[Paper Review] Nuclear modification factor of charged particles and light-flavour hadrons in p--Pb collisions measured by ALICE

G. Bencédi|arXiv (Cornell University)|Sep 19, 2016

High-Energy Particle Collisions Research4 citations

TL;DR

This paper presents ALICE measurements of nuclear modification factors (R_pPb and R_AA) for charged particles and light-flavour hadrons in p–Pb and Pb–Pb collisions at √s_NN = 5.02 TeV and 2.76 TeV, respectively. It demonstrates that high-<i>p</i>_T suppression in Pb–Pb is due to final-state parton energy loss in the quark-gluon plasma, not initial-state effects, as R_pPb is consistent with unity at high <i>p</i>_T, while proton spectra show a Cronin-like enhancement at intermediate <i>p</i>_T in p–Pb.

ABSTRACT

The hot and dense strongly interacting Quark-Gluon Plasma (sQGP) created in ultra-relativistic heavy-ion collisions can be probed by studying high-$p_{\ m T}$ particle production and parton energy loss. Similar measurements performed in p-Pb collisions may help in determining whether initial or final state nuclear effects play a role in the observed suppression of hadron production at high-$p_{\ m T}$ in Pb--Pb collisions. By examining the nuclear modification factors through the comparison of identified hadron yields in different collision systems one can gain insight into particle production mechanisms and nuclear effects.

Motivation & Objective

• To disentangle initial-state and final-state nuclear effects in high-<i>p</i>_T hadron suppression observed in Pb–Pb collisions.
• To investigate whether parton energy loss in the quark-gluon plasma (QGP) is responsible for the suppression of high-<i>p</i>_T particles.
• To examine the role of nuclear medium effects such as Cronin enhancement and shadowing in p–Pb collisions.
• To compare identified hadron yields across collision systems to probe parton fragmentation and energy loss mechanisms.

Proposed method

• Measurement of differential yields of charged particles and identified light-flavour hadrons (π±, K±, p/̄p) in p–Pb and Pb–Pb collisions at √s_NN = 5.02 TeV and 2.76 TeV.
• Calculation of nuclear modification factors R_pPb and R_AA using the formula R = (d²N/dydp_T) / (⟨T⟩ × d²σ^INEL_pp/dydp_T), where ⟨T⟩ is the average nuclear overlap function from the Glauber model.
• Use of ALICE’s particle identification system (ITS, TPC, TOF, HMPID) to separate pions, kaons, and protons via dE/dx and time-of-flight.
• Comparison of R_pPb and R_AA with theoretical models including pQCD, CGC, and HIJING with shadowing effects.
• Analysis of multiplicity dependence of proton-to-pion ratios in p–Pb to isolate collective-like effects.

Experimental results

Research questions

• RQ1 Is the high-<i>p</i>_T suppression of charged hadrons in Pb–Pb collisions due to final-state parton energy loss or initial-state nuclear effects?
• RQ2 Does the nuclear modification factor R_pPb deviate from unity at high <i>p</i>_T, indicating final-state effects in p–Pb?
• RQ3 How do the R_pPb values for different light-flavour hadrons (pions, kaons, protons) compare, and what do they reveal about nuclear medium effects?
• RQ4 Is there evidence of radial flow or mass-dependent suppression at low <i>p</i>_T in Pb–Pb and p–Pb collisions?
• RQ5 To what extent do theoretical models including shadowing, Cronin effects, and jet quenching describe the observed R_pPb and R_AA data?

Key findings

• R_AA for charged particles in central Pb–Pb collisions (0–5%) shows strong suppression at <i>p</i>_T ≈ 6–7 GeV/<i>c</i>, with a rise at higher <i>p</i>_T, indicating reduced relative energy loss.
• R_pPb for charged particles is consistent with unity for <i>p</i>_T > 2 GeV/<i>c</i>, indicating no significant final-state effects in p–Pb, supporting that suppression in Pb–Pb is due to QGP-induced energy loss.
• At <i>p</i>_T ≈ 4 GeV/<i>c</i>, R_pPb for protons exceeds that of charged hadrons by ~3 times, indicating a moderate Cronin-like enhancement, while pions and kaons show no such enhancement.
• For <i>p</i>_T > 10 GeV/<i>c</i>, R_pPb for all identified hadrons is consistent with unity, confirming binary collision scaling and absence of final-state effects at high <i>p</i>_T.
• R_AA for π±, K±, and p/̄p are indistinguishable at <i>p</i>_T > 10 GeV/<i>c</i>, indicating that jet quenching affects all fragments coherently without species-dependent effects.
• The multiplicity dependence of proton-to-pion ratios in p–Pb shows a strong trend at <i>p</i>_T < 10 GeV/<i>c</i>, qualitatively similar to Pb–Pb, suggesting collective-like behavior, while high-<i>p</i>_T ratios match pp data.