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Static quark anti-quark free energy and the running coupling at finite temperature

High Energy Physics - Lattice · Submitted 23 Jun 2004 · 24 references

Pith's verdict: Real lattice data and a quotable r_screen scaling law; the confinement-remnant headline is a gloss on a gauge-fixed correlator, not a derived result.

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arXiv IDhep-lat/0406036 (version 1)
Pith IDV3KJ3KJF, Pith's permanent record of this paper and its review
CitationF. Karsch, F. Zantow, O. Kaczmarek, P. Petreczky. "Static quark anti-quark free energy and the running coupling at finite temperature." arXiv:hep-lat/0406036 (2004). https://pith.science/paper/V3KJ3KJF
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Referee report
The paper

The paper claims

The quark-antiquark force in a hot gluon plasma keeps its zero-temperature strength at short distances and only screens beyond a temperature-set length scale.

The reading

This paper asks how the force between a static quark and antiquark changes when the QCD vacuum is replaced by a gluon plasma, and which length scale controls the force's strength at each distance. Working in quenched SU(3) lattice gauge theory at temperatures from just above $T_c$ up to $12T_c$, the authors extract running couplings from the color-singlet free energy and compare them with the zero-temperature heavy quark potential and with two-loop perturbation theory. They find that below roughly $0.1$ fm the coupling is indistinguishable from its zero-temperature value for temperatures up to about $3T_c$, and that the crossover to screened, temperature-dominated behavior happens at a distance that shrinks inversely with temperature, $r_{screen} = (0.48 +- 0.01)$ fm $T_c/T$. The interpretive payoff is that the growth of the coupling at intermediate distances near $T_c$ is a leftover of the confining force rather than a thermally strengthened Coulomb interaction. That matters because it undercuts a picture in which the near-$T_c$ plasma hosts many Coulombically bound heavy-quark states.

What carries the argument

The load-bearing object is the color-singlet heavy quark free energy $F_1(r,T)$, obtained from the Coulomb-gauge-fixed Polyakov loop correlator $Tr L(x) L^†(0)$ and normalized by matching its short-distance part to the zero-temperature heavy quark potential. Two derived quantities carry the argument: the potential-scheme coupling $α_V = -(3r/4)F_1$, which is sensitive to the sign change that a confining linear term forces, and the force-scheme coupling $α_{qq} = (3r^2/4) dF_1/dr$, which stays positive and whose maximum locates the screening length. A lattice-spacing correction replaces the separation $r$ by $r_I$ defined through the Fourier transform of the tree-level improved gluon propagator, which removes most of the rotational-symmetry breaking at short distances and is what makes the $r < 0.1$ fm comparison with perturbation theory meaningful.

Extended reading notes

Core claim

The authors compute the color-singlet free energy $F_1(r,T)$ of a static quark-antiquark pair in quenched QCD on $32^3 x N_τ$ lattices with $N_τ = 4, 8, 16$ and a tree-level Symanzik-improved gauge action, renormalizing it by matching the short-distance part to the zero-temperature heavy quark potential. From it they build two couplings, $α_V(r,T) = -(3r/4) F_1(r,T)$ and $α_{qq}(r,T) = (3r^2/4) dF_1(r,T)/dr$, and find that for $r <~ 0.1$ fm both agree with the zero-temperature lattice potential and with the two-loop perturbative force, with no visible temperature dependence up to $T ≃ 3T_c$; above that, thermal effects reach even into the short-distance regime. Beyond $r ≃ 0.1$ fm non-perturbative physics takes over: $α_V$ changes sign for $T_c <= T <~ 3T_c$, exactly as a linearly rising confining potential would force it to, while $α_{qq}$ rises and then peaks. The location of that peak defines the screening scale, and across the whole temperature range it obeys $r_{screen} = (0.48 +- 0.01)$ fm $T_c/T$. At large distances the authors fit $F_1$ with a Debye-screened Coulomb form containing two independent couplings, $α(T)$ and $ã(T)$; their ratio approaches one only for $T >~ 6T_c$ and reaches about five near $T_c$, so a temperature-dependent coupling is badly scheme dependent for $T_c <= T <~ 2T_c$. For $T >= 6T_c$ a single rescaled two-loop coupling, $α(T) = 2.095(82) α_{pert}(T)$, describes both the Coulomb strength and the screening mass. The conclusion the authors draw is that the near-$T_c$ plasma is better described by remnants of confinement than by a strong Coulombic force.

Load-bearing premise

The argument depends on the Coulomb-gauge-fixed Polyakov loop correlator being a faithful stand-in for a physical, gauge-invariant singlet free energy at the distances and temperatures studied, since if that identification only approximately holds, the extracted coupling describes a gauge-dependent quantity rather than the plasma.

Editorial extensions

If this is right

  • Short-distance observables at separations below about $0.1$ fm in a quark-gluon plasma up to $3T_c$ can be computed with vacuum QCD input, without thermal corrections to the coupling.
  • The relation $r_{screen} = (0.48 +- 0.01)$ fm $T_c/T$ gives a concrete criterion for when medium effects begin to modify a process of a given spatial extent.
  • For $T >= 6T_c$, a single constant rescaling of the two-loop running coupling describes both the Coulomb strength and the screening mass extracted from the singlet free energy, so higher-order and non-perturbative corrections appear as an overall factor there.
  • Near $T_c$, models that trace the plasma's non-perturbative behavior to a large Coulomb coupling are attributing the effect to the wrong source; the relevant physics is residual confinement at intermediate distances.
  • Any quoted temperature-dependent coupling for $T_c <= T <~ 2T_c$ is scheme dependent at the level of a factor of several, so such couplings should not be transported between different observables without conversion.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • If the remnant-confinement reading is right, fitting $F_1$ near $T_c$ with a linear-plus-Coulomb form should yield an effective string tension that decreases smoothly with temperature and vanishes only gradually, which is a check the paper does not perform directly.
  • The whole short-distance result rests on a gauge-fixed correlator standing in for a gauge-invariant singlet free energy; a cross-check with a genuinely gauge-invariant dressed-Polyakov-loop operator, or in a different gauge, would test whether the reported temperature independence of the slope is physical rather than an artifact of the Coulomb-gauge choice.
  • For quarkonium phenomenology this implies a specific ordering: the Coulombic binding of a heavy quark pair should be essentially unmodified until the screening length falls below the bound-state radius, so medium modification of the lowest-lying states should set in at a calculable temperature rather than gradually from $T_c$.
  • Because the ratio $α(T)/ã(T)$ varies from about five near $T_c$ to one near $6T_c$, anyone extracting a Debye mass or a plasma coupling from lattice free energies in the intermediate temperature range should report which of the two fitted parameters they mean; tabulating the ratio as a function of temperature would make the ambiguity usable rather than merely noted.
Original abstract

We analyze the free energy of a static quark anti-quark pair in quenched QCD at short and large distances. From this we deduce running couplings, g^2(r,T), and determine the length scale that separates at high temperature the short distance perturbative regime from the large distance non-perturbative regime in the QCD plasma phase. Ambiguities in the definition of a coupling beyond the perturbative regime are discussed in their relation to phenomenological considerations on heavy quark bound states in the quark gluon plasma. Our analysis suggests that it is more appropriate to characterize the non-perturbative properties of the QCD plasma phase close to T_c in terms remnants of the confinement part of the QCD force rather than a strong Coulombic force.

The compiled record

The paper's own sentences, arranged, kept beside the review and separate from it. Each label is drawn from a closed vocabulary; the number beside it is the probability the classifier gave that label. Numbers in the tables are the authors', checked by code. No sentence in this record was written by a model.

Compiled by classifier-1.13-20260917 via OpenRouter, question bank abstract-p1-20260923, 2026-09-23: 1 request, 3,717 input tokens, $0.0002 reported.

What the abstract itself states, as probabilities: claims first 0.03 · states limitation 0.67 · human participants 0.01 · controlled comparison 0.04 · randomized assignment 0.01 · states quantitative result 0.08 · compares with prior results 0.13 · mentions code or data availability 0.01.

The abstract, sentence by sentence

arXiv listing abstract v1. Results and the main claim are underlined; .

We analyze the free energy of a static quark anti-quark pair in quenched QCD at short and large distances. From this we deduce running couplings, g^2(r,T), and determine the length scale that separates at high temperature the short distance perturbative regime from the large distance non-perturbative regime in the QCD plasma phase. Ambiguities in the definition of a coupling beyond the perturbative regime are discussed in their relation to phenomenological considerations on heavy quark bound states in the quark gluon plasma. Our analysis suggests that it is more appropriate to characterize the non-perturbative properties of the QCD plasma phase close to T_c in terms remnants of the confinement part of the QCD force rather than a strong Coulombic force.

background contribution method result interpretation main claim

What the classifier reports for each sentence. The number is the probability it gave the label.
#sentencerolethe authors' own workstates the main conclusionstrength
s0 We analyze the free energy of a static quark anti-quark pair in quenched QCD at short and large distances. contribution 0.99 0.94 0.03 asserted 0.89
s1 From this we deduce running couplings, g^2(r,T), and determine the length scale that separates at high temperature the short distance perturbative regime from the large distance non-perturbative regime in the QCD plasma phase. result 0.73 0.96 0.15 asserted 1.00
s2 Ambiguities in the definition of a coupling beyond the perturbative regime are discussed in their relation to phenomenological considerations on heavy quark bound states in the quark gluon plasma. interpretation 0.45 0.42 0.04 unclear 0.54
s3 Our analysis suggests that it is more appropriate to characterize the non-perturbative properties of the QCD plasma phase close to T_c in terms remnants of the confinement part of the QCD force rather than a strong Coulombic force. interpretation 0.99 0.86 0.72 hedged possibility 1.00

What kind of paper the classifier reports this is: simulation study 0.89 · evidence from simulation or computation 0.87.

What was examined, and what was not

sources read
  • arXiv listing abstract v1
not examined
  • full text
  • figures and tables
  • references and citing sentences
  • relations to other papers
levels run
  • P abstract
not run
  • S sections
  • T body sentences
  • C citations
  • R cross-paper relations

Every quoted sentence on this record is the authors'. Every label is from Pith's closed vocabulary, with the probability shown beside it. Every number in the measurements table is the authors', checked by code. Record as JSON

Figures
Figure 1. FIG. 1: Heavy quark free energies in the singlet channel cal
Figure 2. FIG. 2: The running coupling in the
Figure 4. FIG. 4: The location of the maximum in
Figure 5. FIG. 5: The singlet free energy versus

Showing 4 of the paper's 5 figures. See all figures in the PDF ↗ Figures reproduced from arXiv:hep-lat/0406036 by the authors.

References
How later papers used it

51 later papers cite it

Evidence: substantial. 40 of the citing papers share no author with it, and 24 of those carry a confident label from Pith's citation classifier.

Pith read 25 of the 85 citing sentences, choosing only those whose citation markers point at this paper alone, and wrote the paraphrases from them. The summary, the counts, the evidence level, the links and the quotes come from code; the summary counts the papers listed below. A later paper is listed under a label when Pith's citation classifier gives one of its sentences that label at probability 0.8 or more (0.95 for support), Pith's reading finds that the citing paper itself did it, and a second check confirms the paraphrase against the sentence. This section lists uses only. It does not list criticism or disagreement, so their absence here says nothing about how the paper has been received. Papers that share an author with this one are listed apart, under "By the same authors". A quote is the citing authors' own sentence, shown when their paper's licence is CC BY, CC BY-SA or CC0. JSON

Uses its data or values 8 of 14 papers with this label · 2 by the same authors

By the same authors

Cites it as support 1 paper
How the literature uses it

47 arXiv papers cite this in 80 sentences

Each citing sentence is taken from the citing paper's LaTeX source and typed by Pith's classifier. A label appears at probability 0.8 or more (0.95 for support), a contest at 0.7 or more; the quoted words are the citing authors'. How each label was checked · JSON

Papers citing it · 36 sentences without a confident label

Builds on it 1 paper
  • Following [ref] we can generalize this approach to the free energy and singlet free energy and obtain the corresponding effective couplings.

    Color screening in (2+1)-flavor QCD · The effective coupling · 0.94

Uses its method or tool 4 papers
  • In this expression, $μ= G(ξ,θ) m_D$ [ref] is the anisotropic Debye mass, where $G$ is a function which depends on the degree of plasma momentum-space anisotropy $ξ$, the angle of the line connecting the quark-antiquark pair with respect to the beamline direction $θ$, and the isotropic Debye mass $m_D = 1.4 √(1+N_f/6) g_s T$ [ref] .[In the limit $ξ→0$, one has $G = 1$ and the real part of the potential above reduces to the internal energy derived from the original Karsch-Mehr-Satz potential [ref] .].

    Using bottomonium production as a tomographic probe of the quark-gluon plasma · Methodology · 0.87

  • According to the definitions provided in lattice QCD [ref] , the running coupling constant is defined in the so-called qq-scheme as [equation] For Figs. [ref], [ref] (a), [ref] (a), and [ref] (a) of different systems at varying temperatures with $μ=0$, we observed that before reaching the critical temperature of each system, the running coupling constant increases with the quark separation distance.

    The Potential Energy of Heavy Quarkonium in Flavor-Dependent Systems from a Holographic Model · Running coupling constant · 0.81

  • Similarly to papers [ref] we fix the renormalization constant $c'(μ)$ through the matching condition for $Ω_1(r, μ)$ at short distances to the short distance behavior of the interaction potential $V(r)$.

    Lattice study of static quark-antiquark interactions in dense quark matter · The grand potential of a static quark-antiquark pair in dense quark matter · 0.90

Uses its data or values 13 papers
Compares with it 9 papers
Cites it as support 1 paper
Cites it as background 10 papers
  • After proper renormalization the expectation value of the renormalized Polyakov loop is related to the free energy, $F_Q$, of a static quark [ref] [equation] The renormalized Polyakov loop, or equivalently the free energy of a static charge $F_Q$ has been studied in SU(N) gauge theories in a wide temperature interval [ref] .

    Polyakov loop in 2+1 flavor QCD from low to high temperatures · Introduction · 0.85

  • In the literature there are several ways to fix this constant [ref] .

    QCD Thermodynamics · The renormalized static potential in the continuum limit · 0.84

  • A related topic of investigation on the lattice consists of calculating the quark-antiquark potential as a function of temperature [ref] .

    Toward Large N Thermal QCD from Dual Gravity: The Heavy Quarkonium Potential · Introduction · 0.94

Discussion

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