Two-particle azimuthal correlations as a probe of collective behaviour in deep inelastic $\textit{ep}$ scattering at HERA

TL;DR

This study measures two-particle azimuthal correlations in deep inelastic ep scattering at HERA to investigate potential collective effects, finding results consistent with multijet production rather than collective phenomena seen in high-energy hadronic collisions.

Contribution

First measurement of two-particle azimuthal correlations in deep inelastic ep scattering at HERA, providing insights into underlying particle production mechanisms.

Findings

Correlations dominated by multijet production.

No evidence of collective behaviour as seen in high-multiplicity hadronic collisions.

Results align with models tuned for inclusive particle production.

Abstract

Two-particle azimuthal correlations have been measured in neutral current deep $\mbox{inelastic}$ $\textit{ep}$ scattering with virtuality $Q^2 > 5$ GeV$^2$ 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 $-1.5 < \eta < 2.0$ and transverse momentum $0.1 < p_{\textrm{T}}~< 5.0$ GeV and event multiplicities $N_{\textrm{ch}}$ up to six times larger than the average $\left< N_{\textrm{ch}} \right>\approx5$. The two-particle correlations have been measured in terms of the angular observables $c_n\{2\}=\left< \left< \cos{ n \Delta\varphi} \right> \right>$, where $n$ is between 1 and 4 and $\Delta\varphi$ 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.