Dilepton emission in high-energy heavy-ion collisions with viscous hydrodynamics

Vujanovic, Gojko; Young, Clint; Schenke, Bjoern; Rapp, Ralf; Jeon, Sangyong; Gale, Charles

doi:10.1103/physrevc.89.034904

Cited by 89 publications

(109 citation statements)

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“…We use music, a 3+1 D hydrodynamic simulation [9,10]. As the purpose of this study is to determine the global effect of rates in the semi-QGP, versus that in the usual QGP, we also include the hadronic rates for dileptons [16] and photons [15]. We use ideal hydrodynamics for nucleus-nucleus collisions, with A = 200 at RHIC energies, √ s N N = 200 GeV.…”

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“…We compute to leading order in the QCD coupling (for photons, only to leading logarithmic order) and give complete results later [25]. We then use a hydrodynamic model [16] to compute the effect on the number of dileptons and photons produced, and on the elliptic flow, v 2 . The effects on thermal dileptons are modest.…”

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“…Let the sum of the momenta of the dilepton be P µ = (E, p ), p = | p |, where E > p. To leading order in perturbation theory, this arises from the annihilation of a quark anti-quark pair into a virtual photon, which then decays into a dilepton pair. For three colors and Q = 0, the production rate [16] is (1) p ± = (E ± p)/2, and n(E) = 1/(e E/T − 1) is the usual Bose-Einstein distribution function. This includes the contributions of (massless) up, down and strange quarks, where α em = e 2 /4π, and e is the electromagnetic coupling constant.…”

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“…Electromagnetic signals, such as dilepton or photon production, are another valuable probe, since they reflect properties of the quark and gluon distributions of the QGP, and once produced, escape without significant interaction [11][12][13][14][15][16][17][18][19][20][21][22][23][24]. For example, if most photons are emitted at high temperature in the QGP, since the flow at early times is small, one would expect a small net elliptic flow for photons.…”

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Production and Elliptic Flow of Dileptons and Photons in a Matrix Model of the Quark-Gluon Plasma

et al. 2015

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We consider the thermal production of dileptons and photons at temperatures above the critical temperature in QCD. We use a model where color excitations are suppressed by a small value of the Polyakov loop, the semi Quark-Gluon Plasma (QGP). Comparing the semi-QGP to the perturbative QGP, we find a mild enhancement of thermal dileptons. In contrast, to leading logarithmic order in weak coupling there are far fewer hard photons from the semi-QGP than the usual QGP. To illustrate the possible effects on photon and dileptons production in heavy ion collisions, we integrate the rate with a realistic hydrodynamic simulation. Dileptons uniformly exhibit a small flow, but the strong suppression of photons in the semi-QGP tends to bias the elliptical flow of photons to that generated in the hadronic phase.PACS numbers: 11.10. Wx, 12.38.Mh, 25.75.Cj, 25.75.Nq The collisions of heavy nuclei at ultra-relativistic energies can be used to investigate the properties of the Quark-Gluon Plasma (QGP). At both the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC), much of the collision takes place at temperatures which are not that far above that for the transition, T c . This is a difficult region to study: perturbative methods can be used at high temperature, but not nearSimilarly, hadronic models are valid at low temperature, but break down near T c [2]. One model of the region above but near T c is the semi-QGP [3][4][5][6]. This incorporates the results of numerical simulations on the lattice [7], which show that colored excitations are strongly suppressed when T → T + c , as the expectation value of the Polyakov loop decreases markedly.A notable property of heavy ion collisions is elliptic flow, how the initial spatial anisotropy of peripheral collisions is transformed into a momentum anisotropy. The large elliptic flow of hadrons can be well modeled by hydrodynamic models in which the QCD medium is close to an ideal fluid [8][9][10].Electromagnetic signals, such as dilepton or photon production, are another valuable probe, since they reflect properties of the quark and gluon distributions of the QGP, and once produced, escape without significant interaction [11][12][13][14][15][16][17][18][19][20][21][22][23][24]. For example, if most photons are emitted at high temperature in the QGP, since the flow at early times is small, one would expect a small net elliptic flow for photons. However, recently both the PHENIX experiment at RHIC [22] and the ALICE experiment at the LHC [23] have found a large elliptic flow for photons, comparable to that of hadrons. This is most puzzling [17,18,24].In this paper we present the results for the thermal production of hard dileptons and photons in the semi-QGP, and compare them with those of the perturbative QGP. Surprisingly, we find a sharp qualitative difference between the two. In the semi-QGP, the production of dileptons is similar between the deconfined and confined phases, while photon production is strongly suppressed near T c . We compute to leading ord...

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Production and Elliptic Flow of Dileptons and Photons in a Matrix Model of the Quark-Gluon Plasma

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“…But such a fireball parametrization gives a simplified picture of the reaction, with one value for temperature T and baryochemical potential μ B in the whole volume. A more nuanced description is delivered by hydrodynamic models [11,12] respectively a hydro+transport hybrid model [13]. The difficulty here is that one usually needs some initial state for the start of the hydrodynamic evolution and additionally some description of the final-state interactions.…”

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Vector Meson Spectral Functions in a Coarse-Graining Approach

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et al. 2015

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Abstract. Dilepton production in heavy-ion collisions at top SPS energy is investigated within a coarse-graining approach that combines an underlying microscopic evolution of the nuclear reaction with the application of medium-modified spectral functions. Extracting local energy and baryon density for a grid of small space-time cells and going to each cell's rest frame enables to determine local temperature and chemical potential by application of an equation of state. This allows for the calculation of thermal dilepton emission. We apply and compare two different spectral functions for the ρ: A hadronic many-body calculation and an approach that uses empirical scattering amplitudes. Quantitatively good agreement of the model calculations with the data from the NA60 collaboration is achieved for both spectral functions, but in detail the hadronic many-body approach leads to a better description, especially of the broadening around the pole mass of the ρ and for the low-mass excess. We further show that the presence of a pion chemical potential significantly influences the dilepton yield.

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