Recognition: unknown
Local CFTs extremise F
Pith reviewed 2026-05-10 10:25 UTC · model grok-4.3
The pith
Local CFTs lie at the extrema of the sphere free energy along lines of long-range theories.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
Local CFTs lie at the extrema of the (universal part of the) sphere free energy tilde F(Delta) of the long-range CFTs: d tilde F / d Delta at Delta equal to Delta_local equals zero, and for unitary CFTs they locally maximise it. The simple proof uses the fact that the derivative of tilde F with respect to Delta receives contributions only from the nonlocal terms in the action. The nonlocal terms must be absent in the limit Delta to Delta_local, and hence the derivative is zero. Demonstrating maximisation then requires a proof of the generalised F-theorem in conformal perturbation theory to subleading order.
What carries the argument
The universal sphere free energy tilde F(Delta) equal to sin(pi d / 2) times log of the sphere partition function Z_S^d, whose derivative with respect to the parameter Delta vanishes when the long-range theory reaches its local point.
If this is right
- The scaling dimension of the fundamental field is encoded as the zero of d tilde F / d Delta.
- The result explains the derivative structure appearing in the large-N expansion of Delta_phi in the O(N) model.
- It supplies a non-supersymmetric analogue of the known c, F, a extremisation mechanisms.
- The relation is confirmed by direct calculation for the O(N) phi^4 theory and cubic CFTs in both the epsilon expansion and the large-N limit.
Where Pith is reading between the lines
- The same variational property might locate local fixed points inside other families of nonlocal deformations without separate computation of scaling dimensions.
- It could be tested numerically in long-range models studied by bootstrap or Monte Carlo methods.
- Higher derivatives of tilde F or other thermodynamic quantities on the sphere might obey related stationarity conditions at the local point.
Load-bearing premise
The derivative of tilde F with respect to Delta receives contributions only from the nonlocal terms in the action, which disappear as Delta approaches the local value.
What would settle it
An explicit computation of d tilde F / d Delta at the local Delta in any unitary model, such as the 3d Ising CFT or the O(N) vector model in epsilon expansion, that yields a nonzero result would falsify the claim.
read the original abstract
Many CFTs can be extended to lines of nonlocal CFTs parametrised by the scaling dimension $\Delta$ of the fundamental field appearing in the action. $\Delta=\frac{d}{2}-\zeta$ is set by the exponent of the kinetic term $(-\partial^2)^{\zeta}$, which is nonlocal for noninteger $\zeta$. If $\Delta$ is tuned to $\Delta_\mathrm{local}$, the scaling dimension of the fundamental field in the local CFT, arXiv:1703.05325 showed that we recover the conformal data of that CFT (plus a decoupled sector). One natural question is: how is the local point special on this line of nonlocal CFTs? We prove that these local CFTs lie at the extrema of the (universal part of the) sphere free energy $\tilde{F}(\Delta)=\sin(\frac{\pi d}{2}) \log Z_{S^d}$ of the long-range CFTs: $d\tilde{F}/d\Delta|_{\Delta=\Delta_\mathrm{local}}=0$; and for unitary CFTs they locally maximise it. The simple proof uses the fact that the derivative of $\tilde{F}$ with respect to $\Delta$ receives contributions only from the nonlocal terms in the action. The nonlocal terms must be absent in the limit $\Delta \to \Delta_\mathrm{local}$, and hence the derivative is zero. Demonstrating maximisation then requires a proof of the generalised $F$-theorem in conformal perturbation theory to subleading order. We check our result with the O$(N)$ $\phi^4$ and cubic CFTs in the $\epsilon$ expansion and the large-$N$ limit. This result provides a minimal encoding of the scaling dimension of the fundamental field in many CFTs, and also explains the curious derivative structure of the large-$N$ expansion of $\Delta_\phi$ in the O$(N)$ model. Finally, this nonlocal $F$-extremisation can be viewed as a non-supersymmetric version of the known $c,F,a$-extremisation mechanisms.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper claims that local CFTs recovered at a specific value of the parameter Delta in families of nonlocal long-range CFTs (with kinetic term exponent zeta) extremize the universal sphere free energy tilde F(Delta) = sin(pi d / 2) log Z_{S^d}. Specifically, it proves d tilde F / d Delta vanishes at Delta = Delta_local (the local CFT value), with local maximisation for unitary theories. The argument is that the derivative receives contributions only from nonlocal terms in the action, which are absent at the local point; maximisation requires a subleading generalised F-theorem in conformal perturbation theory. The result is checked via epsilon-expansion and large-N limits for O(N) phi^4 and cubic models, and interpreted as a non-supersymmetric analog of c,F,a-extremisation.
Significance. If the central claim holds, the result offers a simple, minimal encoding of the fundamental scaling dimension Delta_phi in many CFTs through extremisation of tilde F, explains the derivative structure of large-N expansions in the O(N) model, and provides a new perspective on F-theorems without supersymmetry. The explicit checks in controlled perturbative regimes add concrete support, and the connection to the referenced nonlocal construction strengthens the conceptual link between local and long-range CFTs.
major comments (2)
- [Abstract and §2] Abstract and §2 (proof sketch): the claim that d tilde F / d Delta receives contributions only from nonlocal terms in the action, which vanish as Delta -> Delta_local, does not yet address the decoupled sector recovered in the construction of arXiv:1703.05325. If this sector's action or sphere free energy retains any Delta dependence (e.g., via its own kinetic term or coupling to zeta), then its contribution to the derivative need not vanish, undermining the proof that d tilde F / d Delta = 0 at the local point.
- [Abstract and §4] Abstract and §4 (maximisation): demonstrating that the stationary point is a local maximum requires a proof of the generalised F-theorem to subleading order in conformal perturbation theory. The manuscript must supply the explicit steps of this argument (including any assumptions on unitarity or the form of the perturbation), as the abstract only states that it is needed; without these details the maximisation claim is not fully established.
minor comments (2)
- [Introduction] Clarify the precise definition of the universal part tilde F versus the full log Z_{S^d} and its relation to the standard F-theorem, especially in the presence of the decoupled sector.
- [§5] In the epsilon-expansion and large-N checks, provide explicit numerical values or a table showing that the derivative vanishes to the computed order, rather than only stating that the result is confirmed.
Simulated Author's Rebuttal
We thank the referee for their careful reading of our manuscript and for the constructive comments. We address each major point below and have revised the text accordingly to strengthen the presentation.
read point-by-point responses
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Referee: [Abstract and §2] Abstract and §2 (proof sketch): the claim that d tilde F / d Delta receives contributions only from nonlocal terms in the action, which vanish as Delta -> Delta_local, does not yet address the decoupled sector recovered in the construction of arXiv:1703.05325. If this sector's action or sphere free energy retains any Delta dependence (e.g., via its own kinetic term or coupling to zeta), then its contribution to the derivative need not vanish, undermining the proof that d tilde F / d Delta = 0 at the local point.
Authors: In the construction of arXiv:1703.05325, the limit Delta → Delta_local recovers the local CFT in the interacting sector together with a decoupled free scalar whose scaling dimension is fixed by the local theory. This decoupled sector has a strictly local kinetic term whose form is independent of the deformation parameter Delta (or zeta). Because the sector is completely decoupled, its contribution to the sphere partition function (and thus to tilde F) does not depend on Delta. The derivative d tilde F / d Delta therefore receives contributions exclusively from the nonlocal kinetic term of the interacting sector, which is absent at the local point. We will add a short clarifying paragraph in the revised §2 making this decoupling explicit. revision: yes
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Referee: [Abstract and §4] Abstract and §4 (maximisation): demonstrating that the stationary point is a local maximum requires a proof of the generalised F-theorem to subleading order in conformal perturbation theory. The manuscript must supply the explicit steps of this argument (including any assumptions on unitarity or the form of the perturbation), as the abstract only states that it is needed; without these details the maximisation claim is not fully established.
Authors: We agree that the maximisation statement requires an explicit subleading argument. The present manuscript notes the necessity of a generalised F-theorem in conformal perturbation theory but does not derive the sign of the second derivative. In the revision we will expand §4 with the following steps: (i) the first-order variation of tilde F vanishes by the stationarity condition already established; (ii) the second-order term is proportional to the integral of the two-point function of the nonlocal perturbing operator; (iii) unitarity implies that this two-point function is positive definite, so the quadratic correction to tilde F is negative. The perturbation is taken to be the long-range deformation, which is relevant or marginal near the local fixed point. These assumptions will be stated clearly. The expanded derivation will be included in the revised manuscript. revision: yes
Circularity Check
No circularity: derivation is self-contained via external citation and independent F-theorem proof
full rationale
The central claim rests on two elements: (1) the cited result from arXiv:1703.05325 that nonlocal terms vanish at Delta_local (recovering the local CFT plus decoupled sector), and (2) a new proof of the generalized F-theorem in conformal perturbation theory to subleading order for the maximisation statement. Neither reduces to a self-definition, a fitted parameter renamed as prediction, nor a load-bearing self-citation chain; the derivative argument follows directly from the action's structure once the external vanishing is granted. The decoupled-sector concern raised in the skeptic note is a potential correctness issue, not a circularity reduction. The paper's checks in epsilon expansion and large-N are independent verifications, not inputs. Overall the derivation chain is non-circular.
Axiom & Free-Parameter Ledger
axioms (1)
- domain assumption The derivative of the sphere free energy tilde F with respect to Delta receives contributions only from the nonlocal terms in the action.
Reference graph
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