Abstract
Medium effects on the production of high-pT particles in nucleus–nucleus (AA) collisions are generally quantified by the nuclear modification factor (RAA), defined to be unity in absence of nuclear effects. Modeling particle production including a nucleon–nucleon impact parameter dependence, we demonstrate that RAA at midrapidity in peripheral AA collisions can be significantly affected by event selection and geometry biases. Even without jet quenching and shadowing, these biases cause an apparent suppression for RAA in peripheral collisions, and are relevant for all types of hard probes and all collision energies. Our studies indicate that calculations of jet quenching in peripheral AA collisions should account for the biases, or else they will overestimate the relevance of parton energy loss. Similarly, expectations of parton energy loss in light–heavy collision systems based on comparison with apparent suppression seen in peripheral RAA should be revised. Our interpretation of the peripheral RAA data would unify observations for lighter collision systems or lower energies where significant values of elliptic flow are observed despite the absence of strong jet quenching.
Highlights
Medium effects on the production of high-pT particles in nucleus– nucleus (AA) collisions are generally quantified by the nuclear modification factor (RAA), defined to be unity in absence of nuclear effects
The number of binary collisions is obtained by assuming that the nucleons move on straight trajectories and a collision is counted if the nucleon–nucleon (NN) impact parameter bNN is below a certain threshold
In variance to the standard Monte Carlo (MC) Glauber approach, the HIJING model [5] takes into account the possibility of multiple hard scatterings in the same NN collision
Summary
Defined as the ratio of the per-event yield YAA measured in nucleus–nucleus (AA) collisions to the yield of an equivalent incoherent superposition of Ncoll binary pp collisions. [7] describes an extension of the optical Glauber model, in which the nuclear overlap function is obtained from a convolution between the product of the thickness functions of the two nuclei and the nucleon– nucleon overlap function These extensions have important consequences for the AA impact parameter dependence of hard processes. A further consequence arises if the yield of hard and soft processes are correlated via the common bNN and centrality selection is based on soft particle production (multiplicity or summed energy) [8,9] In this case for a given centrality class, the NN collisions can be biased towards higher or lower than average impact parameters.
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