Abstract
Using a recently-developed perturbative-QCD approach for jet evolution in a dense quark-gluon plasma, we study the nuclear modification factor for the jet fragmentation function. The qualitative behaviour that we find is in agreement with the respective experimental observations in Pb+Pb collisions at the LHC: a pronounced nuclear enhancement at both ends of the spectrum. Our Monte Carlo simulations are supplemented with analytic estimates which clarify the physical interpretation of the results. The main source of theoretical uncertainty is the sensitivity of our calculations to a low-momentum cutoff which mimics confinement. To reduce this sensitivity, we propose a new observable, which describes the jet fragmentation into subjets and is infrared-and-collinear safe by construction. We present Monte Carlo predictions for the associated nuclear modification factor together with their physical interpretation.
Highlights
One important source of information about the dense partonic matter — the quark-gluon plasma — created in the intermediate stages of ultrarelativistic heavy ion collisions at RHIC and the LHC comes from studies of jets propagating through this dense medium and of the associated modifications of the jet structure and properties
Using a recently-developed perturbative-QCD approach for jet evolution in a dense quark-gluon plasma, we study the nuclear modification factor for the jet fragmentation function
Known as “jet quenching”, these modifications cover a large variety of phenomena and observables, from more inclusive ones, like the energy loss by the jet, to more detailed ones which probe the pattern of the in-medium jet fragmentation or the medium response to the jet
Summary
One important source of information about the dense partonic matter — the quark-gluon plasma — created in the intermediate stages of ultrarelativistic heavy ion collisions at RHIC and the LHC comes from studies of jets propagating through this dense medium and of the associated modifications of the jet structure and properties. This means that its theoretical predictions are strongly sensitive to non-perturbative (confinement) physics like the modelling of the hadronisation mechanism Another potential drawback of the fragmentation function, already recognised in the literature [2, 7], is that the nuclear enhancement seen in the LHC data at x 0.5 is not necessarily an evidence for new physics in the jet fragmentation at large x, but merely a consequence of the overall energy loss by the jet together with the bias introduced by the initial spectrum for jet production via hard (nucleon-nucleon) scatterings.
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