REVIEW
Fractional anomalous determinants and the chiral phase transition
High Energy Physics - Phenomenology · Submitted 30 Sep 2026 · 173 references
Pith's verdict: A conjectural but honest paper from Pisarski: fractional anomalous determinants below T_chi give a new route to a generically second-order chiral transition and a concrete lattice test, but the step that makes it all work (theta/N_c scaling in four dimensions) is assumed and deferred to an unpublished communication.
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Referee report
The paper
The paper claims
This paper claims that below the chiral transition temperature the topological charge is fractional, an integer times 1/N_c, so the effective Lagrangian acquires fractional powers of the anomalous determinant, (det Φ)^(j/N_c), making the…
The reading
What carries the argument
The central object is the fractional anomalous determinant (det Φ)^(j/N_c), where Φ ~ q̄_L q_R is the N_f × N_f linear chiral field and N_c is the number of colors; it carries definite charge under axial U(1)_A rotations because its prefactors F_j depend only on Φ†Φ. Its appearance below T_χ rests on two premises: fractional instantons with topological charge Q = integer × 1/N_c dominate the broken phase, and in the Veneziano limit of large N_f and N_c the free energy is a function of θ/N_c (Eq. 21). Supporting evidence comes from femto-slab and femto-torus studies, where monopole-instantons and Z(N_c) vortices carry charge 1/N_c and yield powers (det U)^(1/N_c), and from lattice measurements of the θ-dependence coefficient b_2 consistent with a dense liquid of fractional instantons. The paper also proposes a baryon-number operator in the symmetric phase built from Ũ = Φ/√(Φ†Φ), which imports an earlier analysis of the transition in the T–θ_B plane into the linear $\sigma$ model.
Extended reading notes
Core claim
In the phase with broken chiral symmetry the complete anomalous Lagrangian is proposed to be L_anom = Σ_j F_j(Φ†Φ)[(det Φ)^(j/N_c) + (det Φ†)^(j/N_c)], a sum over all instanton charges, fractional and integral. Because Φ is a linear, complex field, fractional powers (det Φ)^(1/N_c) are non-standard in the Wilsonian approach and would be ill-defined in the symmetric phase, which is why the model must be syncretic: above T_χ all fractional terms must vanish. For N_f degenerate flavors this generalizes the Witten-Veneziano term, valid at small N_f/N_c, to arbitrary N_f/N_c, by requiring in the Veneziano limit that the free energy be a function of θ/N_c rather than θ alone. The proposal is directly testable: the splitting between the π^a and a_0^a susceptibilities between T_χ and about 2T_χ should scale as $σ_s^{{4/3}}$, $σ_s^{{2/3}}$, and σ_s, where σ_s is the strange-quark chiral condensate.
Load-bearing premise
The construction rests on the premise that below T_χ, in the chiral limit with dynamically massive quarks, fractional instantons dominate so that the topological charge is an integer times 1/N_c and, in the Veneziano limit, the free energy is a function of θ/N_c; this is assumed from lower-dimensional examples rather than derived in four spacetime dimensions.
Editorial extensions
If this is right
- For two and four flavors the chiral transition is second order in the SU(N_f)_L × SU(N_f)_R universality class, but beyond Landau, since the anomalous terms differ below and above T_χ.
- For three flavors the transition is weakly first order, possibly very weakly, because the coefficient ξ_1 of the cubic det Φ term need no longer be large: the η′ mass can be carried by the fractional terms m₁²(det Φ)^{1/3} and m₂²(det Φ)^{2/3} below T_χ instead.
- For one flavor the fractional terms keep the η′ massive at T_χ, so there is no massless isosinglet partner for the σ and, for nonzero ξ_1, the transition is a crossover.
- Lattice measurements of the ratio R_{π,a0} against the strange chiral condensate, fit to σ_s^{4/3}, σ_s^{2/3}, and σ_s, directly determine the parameters F_1 and F_2.
- In the broken phase the θ-dependence is a multi-valued function of θ/N_c, giving a phase transition at θ = π, while above T_χ the free energy is single-valued in θ.
Reading between the lines
- If fractional instantons indeed dominate the broken phase, the scales where the light-quark chiral symmetry restores (~155 MeV) and where the strange condensate evaporates (T_χs) should be well separated; this hierarchy is directly measurable and would test the picture independently of the π–a_0 splitting.
- The paper's Eq. (12), that the topological susceptibility at 2T_χ falls to about one percent of its value at T_χ, is a quantitative anchor that could be cross-checked against a two-loop dilute-instanton-gas computation, which the paper notes has not yet been performed.
- The same fractional-power construction likely transfers to other systems with an axial anomaly, such as Weyl semimetals, where the paper suggests the anomaly would force analogous fractional anomalous terms in the effective theory.
- A four-flavor subtlety: for a Q = 2/3 term built to scale as Φ³, the Halperin-Lubensky-Ma cubic mechanism could drive a weakly first-order transition, so four-flavor lattice data would discriminate regimes of the F_j functions.
Original abstract
At high temperature instantons form a dilute gas, so in QCD-like theories the breaking of the anomalous $U(1)_A$ symmetry is given by integral powers of the anomalous determinant, $\sim (\det \Phi)^{Q}$, where $\Phi\sim \overline{q}_L q_R$ is bilinear in the quark fields, and with untwisted boundary conditions, the topological charge, $Q$, is an integer. A syncretic model is constructed, which is manifestly "beyond Landau". In the chiral limit, at temperatures above the chiral phase transition, $T > T_\chi$, only integral powers of the anomalous determinant appear. Below $T_\chi$, following 't Hooft et al. I assume that the topological charge $Q$ is fractional, as an integer times $1/N_c$, where $N_c$ is the number of colors. I suggest that consequently, fractional powers of the anomalous determinant appear in the chiral effective Lagrangian. For $N_f$ degenerate flavors, this generalizes the Witten-Veneziano term, valid for small $N_f/N_c$, to arbitrary $N_f/N_c$. In this model the chiral phase transition is generically of second order. The two exceptions are for one flavor, where it is probably crossover, and three flavors, where it could well be weakly first order. This can be tested in lattice QCD with $2+1$ flavors by comparing the (known) temperature dependence of the difference of the $\pi^a$ and $a_0^a$ propagators, to the chiral condensate of the strange quark, between $T_\chi$ and $\sim 2 \, T_\chi$. Analogous measurements are possible for one to four degenerate flavors about $T_\chi$. Lastly, I propose an operator for baryon number in the symmetric phase.
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
At high temperature instantons form a dilute gas, so in QCD-like theories the breaking of the anomalous $U(1)_A$ symmetry is given by integral powers of the anomalous determinant, $\sim (\det \Phi)^{Q}$, where $\Phi\sim \overline{q}_L q_R$ is bilinear in the quark fields, and with untwisted boundary conditions, the topological charge, $Q$, is an integer. A syncretic model is constructed, which is manifestly "beyond Landau". In the chiral limit, at temperatures above the chiral phase transition, $T > T_\chi$, only integral powers of the anomalous determinant appear. Below $T_\chi$, following 't Hooft et al. I assume that the topological charge $Q$ is fractional, as an integer times $1/N_c$, where $N_c$ is the number of colors. I suggest that consequently, fractional powers of the anomalous determinant appear in the chiral effective Lagrangian. For $N_f$ degenerate flavors, this generalizes the Witten-Veneziano term, valid for small $N_f/N_c$, to arbitrary $N_f/N_c$. In this model the chiral phase transition is generically of second order. The two exceptions are for one flavor, where it is probably crossover, and three flavors, where it could well be weakly first order. This can be tested in lattice QCD with $2+1$ flavors by comparing the (known) temperature dependence of the difference of the $\pi^a$ and $a_0^a$ propagators, to the chiral condensate of the strange quark, between $T_\chi$ and $\sim 2 \, T_\chi$. Analogous measurements are possible for one to four degenerate flavors about $T_\chi$. Lastly, I propose an operator for baryon number in the symmetric phase.
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | At high temperature instantons form a dilute gas, so in QCD-like theories the breaking of the anomalous $U(1)_A$ symmetry is given by integral powers of the anomalous determinant, $\sim (\det \Phi)^{Q}$, where $\Phi\sim \overline{q}_L q_R$ is bilinear in the quark fields, and with untwisted boundary conditions, the topological charge, $Q$, is an integer. | background 1.00 | 0.06 | 0.04 | asserted 1.00 |
| s1 | A syncretic model is constructed, which is manifestly "beyond Landau". | contribution 0.89 | 0.85 | 0.05 | asserted 0.99 |
| s2 | In the chiral limit, at temperatures above the chiral phase transition, $T > T_\chi$, only integral powers of the anomalous determinant appear. | background 0.60 | 0.31 | 0.07 | asserted 1.00 |
| s3 | Below $T_\chi$, following 't Hooft et al. I assume that the topological charge $Q$ is fractional, as an integer times $1/N_c$, where $N_c$ is the number of colors. | method 0.67 | 0.32 | 0.09 | asserted 0.55 |
| s4 | I suggest that consequently, fractional powers of the anomalous determinant appear in the chiral effective Lagrangian. | result 0.36 | 0.71 | 0.67 | hedged possibility 0.92 |
| s5 | For $N_f$ degenerate flavors, this generalizes the Witten-Veneziano term, valid for small $N_f/N_c$, to arbitrary $N_f/N_c$. | contribution 0.53 | 0.68 | 0.11 | asserted 0.99 |
| s6 | In this model the chiral phase transition is generically of second order. | result 0.97 | 0.72 | 0.26 | asserted 0.90 |
| s7 | The two exceptions are for one flavor, where it is probably crossover, and three flavors, where it could well be weakly first order. | result 0.72 | 0.46 | 0.08 | hedged possibility 0.95 |
| s8 | This can be tested in lattice QCD with $2+1$ flavors by comparing the (known) temperature dependence of the difference of the $\pi^a$ and $a_0^a$ propagators, to the chiral condensate of the strange quark, between $T_\chi$ and $\sim 2 \, T_\chi$. | future work 0.96 | 0.13 | 0.06 | hedged possibility 0.74 |
| s9 | Analogous measurements are possible for one to four degenerate flavors about $T_\chi$. | future work 0.93 | 0.19 | 0.03 | hedged possibility 0.86 |
| s10 | Lastly, I propose an operator for baryon number in the symmetric phase. | contribution 0.67 | 0.68 | 0.05 | speculative 0.46 |
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