We compare results of the $k_t$-factorization approach and the next-to-leading order collinear-factorization approach for photon-jet correlations in $pp$ and $p \bar p$ collisions at RHIC and Tevatron energies. We discuss correlations in the azimuthal angle as well as in the two-dimensional space of transverse momentum of photon and jet. Different unintegrated parton distributions (UPDF) are included in the $k_t$-factorization approach. The results depend on UPDFs used. The standard collinear approach gives cross section comparable to the $k_t$-factorization approach. For correlations of the photon and any jet the NLO contributions dominate at relatively small azimuthal angles as well as for asymmetric transverse momenta. For correlations of the photon with the leading jet (the one having the biggest transverse momentum) the NLO approach gives zero contribution at $ϕ_{-} < π/2$ which opens a possibility to study higher-order terms and/or UPDFs in this region.
The discriminators of quark-gluon jets developed for $pp$ collisions at the LHC can be used to measure the gluon jet fraction in a jet sample. It allows to measure various characteristics of the quark and gluon jets. The methodology of such measurements for modern hadron colliders is discussed.
We report on a recent calculation of single-inclusive high-$p_T$ jet production in unpolarized and longitudinally polarized $pp$ collisions at RHIC, investigating the effect of the algorithm adopted to define the jets on the numerical results for cross sections and spin asymmetries.
The angular structure of collisional and radiative energy losses of a hard parton jet propagating through the quark-gluon plasma is analyzed. The possibility to observe the energy losses of quark- and gluon-initiated jets in dense QCD-matter (jet quenching) measuring the characteristics of real hadronic jets in ultra-relativistic collisions of nuclei is investigated. In particular, using calorimetric studies of jet + jet, gamma + jet and Z + jet channels is discussed.
The LHC data on jet fragmentation function and jet shapes in PbPb collisions at center-of-mass energy 2.76 TeV per nucleon pair are analyzed and interpreted in the frameworks of PYQUEN jet quenching model. A specific modification of longitudinal and radial jet profiles in most central PbPb collisions as compared with pp data is close to that obtained with PYQUEN simulations taking into account wide-angle radiative and collisional partonic energy loss. The contribution of radiative and collisional loss to the medium-modified intra-jet structure is estimated.
The structure of hadronic jets depends not only on the dynamics of QCD but also on the details of the jet finding algorithm and the physical process in which the jet is produced. To study these effects in more detail we calculate the jet cross section and the internal jet structure in e+e- annihilations and compare them to the results found in hadronic collisions using the same jet definition, the cone algorithm. The different structures of the overall events in the two cases are evident in the comparison. For a given cone size and jet energy, the distribution of energy inside the cone is more…
Medium modification of $γ$-tagged jets in high-energy heavy-ion collisions is investigated within a linearized Boltzmann transport model which includes both elastic parton scattering and induced gluon emission. In Pb+Pb collisions at $\sqrt{s}=2.76$ TeV, a $γ$-tagged jet is seen to lose 15\% of its energy at 0-10\% central collisions. Simulations also point to a sizable azimuthal angle broadening of $γ$-tagged jets at the tail of a distribution which should be measurable when experimental errors are significantly reduced. An enhancement at large $z_\text{jet}=p_L/E_{\text{jet}}$ in jet fragmen…
The production scale and internal structure of jets produced in high energy collisions are studied using Jetset7.4 and Herwig5.9 Monte Carlo generators. Two scales are found. One is the jet-development scale, which determines the size of the jet developed from a mother-parton. The other one is the jet-production scale, the jets produced with this scale are the most consistent with QCD jet-production dynamics and will provide the most reliable dynamical information about their mother-partons.
We present next-to-leading order QCD predictions for single-inclusive and di-jet observables, relevant to the collisions of two on-shell photons, obtained with a recently completed computation based upon the subtraction method. We compare these predictions with available theoretical results, with LEP data, and we discuss the various uncertainties which affect them.
We consider three jet production at high $p_T$ in the $γγ\to q \bar q g$ process and calculate quark and gluon jet spectra. The possibility of quark-gluon jet separation is discussed and compared with the $e^+ e^- \to q \bar q g$ case.
The energy dependence of the radii size of jets are studied in detail by the HBT correlation method using Monte Carlo Simulation generator Jetset7.4 to produce 40,000,000 events of e$^+$e$^-$ collisions at $\sqrt s =30$, 50, 70, 91.2, 110, 130, 150 and 170 GeV. The radii of jets are measured using the HBT correlation method with the indistinguishability of identical final state pions. It is found that the average radii of quark-jets and gluon-jets are independent of the c.m. energy of e$^+$e$^-$ collisions. The average radius of quark-jets are obviously larger than that of gluon-jets. The inva…
In peripheral relativistic heavy ion collisions at the Large Hadron Collider, jet+jet and photon+jet final states can be produced when a photon from the virtual photon field surrounding the nucleus interacts with a parton in the opposite nucleus (direct production). The virtual photon may also fluctuate into states with multiple gluons and q-qbar pairs (resolved production), opening more channels for jet photoproduction. We compare the rates for direct and resolved jet+jet and photon+jet production to explore the sensitivity to the nuclear and photon parton distribution functions. We calculate…
We review recent developments related to jet clustering algorithms and jet reconstruction, with particular emphasis on their implications in heavy ion collisions. These developments include fast implementations of sequential recombination algorithms, new IRC safe algorithms, quantitative determination of jet areas and quality measures for jet finding, among many others. We also show how jet reconstruction provides a useful tool to probe the characteristics of the hot and dense medium created in heavy ion collisions, which allows one to distinguish between different models of parton-medium inte…
The method for simulation of medium-induced rescattering and energy loss of hard partons in ultrarelativistic heavy ion collisions is developed. The model is realized as fast Monte-Carlo tool implemented to modify standard PYTHIA jet event.
We present our recent results on jet tomography of AA-collisions at RHIC and LHC. We focus on flavor dependence of the nuclear modification factor. The computations are performed accounting for radiative and collisional parton energy loss with running coupling constant.
We carry out a detailed calculations of W+jets production in Pb+Pb collisions at the Large Hadron Collider (LHC). In our calculations, the production of W+jet in p+p reference is obtained from Sherpa, which matches next-to-leading-order matrix elements to the resummation of parton shower calculations. Jet propagation and medium response in the quark-gluon plasma is simulated with the Linear Boltzmann Transport (LBT) model. We calculate five observables of W+jets productions with jet quenching effect in Pb+Pb collisions: event distribution as a function of the vector sum of the lepton and jets …
We discuss the modification of a jet fragmentation function due to medium-induced partonic energy loss in context of leading particle observables in ultrarelativistic nucleus-nucleus interactions. We also analyze the relation between in-medium softening jet fragmentation function and suppression of the jet rates due to energy loss outside the jet cone. The predicted anti-correlation between two effects allows to probe a fraction of partonic energy loss carried out of the jet cone and truly lost to the jet.
We attempt to deduce simple options of `jet quenching' phenomena in heavy-ion collisions at $\snn=5.5 \tev$ at the LHC from the present knowledge of leading-hadron suppression at RHIC energies. In light of the nuclear modification factor for leading particles we introduce the nuclear modification factor for jets, $\RAA^{jet}$, and for the longitudinal momenta of particles along the jet axis, $\RAA^{p_{\rm L}}$.
We show that a general purpose clusterization algorithm, Deterministic Annealing, can be adapted to the problem of jet identification in particle production by high energy collisions. In particular we consider the problem of jet searching in events generated at hadronic colliders. Deterministic Annealing is able to reproduce the results obtained by traditional jet algorithms and to exhibit a higher degree of flexibility.
Di-jet observables are excellent probes to study the effect of jets in dense systems. Interacting Lund strings will affect jet observables and suggests a new common mechanism responsible for jet modification in p-A and A-A. In this proceeding, we present our new implementation of the string shoving mechanism in PYTHIA8 which lets us study the effects on jet observables in p-p and nuclear collisions. We also present preliminary results showing the effects in hadron-jet correlation studies.
Recent progress in application of higher order QCD calculations to jet and inclusive particle production in photon induced collisions is reviewed. Attention is paid to theoretical uncertainties of such calculations, particularly those coming from the choice of renormalization and factorization scales.
Implications of hadronization as a rapid traversal of the QCD phase boundary are explored for correlation structures in jets, N-N collisions and heavy-ion collisions. Hadronization viewed as a partition of the prehadonic system restricts the scale interval over which power-law jet correlation structures survive. Rapid traversal of the QCD phase boundary may result in a lattice-like residual structure in N-N and A-A conifiguration space accessible to momentum-space correlation analysis via Hubble flow.
Jet finding algorithms, as they are used in $e^+ e^-$ and hadron collisions, are reviewed and compared. It is suggested that a successive combination style algorithm, similar to that used in $e^+ e^-$ physics, might be useful also in hadron collisions, where cone style algorithms have been used previously.
We carry out the first detailed calculations of jet production associated with $W$ gauge bosons in Pb+Pb collisions at the Large Hadron Collider (LHC). In our calculations, the production of $W$+jet in p+p collisions as a reference is obtained by Sherpa, which performs next-to-leading-order matrix element calculations matched to the resummation of parton shower simulations, while jet propagation and medium response in the quark-gluon plasma are simulated with the Linear Boltzmann Transport (LBT) model. We provide numerical predictions on seven observables of $W$+jet production with jet quenchi…
We study jet quenching in $pp$ and $pA$ collisions in the scenario with formation of a mini quark-gluon plasma. We find a significant suppression effect. For light hadrons at $p_{T}\sim 10$ GeV we obtained the reduction of the spectra by $\sim [20-30,25-35,30-40]$\% in $pp$ collisions at $\sqrt{s}=[0.2, 2.76,7]$ TeV. We discuss how jet quenching in $pp$ collisions may change the predictions for the nuclear modification factors in $AA$ collisions for light and heavy flavors. We also give predictions for modification of the photon-tagged and inclusive jet fragmentation functions in high multipli…
A next-to-leading order (NLO) analysis of hadron-jet momentum correlations in p-p collisions at the LHC is carried out. We show that the inclusive charged hadron momentum distributions inside jets is a very sensitive observable which allows one to disentangle among various fragmentation function sets presently available. Correlations with identified hadrons (kaons, protons) are investigated as well.
We study jet final states in diffractive deep-inelastic scattering. We present the QCD factorization formula in terms of diffractive parton distributions and discuss its implementation in NLO Monte Carlo generators. We compute NLO predictions for the diffractive jet cross section. We use this calculational framework to discuss theoretical models of the long-distance behavior.
We examine the intrinsic spin-dependence of the dominant $gg \rightarrow gggg$ subprocess contribution to four-jet production in polarized proton-proton collisions using helicity amplitude techniques. We find that the partonic level, longitudinal spin-spin asymmetry, $\hat{a}_{LL}$, is intrinsically large in the kinematic regions probed in experiments detecting four isolated jets. Such events may provide another qualitative or semi-quantitative test of the spin-structure of QCD in planned polarized $pp$ collisions at RHIC.
The status of CMS jet simulations and physics analysis in heavy ion collisions is presented. Jet reconstruction and high transverse momentum particle tracking in the high multiplicity environment of heavy ion collisions at the LHC using the CMS calorimetry and tracking system are described. The Monte Carlo tools used to simulate jet quenching are discussed.
The suppression of single inclusive hadron spectra in heavy-ion collision as compared to the scaled expectation from proton-proton collisions has long been regarded as an interesting tool to study properties of the bulk matter in heavy-ion collisions. However, the limitations of this class of observables has become increasingly obvious, and both experimental and theoretical efforts are now made to go beyond single hadrons to fully reconstructed jets. Monte-Carlo simulations of in-medium parton showers are currently considered the most promising tool to theoretically access jet physics in heavy…
Transverse momentum imbalance between a jet produced with a hard photon is considered as a signal of medium-induced partonic energy loss in ultrarelativistic heavy ion collisions. We analyze photon-jet correlation in the context of a real experimental situation at the LHC, the problem of neutral pion background being discussed.
Jet deflection by a flowing medium is one of the ideas brought forward to explain the splitting of the correlation function of hadrons associated with a high transverse momentum trigger from a jet-like structure observed in p-p and d-Au collisions to a double-hump structure on the away side in Au-Au collisions. However, just considering the parton kinematics needed to explain the data shows that any attempt of detailed modelling deflected jets either cannot agree with the data or leads to internal contradictions and violations of basic physics principles. The idea that the deflection of jets b…
We study the phenomenon of jet quenching utilizing quark and gluon jet substructures as independent probes of heavy ion collisions. We exploit jet and subjet features to highlight differences between quark and gluon jets in vacuum and in a medium with the jet-quenching model implemented in JEWEL. We begin with a physics-motivated, multivariate analysis of jet substructure observables including the jet mass, the radial moments, the $p_T^D$ and the pixel multiplicity. In comparison, we employ state-of-the-art image-recognition techniques by training a deep convolutional neutral network on jet im…
The method to simulate the rescattering and energy loss of hard partons in ultrarelativistic heavy ion collisions has been developed. The model is a fast Monte-Carlo tool introduced to modify a standard PYTHIA jet event. The full heavy ion event is obtained as a superposition of a soft hydro-type state and hard multi-jets. The model is applied to analysis of the jet quenching pattern at RHIC.
The advent of high-energy hadron colliders necessitates efficient and accurate computation of multi-jet production processes, both as QCD processes in their own right and as backgrounds for other physics. The algorithm that performs these tasks and a brief numerical study of multi-jet processes are presented.
We investigate single-inclusive high-pT jet production in longitudinally polarized pp collisions at RHIC, with particular focus on the algorithm adopted to define the jets. Following and extending earlier work in the literature, we treat the jets in the approximation that they are rather narrow, in which case analytical results for the corresponding next-to-leading order partonic cross sections can be obtained. This approximation is demonstrated to be very accurate for practically all relevant situations, even at Tevatron and LHC energies. We confront results for cross sections and spin-asymme…
Averaged jet charge characterizes the electric charge for the initiating parton, and provides a powerfull tool to distinguish quark jets from gluon jets. In this paper, we give the first prediction for the medium modification of averaged jet charge in heavy-ion collision at the LHC, where jet productions in p+p collisions are simulated by PYTHIA6, and parton energy loss in QGP are 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 participants of…
Parton energy loss in the hot QCD medium will manifest itself not only in leading hadron spectra but also in reconstructed jet productions in high-energy nucleus-nucleus collisions. In this paper we report on recent theoretical efforts in studying full jet observables in relativistic heavy-ion collisions by discussing the modifications of jet shapes, inclusive jet cross section and the vector boson accompanied jet production in the presence of the QGP-induced jet quenching.
We present a short overview of recent progress in the theory of jet quenching in ultra-relativistic nuclear collisions, including phenomenological studies of jet quenching at RHIC and the LHC, development in NLO perburbative QCD calculation of jet broadening and energy loss, full jet evolution and modification, medium response to jet transport, and lattice QCD and AdS/CFT studies of jet quenching.
Medium modification of γ-tagged jets in high-energy heavy-ion collisions is investigated within a Linearized Boltzmann Transport model for jet propagation that includes both elastic parton scattering and induced gluon emission. Inclusion of recoiled medium partons in the reconstruction of partonic jets is found to significantly reduce the net jet energy loss. Experimental data on γ-jet asymmetry and survival rate in Pb + Pb collisions at \sqrt{s}=2.76 TeV can be reproduced. Medium modifications of reconstructed jet fragmentation function, transverse profile and energy flow outside the jet-cone…
The past several years have witnessed important developments in the QCD theory of jet production and jet substructure in hadronic collisions. In the framework of soft-collinear effective theory, semi-inclusive jet functions and semi-inclusive fragmenting jet functions have allowed us to combine higher order calculations with resummation of potentially large logarithms of the jet radius, $\ln R$. Very recently, the semi-inclusive jet functions for partons fragmenting into heavy flavor jets were computed by Dai, Kim and Leibovich. In this paper we show how the formalism can be extended to c-jet …
Measurements of jet substructure in heavy-ion collisions may provide key insight to the nature of jet quenching in the quark-gluon plasma. Jet grooming techniques from high-energy physics have been applied to heavy-ion collisions in order to isolate theoretically controlled jet observables and explore possible modification to the hard substructure of jets. However, the grooming algorithms used have not been tailored to the unique considerations of heavy-ion collisions, in particular to the experimental challenge of reconstructing jets in the presence of a large underlying event. We report a se…
Jet charge characterizes the electric charge distribution inside a jet. In this talk we make the first theoretical study of jet charge in high-energy nuclear collisions and calculate numerically the medium alternations of jet charge due to parton energy loss in the quark-gluon plasma. The parton multiple scattering in hot/dense QCD medium is simulated by a modified version of PYQUEN Monte Carlo model with 3+1D ideal hydrodynamical evolution of the fireball. Our preliminary results show that the averaged jet charge is significant modified in A+A collisions relative to that in p+p. The different…
We present predictions for the inclusive production of charm jets in proton-proton collisions at 7 TeV. Several CTEQ parton distribution functions (PDFs) of the CTEQ6.6M type are employed, where two of the CTEQ6.6 PDFs have intrinsic charm. At large enough jet transverse momentum and large jet rapidity, the intrinsic charm content can be tested.
I review the status and future directions of jet-related measurements in high energy nuclear collisions and their application as a probe of QCD matter.
Jet quenching has been used successfully as a hard probe to study properties of the quark-gluon plasma (QGP) in high-energy heavy-collisions at both the Relativistic Heavy-Ion Collider (RHIC) and the Large Hadron Collider (LHC). We will review recent progresses in theoretical and phenomenological studies of jet quenching with jet transport models. Special emphasis is given to effects of jet-induced medium response on a wide variety of experimental measurements and their implication on extracting transport properties of the QGP in heavy-ion collisions.
The discovery of collectivity in proton-proton collisions, is one of the most puzzling outcomes from the two first runs at LHC, as it points to the possibility of creation of a Quark-Gluon Plasma, earlier believed to only be created in heavy ion collisions. One key observation from heavy ion collisions is still not observed in proton-proton, namely jet-quenching. In this letter it is shown how a model capable of describing soft collective features of proton-proton collisions, also predicts modifications to jet fragmentation properties. With this starting point, several new observables suited f…
Although many observations in high multiplicity p+Pb collisions at the LHC indicate striking similarity to heavy ion collisions, no evidence of jet-medium interactions has been observed till date. We study the effect of jet-medium interaction on the charge dependent three-particle correlation in high multiplicity p+Pb collisions at $\sqrt{s_{NN} }$ = 5.02 TeV using EPOS 3 event generator. The short range component of the three-particle correlation is dominated by the jet fragmentation and has a unique charge dependence as observed in the minimum bias p+Au collisions at the RHIC energy. In EPOS…
Unveiling the jet angular broadening with photon-tagged jets in high-energy nuclear collisions
College of Science, China Three Gorges University（三峡大学理学院）；Center for Astronomy and Space Sciences and Institute of Modern Physics, China Three Gorges University（三峡大学天文与空间科学中心暨现代物理研究所）
Modifications of jets in the existence of a hot and dense QCD medium have recently attracted a lot of attentions. In this talk, we demonstrate how jet-medium interactions change the behavior of jets by offering examples of inclusive jet and dijet productions at $ {\cal O}(α_s^3)$ in heavy ion collisions including initial-state cold nuclear effects and especially the final-state parton energy loss effect. The suppression of inclusive jet spectrum varying with jet radii and a flatter dijet momentum imbalance as compared to those in hadron-hadron collisions are observed in high-energy nuclear col…
In this paper we investigate the medium modifications of girth distributions for inclusive jets and $Z^0$ tagged jets with small radius ($R=0.2$) in Pb+Pb collisions with $\sqrt{s}=2.76$ TeV at the LHC. The partonic spectrum in the initial hard scattering of elementary collisions are obtained by a event generator POWHEG+PYTHIA, which matches the next-to-leading (NLO) matrix elements with parton showering, and energy loss of faster parton traversing in hot/dense QCD medium is calculated by Monte Carlo simulation within Higher-Twist formalism of jet quenching in heavy-ion collisions. We present …
We study the evolution of a partonic jet shower propagating through a quark-gluon plasma. Combining the in-medium evolutions of the leading parton and shower gluons, we compute the depletion of the energy from the jet cone by dissipation through elastic collisions with medium constituents, by scattering of shower partons to larger angles, and by radiation outside the jet cone. Numerical results are presented for the nuclear modification of dijet asymmetry in Pb+Pb collisions at the LHC.
Within a multiphase transport model, medium modifications of differential jet shapes are investigated in Pb+Pb collisions at $\sqrt{s_{_{\rm NN}}}$ = 2.76 TeV. The differential jet shapes are significantly modified by the strong interactions between jets and a partonic medium in Pb+Pb collisions relative to that in p+p collisions. The modifications are slightly weakened by the hadronization of coalescence, but strengthened by resonance decays in hadronic rescatterings. Subleading jets display larger medium modifications than leading jets, especially in central Pb+Pb collisions with large dijet…
Jet energy loss and single inclusive jet suppression in high-energy heavy-ion collisions are studied within a pQCD parton model that includes both elastic and radiative interactions between jet shower and medium partons as they propagate through the quark-gluon plasma. The collisional energy loss of jets with a given cone-size is found to be relatively small comparing with the radiative energy loss. However the effect of transverse momentum broadening due to elastic scattering is significant in the calculation of radiative energy loss within the higher-twist formalism. The nuclear modification…
We calculate photons from jet plasma interaction in a collisional energy loss scenario. It is shown that the PHENIX photon data is well reproduced when photons from initial hard collisions are taken into account.
