Orateur
Description
The azimuthal correlation angle, $\Delta\phi$, between the scattered lepton and the leading jet in deep inelastic $ep$ scattering at HERA has been studied using HERA II data collected with the ZEUS detector. The data set features $e^{\pm}p$ collisions at a centre-of-mass energy of $\sqrt{s} = 318$ GeV, corresponding to an integrated luminosity of 326 pb$^{-1}$. A measurement of jet cross sections was performed within the fiducial region, specified in the laboratory frame, of the photon virtuality 10 GeV$^2 < Q^2 < 350$ GeV$^2$, inelasticity $0.04 < y < 0.7$, outgoing lepton energy $E_e > 10$ GeV, lepton polar angle $140^o < \theta_e < 180^o$, jet transverse momentum 2.5 GeV < $p_\mathrm{T,jet} < 30$ GeV, and jet pseudorapidity $-1.5 < \eta_\mathrm{jet} < 1.8$. Jets were reconstructed using the $k_\mathrm{T}$ algorithm with the radius parameter $R = 1$. Leading jets are defined as jets that carry the highest $p_\mathrm{T,jet}$. Differential cross sections, $d\sigma/d\Delta\phi$, will are presented as a function of the azimuthal correlation angle in various $p_\mathrm{T,jet}$, $Q^2$ and jet-multiplicity ranges. Perturbative calculations at $\mathcal{O}(\alpha_{s}^2)$ accuracy successfully describe the data within the fiducial region, while a lower level of agreement is observed near $\Delta\phi \rightarrow \pi$ for events with high jet-multiplicity due to limitations of the perturbative approach in describing soft QCD phenomena. Monte Carlo predictions that supplement leading-order matrix elements with parton showering describe the data as well as the $\mathcal{O}(\alpha_{s}^2)$ calculations do.
