Heavy-quarks in the QGP: study of medium effects through euclidean propagators and spectral functions

The heavy-quark spectral function in a hot plasma is reconstructed from the corresponding euclidean propagator. The latter is evaluated through a path-integral simulation. A weak-coupling calculation is also performed, allowing to interpret the qualitative behavior of the spectral function in terms of quite general physical processes.Comment: 4 pages, 3 figures - To appear in the conference proceedings for Quark Matter 2009, March 30 - April 4, Knoxville, Tennesse

Medium-induced color flow softens hadronization

Medium-induced parton energy loss, resulting from gluon exchanges between the QCD matter and partonic projectiles, is expected to underly the strong suppression of jets and high-$p_T$ hadron spectra observed in ultra-relativistic heavy ion collisions. Here, we present the first color-differential calculation of parton energy loss. We find that color exchange between medium and projectile enhances the invariant mass of energetic color singlet clusters in the parton shower by a parametrically large factor proportional to the square root of the projectile energy. This effect is seen in more than half of the most energetic color-singlet fragments of medium-modified parton branchings. Applying a standard cluster hadronization model, we find that it leads to a characteristic additional softening of hadronic spectra. A fair description of the nuclear modification factor measured at the LHC may then be obtained for relatively low momentum transfers from the medium

A path integral for heavy-quarks in a hot plasma

We propose a model for the propagation of a heavy-quark in a hot plasma, to be viewed as a first step towards a full description of the dynamics of heavy quark systems in a quark-gluon plasma, including bound state formation. The heavy quark is treated as a non relativistic particle interacting with a fluctuating field, whose correlator is determined by a hard thermal loop approximation. This approximation, which concerns only the medium in which the heavy quark propagates, is the only one that is made, and it can be improved. The dynamics of the heavy quark is given exactly by a quantum mechanical path integral that is calculated in this paper in the Euclidean space-time using numerical Monte Carlo techniques. The spectral function of the heavy quark in the medium is then reconstructed using a Maximum Entropy Method. The path integral is also evaluated exactly in the case where the mass of the heavy quark is infinite; one then recovers known results concerning the complex optical potential that controls the long time behavior of the heavy quark. The heavy quark correlator and its spectral function is also calculated semi-analytically at the one-loop order, which allows for a detailed description of the coupling between the heavy quark and the plasma collective modes

Erratum to: A study of vorticity formation in high energy nuclear collisions

Due to an oversight of ours in proofreading and a communication problem with the publisher, the figures published in F. Becattini et al. Eur. Phys. J. C (2015) 75: 406 were not correct. This Erratum contains the correct figures as in arXiv 1501.04468v2, submitted on March 12 2015, and the post-publication version arXiv 1501.04468v3, submitted on August 17 2015

Relativistic viscous hydrodynamics for heavy-ion collisions with ECHO-QGP

We present ECHO-QGP, a numerical code for $(3+1)$-dimensional relativistic viscous hydrodynamics designed for the modeling of the space-time evolution of the matter created in high energy nuclear collisions. The code has been built on top of the \emph{Eulerian Conservative High-Order} astrophysical code for general relativistic magneto-hydrodynamics [\emph{Del Zanna et al., Astron. Astrophys. 473, 11, 2007}] and here it has been upgraded to handle the physics of the Quark-Gluon Plasma. ECHO-QGP features second-order treatment of causal relativistic viscosity effects in both Minkowskian or Bjorken coordinates; partial or complete chemical equilibrium of hadronic species before kinetic freeze-out; initial conditions based on the optical Glauber model, including a Monte-Carlo routine for event-by-event fluctuating initial conditions; a freeze-out procedure based on the Cooper-Frye prescription. The code is extensively validated against several test problems and results always appear accurate, as guaranteed by the combination of the conservative (shock-capturing) approach and the high-order methods employed. ECHO-QGP can be extended to include evolution of the electromagnetic fields coupled to the plasma.Comment: 25 pages, two column, Final version: accepted for publication in European Physical Journal

Colour-electric spectral function at next-to-leading order

The spectral function related to the correlator of two colour-electric fields along a Polyakov loop determines the momentum diffusion coefficient of a heavy quark near rest with respect to a heat bath. We compute this spectral function at next-to-leading order, O(alpha_s^2), in the weak-coupling expansion. The high-frequency part of our result (omega >> T), which is shown to be temperature-independent, is accurately determined thanks to asymptotic freedom; the low-frequency part of our result (omega << T), in which Hard Thermal Loop resummation is needed in order to cure infrared divergences, agrees with a previously determined expression. Our result may help to calibrate the overall normalization of a lattice-extracted spectral function in a perturbative frequency domain T << omega << 1/a, paving the way for a non-perturbative estimate of the momentum diffusion coefficient at omega -> 0. We also evaluate the colour-electric Euclidean correlator, which could be directly compared with lattice simulations. As an aside we determine the Euclidean correlator in the lattice strong-coupling expansion, showing that through a limiting procedure it can in principle be defined also in the confined phase of pure Yang-Mills theory, even if a practical measurement could be very noisy there.Comment: 38 page

The contribution of medium-modified color flow to jet quenching

Multiple interactions between parton showers and the surrounding QCD matter are expected to underlie the strong medium-modifications of jet observables in ultra-relativistic heavy ion collisions at RHIC and at the LHC. Here, we note that such jet-medium interactions alter generically and characteristically the color correlations in the parton shower. We characterize these effects in a color-differential calculation of the medium-induced gluon radiation spectrum to first and second order in opacity. By interfacing simple branching histories of medium-modified color flow with the Lund hadronization model, we analyze how the medium modification of color correlations can affect the distribution of hadronic fragments in jets. Importantly, we observe that jet-medium interactions give rise to the medium-induced color decoherence of gluons from the parton shower. Since hadronization respects color flow and since each color singlet in a parton shower is hadronized separately, this medium-induced color decoherence leaves characteristic signatures in the jet fragmentation pattern. In particular, it can contribute to the quenching of leading hadron spectra. Moreover, it can increase strongly the yield of soft hadronic fragments from a jet, while the distribution of more energetic hadrons follows naturally the shape of a vacuum-like fragmentation pattern of lower total energy

Thermal width and gluo-dissociation of quarkonium in pNRQCD

The thermal width of heavy-quarkonium bound states in a quark-gluon plasma has been recently derived in an effective field theory approach. Two phenomena contribute to the width: the Landau damping phenomenon and the break-up of a colour-singlet bound state into a colour-octet heavy quark-antiquark pair by absorption of a thermal gluon. In the paper, we investigate the relation between the singlet-to-octet thermal break-up and the so-called gluo-dissociation, a mechanism for quarkonium dissociation widely used in phenomenological approaches. The gluo-dissociation thermal width is obtained by convoluting the gluon thermal distribution with the cross section of a gluon and a 1S quarkonium state to a colour octet quark-antiquark state in vacuum, a cross section that at leading order, but neglecting colour-octet effects, was computed long ago by Bhanot and Peskin. We will, first, show that the effective field theory framework provides a natural derivation of the gluo-dissociation factorization formula at leading order, which is, indeed, the singlet-to-octet thermal break-up expression. Second, the singlet-to-octet thermal break-up expression will allow us to improve the Bhanot--Peskin cross section by including the contribution of the octet potential, which amounts to include final-state interactions between the heavy quark and antiquark. Finally, we will quantify the effects due to final-state interactions on the gluo-dissociation cross section and on the quarkonium thermal width.Comment: 17 pages, 6 figure

Heavy-flavour and quarkonium production in the LHC era: from proton-proton to heavy-ion collisions

This report reviews the study of open heavy-flavour and quarkonium production in high-energy hadronic collisions, as tools to investigate fundamental aspects of Quantum Chromodynamics, from the proton and nucleus structure at high energy to deconfinement and the properties of the Quark-Gluon Plasma. Emphasis is given to the lessons learnt from LHC Run 1 results, which are reviewed in a global picture with the results from SPS and RHIC at lower energies, as well as to the questions to be addressed in the future. The report covers heavy flavour and quarkonium production in proton-proton, proton-nucleus and nucleus-nucleus collisions. This includes discussion of the effects of hot and cold strongly interacting matter, quarkonium photo-production in nucleus-nucleus collisions and perspectives on the study of heavy flavour and quarkonium with upgrades of existing experiments and new experiments. The report results from the activity of the SaporeGravis network of the I3 Hadron Physics programme of the European Union 7th Framework Programme

INFN What Next: Ultra-relativistic Heavy-Ion Collisions

This document was prepared by the community that is active in Italy, within INFN (Istituto Nazionale di Fisica Nucleare), in the field of ultra-relativistic heavy-ion collisions. The experimental study of the phase diagram of strongly-interacting matter and of the Quark-Gluon Plasma (QGP) deconfined state will proceed, in the next 10-15 years, along two directions: the high-energy regime at RHIC and at the LHC, and the low-energy regime at FAIR, NICA, SPS and RHIC. The Italian community is strongly involved in the present and future programme of the ALICE experiment, the upgrade of which will open, in the 2020s, a new phase of high-precision characterisation of the QGP properties at the LHC. As a complement of this main activity, there is a growing interest in a possible future experiment at the SPS, which would target the search for the onset of deconfinement using dimuon measurements. On a longer timescale, the community looks with interest at the ongoing studies and discussions on a possible fixed-target programme using the LHC ion beams and on the Future Circular Collider.Comment: 99 pages, 56 figure