Abstract
Abstract Multiple interactions between parton showers and the surrounding QCD matter are expected to underlie the strong medium-modifications of jet observables in ultra- relativistic heavy ion collisions at RHIC and at the LHC. Here, we note that such jet- medium interactions alter generically and characteristically the color correlations in the parton shower. We characterize these effects in a color-differential calculation of the medium-induced gluon radiation spectrum to first and second order in opacity. By interfacing simple branching histories of medium-modified color flow with the Lund hadronization model, we analyze how the medium modification of color correlations can affect the distribution of hadronic fragments in jets. Importantly, we observe that jet-medium interactions give rise to the medium-induced color decoherence of gluons from the parton shower. Since hadronization respects color flow and since each color singlet in a parton shower is hadronized separately, this medium-induced color decoherence leaves characteristic signatures in the jet fragmentation pattern. In particular, it can contribute to the quenching of leading hadron spectra. Moreover, it can increase strongly the yield of soft hadronic fragments from a jet, while the distribution of more energetic hadrons follows naturally the shape of a vacuum-like fragmentation pattern of lower total energy.
Highlights
Yielding information on the path-length of in-medium propagation), its dependence on kinematic variables including the center of mass energy of the collision and the transverse momentum of the hard process, its approximate independence on the produced hadron species, and the absence of quenching effect for high-pT prompt photons and Z-bosons
In particular, which medium-modified color connections can arise in the interactions of a parton shower with a QCD medium
The term |Mv2irt|2 a accounts for processes in which neither color nor transverse momentum is exchanged between projectile and medium, but where the longitudinal (‘-’ in light-cone coordinates) momentum transferred by the medium is sufficient to open the possibility of gluon radiation for a quark initially on-shell
Summary
To set the stage for the study of medium-modifications of high-pT processes to be carried out in this paper, we start from the ‘vacuum’ baseline of a hard partonic interaction in the absence of medium effects. The Lund model, as implemented e.g. in the Pythia event generator [18], groups the same perturbative information into a set of color strings that start with a quark, follow the color flow by including gluons as kinks, and end on an anti-quark; these color singlet strings are hadronized (through excitation of qqpairs from the vacuum) according to a prescription that requires kinematic information about both the end-points of the string and all the kinks. We shall refer to these color configurations as medium-modified or gluo-decohered We relate this discussion to the diagrams in figure 4 that are usually drawn for the calculation of parton energy loss within an opacity expansion.
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