Induced Gluon Radiation Research Articles

Probing the Short-Distance Structure of the Quark-Gluon Plasma with Energy Correlators.

Energy-energy correlators (EECs) are promising observables to study the dynamics of jet evolution in the quark-gluon plasma (QGP) through its imprint on angular scales in the energy flux of final-state particles. We carry out the first complete calculation of EECs using realistic simulations of high-energy heavy-ion collisions and dissect the different dynamics underlying the final distribution through analyses of jet propagation in a uniform medium. The EECs of γ-jets in heavy-ion collisions are found to be enhanced by the medium response from elastic scatterings instead of induced gluon radiation at large angles. In the meantime, EECs are suppressed at small angles due to energy loss and transverse momentum broadening of jet shower partons. These modifications are further shown to be sensitive to the angular scale of the in-medium interaction, as characterized by the Debye screening mass. Experimental verification and measurement of such modifications will shed light on this scale and the short-distance structure of the QGP in heavy-ion collisions.

Small x physics beyond eikonal approximation: an effective Hamiltonian approach

Understanding the spin structure of hadrons in the small x regime is an important direction to unravel the spin puzzle in hadronic physics. To include spin degrees of freedom in the small x regime requires going beyond the usual eikonal approximation in high energy QCD. We developed an effective Hamiltonian approach to study spin related observables in the small x regime using the shockwave formalism. The small-x effective Hamiltonian incorporates both quark and gluon propagators in the background fields and the background field induced interaction vertices up to next-to-eikonal order. A novel feature of sub-eikonal interactions is the background gluon field induced gluon radiation inside the shockwave. Its relation to chromo-electrically polarized Wilson line correlator is established both in small x helicity evolution and in longitudinal double-spin asymmetry for gluon production.

Improved opacity expansion at NNLO for medium induced gluon radiation

When an energetic parton propagates in a hot and dense QCD medium it loses energy by elastic scatterings or by medium-induced gluon radiation. The gluon radiation spectrum is suppressed at high frequency due to the LPM effect and encompasses two regimes that are known analytically: at high frequencies omega >{omega}_c=hat{q}{L}^2 , where hat{q} is the jet quenching transport coefficient and L the length of the medium, the spectrum is dominated by a single hard scattering, whereas the regime ω < ωc is dominated by multiple low momentum transfers. In this paper, we extend a recent approach (dubbed the Improved Opacity Expansion (IOE)), which allows an analytic (and systematic) treatment beyond the multiple soft scattering approximation, matching this result with the single hard emission spectrum. We calculate in particular the NNLO correction analytically and numerically and show that it is strongly suppressed compared to the NLO indicating a fast convergence of the IOE scheme and thus, we conclude that it is sufficient to truncate the series at NLO. We also propose a prescription to compare the GW and the HTL potentials and relate their parameters for future phenomenological works.

Fully coherent energy loss effects on light hadron production in pA collisions

We single out the role of fully coherent induced gluon radiation on light hadron production in pA collisions. The effect has the same general features as for quarkonium production, however with a richer color structure as the induced radiation depends on the global color charge of the partonic subprocess final state. Baseline predictions for light hadron nuclear suppression in pPb collisions at the LHC are provided, taking into account only the effect of fully coherent energy loss, which proves to be of the same order of magnitude as gluon shadowing or saturation. This underlines the need to include fully coherent energy loss in phenomenological studies of hadron production in pA collisions.

Quenching of Light Hadron Spectra in p-A Collisions from Fully Coherent Energy Loss.

We single out the role of fully coherent induced gluon radiation on light hadron production in p-A collisions. The effect has an interesting color structure, as the induced radiation depends on the global color charge of the partonic subprocess final state. Baseline predictions for light hadron nuclear suppression in p-Pb collisions at the LHC are provided, taking into account only the effect of fully coherent energy loss, which proves of the same order of magnitude as gluon shadowing or saturation. This underlines the need to include fully coherent energy loss in phenomenological studies of hadron production in p-A collisions.

Heavy quark dynamics within a Boltzmann transport model: radiative vs collisional energy loss

We discuss the propagation of heavy quarks (charm and bottom) through the QGP by means of a relativistic Boltzmann transport approach including both collisional and radiative energy loss mechanisms. In particular we investigate the impact of induced gluon radiation by dynamical QCD medium implementing in our transport model a formula for the emitted gluon spectrum calculated in a higher-twist scheme. We notice that in the region of high transverse momentum (pT > 10 GeV) radiative processes play an essential role giving a dominant contribution to the generation of RAA and v2 at momentum values for which the energy loss by collisions is in the perturbative regime.

Medium Recoil and Jet Modification in Heavy Ion Collisions

A complete set of elastic processes and induced gluon radiation within the higher-twist approach have been implemented in the Linear Boltzmann Transport model for jet propagation and interaction with quark-gluon plasma in high-energy heavy-ion collisions. We impose global energy momentum conservation in the 2→n processes of induced radiation which will influence the final gluon spectra. We will compare the elastic and the radiative energy loss of partons and their effects on reconstructed jets. The energy loss of a leading parton is found to have a quadratic distance dependence only for a short distance, but will have much weak distance dependence because of the accumulated energy loss and the strong energy dependence of the local energy loss rate. Since reconstructed jets recover some of the energy lost by the leading parton, the quadratic path length dependence persists for a longer distance. The spatial distribution and time evolution of the jet-induced medium excitation are also discussed.

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.

Medium Modification ofγJets in High-Energy Heavy-Ion Collisions

Two puzzling features in the experimental study of jet quenching in central Pb+Pb collisions at the LHC are explained within a linearized Boltzmann transport model for jet propagation. A γ-tagged jet is found to lose about 15% of its initial energy while its azimuthal angle remains almost unchanged due to rapid cooling of the medium. The reconstructed jet fragmentation function is found to have some modest enhancement at both small and large fractional momenta as compared to that in the vacuum because of the increased contribution of leading particles to the reconstructed jet energy and induced gluon radiation and recoiled partons. A γ-tagged jet fragmentation function is proposed that is more sensitive to jet-medium interaction and the jet transport parameter in the medium. The effects of recoiled medium partons on the reconstructed jets are also discussed.

Coherence and broadening effects in medium induced gluon radiation

Abstract Interferences between different emitters in the multi-parton shower is the building block of QCD jet physics in vacuum. The presence of a hot medium made of quarks and gluons is expected to alter this interference pattern. To study such effects, we derive the gluon emission spectrum off an “asymptotic quark” traversing a hot and dense QCD medium at first order in the medium density. The resulting induced gluon distribution gets modified when the new interference terms between the initial and final quark are included. We comment on the possible phenomenological consequences of this new contribution for jet observables in heavy-ion collisions.

Parton energy loss in cold nuclei

Within the generalized high-twist factorization formalism, we express the contribution from multiple parton scattering and induced gluon radiation to the DY dilepton spectra in terms of nuclear modified e ective beam quark distribution function. We show that beam quark energy loss is characterized by jet transport parameter ˆ q, which is related to the local gluon density of the medium. Using the value of ˆ q determined from the deeply inelastic scattering (DIS) data, we evaluate the nuclear modification factor in the Drell-Yan process in p+A collisions. E ects of parton energy loss in the DY spectra are found negligible in the Fermilab experimental data at Elab = 800 GeV relative to parton shadowing while the predicted suppression of the DY spectra are significant at Elab = 120 GeV.

Parton energy loss and modified beam quark distribution functions in Drell–Yan process in collisions

Within the framework of generalized collinear factorization in perturbative QCD (pQCD), we study the effect of initial multiple parton scattering and induced parton energy loss in Drell-Yan (DY) process in proton-nucleus collisions. We express the contribution from multiple parton scattering and induced gluon radiation to the DY dilepton spectra in terms of nuclear modified effective beam quark distribution functions. The modification depends on the quark transport parameter in nuclear medium. This is similar to the final-state multiple parton scattering in deeply inelastic scattering (DIS) off large nuclei and leads to the suppression of the Drell-Yan cross section in $p+A$ relative to $p+p$ collisions. With the value of quark transport parameter determined from the nuclear modification of single-inclusive DIS hadron spectra as measured by the HERMES experiment, we calculate DY spectra in $p+A$ collisions and find the nuclear suppression due to beam parton energy loss negligible at the Fermilab energy $E_{\rm lab}$=800 GeV in the kinematic region as covered by the E866 experiment. Most of the observed nuclear suppression of DY spectra in E866 experiment can be described well by parton shadowing in target nuclei as given by the EPS08 parameterization. The effect of beam parton energy loss, however, becomes significant for DY lepton pairs with large beam parton momentum fraction $x^{\prime}$ or small target parton momentum fraction $x$. We also predict the DY cross section in $p+A$ collisions at lower beam proton energy $E_{\rm lab}$=120 GeV and show significant suppression due to initial state parton energy loss at moderately large $x^{\prime}$ where the effect of parton shadowing is very small.

Jet energy loss, photon production, and photon-hadron correlations at energies available at the BNL Relativistic Heavy Ion Collider (RHIC)

Jet energy loss, photon production and photon-hadron correlations are studied together at high transverse momentum in relativistic heavy-ion collisions at RHIC energies. The modification of hard jets traversing a hot and dense nuclear medium is evaluated by consistently taking into account induced gluon radiation and elastic collisions. The production of high transverse momentum photons in Au+Au collisions at RHIC is calculated by incorporating a complete set of photon-production channels. Comparison with experimental photon production and photon-hadron correlation data is performed, using a (3+1)-dimensional relativistic hydrodynamic description of the thermalized medium created in these collisions. Our results demonstrate that the interaction between the hard jets and the soft medium is important for the study of photon production and of photon-hadron correlation at RHIC.

Anomalous mass dependence of radiative quark energy loss in a finite-size quark-gluon plasma

We demonstrate that for a finite-size quark-gluon plasma the induced gluon radiation from heavy quarks is stronger than that for light quarks when the gluon formation length becomes comparable with (or exceeds) the size of the plasma. The effect is due to oscillations of the light-cone wave function for the in-medium q→gq transition. The dead cone model by Dokshitzer and Kharzeev neglecting quantum finite-size effects is not valid in this regime. The finite-size effects also enhance the photon emission from heavy quarks.

Flow Effects on Jet Profile

I review the motivation for considering the effect of the expansion of the medium produced in ultra-relativistic heavy ion collisions, on induced gluon radiation. Naive implementations of the effect of flow produce a marked asymmetry in radiation around the jet axis. Such asymmetry should be clearly visible in jet studies at the LHC. They also lead to an increase of the elliptic flow for high p T particles. As additional results, flow may mimic a larger transport coefficient i.e. a larger energy density of the medium, and the surface bias becomes weakened by expansion.

Heavy-quark diffusion, flow and recombination at RHIC

We discuss recent developments in assessing heavy-quark interactions in the quark–gluon plasma (QGP). While induced gluon radiation is expected to be the main energy-loss mechanism for fast-moving quarks, we focus on elastic scattering which prevails towards lower energies, evaluating both perturbative (gluon-exchange) and nonperturbative (resonance formation) interactions in the QGP. The latter are treated within an effective model for D- and B-meson resonances above Tc as motivated by current QCD lattice calculations. Pertinent diffusion and drag constants, following from a Fokker–Planck equation, are implemented into an expanding fireball model for Au–Au collisions at RHIC using relativistic Langevin simulations. Heavy quarks are hadronized in a combined fragmentation and coalescence framework, and resulting electron-decay spectra are compared to recent RHIC data. A reasonable description of both nuclear suppression factors and elliptic flow up to momenta of ∼5 GeV supports the notion of a strongly interacting QGP created at RHIC. Consequences and further tests of the proposed resonance interactions are discussed.

Suppression of high-pT non-photonic electrons in Au+Au collisions at $\sqrt{s_{NN}}$ =200 GeV

The strong suppression of high pT hadrons observed at RHIC has led to the interpretation that energetic partons lose their energy via induced gluon radiation in the hot and dense matter before fragmenting into hadrons. The study of heavy quark production can extend our understanding of this scenario. Due to the dead cone effect, the suppression of heavy quark mesons at high pT is expected to be smaller than that observed for charged hadrons at the same energy. The measurement of non-photonic single electrons up to high pT provides information on charm and beauty production. The semi-leptonic decays of D and B mesons are the dominant contribution to the non-photonic electron spectra. The preliminary spectra from p+p, d+Au and Au+Au collisions at $\sqrt{s_{NN}}$ =200 GeV have been extracted for mid-rapidity non-photonic electrons in the range 1.5<pT (GeV/c)<10. The corresponding nuclear modification factors (RAA) are presented and show a large suppression in central Au+Au collisions, indicating an unexpectedly large energy loss for heavy quarks in the hot and dense matter created at RHIC. This observed suppression is compared to recent theoretical models.

Parton energy loss in hot and dense QCD medium

Induced gluon radiation and energy loss of heavy quark in hot and dense QCD media are discussed. Using the light-cone path integral approach, an analytical formula of the energy loss of heavy quark is derived. The results show that the quark energy loss obviously depends on the mass of quark, i. e. a remarkable suppression occurs in the case of heavy quark comparing to a light one. The radiative energy loss of energetic quark is proportional to L 2, where L is the length of the medium. The dependence of energy loss on L 2 turns to L with decreasing quark energy.

Jet physics at the LHC with ALICE

In central Pb-Pb collisions at the LHC, jet rates are expected to be high at energies at which ALICE can reconstruct jets over the background of the underlying event. This will open the possibility to quantify the effect of partonic energy loss through medium induced gluon radiation, jet quenching, by detailed measurement of the modification of the longitudinal and transverse structure of identified jets. In order to obtain probes sensitive to the properties of the QCD medium, it is mandatory to measure the high-\(p_{\rm T}\) parton fragments together with the low-\(p_{\rm T}\) particles from the radiated gluons. Hence, the excellent charged particle tracking capabilities of ALICE combined with the proposed electromagnetic calorimeter for ALICE, EMCAL, represent an ideal tool for jet quenching studies at the LHC.

Jet physics in heavy-ion collisions at the LHC with the ALICE detector

In central Pb–Pb collisions at the LHC, jet rates are expected to be high at energies at which jets can be fully reconstructed over the background of the underlying event. This will open the possibility of quantifying the effect of partonic energy loss through medium induced gluon radiation, jet quenching, by detailed measurement of the modification of the longitudinal and transverse structure of identified jets. We will present Monte Carlo simulations strongly indicating that, in order to obtain probes sensitive to the properties of the QCD medium, it is mandatory to measure the high-pT parton fragments together with the low-pT particles from the radiated gluons. Hence, the excellent charged particle tracking capabilities of ALICE with Δp/p < 10% from 100 MeV–100 GeV combined with the proposed electromagnetic calorimeter for ALICE, EMCAL, represent an ideal tool for jet quenching studies at the LHC.