Fast Partons Research Articles

Relatingq̂, η/s, andΔEin an expanding quark-gluon plasma

We use linear viscous hydrodynamics to describe the energy and momentum deposited by a fast moving parton in a quark gluon plasma. This energy-momentum is in turn used to compute the probability density for the production of soft partons by means of the Cooper-Frye formula. We use this probability density to render manifest a relation between the average transverse momentum given to the fast moving parton from the medium $\hat{q}$, the entropy density to shear viscosity ratio $\eta/s$ and the energy lost by the fast moving parton $\Delta E$ in an expanding medium under similar conditions to those generated in nucleus-nucleus collisions at the LHC. We find that $\hat{q}$ increases linearly with $\Delta E$ for both trigger and away side partons that have been produced throughout the medium. On the other hand, $\eta/s$ is more stable with $\Delta E$. We also study how these transport coefficients vary with the geometrical location of the hard scattering that produces the fast moving partons. The behavior of $\hat{q}$, with $\Delta E$ is understood as arising from the length of medium the parton traverses from the point where it is produced. However, since $\eta/s$ is proportional to the ratio of the length of medium traversed by the fast parton and the average number of scatterings it experiences, it has a milder dependence on the energy it loses. This study represents a tool to obtain a direct connection between transport coefficients and the description of in-medium energy loss within a linear viscous hydrodynamical evolution of the bulk.

Jets in heavy ion collisions at the LHC

In this document I present a brief review of the concepts of jet physics employed in heavy ion physics. Fast partons originating from scatterings with large momentum transfer are produced at very short time-scales and subsequently propagate through the strongly interacting medium produced in the collisions of heavy nuclei. They feature the only experimental handle available to directly study the interaction of a well defined probe and the medium. Consequently they are ideally suited to investigate the nature of the medium produced in these collisions and the mechanism of interaction between the medium and the partons. The experimental methods necessary to reconstruct jets originating from fragmenting partons in the environment of high particle multiplicity heavy ion collisions will be discussed. Physics observables suited to investigate the parton medium interaction will be introduced and a summary of recent results on jet physics in heavy ion collisions is presented.

Jet structure in heavy ion collisions

We review recent theoretical developments in the study of the structure of jets that are produced in ultra relativistic heavy ion collisions. The core of the review focusses on the dynamics of the parton cascade that is induced by the interactions of a fast parton crossing a quark–gluon plasma. We recall the basic mechanisms responsible for medium induced radiation, underline the rapid disappearance of coherence effects, and the ensuing probabilistic nature of the medium induced cascade. We discuss how large radiative corrections modify the classical picture of the gluon cascade, and how these can be absorbed in a renormalization of the jet quenching parameter [Formula: see text]. Then, we analyze the (wave)-turbulent transport of energy along the medium induced cascade, and point out the main characteristics of the angular structure of such a cascade. Finally, color decoherence of the in-cone jet structure is discussed. Modest contact with phenomenology is presented towards the end of the review.

Angular correlations of jets in lead–lead collisions at 2.76 TeV using the ATLAS detector at the LHC

Highly energetic jets produced in nuclear collisions are considered to be a direct probe of hot and dense QCD medium created in these collisions. Jet measurements performed both at the LHC and RHIC indicate a presence of “jet quenching” – strong energy loss of fast partons in the hot and dense QCD medium. This note reports study of properties of the multi-jet production in heavy ion collisions presented in terms of jet angular correlations . The jet angular correlations are a valuable tool to study the beyond-leading-order effects in QCD and are expected to shed a light on the process of the parton energy loss. The study is performed using Pb + Pb data collected in 2011 by the ATLAS detector at the center of mass energy of 2.76 TeV.

Wakes in viscous quark-gluon plasma

With the dielectric function derived from the chromohydrodynamic approach, we investigate wakes in induced charge density and wake potential induced by a fast parton traveling through the viscous quark-gluon plasma (QGP). When the fast parton moves with a speed v = 0.55c which is less than the phase velocity of plasmon vp in QGP, the equicharge lines show a sign flip in the backward-forward spaces. While for v = 0.99c which is larger than vp, the equicharge lines show an oscillatory behavior. A Lennard-Jones potential and an oscillatory wake potential are found in the backward direction for v = 0.55c and v = 0.99c respectively. In addition, the viscous effect on wakes is also speed-dependent. When v = 0.55c, shear viscosity has a trivial impact on the wakes. While for v = 0.99c, shear viscosity modifies the strength and structure of the wakes significantly.

Jet Physics at the LHC

The production and propagation of high pT probes can explore the mechanisms of parton energy loss, deconfinement in the medium, and shed light on the relevant phys- ical mechanisms and the microscopic properties of the medium formed in the heavy-ion collisions. In particular, partonic energy loss results in significant modification of spectra. The LHC opened a new era in heavy-ion physics bringing, among several, the hard probes to unreached region of the phase-space. Some of the physics results are discussed. High energy partons interact with the medium and loose energy, primarily through induced gluon radiation and, to a smaller extent, elastic scattering (1). The theory of parton energy loss in hot matter has come a long way from the jet quenching predictions by Bjorken and others (2) describing radiative energy loss by a fast parton. Experimentally, several methods are used to address such questions, generally through comparison of the relative production of single particles suppression at RHIC to fully reconstructed jets at the LHC in nuclear collisions to expectations from a superposition of independent nucleon-nucleon collisions. The thermal scale, set by the medium temperature, is very important for the parton energy loss. More specifically, the coupling between partons and medium at large scales, of the order of the initial parton energy, may be weak, governed by perturbative dynamics, and factorize from the medium. At lower scales of the order of 1 GeV, the coupling between the and the medium may be strong. Jet reconstruction and measurements of the properties aim to quantify the in-medium energy loss and capture the dynamics of quenching. At low particle momenta, the underlying event, even if considered as a background contribution to the hard probes because uncorrelated to the hard parton scattering, is an important element of the hadronic environment consisting of complex contributions, spanning over nonperturbative and perturbative QCD and including sensitivities to multiscale and low x physics. Jet reconstruction in heavy-ion collisions proceed against this large background, resulting very challenging. Dedicated algorithms and background subtraction techniques have been developed and optimized to reconstruct the collimated spray of particles associated with the original parton and contained in the cone radius fixed by the specific algorithm (4, 5). At the LHC, fully reconstructed jets are available over a wide pT range at √ sNN = 2.76 TeV confirming and extending the suppression pattern already observed for charged particles both at these energies and at lower collision energies (i.e. RHIC). However, although the original parton energy is better reconstructed in a than by tagging only a fast hadron, the single inclusive suppression is similar to that of single hadrons. This can be understood if parton energy loss is predominantly through radiation outside the cone radius used in the reconstruction algorithm.

Wake potential in collisional anisotropic quark-gluon plasma

Within the framework of Boltzmann transport equation with a Bhatnagar-Gross-Krook (BGK) collisional kernel, we study the wake potential induced by fast partons traveling through the high-temperature QCD plasma which is anisotropic in momentum-space. We calculate the dielectric response function of a collisional anisotropic quark-gluon plasma (AQGP) for small $\xi$ (anisotropic parameter) limit. Using this, the wake potential for various combinations of the anisotropy parameter ($\xi$) and the collision rate ($\nu$) is evaluated both for parallel and perpendicular directions of motion of the fast parton. It is seen that the inclusion of the collision modifies the wake potential and the amount as well as the nature of the potential depends on the combinations of $\xi$ and $\nu$

High pT: Energy Loss Physics at PHENIX

Abstract High transverse momentum measurements are sensitive to the passages of fast partons through the hot and dense medium created during the collisions and can be used to investigate the physical processes resulting in energy loss as well as other properties of the medium. In these proceedings, we review a selection of recent measurements made at high p T with the PHENIX detector and discuss the implications on energy loss mechanisms on fast partons in the collisions of large nuclei.

Wake in anisotropic quark-gluon plasma

We calculate the wake in charge density and the wake potential due to the passage of a fast parton in an anisotropic quark-gluon plasma. For the sake of simplicity, a small $\ensuremath{\xi}$ (anisotropic parameter) limit has been considered. When the velocity ($v$) of the jet is parallel to the anisotropy direction ($\stackrel{^}{n}$) and remains below the phase velocity (${v}_{p}$), the wake in induced charge density shows a little oscillatory behavior in anisotropic quark-gluon plasma, contrary to the isotropic case. With the jet velocity greater than the phase velocity, the oscillatory behavior increases with $\ensuremath{\xi}$. Also for $v>{v}_{p}$ one observes a clear modification of the conelike structure in the presence of anisotropy. For the parallel direction in the backward region, the depth of the wake potential decreases with the increase of $\ensuremath{\xi}$ for $v<{v}_{p}$, and the potential becomes modified Coulomb-like for higher values of $\ensuremath{\xi}$. In the forward region, the potential remains modified Coulomb-like with the change in magnitude for nonzero $\ensuremath{\xi}$, for both $v>{v}_{p}$ and $v<{v}_{p}$. In the perpendicular direction, the wake potential is symmetric in the forward and backward regions. With the increase of $\ensuremath{\xi}$, the depth of negative minimum is moving away from the origin irrespective of the jet velocity. On the other hand, when the jet velocity is perpendicular to the anisotropy direction, we find significant changes in the case of both wake charge density and potential in comparison to the isotropic case. For nonzero $\ensuremath{\xi}$, the oscillatory nature of the color charge wake is reduced at $v>{v}_{p}$. Also the oscillatory behavior of the wake potential along the direction of motion of the parton is attenuated in the backward direction for anisotropic plasma at parton velocity $v>{v}_{p}$. In the presence of anisotropy, for $v<{v}_{p}$, the screening potential along the perpendicular direction of the parton is transformed from the Lennard-Jones type to a modified Coulomb-like potential.

Parton energy loss in glasma

We study the synchrotron-like gluon emission in AA-collisions from fast partons due to interaction with the coherent glasma color fields. Our results show that for RHIC and LHC conditions the contribution of this mechanism to parton energy loss is much smaller than the radiative energy loss in the plasma phase.

Parton showers as sources of energy-momentum deposition in the quark-gluon plasma and their implication for shockwave formation at energies available at the BNL Relativistic Heavy Ion Collider and at the CERN Large Hadron Collider

We derive the distribution of energy and momentum transmitted from a primary fast parton and its medium-induced bremsstrahlung gluons to a thermalized quark-gluon plasma. Our calculation takes into account the important and thus far neglected effects of quantum interference between the resulting color currents. We use our result to obtain the rate at which energy is absorbed by the medium as a function of time and find that the rate is modified by the quantum interference between the primary parton and secondary gluons. This Landau-Pomeranchuk-Migdal type interference persists for time scales relevant to heavy ion phenomenology. We further couple the newly derived source of energy and momentum deposition to linearized hydrodynamics to obtain the bulk medium response to realistic parton propagation and splitting in the quark-gluon plasma. We find that because of the characteristic large angle in-medium gluon emission and the multiple sources of energy deposition in a parton shower, formation of well defined Mach cones by energetic jets in heavy ion reactions is not likely.

The polarized charge distribution induced by a fast parton in the viscous quark–gluon plasma

With the dielectric function derived from the chromohydrodynamic approach, we investigate the polarized charge distribution induced by a fast parton traveling through the viscous quark–gluon plasma. Our results show that viscous effects on the induced color charge density are speed dependent. When the fast parton moves with a speed less than the average speed of plasmon in the quark–gluon plasma, shear viscosity has a trivial impact on the color charge wake. While the fast parton speed is larger than the plasmon speed, viscous effects modify the strength and structure of the induced charge wake significantly. With the increase of η/s, shear viscosity enhances the oscillation of the induced charge density, but makes the cone angle formed by the polarized charges less remarkable.

Systematic analysis of the incoming quark energy loss in cold nuclear matter

The investigation into the fast parton energy loss in cold nuclear matter is crucial for a good understanding of the parton propagation in hot-dense medium. By means of four typical sets of nuclear parton distributions and three parametrizations of quark energy loss, the parameter values in quark energy loss expressions are determined from a leading order statistical analysis of the existing experimental data on nuclear Drell–Yan differential cross section ratio as a function of the quark momentum fraction. It is found that with independence on the nuclear modification of parton distributions, the available experimental data from lower incident beam energy rule out the incident-parton momentum fraction quark energy loss. Whether the quark energy loss is linear or quadratic with the path length is not discriminated. The global fit of all selected data gives the quark energy loss per unit path length α=1.21±0.09 GeV/fm by using nuclear parton distribution functions determined only by means of the world data on nuclear structure function. Our result does not support the theoretical prediction: the energy loss of an outgoing quark is three times larger than that of an incoming quark approaching the nuclear medium. It is desirable that the present work can provide useful reference for the Fermilab E906/SeaQuest experiment.

Parton Energy Loss in Two-Stream Plasma System

The energy loss of a fast parton scattering elastically in a weakly coupled quark-gluon plasma is formulated as an initial value problem. The approach is designed to study an unstable plasma, but it also reproduces the well known result of energy loss in an equilibrium plasma. A two-stream system, which is unstable due to longitudinal chromoelectric modes, is discussed here some detail. In particular, a strong time and directional dependence of the energy loss is demonstrated.

System size dependence of nuclear modification and azimuthal anisotropy of jet quenching

We investigate the system size dependence of jet quenching by analysing transverse momentum spectra of neutral pions in Au+Au and Cu+Cu collisions at =200 GeV for different centralities. The fast partons are assumed to lose energy by radiating gluons as they traverse the plasma and undergo multiple collisions. The energy loss per collision, ε, is taken as proportional to E (where E is the energy of the parton), proportional to , or a constant depending on whether the formation time of the gluon is less than the mean path, greater than the mean-free path but less than the path length, or greater than the path length of the partons, respectively. NLO pQCD is used to evaluate pion production by modifying the fragmentation function to account for the energy loss. We reproduce the nuclear modification factor RAA by treating ε as the only free parameter, depending on the centrality and the mechanism of energy loss. These values are seen to explain the nuclear modification of prompt photons caused by the energy lost by final state quarks before they fragment into photons. These also reproduce the azimuthal asymmetry of transverse momentum distribution for pions within a factor of 2 and for prompt photons in fair agreement with experimental data.

Jet measurements by the CMS experiment in pp and Pb–Pb collisions

The energy loss of fast partons traversing the strongly interacting matter produced in high-energy nuclear collisions is one of the most interesting observables to probe the nature of the produced medium. The multipurpose compact muon solenoid (CMS) detector is well designed to measure these hard scattering processes with its high-resolution calorimeters and high precision silicon tracker. In central Pb–Pb collisions, a center-of-mass energy of 2.76 TeV parton energy loss is observed as a significant dijet transverse momentum imbalance, when comparing to a reference distribution corresponding to pp collisions at the same energy. To gain further understanding of the parton energy loss mechanism, the redistribution of the quenched jet energy was studied using the transverse momentum balance of charged tracks projected onto the direction of the leading jet. In contrast to pp collisions, a large fraction of the momentum balance for asymmetric dijets is found to be carried by low momentum particles at large angular distance to the axis of the recoiling jet. Further, the fragmentation functions for leading and subleading jets were reconstructed and were found to be unmodified compared to measurements in pp collisions. The results yield a detailed picture of parton propagation in the hot QCD medium.

A derivation of the source term induced by a fast parton from the quark energy-momentum tensor

The distribution of energy and momentum deposited by a fast parton in a medium of thermalized quarks, or the source term, is evaluated in perturbative thermal field theory. The calculation is performed by directly evaluating the thermal expectation value of the quark energy-momentum tensor. The fast parton is coupled to the medium by adding an interaction term to the Lagrangian. I show that this approach is very general and can be modified to consider more realistic modeling of fast parton propagation, such as a fast parton created in an initial hard interaction or the evolution of a parton shower due to medium induced radiation. For the scenario considered here, it is found that local excitations fall sharply as a function of the energy of the fast parton. These local excitations couple directly to the sound mode in hydrodynamics and are important for generating an observable shockwave structure. This may have implications for the trigger pT dependence of measurements of azimuthal dihadron particle correlations in heavy-ion collisions. In particular, one would be less likely to observe a conical emission pattern for increasing trigger pT.

Higher Harmonic Anisotropic Flow Measurements of Charged Particles in Pb-Pb Collisions atsNN=2.76 TeV

We report on the first measurement of the triangular v3, quadrangular v4, and pentagonal v5 charged particle flow in Pb-Pb collisions at sqrt(s(NN)) = 2.76 TeV measured with the ALICE detector at the CERN Large Hadron Collider. We show that the triangular flow can be described in terms of the initial spatial anisotropy and its fluctuations, which provides strong constraints on its origin. In the most central events, where the elliptic flow v2 and v3 have similar magnitude, a double peaked structure in the two-particle azimuthal correlations is observed, which is often interpreted as a Mach cone response to fast partons. We show that this structure can be naturally explained from the measured anisotropic flow Fourier coefficients.

The ratios of identified strange associated hadrons in the di-hadron correlation in STAR

Di-hadron correlations will help understand the parton densities near jets traversing the medium, the process of hadron formation and the di-hadron correlation away-side shape. We report on the di-hadron correlations between unidentified charged hadron triggers with identified associated strange particles Λ and K S 0 ) in Au+Au collisions at 200 GeV in STAR. Particle yields and ratios are extracted on the near-side and awayside of the trigger particle. The baryon to meson ratios for associated particles as a function of Δϕ are also extracted. The shape of these ratios on the away-side is studied to understand the away-side’s pattern. The particle-type composition in the hump regions in away-side is compared to that in the near-side peak. These ratios may help to elucidate the particle composition in the away-side and near-side of the correlation pattern arising due to a fast parton traversing the medium.

Thermal field theory derivation of the source term induced by a fast parton from the quark energy-momentum tensor

I derive the distribution of energy and momentum transmitted from a fast parton to a medium of thermalized quarks, or the source term, in perturbative thermal field theory directly from the quark energy-momentum tensor. The fast parton is coupled to the medium by adding an interaction term to the Lagrangian. The thermal expectation value of the energy-momentum tensor source term is then evaluated using standard Feynman rules at finite temperature. It is found that local excitations, which are important for exciting an observable Mach cone structure, fall sharply as a function of the energy of the fast parton. This may have implications for the trigger $p_T$ dependence of measurements of azimuthal dihadron particle correlations in heavy-ion collisions. In particular, a conical emission pattern would be less likely to be observed for increasing trigger $p_T$. I show that the results presented in this paper can be generalized to more realistic modeling of fast parton propagation, such as through a time dependent interaction term, in future studies.