Jet Nuclear Modification Factor Research Articles

New metric improving Bayesian calibration of a multistage approach studying hadron and inclusive jet suppression

We study parton energy-momentum exchange with the quark gluon plasma (QGP) within a multistage approach composed of in-medium Dokshitzer-Gribov-Lipatov-Altarelli-Parisi evolution at high virtuality, and (linearized) Boltzmann transport formalism at lower virtuality. This multistage simulation is then calibrated in comparison with high-pT charged hadrons, D mesons, and the inclusive jet nuclear modification factors, using Bayesian model-to-data comparison, to extract the virtuality-dependent transverse momentum broadening transport coefficient q̂. To facilitate this undertaking, we develop a quantitative metric for validating the Bayesian workflow, which is used to analyze the sensitivity of various model parameters to individual observables. The usefulness of this new metric in improving Bayesian model emulation is shown to be highly beneficial for future such analyses. Published by the American Physical Society 2024

Recent Jet Measurements in ALICE

The precise measurement of jets in pp collisions plays a critical role in the study of quantum chromodynamics (QCD). Measurements of jets in relativistic heavy-ion collisions are additionally useful for probing QCD matter in the regime of high densities and temperatures since the partons in the jet evolution interact strongly with the medium. This phenomenon is called jet quenching and is characterized by both jet energy loss and modification of the internal structure of the jet. The ALICE experiment is well suited for studies of jets and their corresponding internal structure due to the high precision tracking system. These proceedings report on an overview of recent ALICE jet results in both pp and Pb–Pb collisions. Results in pp collisions include measurements of generalized angularities of groomed and inclusive jets, a measurement of the Lund jet plane, and a measurement of jet axis differences. Results from Pb–Pb collisions include measurements of jet splittings, studies of subjet fragmentation, as well as a measurement of the jet nuclear modification factor made with machine learning techniques. These new results, when combined with the corresponding theoretical comparisons, provide information regarding the QCD processes of the parton shower and hadronization, as well as the underlying physics of jet quenching.

First measurement of large area jet transverse momentum spectra in heavy-ion collisions

Jet production in lead-lead (PbPb) and proton-proton (pp) collisions at a nucleon-nucleon center-of-mass energy of 5.02 TeV is studied with the CMS detector at the LHC, using PbPb and pp data samples corresponding to integrated luminosities of 404 μb−1 and 27.4 pb−1, respectively. Jets with different areas are reconstructed using the anti-kT algorithm by varying the distance parameter R. The measurements are performed using jets with transverse momenta (pT) greater than 200 GeV and in a pseudorapidity range of |η| < 2. To reveal the medium modification of the jet spectra in PbPb collisions, the properly normalized ratio of spectra from PbPb and pp data is used to extract jet nuclear modification factors as functions of the PbPb collision centrality, pT and, for the first time, as a function of R up to 1.0. For the most central collisions, a strong suppression is observed for high-pT jets reconstructed with all distance parameters, implying that a significant amount of jet energy is scattered to large angles. The dependence of jet suppression on R is expected to be sensitive to both the jet energy loss mechanism and the medium response, and so the data are compared to several modern event generators and analytic calculations. The models considered do not fully reproduce the data.

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.

Deciphering the \U0001d4cfg distribution in ultrarelativistic heavy ion collisions

Within perturbative QCD, we develop a new picture for the parton shower generated by a jet propagating through a dense quark-gluon plasma. This picture combines in a simple, factorised, way multiple medium-induced parton branchings and standard vacuum-like emissions, with the phase-space for the latter constrained by the presence of the medium. We implement this picture as a Monte Carlo generator that we use to study two phenomenologically important observables: the jet nuclear modification factor RAA and the \U0001d4cfg distribution reflecting the jet substructure. In both cases, the outcome of our Monte Carlo simulations is in good agreement with the LHC measurements. We provide basic analytic calculations that help explaining the main features observed in the data. We find that the energy loss by the jet is increasing with the jet transverse momentum, due to a rise in the number of partonic sources via vacuum-like emissions. This is a key element in our description of both RAA and the \U0001d4cfg distribution. For the latter, we identify two main nuclear effects: incoherent jet energy loss and hard medium-induced emissions. As the jet transverse momentum increases, we predict a qualitative change in the ratio between the \U0001d4cfg distributions in PbPb and pp collisions: from increasing at small \U0001d4cfg, this ratio becomes essentially flat, or even slightly decreasing.

Exploring jet profiles in Pb-Pb collisions at 5.02 TeV with the ALICE detector

In this contribution, we present measurements of inclusive jet production with the ALICE detector in Pb-Pb collisions at sNN=5.02TeV. Production cross sections of charged particle jets and fully reconstructed jets are measured with jet resolution parameters R = 0.2 and 0.3. To quantify the medium induced parton energy loss, jet nuclear modification factors (RAA) are measured by utilizing POWHEG+Pythia8 predictions as reference cross section in pp collisions. We also calculate the ratio of jet production cross sections for jets reconstructed with different cone radii to explore the jet radial profile in Pb-Pb collisions. The ratios of jet cross sections are compared to a pp reference predicted by POWHEG+Pythia8 and in-medium energy loss model prediction by the JEWEL Monte Carlo event generator.

Measurements of heavy-flavour correlations and jets with ALICE at the LHC

In this work, the latest heavy-flavour correlation and jet measurements at mid-rapidity with the ALICE detector in pp, p–Pb and Pb–Pb collisions from the LHC Run 2 are reported. In particular, the results of azimuthal correlation measurements of D mesons with charged particles in pp collisions at s=13TeV and in p–Pb collisions at sNN=5.02TeV are presented. Studies on the centrality-dependent azimuthal correlations of heavy-flavour hadron decay electrons with charged particles in Pb–Pb collisions at sNN=5.02TeV are shown. In addition, measurements of D-meson tagged jet production in pp collisions at s=7TeV and in p–Pb collisions at sNN=5.02TeV are presented, including studies in pp collisions of the jet momentum fraction carried by the D meson. A first result on the D-meson tagged jet nuclear modification factor in central Pb–Pb collisions at sNN=5.02TeV is also reported.

Sudakov suppression of jets in QCD media

We compute modifications to the jet spectrum in the presence of a dense medium. We show that in the large-$N_c$ approximation and at leading logarithmic accuracy the jet nuclear modification factor factorizes into a quenching factor associated to the total jet color charge and a Sudakov suppression factor which accounts for the energy loss of jet substructure fluctuations. This factor, called the jet collimator, implements the fact that subjets, that are not resolved by the medium, lose energy coherently as a single color charge, whereas resolved large angle fluctuations suffer more quenching. For comparison, we show that neglecting color coherence results in a stronger suppression of the jet nuclear modification factor.

HYDJET++ model as a hot mixture of jets and hydrodynamics at ultra-relativistic heavy-ion collisions

HYDJET++ model combines the description of soft processes with the treatment of hard partons propagating hot and dense nuclear medium. The model is employed for the analysis of PbPb collisions at LHC energies. The cross-talk of elliptic v2 and triangular v3 flow in the model generates both even and odd harmonics of higher order, as well as long-range azimuthal dihadron correlations (“ridge” structure). The simple modification of the model via introducing the distribution over spatial anisotropy parameters permits HYDJET++ to reproduce both v2 and v3 flow fluctuations and related to it eccentricity fluctuations of the initial state. The data on momentum spectra and elliptic flow of charmed mesons are reproduced by HYDJET++ including thermal and non-thermal charm production mechanisms. A tolerable agreement of HYDJET++ predictions with the data for hard probes (including such observables as high-pT hadron and jet nuclear modification factors, and specific modification of longitudinal and radial jet profiles) testifies in favour of successful treatment of the hard component based on PYQUEN event generator.

Measurement of inclusive jet cross sections in pp and PbPb collisions at sNN=2.76 TeV

Inclusive jet spectra from pp and PbPb collisions at a nucleon-nucleon center-of-mass energy of 2.76 TeV, collected with the CMS detector at the LHC, are presented. Jets are reconstructed with three different distance parameters (R = 0.2, 0.3, and 0.4) for transverse momentum (pT) greater than 70 GeV and pseudorapidity abs(eta) < 2. Next-to-leading-order quantum chromodynamic calculations with non-perturbative corrections are found to over-predict jet production cross sections in pp for small distance parameters. The jet nuclear modification factors for PbPb compared to pp collisions, show a steady decrease from peripheral to central events, along with a weak dependence on the jet pT. They are found to be independent of the distance parameter in the measured kinematic range.

Effect of a meson cloud on the jet nuclear modification factor in pA collisions

We study the effect of the nucleon meson cloud on centrality dependence of the jet nuclear modification factor $R_{pA}$. We find that the meson-baryon Fock components may lead to a noticeable deviation of $R_{pA}$ from unity. Our results for $R_{pA}$ show the same tendency as that observed by ATLAS in $p+Pb$ collisions at $\sqrt{s}=5.02$ TeV. The meson cloud suppresses the central jet events and enhances the peripheral jet events. But quantitatively the effect is somewhat smaller than in the data.

Measurements of hard probes of the quark-gluon plasma with the ATLAS experiment at the LHC

ATLAS results on the production of high-transverse momentum probes in Pb+Pb and p+Pb collisions at the LHC are presented. The focus is on the jet measurements, which provide a useful tool to study the hot, dense and coloured matter created in ultra-relativistic heavy ion collisions. The ATLAS experiment has measured inclusive jet yields in pp and Pb+Pb collisions at sNN=2.76 TeV and in p+Pb collisions at sNN=5.02 TeV. The jet nuclear modification factor, RAA, is shown as a function of jet pT, rapidity and collision centrality. The RAA weakly increases with pT, shows no dependence on rapidity and smoothly decreases with the collision centrality. In 10% of the most central collisions, jet production is suppressed by a factor of two relative to pp yields scaled by the number of binary nucleon-nucleon collisions. Charged-particle fragmentation functions of jets are also measured and ratios of fragmentation functions between different centrality intervals show a centrality-dependent modification. The jet measurements in p+Pb collisions show jet enhancement in peripheral collisions and suppression in central collisions as compared to the scaled pp jet yields. These modifications are most pronounced at forward rapidities and at large jet transverse momenta. Jet production at forward rapidities shows a scaling in the total jet energy, suggesting that the modification of jet production in p+Pb collisions may depend on the kinematics of the initial hard parton-parton scattering.

Sensitivity of Pion versus Parton-Jet Nuclear Modification Factors to the Path-Length Dependence of Jet-Energy Loss at RHIC and LHC**Supported by the Helmholtz International Centre for FAIR within the Framework of the LOEWE Program, and the US-DOE Nuclear Science under Grant Nos DE-FG02-93ER40764 and DE-AC02-05CH11231.

We compare the jet-path length and beam-energy dependence of the pion nuclear modification factor and a parton-jet nuclear modification factor at RHIC and LHC, and contrast the predictions based on a linear pQCD and a highly non-linear hybrid AdS holographic model of jet-energy loss. It is found that both models require a reduction of the jet-medium coupling from RHIC to LHC to account for the measured pion nuclear modification factor. In the case of the parton-jet nuclear modification factor, however, which serves as a lower bound for the LO jet nuclear modification factor of reconstructed jets, the extracted data can be characterized without a reduced jet-medium coupling at LHC energies. It is concluded that when the reconstructed jets are sensitive to both quarks and gluons and thus provide more information than the pion nuclear modification factor, their information regarding the jet-medium coupling is limited due to the superposition with NLO and medium effects. Hence, a detailed description of the underlying physics requires both the leading hadron and the reconstructed jet nuclear modification factor. Unfortunately, the results for both the pion and the parton-jet nuclear modification factor are insensitive to the jet-path dependence of the models considered.

Jet (de)coherence in Pb–Pb collisions at the LHC

We study the modifications of jets created in heavy-ion collisions at LHC energies. The inherent hierarchy of scales governing the jet evolution allows to distinguish a leading jet structure, which interacts coherently with the medium as a single color charge, from softer sub-structures that will be sensitive to effects of color decoherence. We argue how this separation comes about and show that this picture is consistent with experimental data on reconstructed jets at the LHC, providing a quantitative description simultaneously of the jet nuclear modification factor, the missing energy in di-jet events and the modification of the fragmentation functions. In particular, we demonstrate that effects due to color decoherence are manifest in the excess of soft particles measured in fragmentation functions in Pb–Pb compared to proton–proton collisions.

Strong-coupling jet energy loss from AdS/CFT

We propose a novel definition of a holographic light hadron jet and consider the phenomenological consequences, including the very first fully self-consistent, completely strong-coupling calculation of the jet nuclear modification factor $R_{AA}$, which we find compares surprisingly well with recent preliminary data from LHC. We show that the thermalization distance for light parton jets is an extremely sensitive function of the \emph{a priori} unspecified string initial conditions and that worldsheets corresponding to non-asymptotic energy jets are not well approximated by a collection of null geodesics. Our new string jet prescription, which is defined by a separation of scales from plasma to jet, leads to the re-emergence of the late-time Bragg peak in the instantaneous jet energy loss rate; unlike heavy quarks, the energy loss rate is unusually sensitive to the very definition of the string theory object itself. A straightforward application of the new jet definition leads to significant jet quenching, even in the absence of plasma. By renormalizing the in-medium suppression by that in the vacuum we find qualitative agreement with preliminary CMS $R_{AA}^{jet}(p_T)$ data in our simple plasma brick model. We close with comments on our results and an outlook on future work.

Jet (de)coherence in Pb–Pb collisions at the LHC

Jets are collimated QCD multi-particle states that are abundantly produced in heavy-ion collisions at the LHC. Their description in the vacuum is governed by the hardest scale of the problem, typically the jet virtuality. In the presence of an interacting background field, e.g. such as expected in a quark–gluon plasma, one also has to consider a hard scale arising from the medium interactions. We show how a factorization of small-angle jet evolution and large-angle medium-induced emissions can be realized and calculate three key observables: the jet nuclear modification factor, modification of the intra-jet structure and the amount of out-of-cone radiation.

Results on Jet Spectra and Structure from ALICE

Full jet reconstruction in ALICE uses the combined information from charged and neutral particles. Essentially all jet constituents can be measured with large efficiency down to very low transverse momenta (pT > 150 MeV/c). This has the advantage to introduce a minimum bias on the jet fragmentation, in particular for low jet momenta and in the presence of quenching. In this article, we present preliminary results from reconstruction of charged jets in Pb-Pb collisions at sqrt(s_NN) = 2.76 TeV. The inclusive charged jet spectrum, the jet nuclear modification factors (R_AA, R_CP), the ratio of spectra measured with different resolution parameters and hadron-jet correlations are discussed. For pp data at the same center of mass energy, the inclusive spectrum of fully reconstructed jets and its resolution parameter dependence are reported.