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From Z to all: cyclic sum rules for thermal CFT partition functions

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Abstract

The high-temperature expansion of twisted Sd−1×S1S^{d-1}\times S^1 partition functions of CFTd_d, d=2κd = 2κ, has recently been shown to obey an infinite tower of universal constraints. We determine these constraints explicitly and demonstrate that they can be packaged into a single equation involving a large conformal diffeomorphism isomorphic to Zκ+1\mathbb{Z}_{κ+1}, which cyclically permutes the thermal circle and the rotation angles. Our equation asserts that the Zκ+1\mathbb{Z}_{κ+1}-invariant part of ln⁡Z/β\ln Z/β is a constant fixed by the a-anomaly. Our derivation assumes CP-invariance and locality of the thermal effective action, whose structure we constrain using an algebraic formalism starting from embedding space.

Introduction

By the state-operator correspondence, the thermal partition function of a CFTd\mathrm{CFT}_d on a twisted Sd−1×S1S^{d-1} \times S^1

Z(β;ωi):=Tr⁡Sd−1e−β(D−ω1J1−⋯−ωκJκ),κ:=⌊d2⌋(1)Z(\beta;\omega_i) := \operatorname{Tr}_{S^{d-1}} e^{-\beta(D-\omega_1J_1-\cdots-\omega_\kappa J_\kappa)}, \qquad\kappa:= \left\lfloor\frac{d}{2}\right\rfloor \tag*{(1)}

fully encodes the scaling dimensions Δ\Delta, spins JiJ_i and degeneracies of all local operators. While the detailed structure of Z(β;ωi)Z(\beta;\omega_i) depends sensitively on the particular CFT, it was shown almost two decades ago [1] that its high-temperature behavior always takes the hydrodynamic form

ln⁡Z=c′βd−1∏i=1κ(1−ωi2)+O(β3−d),β→0+, ωi fixed.(2)\ln Z = \frac{c'}{\beta^{d-1}\prod_{i=1}^{\kappa}(1-\omega_i^2)}+O\left(\beta^{3-d}\right),\qquad\beta\to0^+,\ \omega_i\ \text{fixed}. \tag*{(2)}

This remarkably universal result was subsequently rederived using the formalism of ‘equilibrium partition functions’ [2, 3, 4], an approach that was further developed and named thermal effective theory in [5].

A systematic analysis of subleading corrections to Eq. (2) was presented in the recent paper [6]. The analysis of [6] was largely performed in d=4d=4; the generalization to CP-preserving1 CFTs in arbitrary even dimensions

d=2κd=2\kappa is presented below. One finds

ln⁡Z=ln⁡Zrot+ln⁡Zsh,\ln Z=\ln Z^{\mathrm{rot}}+\ln Z^{\mathrm{sh}},
ln⁡Zrot=−1βd−1x1x2…xκ(∑q=0∞β2qSq(xi)),\ln Z^{\mathrm{rot}}=-\frac{1}{\beta^{d-1}x_1x_2\ldots x_\kappa}\left(\sum_{q=0}^{\infty}\beta^{2q}S_q(x_i)\right),
ln⁡Zshβ=κ!a2κ[(−1)κ+122κ−1∑i=1κ∫01ds∏j=1j≠iκ(sxi−2xj)2xj(xi−xj)].(3)\frac{\ln Z^{\mathrm{sh}}}{\beta}=\kappa!a_{2\kappa}\left[(-1)^\kappa+\frac{1}{2^{2\kappa-1}}\sum_{i=1}^{\kappa}\int_0^1ds\prod_{\substack{j=1\\j\ne i}}^\kappa\frac{(sx_i-2x_j)^2}{x_j(x_i-x_j)}\right]. \tag*{(3)}

ln⁡Zsh\ln Z^{\mathrm{sh}}, the universal ‘anomalous shift’ of the partition function, is proportional to the aa anomaly a2κa_{2\kappa}.12 The ‘non-anomalous’ part, ln⁡Zrot\ln Z^{\mathrm{rot}}, is non-universal. It is characterized by the coefficient functions SqS_q, which turn out to be (not necessarily homogeneous) symmetric degree-qq polynomials13 in the variables ωi2\omega_i^2, or equivalently in

xi:=1−ωi2,(i=1…κ).(4)x_i:=1-\omega_i^2,\qquad(i=1\ldots\kappa). \tag*{(4)}

While the polynomials SqS_q are theory dependent, their coefficients are constrained, by the locality of the thermal effective action, to obey several additional sharp universal constraints [6]. These constraints force SqS_q to lie in a dqd_q codimensional subspace [6] (of the space of arbitrary symmetric degree qq polynomials), where dqd_q is defined by14

tκ∏i=1κ+1(1−ti)=∑q=0∞dqtq .(5)\frac{t^\kappa}{\prod_{i=1}^{\kappa+1}(1-t^i)}=\sum_{q=0}^{\infty}d_qt^q \, . \tag*{(5)}

at large qq, dq∼qκκ!(κ+1)!d_q\sim\frac{q^\kappa}{\kappa!(\kappa+1)!}. While the precise form of the constraints on SqS_q was worked out in [6] at the first nontrivial value, q=κq=\kappa, the detailed structure of the infinite tower of constraints at q≥κ+1q\geq\kappa+1 was left undetermined. A central result of this paper is the following. The entire infinite tower of constraints is encapsulated, at every κ≥2\kappa\geq2, by a strikingly simple universal equation involving a Zκ+1\mathbb{Z}_{\kappa+1} action, generated by Ω\Omega, which cyclically permutes the thermal circle and angular directions on S2κ−1×S1S^{2\kappa-1}\times S^1, via

Ω:(b0,b1,…,bκ)⟶(b1,…,bκ,b0),\Omega:(b_0,b_1,\ldots,b_\kappa)\longrightarrow(b_1,\ldots,b_\kappa,b_0),

where $(b_0,b_1,\ldots,b_\kappa):=(\beta,\beta\omega_1,\ldots,\beta\omega_\kappa).

{#eq:6}

Our central equation asserts the vanishing of the Zκ+1\mathbb{Z}_{\kappa+1}-invariant part of ln⁡Zrotβ\frac{\ln Z^{\mathrm{rot}}}{\beta},

(1+Ω+Ω2+…+Ωκ)∘[ln⁡Zrotβ]=0.(6)(1+\Omega+\Omega^2+\ldots+\Omega^\kappa)\circ\left[\frac{\ln Z^{\mathrm{rot}}}{\beta}\right]=0. \tag*{(6)}

Eq. (6), together with our (conjectured) expression for ln⁡Zsh\ln Z^{\mathrm{sh}}, then implies

(1+Ω+Ω2+…+Ωκ)∘[ln⁡Zβ]=(−1)κκ!a2κ.(7)(1+\Omega+\Omega^2+\ldots+\Omega^\kappa)\circ\left[\frac{\ln Z}{\beta}\right]=(-1)^\kappa\kappa!a_{2\kappa}. \tag*{(7)}

Our demonstration of Eq. (7) applies only in an order-by-order sense in β2\beta^2. Eq. (7) is thus best viewed as a generating function for an infinite tower of constraints on coefficient polynomials (see Eq. (3))

Tq:=β2qSq,(8)T_q:=\beta^{2q}S_q, \tag*{(8)}

that are homogeneous of degree qq in (b02,b12,…,bκ2)(b_0^2,b_1^2,\ldots,b_\kappa^2). While TqT_q are symmetric under the interchange bi2↔bj2b_i^2\leftrightarrow b_j^2 for i,j≠0i,j\neq0, they are not necessarily invariant under permutations involving b02b_0^2, and so can be rewritten (see Appendix A) as

Tq=Iq(0)+b02Iq(1)+b04Iq(2)+…+b02κIq(κ)(9)T_q=I_q^{(0)}+b_0^2I_q^{(1)}+b_0^4I_q^{(2)}+\ldots+b_0^{2\kappa}I_q^{(\kappa)} \tag*{(9)}

where Iq(r)I_q^{(r)} is a completely symmetric polynomial of degree (q−r)(q-r) in (b02,b12,…,bκ2)(b_0^2,b_1^2,\ldots,b_\kappa^2) and absent for r>qr>q. Eq. (6) is equivalent to

Iq(κ)=0,∀q≥κ,(10)I_q^{(\kappa)}=0,\quad\forall q\geq\kappa, \tag*{(10)}

effectively imposing dqd_q constraints on SqS_q, in agreement with the expectations described around Eq. (5). We demonstrate the constraining power below explicitly for κ=2\kappa=2 and delegate the derivation of Eq. (10) for κ>2\kappa>2 to Appendix A and the SM, and the counting of constraints to Appendix D.

As a highly nontrivial check on the results of this paper, we verify that the partition function of Kerr-AdS2κ+1_{2\kappa+1} black holes15 obeys the central equation Eq. (7) as an exact non-perturbative identity between functions of (b02,b12,…,bκ2)(b_0^2,b_1^2,\ldots,b_\kappa^2). In the rest of this paper, we first present details of our derivations for the simplest nontrivial dimension d=4d=4, and then sketch generalizations to arbitrary dimensions.

Derivation of the central equation

Simplifying the measure

As explained in [6], the most general local thermal effective action for a d=2κd=2\kappa dimensional CFT takes the form Eq. (3), with

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