[Paper Review] Centrality dependence of charged jets in p-Pb collisions at $\sqrt{s_\mathrm{NN}} = 5.02$ TeV measured with the ALICE detector

Rüdiger Haake|arXiv (Cornell University)|Mar 22, 2015

High-Energy Particle Collisions Research3 citations

TL;DR

This study presents the first measurement of centrality-dependent charged jet production in p–Pb collisions at √sNN = 5.02 TeV using the ALICE detector. By applying a hybrid centrality approach and measuring jet cross sections and nuclear modification factors (QpPb) for R = 0.2 and R = 0.4, the analysis finds no significant centrality dependence, indicating minimal cold nuclear matter effects on jet production.

ABSTRACT

Highly energetic jets are sensitive probes for the kinematics and the topology of nuclear collisions. Jets are collimated sprays of charged and neutral particles, which are produced in the fragmentation of hard scattered partons in an early stage of the collision. The measurement of jet spectra in p-Pb collisions provides an important way of quantifying the effects of cold nuclear matter in the initial state on jet production, fragmentation, and hadronization. Unlike in Pb-Pb collisions, strong hot nuclear matter effects - e.g. from quark-gluon plasma formation - are not expected to occur in p-Pb collisions. Hence, cold nuclear matter effects can be investigated in isolation. The impact of cold nuclear matter effects on charged jet spectra is expected to depend on the event centrality. Higher event centralities are principally connected to a higher probability for an interaction of proton and lead-nucleus and therefore also for a possible nuclear modification. This article is the conference proceeding of a talk, in which centrality-dependent properties of charged jets in p-Pb measured by ALICE were shown for the first time. The focus is here on the fully corrected jet production cross sections and the nuclear modification factors. Additionally, the jet radial structure is explored by comparing jet spectra reconstructed with different resolution parameters.

Motivation & Objective

• To investigate the impact of cold nuclear matter effects on jet production in p–Pb collisions by studying centrality dependence.
• To provide a reference measurement for Pb–Pb collisions by isolating cold nuclear matter effects in p–Pb systems.
• To explore jet radial structure using cross section ratios between different resolution parameters (R = 0.2 and R = 0.4).
• To apply a novel hybrid centrality estimation method combining multiplicity and Pb-side collision counts (Ncoll) for improved jet analysis.

Proposed method

• Jet reconstruction using the anti-kT algorithm on charged particle tracks from the ITS and TPC within |η| < 0.9 and pT > 150 MeV/c.
• Background correction via the kT algorithm to estimate and subtract event-wise background density (ρ), scaled by track density to account for acceptance effects.
• Application of detector response corrections to raw jet spectra to obtain fully corrected jet production cross sections.
• Nuclear modification factor QpPb calculated using two Ncoll estimators: Ncoll^mult (from multiplicity) and Ncoll^Pb-side (from Pb-side geometry), with pp reference scaled from 7 TeV data.
• Jet cross section ratio R(0.2, 0.4) defined as the ratio of dσ/dpT at R = 0.2 to R = 0.4 to probe radial jet structure.
• Centrality classes selected using VZERO and ZDC signals, with hybrid centrality approach combining multiplicity and geometric estimates.

Experimental results

Research questions

• RQ1 Does the production rate of charged jets in p–Pb collisions depend on event centrality, indicating cold nuclear matter effects?
• RQ2 How do different centrality estimators (Ncoll^mult vs. Ncoll^Pb-side) affect the interpretation of jet modification?
• RQ3 Is there a measurable change in jet radial structure (collimation) with increasing centrality?
• RQ4 To what extent do jet spectra in p–Pb deviate from expectations in pp collisions, as quantified by QpPb?

Key findings

• The fully corrected charged jet production cross sections for R = 0.2 and R = 0.4 show no significant variation across centrality intervals.
• The nuclear modification factor QpPb is consistent with unity across all centrality classes for both Ncoll estimators, indicating no strong cold nuclear matter effects.
• No significant centrality dependence is observed in QpPb when using either Ncoll^mult or Ncoll^Pb-side, with uncertainties overlapping across classes.
• The jet cross section ratio R(0.2, 0.4) shows no measurable centrality dependence, suggesting no modification in jet radial structure.
• The small differences between QpPb values from the two Ncoll estimators are interpreted as a systematic uncertainty of the hybrid centrality method.
• The results are fully compatible with the absence of jet suppression or enhancement in p–Pb collisions, consistent with expectations from pQCD and minimal nuclear effects.