
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