Remarks on transient effects for photon production in heavy ion collisions

Fraga, Eduardo S.; Gelis, François; Schiff, D.

doi:10.1103/physrevd.71.085015

Cited by 9 publications

(25 citation statements)

References 32 publications

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“…The result was that there are important transient effects that make the yield much larger than what would have been expected by simply multiplying the equilibrium rates by the corresponding amount of time. This unexpectedly large photon yield was the starting point of many discussions regarding the validity of this approach [3,4].In this note we show in a systematic and simple way that the expression of the photon yield obtained in [2] relies on unphysical assumptions. We show that the standard canonical formalism in the S-matrix approach leads to this expression, provided the electromagnetic interactions are unduly turned on and off at finite initial and final times.…”

mentioning

confidence: 96%

“…The simplicity of this derivation allows us to exhibit the illegitimate character of the expression used to predict the transient effects (For details, see Ref. [4]). …”

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confidence: 99%

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confidence: 99%

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Transient photon production in a QGP

Fraga¹

2004

AIP Conference Proceedings

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Abstract.We discuss the shortcomings of a formula that has been used in the literature to compute the number of photons emitted by a hot or dense system during a finite time, and show that the transient effects it predicts for the photon rate are unphysical.Electromagnetic radiation has long been thought to be a good probe of the early stages of heavy ion collisions [1]. In recent years, a new "real-time" approach has been proposed in order to compute out-of-equilibrium effects for photon production in a dense equilibrated quark-gluon system, which originate in its finite life-time [2]. The result was that there are important transient effects that make the yield much larger than what would have been expected by simply multiplying the equilibrium rates by the corresponding amount of time. This unexpectedly large photon yield was the starting point of many discussions regarding the validity of this approach [3,4].In this note we show in a systematic and simple way that the expression of the photon yield obtained in [2] relies on unphysical assumptions. We show that the standard canonical formalism in the S-matrix approach leads to this expression, provided the electromagnetic interactions are unduly turned on and off at finite initial and final times. The simplicity of this derivation allows us to exhibit the illegitimate character of the expression used to predict the transient effects (For details, see Ref. [4]).Let us consider a system of quarks and gluons, and denote by L QCD its Lagrangian. We couple the quarks to the electromagnetic field in order to study photon emission by this system, and denote L e.m. the Lagrangian of the electromagnetic field, and L qγ the term that couples the quarks to the photons. The complete Lagrangian is thereforeHere G µ and G a µν are respectively the gluon field and field strength, A µ and F µν the photon field and field strength, and ψ the quark field (only one flavor is considered here). g is the strong coupling constant, and e is the quark electric charge. We denote collectively by L int the sum of all the interaction terms. The number of photons measured in the system at some late time is given by 1 Presented at the Hadron Physics

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confidence: 96%

“…The simplicity of this derivation allows us to exhibit the illegitimate character of the expression used to predict the transient effects (For details, see Ref. [4]). …”

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confidence: 99%

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Transient photon production in a QGP

Fraga¹

2004

AIP Conference Proceedings

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“…Recently a criticism has been leveled at the real time formulation for studying photon production from a transient QGP [37] that we propose. The authors in this reference claim that the unitary time evolution operator U (t, t i ) in the interaction picture in eq.…”

Section: A Beyond Htlmentioning

confidence: 99%

“…[37] do not attempt to scrutinize the usual assumptions with their glaring inconsistencies, nor do they provide a reliable framework to study the physical situation at hand, vi) the non-perturbative aspects associated with the propagation of photons in the medium (see discussion below) are not included in ref. [37], which our formulation in terms of the real time effective action describes consistently and systematically. Hence, these criticisms not only do not help to understand (or even address) the important and very relevant physical questions but disguise these fundamental aspects of URHIC physics.…”

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confidence: 99%

Photon production from a thermalized quark–gluon plasma: quantum kinetics and non-perturbative aspects

Boyanovsky

Vega

2005

Nuclear Physics A

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We study the production of photons from a quark gluon plasma in local thermal equilibrium by introducing a non-perturbative formulation of the real time evolution of the density matrix. The main ingredient is the real time effective action for the electromagnetic field to O(αem) and to all orders in αs. The real time evolution is completely determined by the solution of a classical stochastic non-local Langevin equation which provides a Dyson-like resummation of the perturbative expansion. The Langevin equation is solved in closed form by Laplace transform in terms of the thermal photon polarization. A quantum kinetic description emerges directly from this formulation. We find that photons with k 200 Mev thermalize as plasmon quasiparticles in the plasma on time scales t ∼ 10 − 20 fm/c which is of the order of the lifetime of the QGP expected at RHIC and LHC. We then obtain the direct photon yield to lowest order in αem and to leading logarithmic order in αs in a uniform expansion valid at all time. The yield during a QGP lifetime t ∼ 10 fm/c is systematically larger than that obtained with the equilibrium formulation and the spectrum features a distinct flattening for k 2.5 Gev. We discuss the window of reliability of our results, the theoretical uncertainties in any treatment of photon emission from a QGP in LTE and the shortcomings of the customary S-matrix approach.

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Off-equilibrium photon production during the chiral phase transition

et al. 2013

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In the early stage of ultrarelativistic heavy-ion collisions chiral symmetry is restored temporarily. During this so-called chiral phase transition, the quark masses change from their constituent to their bare values. This mass shift leads to the spontaneous non-perturbative creation of quark-antiquark pairs, which effectively contributes to the formation of the quark-gluon plasma. We investigate the photon production induced by this creation process. We provide an approach that eliminates possible unphysical contributions from the vacuum polarization and renders the resulting photon spectra integrable inthe ultraviolet domain. The off-equilibrium photon numbers are of quadratic order in the perturbative coupling constants while a thermal production is only of quartic order. Quantitatively, we find, however, that for the most physical mass-shift scenarios and for photon momenta larger than 1 GeV the off-equilibrium processes contribute less photons than the thermal processes.Comment: 70 pages, 29 figures, added reference

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Remarks on transient effects for photon production in heavy ion collisions

Cited by 9 publications

References 32 publications

Transient photon production in a QGP

Transient photon production in a QGP

Photon production from a thermalized quark–gluon plasma: quantum kinetics and non-perturbative aspects

Off-equilibrium photon production during the chiral phase transition

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