Analysis of the apparent nuclear modification in peripheral Pb-Pb collisions at 5.02 TeV
Abstract
Charged-particle spectra at midrapidity are measured in Pb-Pb collisions
at the centre-of-mass energy per nucleon-nucleon pair root s(NN) = 5.02
TeV and presented in centrality classes ranging from most central
(0-5\%) to most peripheral (95-100\%) collisions. Possible medium
effects are quantified using the nuclear modification factor (R-AA) by
comparing the measured spectra with those from proton-proton collisions,
scaled by the number of independent nucleon-nucleon collisions obtained
from a Glauber model. At large transverse momenta (8 < p(T) < 20 GeV/c),
the average R-AA is found to increase from about 0.15 in 0-5\% central
to a maximum value of about 0.8 in 75-85\% peripheral collisions, beyond
which it falls off strongly to below 0.2 for the most peripheral
collisions. Furthermore, R-AA initially exhibits a positive slope as a
function of p(T) in the 8-20 GeV/c interval, while for collisions beyond
the 80\% class the slope is negative. To reduce uncertainties related to
event selection and normalization, we also provide the ratio of R-AA in
adjacent centrality intervals. Our results in peripheral collisions are
consistent with a PYTHIA-based model without nuclear modification,
demonstrating that biases caused by the event selection and collision
geometry can lead to the apparent suppression in peripheral collisions.
This explains the unintuitive observation that R-AA is below unity in
peripheral Pb-Pb, but equal to unity in minimum-bias p-Pb collisions
despite similar charged-particle multiplicities. (C) 2019 Conseil
Europeen pour la Recherche Nucleaire. Published by Elsevier B.V.
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