Jet charge modification in finite QCD matter

Abstract

Jet production and jet substructure modification in heavy-ion collisions have played an essential role in revealing the in-medium evolution of parton showers and the determination of the properties of strongly-interacting matter under extreme conditions. It is imperative to extend these studies to include flavor tagging and to devise observables that are sensitive to the partonic origin of jets. The average jet charge, defined as the momentum-weighted sum of the electric charges of particles inside the jet, is a proxy of the electric charge of the quark or gluon that initiates the jet. We demonstrate how the factorization framework of soft-collinear effective theory can be generalized to evaluate the jet charge in a dense strongly-interacting matter environment, such as the one produced in nuclear reactions at collider energies. Observables that can separate the contribution of in-medium branching from the trivial isospin effects are identified and their connection to established jet quenching effects is elucidated. We present predictions for the transverse momentum dependence of the jet charge distribution in nucleus-nucleus collisions and its modification relative to the proton case. * haitaoli@lanl.gov † ivitev@lanl.gov

Measurement of quark- and gluon-like jet fractions using jet charge in PbPb and pp collisions at 5.02 TeV

“…To obtain an optimal number of iterations, reconstructed jet charge distributions from modified samples of pythia6 and pythia6+hydjet are unfolded using the nominal response matrices. Quark and gluon jet fractions are varied by 50% in the modified simulation samples, which is expected to give a good bound on the potential modification of the jet charge distribution in data [23]. Based on these studies, three to four iterations are used in the unfolding procedure for different selections of threshold track p T and κ.…”

“…More recently, jet charge was measured at the LHC in pp collisions by the ATLAS [21] and CMS [22] collaborations, characterizing the contributions of quark and gluon fragmentation to jet production. At LHC energies, gluon contributions dominate jet production at lower transverse momenta, while the valence quark contributions overtake at higher jet p T [23]. According to predictions from the pythia event generator (version 6.424 [24], tune Z2 [25]) for pp collisions at 5.02 TeV, gluon jets are expected to constitute about 59% of a sample of jets with transverse momenta above 120 GeV.…”

“…Similarly, up and down (anti)quark jets are predicted to make up about 32% of the sample with the other 9% arising from charm, strange and bottom (anti)quark jet contributions. A detailed investigation of jet charge and its applications in heavy ion collisions is motivated by extensive theoretical calculations [23,26,27]. The dependence of the mean and width (standard deviation) of the jet charge distribution on both jet energy and size, can be calculated independently of Monte Carlo (MC) fragmentation models despite the large experimental uncertainty in fragmentation functions [28].…”

Measurements of photon- and Z-tagged jet quenching by ATLAS

“…This calculation includes particles originating in the wake of the parton as it traverses the medium. In green, a calcuation within the framework of soft collinear effective theory (SCET G , labelled "Li & Vitev") models parton-medium interaction with Glauber gluon interactions [11,12], with systematic uncertainty on the parton-medium coupling = 2.0 ± 0.2 shown by shaded bands. Both models reproduce the suppression of high-hZ charged particles observed in data, and the hybrid model produces a similar level of particle enhancement that is observed in data, rising to AA ∼ 2 around hZ ∼ 0.02 − 0.04.…”

Transverse-energy-energy correlations in deep inelastic scattering

“…Soft collinear effective theory with Glauber gluon interactions has provided a mean to evaluate the contribution in-medium parton showers [67,68] to a variety of observables in reactions with nuclei. The most recent examples include the modification of jet cross sections and jet substructure ranging from the jet splitting functions to the jet charge [69][70][71]. To obtain predictions for the TEEC event shape observable in DIS on nuclei will require a computation of the contributions from parton branching in strongly-interacting matter to the terms in the master factorization formula.…”