# A note on the Virasoro blocks at order 1 / *c*

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## Abstract

We derive an explicit expression for the 1 / *c* contribution to the Virasoro blocks in 2D CFT in the limit of large *c* with fixed values of the operators’ dimensions. We follow the direct approach of orthonormalising, at order 1 / *c*, the space of the Virasoro descendants to obtain the blocks as a series expansion around \(z=0\). For generic conformal weights this expansion can be summed in terms of hypergeometric functions and their first derivatives with respect to one parameter. For integer conformal weights we provide an equivalent expression written in terms of a finite sum of undifferentiated hypergeometric functions. These results make the monodromies of the blocks around \(z=1\) manifest.

## 1 Introduction

Conformal symmetry is a powerful tool to constrain the correlators in two-dimensional CFT. It implies that four-point functions of primary fields on the sphere are given by (possibly infinite) sums of conformal blocks,^{1} which are functions of the complex harmonic ratio: each block encodes the contributions of all the Virasoro descendants of a primary field. Given the central charge *c* and the conformal dimensions \(h_i\) and *h* of the external and internal primary fields, conformal symmetry determines in principle the full functional dependence of the blocks on the harmonic ratio *z*. The infinite dimensionality of the 2D conformal algebra, which makes this powerful statement possible, also makes the task of computing conformal blocks particularly difficult. Multiple efforts have been made devoted to this task since the seminal work of [1], but the general form of the Virasoro blocks for generic values of *c* remains unknown. Various perturbative expansions of the blocks can be generated via recursion relations [2, 3, 4]. Combinatorial formulae for the coefficients in the *z*-expansion of the blocks have been found in [5], based on the AGT correspondence [6] between 4d supersymmetric gauge theories and 2d CFT.

When the CFT admits a holographic dual, it is interesting to study the conformal blocks in the limit of large central charge. It is well known that in the \(c\rightarrow \infty \) limit with fixed \(h_i\) and *h*—a limit that we will call the LLLL limit—the conformal blocks reduce to the global ones, associated with the projective subgroup of the local conformal group. This result can be extended in various directions: one can consider the so called HHLL limit, in which one keeps fixed the dimensions of the internal and of two (light) external operators and sends to infinity the dimensions \(h_L\) of the two remaining (heavy) external operators, keeping the ratio \(h_H/c\) fixed when \(c\rightarrow \infty \). The leading contribution to the blocks in this regime has been derived in [7, 8], and the subleading corrections have been studied in [9, 10, 11, 12]. One could also consider a classical limit in which all the dimensions \(h_i\) and *h* are rescaled together with *c* [13, 14, 15]. Conformal blocks have also been analysed from a bulk perspective exploiting their connection with geodesic Witten diagrams and Wilson lines [16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26].

In this technical note we focus on the LLLL regime and derive exact expressions for the correction to the Virasoro blocks at order 1 / *c*. Our main motivation comes from holography, since this correction contributes to the connected part of correlators in the supergravity approximation. In Sect. 2 we compute the blocks by a direct method, summing over the Virasoro descendants that contribute at the desired order in the \(c\rightarrow \infty \) limit. This produces the result (2.21) given as a series expansion in the “direct channel” \(z\rightarrow 0\). In Sect. 3 we also make various attempts at summing the *z*-series to have access to the behaviour of the blocks also away from \(z=0\). We first decompose the blocks into three contributions—denoted as \(f_a\), \(f_b\), \(f_c\) in (3.1)—according to their dependence on the external dimensions \(h_i\). Two first two terms, \(f_a\) and \(f_b\), were computed with the Wilson-line approach in [24], and we verify the agreement with our results. An explicit expressions for the last term was recently derived in [26] by analytically continuing to a generic dimension *h* the result for the \(W_2\) minimal model obtained via the Wilson-line approach developed in [25]. Here we provide two alternative exact expressions for \(f_c\). Equation (3.13) applies to generic real values of the internal dimension *h*, but involves derivatives of generalised hypergeometric functions with respect to one of their parameters. We also check that this result agrees with that of [26], thus providing a further non-trivial confirmation of the Wilson-line approach at the quantum level. Then we find that for integer values of *h* the block can be written as a finite sum of undifferentiated hypergeometric functions (see (3.16)). We also make some comments on the somewhat surprising singularity structure of the result: we find that the 1 / *c* correction to the block has a leading singularity around \(z=1\) that goes like \(\log ^2(1-z)\), as opposed to the \(\log (1-z)\)-singularity of the \(c^0\) contribution. Since terms proportional to \(\log ^2(1-z)\) cannot arise in the expansion of the correlator in the crossed (\(z\rightarrow 1\)) channel, we speculate that these singularities have to cancel when an infinite series of conformal blocks is summed to produce a physical correlator.

## 2 Perturbative Virasoro blocks at large *c*

*z*is the following projective invariant cross ratio:

*c*for each Virasoro generator with mode lower than \(-1\). Thus in the strict \(c\rightarrow \infty \) limit, it is sufficient to focus on the descendants obtained by acting with \(L_{-1}\), which implies that at leading order in

*c*the Virasoro blocks reduce to the global conformal blocks. Here we are interested in the first subleading correction, so we need to consider the space spanned by descendants that have at most one \(L_{-s}\) generator with \(s\ge 2\). At level

*q*we have to deal with the following states:

*c*except for \(j=1\) where we have a coefficient \(\alpha _{(q,s)}^{(1)}\) of order 1. So at leading order the norm of \(|s\rangle _q\) comes from its first term in (2.6)

*z*, the operators \(L_{-n}\) can be replaced by the differential operators \(\mathcal {L}_{-n}\),

*c*correction \({\mathcal {V}}_h^{(1)}\)

*O*(

*c*); the subleading correction in any of the two will contribute to \({\mathcal {V}}_h\) at order \(O(c^{-2})\) and so can be neglected. To compute the three-point function

*c*limit

*c*correction \({\mathcal {V}}_h^{(1)} (z)\) to the full Virasoro block as follows:

*z*-expansion of \({\mathcal {V}}_h^{(1)} (z)\) was obtained recently in [26] (see Eqs. (4.40) and (4.42)), based on the approach developed in [25]. It is straightforward to check by using Mathematica that (2.21) agrees with the result of [26] as an expansion around \(z=0\).

## 3 Exact Virasoro blocks at large *c*

The result (2.21) allows one to easily derive the behaviour of the conformal block around \(z=0\). It might be useful, in particular when applying the conformal bootstrap method, to have a non-perturbative control over \({\mathcal {V}}_h^{(1)} (z)\) away from \(z=0\), and for this purpose one needs to sum the double series in (2.21). This is easily done for the identity block (\(h=0\)) and one obtains the result in Eq. (2.36) of [4]: this is nothing but the global block of the stress tensor *T*, which is the only quasi-primary among the descendants of the identity that contributes at order \(c^{-1}\).

*h*performing the summation in closed form is non-trivial. One can distinguish three terms in (2.21), according to their dependence on the external conformal dimensions \(h_1\) and \(h_2\):

*m*-series can be summed in terms of \({}_3F_2\) for the case of \(f^{(3)}_c(h;z)\) and in terms of \({}_2F_1\) for all other cases. The remaining sum over

*s*can also easily be done for \(f_a(h;z)\) and \(f^{(1)}_b(h;z)\), where the

*m*and

*s*-dependence factorise, and for \(f^{(2)}_b(h;z)\), where one exploits a partial cancellation between the two terms in the square bracket in (3.3c). One thus obtains

^{2}with Eqs. (3.22) and (3.23) of [24], which were derived with the Wilson-line approach. The same expressions can be derived by expanding at first order in \(h_H/c\) the HHLL blocks at order \(O(c^0)\) computed in [8].

*s*containing hypergeometrics of the \({}_2F_1\) and \({}_3F_2\)-type. All the series are of the form

*h*. To sum these series we can use the identity [28]

*h*; this guarantees that the results of the Wilson-line approach are also valid non-perturbatively in

*z*.

*h*. The fact that \(B_{(q,s)} (h_i=0 ; h=1)=0\) implies that \(f_c(h=1,z)=0\). Inspecting the behaviour of \(f_c\) for various integer values of \(h\ge 2\), one finds that \(f_c\) contains four type of terms: terms containing, respectively, \(\log ^2(1-z)\), \(\mathrm {Li}_2(z)\), \(\log (1-z)\) times a rational function of

*z*, or terms containing only rational functions. The \(\log ^2\)-terms only come from the last term in the first line of (3.13). The \(\mathrm {Li}_2\)-terms come from the functions \({}_3\hat{F}_2\) and they can always be expressed as linear combinations of undifferentiated hypergeometric functions \({}_3F_2\), with constant coefficients: a closed form expression for these coefficients, for generic integer

*h*, can be obtained by trial and error. Subtracting this linear combination from \(f_c\), one can similarly generate the \(\log \)-terms of the remainder by a linear combination of \({}_2 F_1\): an educated guess is sufficient to determine all coefficients but one, which can be inferred by looking at the \(z\rightarrow 0\) limit. Remarkably, one verifies that also the rational part of \(f_c\) is reproduced. Finally, we arrive at the following explicit expression for \(f_c\), which we conjecture to be valid for any integer \(h\ge 2\):

^{3}that (3.13) and (3.16) agree up to \(h=10\) exactly in

*z*. We have also checked that the expansion of (3.16) around \(z=0\) is in agreement with (3.4) for generic

*h*up to order \(z^{20}\).

*c*from the knowledge of the non-analytic behaviour of the hypergeometric functions. This is particularly straightforward in the case of integer conformal weights where we can use (3.16) (together with (3.6) and (3.8) which are valid in general). A new feature of the 1 /

*c*corrected Virasoro block is the presence of terms proportional to \(\log ^2 (1-z)\), which are absent in the global blocks. Using well-known results for the non-analytic behaviour of \({}_2F_1\) [31], one finds that the \(\log ^2(1-z)\) terms of \(f_a\), \(f_b\) and \(f_c\) are

*c*expansion of a CFT correlator generically contains terms with logarithms, which are related to the 1 /

*c*corrections to the conformal dimensions of multiparticle operators. However, there is no room, at order 1 /

*c*, for terms with the logarithm squared such as the ones appearing in (3.18). This probably means that in a large

*c*2D CFT the correlators always involve an infinite number of Virasoro primaries so as to avoid the appearance of the \(\log ^2\) terms that are present in each block. If possible, it would of course be very interesting to generalise what has been done by using the global blocks (see for instance [32, 33]) to the case of the Virasoro blocks and exploit the full Virasoro algebra in the holographic reconstruction of AdS\(_3\) physics.

## Footnotes

- 1.
We use the terms conformal and Virasoro block as synonymous.

- 2.
- 3.
The interested reader might find a Mathematica notebook useful to verify these identities at the following link: https://arxiv.org/abs/1807.07886.

- 4.
Equation (3.20) of [24] should have an extra factor of \(z_{21}^{-1} z_{43}^{-1}\) and the overall factor of (3.24) should read 1 / 24 instead of 1 / 2.

## Notes

### Acknowledgements

We would like to thank L. F. Alday and A. Bissi for discussions and correspondence. This work was supported in part by the Science and Technology Facilities Council (STFC) Consolidated Grant ST/L000415/1 *String theory, gauge theory & duality*.

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