Jets In Heavy-ion Collisions Research Articles

Separation of quark and gluon jets in heavy ion collisions at high energies

The possibility of separating quark and gluon jets in heavy ion collisions, both in the presence of the jet quenching effect and in its absence, has been investigated. Using machine learning methods, a technique has been developed to recover the fraction of quark and gluon jets, using variables characterizing the shape, multiplicity, and charge of the jet. The potential of using these variables to study the characteristics of the dense medium that arises during high-energy ion collisions has been demonstrated.

Quantifying the jet energy loss in Pb+Pb collisions at LHC

In this work, we give a method to study the energy loss of jets in the medium using a variety of jet energy loss observables such as nuclear modification factor and transverse momentum asymmetry in dijets and γ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\gamma $$\\end{document}-jets in heavy ion collisions. The energy loss of jets in the medium depends mainly on the size and the properties of medium viz. temperature and is a function of energy of the jets as predicted by various models. A Monte Carlo (MC) method is employed to generate the transverse momentum and path-lengths of the initial jets that undergo energy loss. Using different scenarios of energy loss, the transverse momentum and system size dependence of nuclear modification factors and different measures of dijet momentum imbalance at energies sNN\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sqrt{s_{\ extrm{NN}}}$$\\end{document} = 2.76 TeV and 5.02 TeV and γ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\gamma $$\\end{document}-jet asymmetry at sNN\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sqrt{s_{\ extrm{NN}}}$$\\end{document} =2.76 TeV in Pb+Pb collisions are simulated. The results are compared with the measurements by ATLAS and CMS experiments as a function of transverse momentum and centrality. The study demonstrates how the system size and energy dependence of jet energy loss can be quantified using various experimental observables.

Deciphering yield modification of hadron-triggered semi-inclusive recoil jets in heavy-ion collisions

In relativistic heavy-ion collisions, a hot and dense state of matter, called the Quark-Gluon Plasma (QGP), is produced. Semi-inclusive jets recoiling from trigger hadrons of high transverse momenta (pT) can serve as an effective probe of the QGP properties, as they are expected to experience jet quenching when traversing the QGP. Recent experimental results on the ratio of recoil jet yields normalized by the trigger counts in heavy-ion collisions to that in p+p collisions (IAA) pose an unexpected challenge in its interpretation. It is observed that IAA rises with the jet pT and possibly exceeds unity at high pT, while traditionally it is expected that jet quenching would lead to IAA<1. To address this challenge, we utilize the Linear Boltzmann Transport (LBT) model to simulate jet transport in the QGP, and study the effect of jet quenching for high-pT triggers and recoil jets separately on IAA. We find that the quenching of the colored triggers alone is responsible for the rising trend and larger-than-unity value observed experimentally.

Separation of Quark and Gluon Jets in Heavy Ion Collisions at High Energies

Separation of Quark and Gluon Jets in Heavy Ion Collisions at High Energies

Identifying quenched jets in heavy ion collisions with machine learning

Measurements of jet substructure in ultra-relativistic heavy ion collisions suggest that the jet showering process is modified by the interaction with the quark-gluon plasma. Modifications of the hard substructure of jets can be explored with modern data-driven techniques. In this study, a machine learning approach to the identification of quenched jets is designed. Jet showering processes are simulated with a jet quenching model Jewel and a non-quenching model Pythia 8. Sequential substructure variables are extracted from the jet clustering history following an angular-ordered sequence and are used in the training of a neural network built on top of a long short-term memory network. We show that this approach successfully identifies the quenching effect in the presence of the large uncorrelated background of soft particles created in heavy-ion collisions.

Heavy-flavor jets in heavy-ion collisions

Heavy-flavor jets are a unique tool to study dynamical evolution of QCD systems via heavy-flavor particles, allowing one to access to the original parton kinematics and to separate out production and fragmentation effects. In these proceedings, various heavy-flavor jet results in pp, pA, and AA collisions are presented, extending the scope of heavy-flavor studies discussed at this conference series, the Strangeness in Quark Matter.

Improved background subtraction and a fresh look at jet sub-structure in JEWEL

Interactions of hard partons in the Quark Gluon Plasma (QGP) created with relativistic heavy ion collisions lead to characteristic modifications of the internal structure of reconstructed jets. A large part of the observed jet sub-structure modifications stem from the QGP’s response to energy and momentum deposited by hard partons. Good control over medium response in theoretical calculations is thus instrumental to a quantitative understanding of medium modified (quenched) jets in heavy ion collisions. We present an improved way of handling the medium response in the jet quenching model Jewel and present results for a variety of jet sub-structure observables. The new recoil handling is more versatile and robust than the old scheme, giving a better control over many observables and, in particular, greatly improves the description of the jet mass.

Model studies of fluctuations in the background for jets in heavy ion collisions

Jets produced in high energy heavy ion collisions are quenched by the quark gluon plasma. Measurements of these jets are influenced by the methods used to suppress and subtract the large, fluctuating background and the assumptions inherent in these methods. We compare the measurements of the background in Pb+Pb collisions at $\sqrt{s_{NN}}$ = 2.76 TeV by the ALICE Collaboration $\underline{(B. Abelev \textit{et al.}, J. High Energy Phys. \mathbf{2012}, 053)}$ to calculations in TennGen (a data-driven random background generator) and PYTHIA Angantyr. A detailed understanding of the width of these fluctuations is important for reducing uncertainties due to unfolding and extending measurements to lower momenta and larger resolution parameters. The standard deviation of the energy in random cones in TennGen is approximately in agreement with the form predicted in the ALICE paper, with deviations of 1--6%. The standard deviation of energy in random cones in Angantyr exceeds the same predictions by approximately 13%. Deviations in both models can be explained by the assumption that the single-particle $d^2N/dy$ $dp_T$ is a gamma distribution in the derivation of the prediction, whereas the model uses a different distribution. This indicates that model comparisons are potentially sensitive to the treatment of the background. We demonstrate that unfolding methods used to remove background fluctuations from jets can affect the comparisons between models and data, $\textit{even in the absence of detector effects}$. Our findings suggest the need to more carefully consider methods for comparing simulations and data.

P T dispersion of inclusive jets in high-energy nuclear collisions* *Supported in part by Guangdong Major Project of Basic and Applied Basic Research (2020B0301030008), the National Natural Science Foundation of China (NSFC) (11935007, 12035007), the MOE Key Laboratory of Quark and Lepton Physics (CCNU) (QLPL2020P01)

In this study, we investigate the impact of jet quenching on the dispersion () of inclusive jets () in central Pb+Pb (0%–10%) collisions at TeV. The partonic spectrum in the initial hard scattering of elementary collisions is obtained by an event generator POWHEG+PYTHIA, which matches the next-to-leading order (NLO) matrix elements with parton showering, and the energy loss of a fast parton traversing through hot/dense QCD medium is calculated using Monte Carlo simulation within the Higher-Twist formalism of jet quenching in heavy-ion collisions. We present model calculations of the normalized distributions of inclusive jets in p+p and central Pb+Pb collisions at TeV, which offer good descriptions of ALICE measurements. It is shown that the distributions of inclusive jets in central Pb+Pb collisions shift significantly to a higher region relative to those in p+p collisions. Thus, the nuclear modification ratio of the distributions of inclusive jets is smaller than unity in the small region and larger than one in the large region. This behavior is caused by a more uneven distribution for jet constituents as well as the fraction alteration of quark/gluon initiated jets in heavy-ion collisions. The difference in distribution between groomed and ungroomed jets in Pb+Pb collisions is also discussed.

Disentangling jet modification in jet simulations and in Z+jet data

The selection of jets in heavy-ion collisions based on their pT after jet quenching is known to bias towards jets that lost little energy in the quark-gluon plasma. In this work, we study and quantify the impact of this selection bias on jet substructure observables so as to isolate effects caused by the modification of the substructure of jets by quenching. We do so at first in a simplified Monte Carlo study in which it is possible to identify the same jet before and after quenching. We show explicitly that jets selected based on their quenched (i.e. observable) pT have substantially smaller fractional energy loss than those selected based on the pT that they would have had in the absence of any quenching. This selection bias has a large impact on jet structure and substructure observables. As an example, we consider the angular separation ∆R of the hardest splitting in each jet, and find that the ∆R distribution of the (biased) sample of jets selected based upon their quenched pT is almost unmodified by quenching. In contrast, quenching causes dramatic modifications to the ∆R distribution of a sample of jets selected based upon their unquenched pT, with a significant enhancement at larger ∆R coming from the soft particles originating from the wake of the jet in the quark-gluon plasma. The jets which contribute to this enhancement are those which have lost the most energy and which were, therefore, left out of the sample selected after quenching. In a more realistic study, we then show that the same qualitative effects can all be observed in Z+jet events. Selecting jets in such events based on either the jet pT or the Z-boson pT provides an experimentally accessible way to quantify the effects of selection biases in jet observables and separate them from the modification of jet substructure caused by quenching. Selecting Z+jet events based upon the jet pT yields a ∆R distribution that appears almost unmodified whereas selecting Z+jet events based upon the Z-boson pT reveals a significant modification to the ∆R-distribution caused by quenching, once again arising from the wakes of those jets that lose more energy.

Cone-Size Dependence of Jet Suppression in Heavy-Ion Collisions.

The strong suppression of high-p_{T} jets in heavy-ion collisions is a result of elastic and inelastic energy loss suffered by the jet multiprong collection of color charges that are resolved by medium interactions. Hence, quenching effects depend on the fluctuations of the jet substructure that are probed by the cone-size dependence of the spectrum. In this Letter, we present the first complete, analytic calculation of the inclusive R-dependent jet spectrum in PbPb collisions at LHC energies, including resummation of energy loss effects from hard, vacuumlike emissions occurring in the medium and modeling of soft energy flow and recovery at the jet cone. Both the geometry of the collision and the local medium properties, such as the temperature and fluid velocity, are given by a hydrodynamic evolution of the medium, leaving only the coupling constant in the medium as a free parameter. The calculation yields a good description of the centrality and p_{T} dependence of jet suppression for R=0.4 together with a mild cone-size dependence, which is in agreement with recent experimental results. Gauging the theoretical uncertainties, we find that the largest sensitivity resides in the leading logarithmic approximation of the phase space of resolved splittings, which can be improved systematically, while nonperturbative modeling of the soft-gluon sector is of relatively minor importance up to large cone sizes.

First measurements of N-subjettiness in central Pb-Pb collisions at sqrt{s_{mathrm{NN}}} = 2.76 TeV

The ALICE Collaboration reports the first fully-corrected measurements of the N-subjettiness observable for track-based jets in heavy-ion collisions. This study is performed using data recorded in pp and Pb-Pb collisions at centre-of-mass energies of sqrt{s} = 7 TeV and sqrt{s_{mathrm{NN}}} = 2.76 TeV, respectively. In particular the ratio of 2-subjettiness to 1-subjettiness, τ2/τ1, which is sensitive to the rate of two-pronged jet substructure, is presented. Energy loss of jets traversing the strongly interacting medium in heavy-ion collisions is expected to change the rate of two-pronged substructure relative to vacuum. The results are presented for jets with a resolution parameter of R = 0.4 and charged jet transverse momentum of 40 ≤ pT,jet ≤ 60 GeV/c, which constitute a larger jet resolution and lower jet transverse momentum interval than previous measurements in heavy-ion collisions. This has been achieved by utilising a semi-inclusive hadron-jet coincidence technique to suppress the larger jet combinatorial background in this kinematic region. No significant modification of the τ2/τ1 observable for track-based jets in Pb-Pb collisions is observed relative to vacuum PYTHIA6 and PYTHIA8 references at the same collision energy. The measurements of τ2/τ1, together with the splitting aperture angle ∆R, are also performed in pp collisions at sqrt{s} = 7 TeV for inclusive jets. These results are compared with PYTHIA calculations at sqrt{s} = 7 TeV, in order to validate the model as a vacuum reference for the Pb-Pb centre-of-mass energy. The PYTHIA references for τ2/τ1 are shifted to larger values compared to the measurement in pp collisions. This hints at a reduction in the rate of two-pronged jets in Pb-Pb collisions compared to pp collisions.

Deep learning jet modifications in heavy-ion collisions

Jet interactions in a hot QCD medium created in heavy-ion collisions are conventionally assessed by measuring the modification of the distributions of jet observables with respect to the proton-proton baseline. However, the steeply falling production spectrum introduces a strong bias toward small energy losses that obfuscates a direct interpretation of the impact of medium effects in the measured jet ensemble. Modern machine learning techniques offer the potential to tackle this issue on a jet-by-jet basis. In this paper, we employ a convolutional neural network (CNN) to diagnose such modifications from jet images where the training and validation is performed using the hybrid strong/weak coupling model. By analyzing measured jets in heavy-ion collisions, we extract the original jet transverse momentum, i.e., the transverse momentum of an identical jet that did not pass through a medium, in terms of an energy loss ratio. Despite many sources of fluctuations, we achieve good performance and put emphasis on the interpretability of our results. We observe that the angular distribution of soft particles in the jet cone and their relative contribution to the total jet energy contain significant discriminating power, which can be exploited to tailor observables that provide a good estimate of the energy loss ratio. With a well-predicted energy loss ratio, we study a set of jet observables to estimate their sensitivity to bias effects and reveal their medium modifications when compared to a more equivalent jet population, i.e., a set of jets with similar initial energy. Finally, we also show the potential of deep learning techniques in the analysis of the geometrical aspects of jet quenching such as the in-medium traversed length or the position of the hard scattering in the transverse plane, opening up new possibilities for tomographic studies.

Jet substructure and parton splitting: an experimental overview

It has been established that experimental jets in heavy ion collisions have less energy as compared to pp collisions on average, where the lost energy is manifested as soft particles at large angles with respect to the jet axis. It is known as jet quenching. Various experiments have reported new measurements of jet nuclear modification factor extending to large radius and low momentum, which provide unique information on how the energy is redistributed. Substructure measurements aim to study this phenomenon in a greater detail, and one can hope to unravel the underlying mechanisms from which the parton interacts with the hot QCD matter created in the collisions. In this talk, most recent results on jet substructures and jet nuclear modification factors from experiments in LHC and RHIC are summarized.

Medium modifications of girth distributions for inclusive jets and Z 0+jets in relativistic heavy-ion collisions at the LHC * *Supported by Natural Science Foundation of China (11935007, 11805167)

In this paper, we investigate the medium modifications of girth distributions for inclusive jets and tagged jets with a small radius ( ) in Pb+Pb collisions with TeV at the LHC. The partonic spectra in the initial hard scattering of elementary collisions are obtained by an event generator POWHEG+PYTHIA, which matches the next-to-leading order (NLO) matrix elements with parton showering, and the energy loss of a fast parton traversing the hot/dense QCD medium is calculated by Monte Carlo simulation within the higher-twist formalism of jet quenching in heavy-ion collisions. We present the model calculations of event normalized girth distributions for inclusive jets in p+p and Pb+Pb collisions at TeV, which give good descriptions of ALICE measurements. It is shown that the girth distributions of inclusive jets in Pb+Pb are shifted to lower girth regions relative to those in p+p. Thus, the nuclear modification factor of girth distributions for inclusive jets is larger than unity at small girth regions and smaller than unity at large girth regions. This behavior results from softer fragments inside a jet as well as the fraction alteration of gluon/quark initiated jets in heavy-ion collisions. We further predict the girth distributions for boson tagged jets in Pb+Pb collisions at TeV and demonstrate that the medium modification on girth distributions for tagged jets is less pronounced compared to that for inclusive jets because the dominant components of tagged jets are quark-initiated jets.

Medium modification of γ-jet fragmentation functions in Pb+Pb collisions at LHC

Coupled linear Boltzmann transport and hydrodynamic (CoLBT-hydro) model has been developed for simultaneous simulations of jet propagation and jet-induced medium excitation in heavy-ion collisions. Within this coupled approach, the final reconstructed jets in heavy-ion collisions include not only hadrons from the hadronization of medium modified jet shower partons from the linear Boltzmann transport (LBT) but also hadrons from the freeze-out of the jet-induced medium excitation in the hydrodynamic evolution of the bulk medium. Using the CoLBT-hydro model, we study medium modification of the fragmentation functions of γ-triggered jets in high-energy heavy-ion collisions at the Large Hadron Collider. The CoLBT-hydro model is shown to describe the experimental data not only on the suppression of leading hadrons within the jet cone at large momentum fraction zγ=pTh/pTγ relative to the transverse momentum of the trigger photon due to parton energy loss but also the enhancement of soft hadrons at small zγ and zjet=pTh/pTjet due to jet-induced medium excitation. There is no suppression of the fragmentation function, however, at large momentum fraction zjet relative to the transverse momentum of the reconstructed jet due to trigger bias and medium modification of quark to gluon jet fraction. For jets whose final transverse momenta are comparable to or larger than that of the trigger photon, the trigger bias can lead to enhancement of the jet fragmentation function at large zjet.

Can we observe jet PT-broadening in heavy-ion collisions at the LHC?

We investigate the effect of PT-broadening on jet substructure observables in heavy-ion collisions at the LHC. As an example, we focus on the opening angle of the two branches that satisfy the soft drop grooming condition in a highly energetic jet. The medium modification of the angular distribution can provide important information on the jet transport properties of hot QCD matter. In addition, we take into account a change of the overall fraction of quark and gluon jets in heavy-ion collisions. We comment on the comparison to a recent measurement from the ALICE Collaboration.

Jet charge in high-energy nuclear collisions * * This research is supported in part by the NSFC of China with Project Nos. 11935007 and 11435004

The averaged jet charge characterizes the electric charge of the initiating parton and provides a powerful tool to distinguish quark jets from gluon jets. We predict, for the first time, the medium modification of the averaged jet charge in the heavy-ion collisions at the LHC, where jet productions in p+p collisions are simulated by PYTHIA6, and the parton energy loss in QGP is calculated with two Monte Carlo models of jet quenching: PYQUEN and JEWEL. We found that the distribution of averaged jet charge is significantly suppressed by initial state isospin effects due to the participation of neutrons with zero electric charge during nuclear collisions. The considerable enhancement of the averaged jet charge in central Pb+Pb collisions is observed relative to peripheral collisions, since the jet quenching effect is more pronounced in central collisions. The distinct feature of the averaged jet charge between quark and gluon jets, along with the sensitivity of medium modifications on the jet charge to flavor dependence of the parton energy loss, could be very useful to discriminate the energy loss pattern between quark and gluon jets in heavy-ion collisions.

Nuclear jets in heavy-ion collisions

Head-on collisions between nuclei of different size at Fermi energies may give rise to extremely deformed dynamical regimes and patterns. Those latter, may suddenly turn into a stream of nuclear clusters, resembling collimated jets. This mechanism, which could be easily addressed in experiments, is simulated in the framework of stochastic one-body approaches. We employ the Boltzmann-Langevin equation to specifically address out-of-equilibrium conditions and handle dynamical fluctuations. An interesting interplay between surface and volume instabilities is discussed for the first time, leading to the appearing of stable and rather regular patterns of streaming clusters.

Sorting Out Quenched Jets.

We introduce a new "quantile" analysis strategy to study the modification of jets as they traverse through a droplet of quark-gluon plasma. To date, most jet modification studies have been based on comparing the jet properties measured in heavy-ion collisions to a proton-proton baseline at the same reconstructed jet transverse momentum (p_{T}). It is well known, however, that the quenching of jets from their interaction with the medium leads to a migration of jets from higher to lower p_{T}, making it challenging to directly infer the degree and mechanism of jet energy loss. Our proposed quantile matching procedure is inspired by (but not reliant on) the approximate monotonicity of energy loss in the jet p_{T}. In this strategy, jets in heavy-ion collisions ordered by p_{T} are viewed as modified versions of the same number of highest-energy jets in proton-proton collisions, and the fractional energy loss as a function of jet p_{T} is a natural observable (Q_{AA}). Furthermore, despite nonmonotonic fluctuations in the energy loss, we use an event generator to validate the strong correlation between the p_{T} of the parton that initiates a heavy-ion jet and the p_{T} of the vacuum jet which corresponds to it via the quantile procedure (p_{T}^{quant}). We demonstrate that this strategy both provides a complementary way to study jet modification and mitigates the effect of p_{T} migration in heavy-ion collisions.