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 Nch ≥ 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.
Highlights
Experiment at HERA [15]
Products of separate multiparton interactions (MPI) subprocesses may further interact through the colour reconnection (CR) framework in PYTHIA
All distributions and correlation functions in this article are shown for high-multiplicity events, which are defined by a weighted sum over the number of reconstructed tracks (Nrec) passing the selection criteria: Nch =
Summary
The photoproduction and NC DIS data used in this analysis were recorded with the ZEUS detector at HERA during 2003–2007 (HERA II) During this period, the HERA accelerator collided 27.5 GeV electron/positron beams with 920 GeV proton beams, which yields a. For the NC DIS part of this analysis, events were selected at the first level if they had an energy deposit in the CAL consistent with an isolated scattered electron. A requirement on the total recorded energy and longitudinal momentum of the event was used to select NC DIS event candidates. The location of the electron-proton scattering, defined as the primary vertex, was required to be near the centre of the detector, |VZ| < 30 cm. The track associated with the scattered electron candidate used to identify the NC DIS event was excluded from the correlation analysis. A more detailed description of other event- and track-selection criteria can be found in an earlier ZEUS publication on this subject [15]
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