REVIEW
Chiral Symmetry Restoration and $Z_N$ Symmetry
High Energy Physics - Lattice · Submitted 7 Dec 1995 · 12 references
Pith's verdict: The Z_3 sign mechanism is correct and cheap; the quantitative model around it is standard NJL scaffolding whose strength depends on an externally imposed, uncomputed Polyakov loop.
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Referee report
The paper
The paper claims
Chiral symmetry restoration in hot QCD depends on which $Z_3$ phase the Polyakov loop sits in, because the leading finite-temperature correction to the chiral effective potential changes sign with the real part of the colour-traced loop.
The reading
What carries the argument
The load-bearing device is the image expansion of the finite-temperature quark determinant, $V(sigma, A_0, T) = sum_f [2Gsigma_f^2 - 2N_c int d^3k,(2pi)^{-3} omega_{fvec{k}} - 2beta sum_{n=1}^{infty} (-1)^n/n, trit{c}(mathcal{P}^n + mathcal{P}^{dagger n}) int d^3k,(2pi)^{-3} e^{-nbetaomega_{fvec{k}}}]$, in which each order $n$ counts quark paths winding $n$ times around the Euclidean time direction. Truncating at $n=1$ gives a modified Bessel function $K_2(M/T)$ multiplying $trit{c}(mathcal{P} + mathcal{P}^dagger)$, which makes the sign dependence transparent. To obtain a closed form for numerical work the authors further approximate $trit{c}(mathcal{P}^n) approx N_c (trit{c}mathcal{P}/N_c)^n$ and regulate with a non-covariant cut-off $Lambda$, resumming the series into a logarithmic Fermi-style factor containing $varphi + varphi^*$ and $|varphi|^2/N_c^2$.
Extended reading notes
Core claim
The paper claims that the sign of the leading finite-temperature correction to the effective potential for the chiral condensate is controlled by the sign of $trit{c}(mathcal{P} + mathcal{P}^dagger)$, and that this single fact explains the phase dependence seen in quenched lattice simulations. Working in a chirally invariant Nambu-Jona-Lasinio model with $U(N_f) times U(N_c)$ symmetry, the authors couple the quarks to a uniform, temperature-dependent temporal gauge field $A_0$ and compute the effective potential $V(sigma, A_0, T)$ in the Hartree approximation. Expanding the quark determinant as a sum over winding numbers, the $n$-th term is weighted by $trit{c}(mathcal{P}^n + mathcal{P}^{dagger n})$; the $n=1$ term dominates when $T ll M$ and carries the full sign dependence. In the phase where $trit{c}(mathcal{P})$ is real and positive, that term raises the massive minimum relative to the massless one, so the first-order deconfining transition induces chiral symmetry restoration, as simulations find. In the two phases where $mathrm{Re}[trit{c}(mathcal{P})] < 0$, the correction changes sign and instead deepens the broken minimum, so chiral symmetry is not restored at deconfinement; the authors report this agrees with the Chandrasekharan-Christ simulations. For $SU(N)$ the same argument yields $N+1$ distinct chiral behaviors, with only the real-positive phase behaving conventionally.
Load-bearing premise
The Polyakov loop is not computed but imposed by hand as a step function that is zero below the deconfinement temperature and jumps to an adjustable magnitude, so the model cannot predict where the chiral transition sits relative to deconfinement and the agreement with lattice data is a sign check rather than a quantitative test.
Editorial extensions
If this is right
- The chiral and deconfining transitions of quenched QCD are coupled through the $Z_3$ phase of the Polyakov loop, so the chiral condensate is not a function of temperature alone.
- In the two phases with $mathrm{Re}[trit{c}(mathcal{P})] < 0$, chiral symmetry survives to temperatures well above the critical temperature of the real-positive phase.
- For $SU(N)$ gauge theory there are $N+1$ distinct patterns of chiral behavior, with phases related by complex conjugation behaving identically.
- In full QCD, where quark effects lift the $Z_N$ degeneracy, the persistence of chiral symmetry breaking in the metastable phases should lower the bounce action and lengthen the lifetime of those metastable states.
Reading between the lines
- The mechanism suggests a sharper lattice test than the authors perform: measure the chiral condensate as a function of temperature separately in each $Z_3$ phase and check whether the negative-real phases show a chiral transition temperature measurably above the real phase's $T_c$, not merely a suppressed condensate.
- Because the sign flip lives in the $n=1$ winding term, the effect should weaken as the constituent mass grows relative to temperature; a heavy-quark regime ought to show the phase dependence fading, which is a checkable trend rather than a binary.
- If chiral symmetry breaking really persists in the metastable $Z_3$ phases of full QCD, then those phases carry a different equation of state, which in principle leaves a signature in the early-universe QCD transition and in heavy-ion fluctuation observables sensitive to the Polyakov loop.
- The same winding-sign argument should apply to any theory where a fermion determinant couples to a centre-symmetry phase, so analogous phase-dependent chiral behavior may be worth looking for in QCD-like theories with adjoint or two-index fermions.
Original abstract
We demonstrate that chiral symmetry restoration in quenched finite temperature QCD depends crucially on the $Z_3$ phase of the Polyakov loop ${\cal P}$. This dependence is a general consequence of the coupling of the chiral order parameter to the Polyakov loop. We construct a model for chiral symmetry breaking and restoration which includes the effect of a nontrivial Polyakov loop by calculating the effective potential for the chiral condensate of a Nambu-Jona-Lasinio model in a uniform temperature dependent $A_0$ gauge field background. Above the deconfinement temperature there are three possible phases corresponding to the $Z_3$ symmetric phases of the Polyakov loop in the pure gauge theory. In the phase in which ${\rm tr_c}({\cal P})$ is real and positive the first order deconfining transition induces chiral symmetry restoration in agreement with simulation results. In the two phases where $Re[{\rm tr_c}({\cal P})] < 0$ the sign of the leading finite temperature correction to the effective potential is reversed from the normal phase, and chiral symmetry is not restored at the deconfinement transition; this agrees with the recent simulation studies of Chandrasekharan and Christ. In the case of $SU(N)$ a rich set of possibilites emerges. The generality of the mechanism makes it likely to occur in full QCD as well; this will increase the lifetimes of metastable $Z_3$ phases.
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
We demonstrate that chiral symmetry restoration in quenched finite temperature QCD depends crucially on the $Z_3$ phase of the Polyakov loop ${\cal P}$. This dependence is a general consequence of the coupling of the chiral order parameter to the Polyakov loop. We construct a model for chiral symmetry breaking and restoration which includes the effect of a nontrivial Polyakov loop by calculating the effective potential for the chiral condensate of a Nambu-Jona-Lasinio model in a uniform temperature dependent $A_0$ gauge field background. Above the deconfinement temperature there are three possible phases corresponding to the $Z_3$ symmetric phases of the Polyakov loop in the pure gauge theory. In the phase in which ${\rm tr_c}({\cal P})$ is real and positive the first order deconfining transition induces chiral symmetry restoration in agreement with simulation results. In the two phases where $Re[{\rm tr_c}({\cal P})] < 0$ the sign of the leading finite temperature correction to the effective potential is reversed from the normal phase, and chiral symmetry is not restored at the deconfinement transition; this agrees with the recent simulation studies of Chandrasekharan and Christ. In the case of $SU(N)$ a rich set of possibilites emerges. The generality of the mechanism makes it likely to occur in full QCD as well; this will increase the lifetimes of metastable $Z_3$ phases.
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | We demonstrate that chiral symmetry restoration in quenched finite temperature QCD depends crucially on the $Z_3$ phase of the Polyakov loop ${\cal P}$. | result 0.86 | 0.91 | 0.86 | asserted 1.00 |
| s1 | This dependence is a general consequence of the coupling of the chiral order parameter to the Polyakov loop. | interpretation 0.89 | 0.34 | 0.20 | asserted 1.00 |
| s2 | We construct a model for chiral symmetry breaking and restoration which includes the effect of a nontrivial Polyakov loop by calculating the effective potential for the chiral condensate of a Nambu-Jona-Lasinio model in a uniform temperature dependent $A_0$ gauge field background. | method 0.86 | 0.96 | 0.06 | asserted 0.99 |
| s3 | Above the deconfinement temperature there are three possible phases corresponding to the $Z_3$ symmetric phases of the Polyakov loop in the pure gauge theory. | result 0.86 | 0.49 | 0.08 | asserted 0.96 |
| s4 | In the phase in which ${\rm tr_c}({\cal P})$ is real and positive the first order deconfining transition induces chiral symmetry restoration in agreement with simulation results. | result 1.00 | 0.71 | 0.20 | asserted 0.98 |
| s5 | In the two phases where $Re[{\rm tr_c}({\cal P})] < 0$ the sign of the leading finite temperature correction to the effective potential is reversed from the normal phase, and chiral symmetry is not restored at the deconfinement transition; this agrees with the recent simulation studies of Chandrasekharan and Christ. | result 0.99 | 0.50 | 0.15 | asserted 0.99 |
| s6 | In the case of $SU(N)$ a rich set of possibilites emerges. | result 0.54 | 0.36 | 0.03 | asserted 0.90 |
| s7 | The generality of the mechanism makes it likely to occur in full QCD as well; this will increase the lifetimes of metastable $Z_3$ phases. | interpretation 0.99 | 0.16 | 0.12 | hedged possibility 0.99 |
What was examined, and what was not
- arXiv listing abstract v1
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- P abstract
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Figures
Figures reproduced from arXiv:hep-lat/9512011 by the authors.
References
How the literature uses it
39 arXiv papers cite this in 43 sentences
Papers citing it · 23 sentences without a confident label
Uses its method or tool 4 papers
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The first one is the Chiral Mean Field (CMF) model [ref] and the second one is the Polyakov-loop extended Nambu–Jona-Lasinio (PNJL) model [ref] , both of which will be described in the next section, after discussing different conditions related to conservation laws and different kinds of equilibria.
Temperature and Strong Magnetic Field Effects in Dense Matter · Introduction · 0.90
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To describe strongly interacting matter we adopt the framework of the Polyakov-Quark-Meson model [ref] that we enhance by applying the unquenched Polyakov-loop potential [ref] .
Thermodynamics of (2+1)-flavor strongly interacting matter at nonzero isospin · Introduction · 0.92
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In our calculations, we use the Nambu-Jona Lasinio model with the Polyakov loop (PNJL model in what follows) [ref] as an effective model of QCD.
Probing the QCD vacuum with an abelian chromomagnetic field: A study within an effective model · Introduction · 0.97
Cites it as background 16 papers
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At low energy effective theory, the mesons are treated as Goldstone boson of the broken chiral $SU(3)_L ×SU(3)_R$ symmetry, in that way some phenomenological relation of meson masses can be obtained [ref] .
On the masses of light pseudoscalar mesons · Introduction · 0.87
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QCD phase diagram has been studied in QCD inspired models, e.g. instanton models, linear sigma model, Nambu-Jona-Lasinio (NJL) or Polyakov loop extended Nambu-Jona-Lasinio models [ref] , [ref] , [ref] , [ref] , [ref] , [ref] .
The phase diagram of QCD in a BAG+HRG based equation of state: appearance of a pseudo-critical point · Introduction · 0.99
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Significant efforts have been put in for building effective models with Polyakov loops taken as order parameters for a spontaneous Z(3) symmetry breaking using Landau type of polynomial potentials [ref] .
Consistent approach to study gluon quasi-particles · Introduction · 0.94
Discussion
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