Medium-induced color flow softens hadronization

Abstract

Medium-induced parton energy loss, resulting from gluon exchanges between the QCD matter and partonic projectiles, is expected to underlie the strong suppression of jets and high-p T hadron spectra observed in ultrarelativistic heavy-ion collisions. Here, we present the first color-differential calculation of parton energy loss. We find that color exchange between medium and projectile decoheres the color of radiated gluons from that of the most energetic fragments and enhances parametrically the invariant mass of energetic color singlet clusters in the parton shower. These effects are seen in more than half of the medium-modified parton branchings. As shown for a cluster hadronization model and as argued for a Lund string fragmentation model, this leads to additional softening of hadronic spectra. Compared to standard parton energy loss calculations, a lower density of the QCD matter is then needed to account for the nuclear modification factor at the Large Hadron Collider. High-transverse-momentum partons (p T 10 GeV) produced in heavy-ion collisions interact with the QCD matter in the collision region while branching. This interaction is thought to cause the strong medium modification of hadronic spectra and jets measured in heavy-ion collisions at the Large Hadron Collider (LHC) and at the Relativistic Heavy Ion Collider. The modeling of this jet quenching phenomenon has focused so far on medium-induced parton energy loss prior to hadronization [1], assuming that, for sufficiently high p T , hadronization occurs time delayed and thus outside the medium. However, if the color flow of a parton shower is modified within the medium, then hadronization can be affected irrespective of when it occurs. Here, we analyze the medium-induced color flow in a standard parton-energy-loss calculation. Compared to the current modeling of parton energy loss [2][3][4][5][6][7], this will be seen to result in a characteristic softening of the ensuing hadronization. It may, thus, affect significantly the extraction of medium properties from the measured nuclear modification factor at the LHC [8][9][10].We start by considering an elementary building block of a parton shower, the q → q g parton splitting. For a small light-cone energy k + of the gluon compared to the parent parton, x ≡ k + /p + 1, and for transverse gluon momentum k with K 0 ≡ k/k 2 , the gluon spectrum reads, to leading order in α s ,In the presence of QCD matter, interactions between projectile parton and medium result in a modified spectrum dI rad ≡ dI vac + dI med that can be written as an expansion in powers of the ratio ζ ≡ L + /λ + el of the in-medium path length L + and an elastic mean free path λ + el (for details, see Ref.[1]). To first order in this opacity expansion, the medium-induced radiation spectrum readswhere · · · denotes averaging over the transverse-momentum transfer q of a single interaction andMedium-induced interference effects enter via the factorThus, the medium-modification dI med can occur only for quanta of sufficiently small formation...

Angular structure of jet quenching within a hybrid strong/weak coupling model

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Nuclear modification factor for light and heavy flavors within pQCD and recent data from the LHC

“…For gluon emission in vacuum a reasonable choice is α The approximation (4) ignores creation in the QGP of the anomalous jet color states, which may be important for the baryon R AA [22] at not very high p T and the jet structure in the soft region [23]. But it should be reasonable for evaluating the R AA for charged hadrons, which is dominated by the charged pions, and the R AA for heavy flavors.…”

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