Medium-induced Gluon Radiation Research Articles

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.

Large angle hadron correlations from medium-induced gluon radiation

Final state medium-induced gluon radiation in ultradense nuclear matter is shown to favor large angle emission when compared to vacuum bremsstrahlung due to the suppression of collinear gluons. In a simple model of momentum transfers from the medium to the jet+gluon system the phase space distribution of non-Abelian bremsstrahlung is examined to first order in opacity. Perturbative expression for the contribution of the hadronic fragments from mediuminduced gluons to the back-to-back particle correlations is derived. It is found that in the limit of large jet energy loss gluon radiation determines the yield and angular distribution of |Δφ| ⩾ π/2 di-hadrons to high transverse momenta pT2 of the associated particles. Clear transition from enhancement to suppression of the away-side di-hadron correlations is established at moderate pT2 and its experimentally accessible features are predicted versus the trigger particle momentum pT1. The measured away-side correlation function is shown to be sensitive to the broad rapidity distribution of di-jets and their vacuum and medium-induced acoplanarity.

Measuring the Collective Flow with Jets

In nucleus-nucleus collisions, high-p(T) partons interact with a dense medium, which possesses strong collective flow components. Here, we demonstrate that the resulting medium-induced gluon radiation does not depend solely on the energy density of the medium, but also on the collective flow. Both components can be disentangled by the measurement of particle production associated with high-p(T) trigger particles, jetlike correlations, and jets. In particular, we show that flow effects lead to a characteristic breaking of the rotational symmetry of the average jet energy and jet multiplicity distribution in the eta x phi plane. We argue that data on the medium-induced broadening of jetlike particle correlations in Au + Au collisions at the Relativistic Heavy-Ion Collider may provide evidence for a significant distortion of parton fragmentation due to the longitudinal collective flow.

Medium-induced gluon radiation off massive quarks fills the dead cone

We calculate the transverse momentum dependence of the medium-induced gluon energy distribution radiated off massive quarks in spatially extended QCD matter. In the absence of a medium, the distribution shows a characteristic mass-dependent depletion of the gluon radiation for angles $\ensuremath{\theta}lm/E,$ the so-called dead cone effect. Medium modifications of this spectrum are calculated as a function of quark mass m, initial quark energy E, in-medium path length and density. Generically, medium-induced gluon radiation is found to fill the dead cone, but it is reduced at large gluon energies compared to the radiation off light quarks. We quantify the resulting mass dependence for momentum-averaged quantities (gluon energy distribution and average parton energy loss), compare it to simple approximation schemes and discuss its observable consequences for nucleus-nucleus collisions at the BNL RHIC and CERN LHC. In particular, our analysis does not favor the complete disappearance of energy loss effects from leading open charm spectra at the RHIC.

Finding the remnants of lost jets at RHIC

A fast parton propagating through partonic matter loses energy by medium-induced gluon radiation. We propose a method to detect the energy loss of jets and the final-state interaction of the radiated gluons and the jets with the partonic medium. By embedding jets and radiated energy into a parton cascade, we find that the fate of lost jets can be studied by analyzing the momentum distribution of soft hadrons relative to the jet axis.

Calculating quenching weights

We calculate the probability (``quenching weight'') that a hard parton radiates an additional energy fraction due to scattering in spatially extended QCD matter. This study is based on an exact treatment of finite in-medium path length, it includes the case of a dynamically expanding medium, and it extends to the angular dependence of the medium-induced gluon radiation pattern. All calculations are done in the multiple soft scattering approximation (Baier-Dokshitzer-Mueller-Peign\'e-Schiff--Zakharov ``BDMPS-Z''-formalism) and in the single hard scattering approximation (N=1 opacity approximation). By comparison, we establish a simple relation between transport coefficient, Debye screening mass and opacity, for which both approximations lead to comparable results. Together with this paper, a CPU-inexpensive numerical subroutine for calculating quenching weights is provided electronically. To illustrate its applications, we discuss the suppression of hadronic transverse momentum spectra in nucleus-nucleus collisions. Remarkably, the kinematic constraint resulting from finite in-medium path length reduces significantly the transverse momentum dependence of the nuclear modification factor, thus leading to consistency with the data measured at the Relativistic Heavy Ion Collider (RHIC).

Determining the BDMPS transport coefficient via medium-modified fragmentation functions

In nucleus-nucleus collisions at RHIC and LHC, partons produced at high transverse momentum can undergo multiple scattering within the collision region prior to fragmenting into hadrons. We have studied the resulting medium-modified fragmentation function based on a calculation of the BDMPS-Z medium-induced gluon radiation for a dense, expanding medium of small finite extension. Here we explain how the BDMPS transport coefficient which measures the energy density attained in the collision, can be extracted from the observed modification of high-pt hadroproduction. We also comment on the significant remaining uncertainties in extracting the transport coefficient from data.

High-p(T) Tomography of d+Au and Au+Au at SPS, RHIC, and LHC.

The interplay of nuclear effects on the p(T)>2 GeV inclusive hadron spectra in d+Au and Au+Au reactions at sqrt[s(NN)]=17, 200, and 5500 GeV is compared to leading order perturbative QCD calculations for elementary p+p (p+p) collisions. The competition between nuclear shadowing, Cronin effect, and jet energy loss due to medium-induced gluon radiation is predicted to lead to a striking energy dependence of the nuclear suppression/enhancement pattern in A+A reactions. We show that future d+Au data can be used to disentangle the initial and final state effects.

Energy loss of hard partons in nuclear matter

We report on recent calculations of the medium-induced gluon radiation off hard partons. The employed path-integral formalism reduces in limiting cases to the main ``jet quenching'' results of the existing literature. Moreover, it describes destructive interference effects between medium-independent and medium-induced radiation amplitudes. These affect the angular distribution and $L$-dependence of the medium-induced gluon bremsstrahlung spectrum significantly.

Gluon radiation off hard quarks in a nuclear environment: opacity expansion

We study the relation between the Baier-Dokshitzer-Mueller-Peigne-Schiff (BDMPS) and Zakharov formalisms for medium-induced gluon radiation off hard quarks, and the radiation off very few scattering centers. Based on the non-abelian Furry approximation for the motion of hard partons in a spatially extended colour field, we derive a compact diagrammatic and explicitly colour trivial expression for the N-th order term of the kt-differential gluon radiation cross section in an expansion in the opacity of the medium. Resumming this quantity to all orders in opacity, we obtain Zakharov's path-integral expression (supplemented with a regularization prescription). This provides a new proof of the equivalence of the BDMPS and Zakharov formalisms which extends previous arguments to the kt-differential cross section. We give explicit analytical results up to third order in opacity for both the gluon radiation cross section of free incoming and of in-medium produced quarks. The N-th order term in the opacity expansion of the radiation cross section is found to be a convolution of the radiation associated to N-fold rescattering and a readjustment of the probabilities that rescattering occurs with less than N scattering centers. Both informations can be disentangled by factorizing out of the radiation cross section a term which depends only on the mean free path of the projectile. This allows to infer analytical expressions for the totally coherent and totally incoherent limits of the radiation cross section to arbitrary orders in opacity.