Near-side jet peak broadening in Pb–Pb collisions atsNN=2.76 TeV

Abstract

Two-particle angular correlation measurements are sensitive probes of the interactions of particles with the medium formed in heavy-ion collisions. Such measurements are done by determining the distribution of the relative pseudorapidity (∆η) and azimuthal angle (∆ϕ) of particles with respect to a higher p T trigger particle (1 < p T,trig < 8 GeV/c). The near-side peak is fitted with a function, which includes both the near-side jet peak and also accounts for the ∆η-independent long-range correlations. The centrality evolution of the width (variance) of the fitted distribution is investigated. In Pb-Pb collisions a significant broadening of the near-side peak in the ∆η direction is observed from peripheral to central collisions, while in the ∆ϕ direction the peak is almost independent of centrality. For the 10% most central events, a departure from the Gaussian shape is also observed at low transverse momentum (1 < p T,assoc < 2 GeV/c, 1 < p T,trig < 3 GeV/c). In this contribution the results obtained by the ALICE experiment in Pb-Pb and pp collisions at √ s NN = 2.76 TeV are shown, and they are interpreted in terms of radial and elliptic flow by comparing them to AMPT model simulations.Keywords: Angular correlations, two-particle correlations, near-side peak, Pb-Pb, 2.76 TeV, ALICE MotivationIn high-energy collisions, the scattered high p T partons produce parton showers, which hadronize into collimated jets of hadrons. As the partons propagate through the expanding medium, they lose energy by medium induced gluon radiation and elastic scatterings, which is commonly referred to as jet quenching. This can result in the energy imbalance of back to back jets, with the radiated energy reappearing at large angles from the jet axis [1,2].At low p T , jets are difficult to identify in the large fluctuating background, and instead statistical analysis techniques are used to study the interaction of the propagating partons with the medium. One such technique is the measurement of two-particle angular correlations, where the distribution of the azimuthal angle difference Email address: monika.kofarago@cern.ch (Monika Kofarago) 1 for the ALICE collaboration 2 CERN and MTA Wigner RCP (∆ϕ) and pseudorapidity difference (∆η) is studied on a statistical basis. In these measurements jets manifest themselves as a peak around (∆ϕ, ∆η) = (0, 0) on the near-side, while as an approximately ∆η independent structure on the away-side (around ∆ϕ = π). The interaction of jets with the medium can modify the shape of the near-side jet peak [3,4,5], and such a centrality dependent broadening was seen by the STAR collaboration in Au-Au collisions atIn the present contribution, an extension of these measurements to LHC energies by the ALICE detector is discussed. The full details of the presented analysis can be found in [7,8]. AnalysisIn the current analysis Pb-Pb and pp data at √ s NN = 2.76 TeV recorded by the ALICE detector [9] are studied. The Inner Tracking System (ITS) and the Time Projection Chamber (TPC) are used for trac...