Transverse momentum broadening and energy loss of a propagating parton are dictated by the space-time profile of the jet transport coefficient $\hat q$ in a dense QCD medium. The spatial gradient of $\hat q$ perpendicular to the propagation direction can lead to a drift and asymmetry in parton transverse momentum distribution. Such an asymmetry depends on both the spatial position along the transverse gradient and path length of a propagating parton as shown by numerical solutions of the Boltzmann transport in the simplified form of a drift-diffusion equation. In high-energy heavy-ion collisions, this asymmetry with respect to a plane defined by the beam and trigger particle (photon, hadron or jet) with a given orientation relative to the event plane is shown to be closely related to the transverse position of the initial jet production in full event-by-event simulations within the linear Boltzmann transport model. Such a gradient tomography can be used to localize the initial jet production position for more detailed study of jet quenching and properties of the quark-gluon plasma along a given propagation path in heavy-ion collisions.
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.
We illustrate with both a Boltzmann diffusion equation and full simulations of jet propagation in heavy-ion collisions within the Linear Boltzmann Transport (LBT) model that the spatial gradient of the jet transport coefficient perpendicular to the propagation direction can lead to a drift and asymmetry in the transverse momentum distribution. Such an asymmetry depends on both the spatial position along the transverse gradience and the propagating length. It can be used to localize the initial jet production positions for more detailed studies of jet quenching and properties of the quark-gluon…
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.
Significant jet quenching in central $Au+Au$ collisions has been discovered at RHIC. This paper provides theoretical arguments and lists experimental evidence that the observed jet quenching at RHIC is due to parton energy loss instead of hadron rescattering or absorption in a hadronic medium. These include: (1) hadron formation time based on the uncertainty principle, (2) $p_T$ dependence and (3) centrality dependence of the observed jet quenching, (4) jet-like leading hadron correlations (5) high-$p_T$ azimuthal anisotropy and (6) experimental data from $Pb+Pb$ collisions at SPS and $e+A$ co…
Concept of transverse deflection probability of a parton that travels through strongly interacting medium, recently introduced by D'Eramo, Liu and Rajagopal, have been used to derive high energy evolution equation for the jet quenching parameter in stochastic multiple scatterings regime. Jet quenching parameter, ${\hat q}(x)$, appears to evolve with $x$, with an exponent $0.9{\bar α}_s$, which is slightly less than that of $x {\cal G} (x)$ where ${\cal G} (x)$ is the gluon distribution function.
We compute the inclusive jet spectrum in the presence of a dense QCD medium by going beyond the single parton energy loss approximation. We show that higher-order corrections are important yielding large logarithmic contributions that must be resummed to all orders. This reflects the fact that jet quenching is sensitive to fluctuations of the jet substructure.
High transverse momentum single (non-photonic) electrons are shown to be sensitive to the stopping power of both bottom, b, and charm, c, quarks in AA collisions. We apply the DGLV theory of radiative energy loss to predict c and b quark jet quenching and compare the FONLL and PYTHIA heavy flavor fragmentation and decay schemes. We show that single electrons in the p_T=5-10 GeV range are dominated by the decay of b quarks rather than the more strongly quenched c quarks in Au+Au collisions at sqrt{s}=200 AGeV. The {smaller} b quark energy loss, even for extreme opacities with gluon rapidity den…
We calculate higher-order corrections to the quenching factor of heavy-quark jets due to hard, in-medium splittings in the framework of the BDMPS-Z formalism. These corrections turn out to be sensitive to a single mass-scale $m_\ast = (\hat q L)^{1/2}$, where $\hat q$ is the medium transport coefficient and $L$ the path length, and allow to draw a distinction between the way light, with $m < m_\ast$ (in contrast to massless $m=0$), and genuinely heavy, with $m > m_\ast$, quark jets are quenched in the medium. We show that the corrections to the quenching factor at high energies are double-loga…
Jet quenching has successfully served as a hard probe to study the properties of Quark-Gluon Plasma (QGP). As a multi-particle system, jets take time to develop from a highly virtual parton to a group of partons close to mass shells. In this study, we present a systematical study on the effects of this formation time on jet quenching in relativistic nuclear collisions. Jets from initial hard scatterings were simulated with Pythia, and their interactions with QGP were described using a Linear Boltzmann Transport (LBT) model that incorporates both elastic and inelastic scatterings between jet pa…
We perform an analysis of jet quenching in heavy ion collisions at RHIC and LHC energies with the temperature dependent running QCD coupling. Our results show that the $T$-dependent QCD coupling largely eliminates the difference between the optimal values of $α_s$ for the RHIC and LHC energies. It may be viewed as direct evidence of the increase of the thermal suppression of $α_s$ with rising temperature.
The azimuthal anisotropy of high-pt particle production in non-central heavy ion collisions is among the most promising observables of partonic energy loss in an azimuthally non-symmetric volume of quark-gluon plasma. We discuss the implications of nuclear geometry for the models of partonic energy loss in the context of recent RHIC data and consequences for observation of jet quenching at the LHC.
The modification of hard jets in dense matter has so far been described by four different formalisms based on perturbative QCD (pQCD). In these proceedings, we compare the various approximations made in these different schemes, especially those regarding the structure of the medium through which jets propagate. Following this, we highlight some of the major differences in the various physical processes contained in the different approaches.
Jet quenching in high-energy heavy-ion collisions can be used to probe properties of hot and dense quark-gluon plasma. We provide a brief introduction to the concept and framework for the study of jet quenching. Different approaches and implementation of multiple scattering and parton energy loss are discussed. Recent progresses in the theoretical and phenomenological studies of jet quenching in heavy-ion collisions at RHIC and LHC are reviewed.
Theoretical understanding of the observed jet quenching measurements at RHIC and LHC is challenging in QCD because it requires understanding of parton to hadron fragmentation function in non-equilibrium QCD. In this paper, by using closed-time path integral formalism, we derive the gauge invariant definition of the gluon to hadron fragmentation function in non-equilibrium QCD which is consistent with factorization theorem in non-equilibrium QCD from first principles.
It was argued recently that loop corrections to tree-level amplitudes are essential in the discussion of the collisional energy loss of energetic partons in the hot quark gluon plasma: Instead of $dE_{\rm coll}^B/dx \sim α^2 T^2 \ln(ET/m_D^2)$, as first derived by Bjorken (assuming a constant $α$), the mean energy loss actually behaves as $dE_{\rm coll}/dx \sim α(m_D^2)T^2$. Here we calculate, within this resummation-improved framework, the probability distribution functions (`quenching weights') of the collisional energy loss. First results from a Monte Carlo implementation of this probabilis…
We perform an analysis of jet quenching in heavy and light ion collisions for scenarios without and with quark-gluon plasma formation in $pp$ collisions. We find that the results for these scenarios are very similar, and both of them are in reasonable agreement with data for heavy ion collisions. However, their results become differ significantly for light nuclei. Using the parameters fitted to heavy ion data on the nuclear modification factor $R_{AA}$, we make predictions for 0.2 and 7 TeV O+O collisions that can be verified by future experiments at RHIC and the LHC.
The gluonic contribution to the nuclear modification factor R_AA is investigated for central Au + Au collisions at sqrt(s) = 200 GeV employing a perturbative QCD-based parton cascade including radiative processes. A flat quenching pattern is found up to transverse momenta of 30 GeV which is slightly smaller compared with results from the Gyulassy-Levai-Vitev formalism. We demonstrate that the present microscopic transport description provides challenging means of investigating both jet-quenching and a strong buildup of elliptic flow in terms of the same standard perturbative QCD interactions.
We review recent finite opacity approaches (GLV, WW, WOGZ) to the computation of the induced gluon radiative energy loss and their application to the tomographic studies of the density evolution in ultra-relativistic nuclear collisions.
A new model potential in the presence of collective flow describing the interaction of the hard jet with scattering centers is derived based on the static color-screened Yukawa potential. The flow effect on jet quenching with detailed balance is investigated in pQCD. It turns out that the collective flow changes the emission current and the LPM destructive interference comparing to that in the static medium. Considering the collective flow with velocity v_z along the jet direction, the energy loss is (1 - v_z) times that in the static medium to the first order of opacity. The flow dependence o…
We study the propagation of an ultrarelativistic light quark jet inside a shock wave using the holographic principle. The maximum stopping distance and its dependency on the energy of the jet is obtained.
We perform a joint jet tomographic analysis of the data on the nuclear modification factor $R_{AA}$ from PHENIX at RHIC and ALICE at LHC. The computations are performed accounting for radiative and collisional parton energy loss with running coupling constant. Our results show that the observed slow variation of $R_{AA}$ from RHIC to LHC indicates that the QCD coupling constant is suppressed in the quark-gluon plasma produced at LHC.
Medium modification of jet fragmentation function due to multiple scattering and induced gluon radiation leads directly to jet quenching or suppression of leading particle distribution from jet fragmentation. One can extract an effective total parton energy loss which can be related to the total transverse momentum broadening. For an expanding medium, both are shown to be sensitive to the initial parton density and formation time. Therefore, one can extract the initial parton density and formation time from simultaneous measurements of parton energy loss and transverse momentum broadening. Imp…
Electroweak boson-tagged jet measurements provide a promising experimental channel to accurately study the physics of jet production and propagation in dense QCD medium. In this talk, we present theoretical predictions for the nuclear-induced attenuation of the differential cross section for isolated photon-tagged and $\rm Z^0$-tagged jet production in heavy ion collisions, and provide theoretical interpretations to the recent LHC data. We demonstrate quantitatively the significance of collisional and radiative energy losses, as revealed in the strong momentum asymmetry $dσ/dx_{VJ}$ and nuclea…
A new Monte Carlo implementation of Djordjevic's dynamical scattering generalization of the DGLV radiative energy loss opacity series is used with a hybrid interpolation scheme to compute both light and heavy quark jet quenching up to third order in opacity. The enhancement of the ratio of bottom to charm quark energy loss due to perturbative long range color magnetic effects in nonuniform Bjorken expanding geometries is found to reduce the significance of the heavy quark jet puzzle posed by the observed near equality (within sizeable errors) of pion and nonphotonic electron nuclear modificati…
Motivated by the new results obtained in heavy-ion collision experiments at the LHC, several extensions of the standard calculations of energy loss have been made recently. In this manuscript, I provide a short overview of some of the recent developments in jet quenching theory. First, I discuss some improvements computed by different groups to implement energy-momentum conservation in a rigorous way, relaxing some of the assumptions that were made in the standard jet quenching calculations. Second, quantum interference effects between different parton emitters when propagating through an exte…
We propose a new description of the jet quenching phenomenon observed in nuclear collisions at high energies in which coherent parton branching plays a central role. This picture is based on the appearance of a dynamically generated scale, the jet resolution scale, which controls the transverse resolution power of the medium to simultaneously propagating color probes. Since from the point of view of the medium all partonic jet fragments within this transverse distance act coherently as a single emitter, this scale allows us to rearrange the jet shower into effective emitters. We observe that i…
SUBATECH UMR 6457 (IMT Atlantique, Université de Nantes, IN2P3/CNRS)（苏巴特克联合研究实验室（大西洋高等矿业学院，南特大学，法国国家科学研究中心/国家原子能研究所））；Physics Department, Brookhaven National Laboratory（布鲁海文国家实验室物理系）
We calculate the jet quenching parameter in medium with chemical potential from AdS/CFT correspondence. Our result is summarized in a plot. Moreover, we extract the explicit form of the jet quenching parameter of medium with small chemical potential for phases of dual SYM corresponding to large and small black holes. For the former phase, the jet quenching is increased as the charge density increases, however, for the latter it is the opposite though the background is thermodynamically unstable.
The ATLAS Collaboration recently reported strong modifications of dijet properties in heavy ion collisions. In this work, we discuss to what extent these first data constrain already the microscopic mechanism underlying jet quenching. Simple kinematic arguments lead us to identify a frequency collimation mechanism via which the medium efficiently trims away the soft components of the jet parton shower. Through this mechanism, the observed dijet asymmetry can be accomodated with values of $\hat{q}\, L$ that lie in the expected order of magnitude.
A thorough understanding of jet quenching on the basis of multi-particle final states and jet observables requires new theoretical tools. This talk summarises the status and propects of the theoretical description of jet quenching in terms of Monte Carlo generators.
Preliminary PHENIX data on $Au+Au$ reactions at $\sqrt{s}= 130$ AGeV suggest that $\bar{p}$ yields may exceed $π^-$ at high $p_{\rm T} > 2$ GeV/c. We propose that jet quenching in central collisions suppresses the hard PQCD component of the spectra in central $A+A$ reactions, thereby exposing a novel non-perturbative component of baryon dynamics. We suggest that baryon junctions provide a possible explanation of the anomalous component. We predict that the $\bar{p} \geq π^-$ anomaly is limited to a finite $p_{\rm T}$ window and decreases with increasing impact parameter.
The strong modifications of dijet properties in heavy ion collisions measured by ATLAS and CMS provide important constraints on the dynamical mechanisms underlying jet quenching. In this work, we show that the transport of soft gluons away from the jet cone - jet collimation - can account for the observed dijet asymmetry with values of $\hat{q}\, L$ that lie in the expected order of magnitude. Further, we show that the energy loss attained through this mechanism results in a very mild distortion of the azimuthal angle dijet distribution.
PHENIX data on Au+Au at root(s)_= 130 AGeV suggest that p-bar yields may exceed pi- at high p_T > 2 GeV/c. We propose that jet quenching in central collisions suppresses the hard PQCD component of the spectra in central A+A reactions, thereby exposing a novel component of baryon dynamics that we attribute to (gluonic) baryon junctions. We predict that the observed p-bar >= pi- and the p > pi+ anomaly at p_T ~ 2 GeV/c is limited to a finite p_T window that decreases with increasing impact parameter.
Medium modification of jet fragmentation functions and parton energy loss in cold and hot matter are reviewed. The predicted nuclear modification of the jet fragmentation function agrees well with the recent HERMES data with a resultant energy loss dE/dx approximately 0.5 GeV/fm. From the the recent PHENIX data of high p_T pi_0 spectra in central Au+Au collisions at sqrt(s)=130 GeV, one extracts an energy loss for a 10 GeV parton that is equivalent to dE/dx=7.3 GeV/fm in a static medium with the same gluon density as in the initial stage of the collision at tau_0=0.2 fm/c. Constraints on jet q…
The phase transition and jet quenching parameter ${\hat q}$ have been investigated in the framework of dynamical holographic QCD model. It is found that both the trace anomaly and the ratio of the jet quenching parameter over cubic temperature ${\hat q}/T^3$ show a peak around the critical temperature $T_c$, and the ratio of jet quenching parameter over entropy density ${\hat q}/s$ sharply rises at $T_c$. This indicates that the jet quenching parameter can characterize the phase transition. The effect of jet quenching parameter enhancement around phase transition on nuclear modification factor…
In this work, we propose a new class of jet substructure observables which, unlike fragmentation functions, are largely insensitive to the poorly known physics of hadronization. We show that sub-jet structures provide us with a large discriminating power between different jet quenching Monte Carlo implementations.
We consider the backreaction of the fundamental flavor degrees of freedom on the AdS$_5$-Schwarz background, and calculate their contributions to the shear viscosity and jet-quenching parameter of the thermal quark-gluon plasma.
We present a comprehensive review of the physics of hadron and jet production at large transverse momentum in high-energy nucleus-nucleus collisions. Emphasis is put on experimental and theoretical "jet quenching" observables that provide direct information on the (thermo)dynamical properties of hot and dense QCD matter.
A calculation of the $R^2$ corrections to the jet quenching parameter from AdS/CFT correspondence is presented. These corrections are related to curvature-squared corrections in the corresponding gravity dual. It is shown that the corrections will increase or decrease the jet quenching parameter depending on the coefficients of the high curvature terms.
Using the AdS/CFT correspondence, we study the heavy quark potential and the jet quenching parameter in the near horizon limit of D3-D(-1) background. The results are compared with those of conformal cases. It is shown that the presence of instantons tends to suppress the heavy quark potential and enhance the jet quenching parameter.
We compare fully perturbative and fully nonperturbative pictures of high-pT energy loss calculations to the first results from LHC. While over-suppressed compared to published ALICE data, parameter-free pQCD predictions based on the WHDG energy loss model constrained to RHIC data simultaneously describe well the preliminary CMS hadron suppression, ATLAS charged hadron v2, and ALICE D meson suppression; we also provide for future reference WHDG predictions for B meson RAA. However, energy loss calculations based on AdS/CFT also qualitatively describe well the RHIC pion and non-photonic electron…
The leading order $α_s$ effect in jet quenching corresponds to the broadening of the jet's transverse momentum, due to the multiple interactions with the underlying medium. A complete understanding of momentum broadening is critical for the success of jet quenching phenomenology. In this talk, we introduce a strategy to quantum simulate single particle momentum broadening in a QCD background medium. We argue that it is, in principle, possible to extract the jet quenching parameter $\hat{q}$ from such an algorithm. More importantly, this corresponds to the first step towards simulating full med…
Recent observation of high-$p_T$ hadron spectra suppression and mono-jet production in central $Au+Au$ collisions and their absence in $d+Au$ collisions at RHIC have confirmed the long predicted phenomenon of jet quenching in high-energy heavy-ion collisions. Detailed analyses of the experimental data also show parton energy loss as the mechanism for the discovered jet quenching. Using a pQCD parton model that incorporates medium modified parton fragmentation functions and comparing to experimental data from deeply inelastic scattering off nuclei, one can conclude that the initial gluon (energ…
A model for the Color Glass Condensate as opposed to jet quenching is proposed for the explanation of the presently available RHIC data. Good fits to these data are presented. A clear way to distinguish between the two possible explanations is also given.
In order to assess the ability of jet observables to constrain the characteristics of the medium produced in heavy-ion collisions at the LHC, we investigate the influence of background subtraction and jet quenching on jet reconstruction, with focus on the dijet asymmetry as currently studied by ATLAS and CMS. Using a toy model, we examine the influence of different background subtraction methods on dijet momentum imbalance and azimuthal distributions. We compare the usual jet-area based background subtraction technique and a variant of the noise-pedestal subtraction method used by CMS. The pur…
This contribution discusses the geometric tomography by highly energetic jets penetrating the hot QCD matter in heavy ion collisions from RHIC to LHC energies. In particular the geometric data on the azimuthal anisotropy of high $p_t$ hadrons discriminate different models and strongly hint at energy loss mechanism beyond those based on perturbation theory. Taking together the RHIC and LHC data, the comparison with models is in favor of the model with strong enhancement of jet quenching in near-$T_c$ matter.
We study the jet quenching effect in heavy ion collisions, based on medium-induced splitting functions calculated from Soft Collinear Effective Theory with Glauber Gluons. Our method is formulated in the language of DGLAP evolution equations with medium-induced splitting functions. In the small-$x$ soft gluon approximation we analytically solve the evolution equations and find an intuitive connection to the energy loss approach. For central Pb+Pb collisions at the LHC we quantify the effect of finite-$x$ corrections for the nuclear modification factor and compare to data.
