Ivan Vitev

Reaction operator approach to non-abelian energy loss

Abstract A systematic expansion of induced gluon radiation associated with jet production in a dense QCD plasma is derived using a reaction operator formalism. Analytic expressions for the induced inclusive gluon transverse momentum and light-cone momentum distributions are derived to all orders in powers of the opacity of the medium for gluons, Nσ g /A ⊥ =L/λ g . The reaction operator approach leads to a simple algebraic proof of the “color triviality” of single inclusive distributions and leads to a solvable set of recursion relations. The analytic solution to all orders in opacity generalizes previous continuum solutions (BDMPS) by allowing for arbitrary correlated nuclear geometry and evolving screening scales as well as the inclusion of finite kinematic constraints. The latter is particularly important because below LHC energies the kinematic constraints significantly decrease the non-abelian energy loss. Our solution for the inclusive distribution turns out to have a much simpler structure than the exclusive (tagged) distribution case that we studied previously (GLV1). The analytic expressions are also obtained in a form suitable for numerical implementation in Monte Carlo event generators to enable more accurate calculations of jet quenching in ultra-relativistic nuclear collisions. Numerical results illustrating the first three orders in opacity are compared to the “self-quenching” hard radiation intensity that always accompanies jet production in the vacuum. A surprising result is that the induced gluon radiation intensity is dominated by the (quadratic in L ) first order opacity contribution for realistic geometries and jet energies in nuclear collisions.

Non-Abelian Energy Loss at Finite Opacity

A systematic expansion in opacity, L/lambda, is used to clarify the nonlinear behavior of induced gluon radiation in quark-gluon plasmas. The inclusive differential gluon distribution is calculated up to second order in opacity and compared to the zeroth order (factorization) limit. The opacity expansion makes it possible to take finite kinematic constraints into account that suppress jet quenching in nuclear collisions below RHIC (square root of s = 200 AGeV) energies.

High

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.

p_{T}

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.

tomography of

Abstract The modification and amplification of the gluon angular distribution produced along with hard jets in nuclear collisions is computed. We consider the limit of a thin quark–gluon plasma, where the number of rescatterings of the jet and gluons is small. The focus is on jet quenching associated with the formation of highly off-shell partons in hard scattering events involving nuclei. The interference between the initial hard radiation amplitude, the multiple induced Gunion–Bertsch radiation amplitudes, and gluon rescattering amplitudes leads to an angular distribution that differs considerably from both the standard DGLAP evolution and from the classical limit parton cascading. The cases of a single and double rescattering are considered in detail, and a systematic method to compute all matrix elements for the general case is developed. A simple power law scaling of the angular distribution with increasing number of rescatterings is found and used for estimates of the fractional energy loss as a function of the plasma thickness.

d

Predictions for charged hadron, identified light hadron, quarkonium, photon, jet and gauge bosons in p+Pb collisions at sqrt s_NN = 5 TeV are compiled and compared. When test run data are available, they are compared to the model predictions.

+ Au and Au+Au at SPS, RHIC, and LHC

Abstract Jet tomography is the analysis of the attenuation pattern of high transverse momentum hadrons to determine certain line integral transforms of the density profile of the QCD matter produced in ultra-relativistic nuclear collisions. In this Letter, we calculate the distortion of jet tomography due to multi-gluon fluctuations within the GLV radiative energy loss formalism. We find that fluctuations of the average gluon number, 〈 N g 〉∼3 for RHIC initial conditions, reduce the attenuation of pions by approximately a factor Z ≈0.4–0.5. Therefore the plasma density inferred from jet tomography without fluctuations must be enhanced by a factor 1/ Z ∼2.

Jet quenching and radiative energy loss in dense nuclear matter

Abstract The impact parameter and rapidity dependence of the Cronin effect for massless pions in d+Au reactions at s NN =200 xa0GeV at RHIC is computed in the framework of pQCD multiple elastic scattering on a nuclear target. We introduce a formalism to incorporate initial state energy loss in perturbative calculations and take into account the elastic energy loss in addition to the transverse momentum broadening of partons. We argue that the centrality dependence of the Cronin effect can distinguish between different hadron production scenarios at RHIC. Its magnitude and rapidity dependence are shown to carry important experimental information about the properties of cold nuclear matter up to the moderate- and large-x antishadowing/EMC regions.

Jet quenching in thin quark–gluon plasmas I: formalism

Abstract Rapid (3+1)D transverse plus Bjorken collective expansion in A + A collisions at ultra-relativistic energies is shown to reduce substantially the azimuthal asymmetry resulting from jet quenching. While the azimuthal asymmetry in non-central collisions, v 2 ( p T >2xa0GeV/ c )∼0.15 reported by STAR at RHIC, can be accounted for by spatially anisotropic jet energy loss through a (1+1)D expanding gluon plasma with dN g / dy ∼1000, we show that if rapid transverse collective expansion of the plasma is assumed, then the asymmetry due to jet quenching may be reduced below the observed level. Possible implications of this effect are discussed.

Predictions for

We calculate and resum a perturbative expansion of nuclear enhanced power corrections to the structure functions measured in deeply inelastic scattering of leptons on a nuclear target. Our results for the Bjorken x, Q2 and A dependence of nuclear shadowing in F2A(x,Q2) and the nuclear modification to FAL(x,Q2), obtained in terms of the QCD factorization approach, are consistent with the existing data. We demonstrate that the low Q2 behavior of these data and the measured large longitudinal structure function point to a critical role for the power corrections when compared to other theoretical approaches.