REVIEW
Critical point of QCD at finite T and \mu, lattice results for physical quark masses
High Energy Physics - Lattice · Submitted 8 Feb 2004 · 30 references
Pith's verdict: A real physics update — the endpoint drops from 720 to 360 MeV at physical quark masses — but the reweighting overlap at the endpoint is argued, not measured, and the scale systematic is wider than the quoted error.
T0 · AI review
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Referee report
The paper
The paper claims
The paper locates the QCD critical endpoint at $T_E=162$ MeV and baryon chemical potential $200$ MeV using lattice QCD with physical quark masses.
The reading
What carries the argument
The method is overlap-improving multi-parameter reweighting combined with Lee-Yang zeros. Configurations are generated at $200$ and at the zero-density transition temperature, where importance sampling works; each configuration is then given a Boltzmann weight for nonzero $200$ and for the lower transition temperature at that $200$, with the gauge coupling reweighted by the Ferrenberg-Swendsen procedure. The partition function is continued into the complex $200$ plane, and its Lee-Yang zeros are located. Their finite-volume scaling carries the argument: zeros that stay away from the real axis mean a crossover, zeros that approach it as $1/V$ mean a first-order transition, and the endpoint is the $200$ at which the extrapolated imaginary part vanishes. The physical scale is set from the Sommer scale $R_0$ and the $200$ mass.
Extended reading notes
Core claim
The central claim is that the QCD critical endpoint sits at $T_E=162$ MeV and $200$ MeV for 2+1-flavour QCD with physical quark masses on $L_t=4$ lattices. In lattice units the endpoint is $200$. The evidence comes from the infinite-volume extrapolation of Lee-Yang zeros of the reweighted partition function: at small $200$ the extrapolated imaginary part of the zero, $200$, is inconsistent with zero and the transition is a crossover; as $200$ increases it falls, and at the endpoint the crossover disappears and finite-volume scaling becomes consistent with a first-order transition. The paper also reports a crossover-line curvature $T/T_c=1-C200^2/T_c^2$ with $C=0.0032(1)$. The authors explicitly note that the continuum extrapolation has not been done.
Load-bearing premise
The load-bearing premise is that the reweighting from $200$ ensembles still samples the nonzero-density transition well enough at the endpoint for the Lee-Yang zeros to be reliable; the authors also acknowledge that the continuum extrapolation is still missing.
Editorial extensions
If this is right
- Heavy-ion searches for critical-point signatures can target the region around $T\approx162$ MeV and baryon chemical potential $\approx360$ MeV rather than the much larger $\mu_E\approx700$ MeV suggested by earlier large-mass estimates.
- The crossover line's small curvature, $C=0.0032(1)$, gives a concrete input for the QCD equation of state at small baryon density.
- Because lowering the light quark masses moved $\mu_E$ down from about 720 MeV to 360 MeV, quark-mass effects must be controlled in any comparison between lattice endpoints and experiment.
- The insensitivity to a threefold volume increase supports the reliability of the finite-volume Lee-Yang analysis, leaving the lattice spacing as the main systematic uncertainty.
Reading between the lines
- A quantitative overlap diagnostic at the endpoint, such as the distribution of reweighting weights or an effective sample size, would test the method independently of the statistical error bars on the zeros.
- Agreement between this reweighting result and an imaginary-$\mu$ analytic continuation at the same quark masses would be a strong cross-check, because the two methods fail in different ways.
- Since $T_E$ and $T_c$ differ by only about 2 MeV, within errors, the data do not yet establish whether the endpoint lies exactly on the zero-density transition line or slightly below it.
- Mapping $T$ and $\mu_B$ to heavy-ion collision energies requires a freeze-out model; the endpoint's location is a thermodynamic statement, not by itself a prediction of which beam energy will see critical fluctuations.
Original abstract
A critical point (E) is expected in QCD on the temperature (T) versus baryonic chemical potential (\mu) plane. Using a recently proposed lattice method for \mu \neq 0 we study dynamical QCD with n_f=2+1 staggered quarks of physical masses on L_t=4 lattices. Our result for the critical point is T_E=162 \pm 2 MeV and \mu_E= 360 \pm 40 MeV. For the critical temperature at \mu=0 we obtained T_c=164 \pm 2 MeV. This work extends our previous study [Z. Fodor and S.D.Katz, JHEP 0203 (2002) 014] by two means. It decreases the light quark masses (m_{u,d}) by a factor of three down to their physical values. Furthermore, in order to approach the thermodynamical limit we increase our largest volume by a factor of three. As expected, decreasing m_{u,d} decreased \mu_E. Note, that the continuum extrapolation is still missing
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.
The abstract, sentence by sentence
A critical point (E) is expected in QCD on the temperature (T) versus baryonic chemical potential (\mu) plane. Using a recently proposed lattice method for \mu \neq 0 we study dynamical QCD with n_f=2+1 staggered quarks of physical masses on L_t=4 lattices. Our result for the critical point is T_E=162 \pm 2 MeV and \mu_E= 360 \pm 40 MeV. For the critical temperature at \mu=0 we obtained T_c=164 \pm 2 MeV. This work extends our previous study [Z. Fodor and S.D.Katz, JHEP 0203 (2002) 014] by two means. It decreases the light quark masses (m_{u,d}) by a factor of three down to their physical values. Furthermore, in order to approach the thermodynamical limit we increase our largest volume by a factor of three. As expected, decreasing m_{u,d} decreased \mu_E. Note, that the continuum extrapolation is still missing
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | A critical point (E) is expected in QCD on the temperature (T) versus baryonic chemical potential (\mu) plane. | background 1.00 | 0.05 | 0.07 | hedged possibility 0.88 |
| s1 | Using a recently proposed lattice method for \mu \neq 0 we study dynamical QCD with n_f=2+1 staggered quarks of physical masses on L_t=4 lattices. | method 0.85 | 0.89 | 0.04 | asserted 0.94 |
| s2 | Our result for the critical point is T_E=162 \pm 2 MeV and \mu_E= 360 \pm 40 MeV. | result 1.00 | 0.97 | 0.86 | asserted 1.00 |
| s3 | For the critical temperature at \mu=0 we obtained T_c=164 \pm 2 MeV. | result 1.00 | 0.97 | 0.09 | asserted 1.00 |
| s4 | This work extends our previous study [Z. | contribution 0.58 | 0.43 | 0.02 | asserted 0.82 |
| s5 | Fodor and S.D.Katz, JHEP 0203 (2002) 014] by two means. | background 0.51 | 0.32 | 0.02 | asserted 0.67 |
| s6 | It decreases the light quark masses (m_{u,d}) by a factor of three down to their physical values. | contribution 0.61 | 0.89 | 0.03 | asserted 0.99 |
| s7 | Furthermore, in order to approach the thermodynamical limit we increase our largest volume by a factor of three. | method 0.59 | 0.93 | 0.03 | asserted 0.99 |
| s8 | As expected, decreasing m_{u,d} decreased \mu_E. | interpretation 0.67 | 0.85 | 0.06 | asserted 0.97 |
| s9 | Note, that the continuum extrapolation is still missing | limitation 1.00 | 0.37 | 0.03 | asserted 0.98 |
What was examined, and what was not
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Figures
Figures reproduced from arXiv:hep-lat/0402006 by the authors.
References
How later papers used it
160 later papers cite it
2 later papers by other authors are listed below: 2 used its data or values.
Uses its data or values 2 of 20 papers with this label
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Chose its mixed temperature and chemical potential parameter to reproduce a critical endpoint at μ_c=354 MeV and T_c=167 MeV for symmetric matter and zero net strangeness in accordance with this paper's lattice data.
Chiral Symmetry Restoration and Deconfinement to Quark Matter in Neutron Stars (2009) · 0.98 · paraphrased: the licence does not allow quoting
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Took the position of the critical point in μ (at μ=360 MeV) from this paper's lattice QCD calculations.
Event-by-Event Fluctuations and the Search for the Critical Point within the NA49 Experiment (2009) · 0.92 · paraphrased: the licence does not allow quoting
How the literature uses it
130 arXiv papers cite this in 184 sentences
Papers citing it · 107 sentences without a confident label
Contests it 1 paper
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Our results about the critical surface appear to be in qualitative contradiction with those of [ref] , [ref] , which both conclude for the existence of a critical point $(μ_E,T_E)$ at small chemical potential $μ_E/T_E 1$.
Exploring the QCD phase diagram · Discussion · contest 0.72
Builds on it 1 paper
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Generalization to multi-parameter and joint expansion in $T,$.– Akin to multi-parameter reweighting [ref] in bare gauge coupling, $Δβ=β-β_0$, and quark mass, $Δm=m-m_0$, our resummation scheme also can be extended to obtain $_N(T,)$ starting from a different temperature $T_0(β_0)$ and bare quark mass $m_0$,
Lattice QCD Equation of State for Nonvanishing Chemical Potential by Resumming Taylor Expansion · Introduction · 0.84
Uses its method or tool 8 papers
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In this manuscript we study Lee-Yang zeros, which offer a different approach to the study of the phase diagram [ref] , by means of a two-step extrapolation (see Ref. [ref] for a detailed explanation).
High-precision baryon number cumulants from lattice QCD in a finite box: cumulant ratios, Lee-Yang zeros and critical endpoint predictions · Introduction · 0.80
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Using the multi-parameter reweighting technique [ref] we studied dynamical QCD with $n_f$=2+1 staggered quarks of physical masses on $N_t$=4 lattices [ref] .
Recent Result in QCD Thermodynamics from the Lattice · Results at non-vanishing baryonic chemical potential · 0.97
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In this paper, as in Refs. [ref] , we adopt the following prescription.
Lattice simulations of the QCD chiral transition at real baryon density · B.1. Staggered rooting at finite chemical potential · 0.99
Uses its data or values 17 papers
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There is as yet no complete agreement as to the location of the quark matter critical endpoint, as obtained from lattice studies: in [ref] one finds a discussion of various proposed points in the phase diagram, such as one (see [ref] ) with coordinates $(μ_B^CEP/T^CEP, T^CEP/T^C_0) ≈(1.68, 0.94)$, where $T^C_0$ is the crossover temperature at $μ_B = 0$, for which we follow [ref] and take the value $≈$ 150 MeV; but also another with coordinates $(μ_B^CEP/T^CEP, T^CEP/T^C_0) ≈(2.2, 0.99)$ (see [ref] ).
The Trajectory of the Cosmic Plasma Through the Quark Matter Phase Diagram · Section · 0.80
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The lattice data includes a first order phase transition at $T=270$ MeV and a pressure functional P(T) similar to Refs. [ref] at $μ=0$ for pure gauge, a crossover at vanishing chemical potential with a transition temperature of $171$ MeV (determined as the peak of the change of the chiral condensate and $Φ$) and the location of the critical end-point (at $μ_c=354$ MeV , $T_c=167$ MeV for symmetric matter in accordance with one of the existent calculations [ref] ).
A Novel Approach to Model Hybrid Stars · Introduction · 0.97
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The case of physical quark masses, after conversion to continuum units, is shown in [ref] (right) [ref] .
Status of Lattice Studies of the QCD Phase Diagram · Calculations at finite density · 0.92
Compares with it 9 papers
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As can be seen in that Figure, the leading Fisher zero extrapolates to a point on the real line at around $μ_B/T=2.4$, in agreement with the result of Ref. [ref] .
New approach to lattice QCD at finite density: reweighting without an overlap problem · Unimproved action at $N_τ=4$ · 0.99
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The obtained expression is in excellent agreement with numerical computations [ref] .
Phase Transitions, theta Behavior and Instantons in QCD and its Holographic Model · conclusion · 0.96
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Indeed, this must happen if one wishes to reconcile our result with that of Fodor and Katz [ref] , whose critical point Fig. [ref] lies at $μ/T ∼0.7$ for mass parameters similar to eq.
Simulating QCD at finite density · The curvature of the critical surface · 0.94
Cites it as support 1 paper
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In particular, it includes: $∙$ lattice computations of topological susceptibility in the vicinity of the phase transition [ref] ; $∙$ lattice computations of the critical temperature $T_c(μ)$ as a function of $μ$ at small $μ$ [ref] ; $∙$ analysis of the holographic models of QCD in vicinity of the phase transition at nonzero temperature [ref] .
Confinement- Deconfinement Phase Transition in Hot and Dense QCD at Large N · Confinement- Deconfinement Phase Transition in hot QCD at large $N$. · 1.00
Cites it as background 39 papers
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A number of approaches have been proposed to side-step this problem, such as reweighting [ref] , Taylor expansion around zero chemical potential [ref] , analytic continuation from purely imaginary chemical potentials [ref] , or the complex Langevin approach [ref] .
New approach to lattice QCD at finite density: reweighting without an overlap problem · Introduction · 0.98
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It is well known that at vanishing chemical potentials, QCD exhibits a cross-over phase transition [ref] near the pseudo-critical temperature $T_c∼155$ MeV, passing gradually from hadronic degrees of freedom below $T_c$ to quarks and gluons above.
Magnetic susceptibility of a hot hadronic medium and quark degrees of freedom near the QCD cross-over point · Introduction · 0.90
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Though there have been attempts in extracting information about the location of the supposed CEP from lattice simulations [ref] , these attempts face great difficulties, as extrapolation-type methods have the property that they give reliable results mostly in the immediate vicinity of $μ_B=0$.
The QCD crossover at finite chemical potential from lattice simulations · Introduction · 0.91
Discussion
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