Abstract
Two-particle azimuthal correlations have been measured in neutral current deep inelastic ep scattering with virtuality Q2> 5 GeV2 at a centre-of-mass energy sqrt{s} = 318 GeV recorded with the ZEUS detector at HERA. The correlations of charged particles have been measured in the range of laboratory pseudorapidity −1.5 < η < 2.0 and transverse momentum 0.1 < pT< 5.0 GeV and event multiplicities Nch up to six times larger than the average 〈Nch〉 ≈ 5. The two-particle correlations have been measured in terms of the angular observables cn{2} = 〈〈cosnΔφ〉〉, where n is between 1 and 4 and ∆φ is the relative azimuthal angle between the two particles. Comparisons with available models of deep inelastic scattering, which are tuned to reproduce inclusive particle production, suggest that the measured two-particle correlations are dominated by contributions from multijet production. The correlations observed here do not indicate the kind of collective behaviour recently observed at the highest RHIC and LHC energies in high-multiplicity hadronic collisions.
Highlights
Recent measurements [10,11,12,13,14,15,16] at the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC) have revealed similar collective behaviour in lighter colliding systems at high multiplicity, such as proton nucleus (p+A) and even pp, compared to heavy-ion systems
At present it is unclear whether the collectivity observed in different colliding systems is of the same fluid-dynamic origin and how small the interaction region can be until such a description of soft QCD breaks down
There is no indication of a near-side ridge with or without the subtraction of C(∆η, ∆φ) at low Nch from that at high Nch, which would be an indication of hydrodynamic collectivity
Summary
The NC DIS data sample used in this analysis was taken with the ZEUS detector at HERA during 2003–2007 (HERA II). In the kinematic range of the analysis, charged particles were mainly tracked in the central tracking detector (CTD) [32,33,34] and the microvertex detector (MVD) [35]. These components operated in a magnetic field of 1.43 T provided by a thin superconducting solenoid. (DCA) to the nominal vertex in X–Y was measured to have a resolution, averaged over the azimuthal angle, of (46 ⊕ 122/pT) μm, where ⊕ indicates that the values are added in quadrature, and with pT in GeV denoting the momentum transverse to the beam axis. The luminosity was measured using the Bethe-Heitler reaction ep → eγp by a luminosity detector which consisted of independent lead-scintillator calorimeter [40,41,42] and magnetic spectrometer [43, 44] systems
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