CERN-PH-TH/2010-177 HU-EP-10/47 sl Affine (N) conformal blocks from = 2 SU(N) gauge theories N Can Kozc¸az , Sara Pasquetti , Filippo Passerini and Niclas Wyllard † † § † PH-TH division, CERN, CH-1211 Geneva, Switzerland 1 § Institut fu¨r Physik, Humboldt-Universit¨at zu Berlin, 1 0 Newtonstraße 15, D-12489 Berlin, Germany 2 n [email protected], [email protected], a [email protected], [email protected] J 6 ] h Abstract t - p e Recently Alday and Tachikawa [1] proposed a relation between conformal blocks in a h two-dimensional theory with affine sl(2) symmetry and instanton partition functions [ in four-dimensional conformal = 2 SU(2) quiver gauge theories in the presence of a 2 N v certain surface operator. In this paper we extend this proposal to a relation between 2 conformal blocks in theories with affine sl(N) symmetry and instanton partition func- 1 4 tions in conformal = 2 SU(N) quiver gauge theories in the presence of a surface N 1 operator. We also discuss the extension to non-conformal = 2 SU(N) theories. . N 8 0 0 1 Table of contents : v i X r 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 a 2 Surface operators and instanton counting . . . . . . . . . . . . . . . . 3 3 Affine sl(2) and surface operators in SU(2) gauge theories . . . . . . . 10 4 Affine sl(N) and surface operators in SU(N) gauge theories . . . . . 17 5 Asymptotically free SU(N) gauge theories and affine sl(N) . . . . . . 26 6 Summary and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 A Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 1 1 Introduction Ever since the groundbreaking work of Seiberg and Witten [2], the study of four- dimensional gaugetheories with = 2supersymmetry hasbeen animportant research N topic. Such theories have a very rich structure and have many remarkable connections to other areas of both physics and mathematics. Last year, building on earlier work by Witten [3], Gaiotto [4] introduced a new way of analysing = 2 theories by viewing them as arising from a six-dimensional N theory compactified on a two-dimensional Riemann surface with punctures. In this approach one naturally expects connections between the 4d = 2 gauge theory and N some conformal theory on the 2d Riemann surface. The AGT relation [5] is a precise realisation of this expectation. It encompasses a relation between instanton partition functions in conformal = 2 theories and N conformal blocks in two-dimensional conformal field theories. The original work [5] proposed a relation between conformal 4d SU(2) quiver gauge theories and the 2d Liouville theory. This relation was subsequently extended [6] to a relation between conformal 4d = 2 SU(N) theories and 2d A Toda field theories. Non-conformal N−1 N = 2 theories have also been considered and related to two-dimensional CFT [7,8]. N A natural way to extend the AGT relation is to consider the inclusion of various defects in the gauge theory. Examples include one-dimensional (line) defects (e.g. Wil- son and ’t Hooft loops), and three-dimensional (domain wall) defects. Such defects have been considered in [9,10], and [11], respectively. In this paper we focus on defects which are supported on two-dimensional submani- folds, i.e. surfaceoperators. Surfaceoperatorsin = 4gaugetheorieswere extensively N studied in [12] (see [13] for some similar work in = 2 theories). In the context of the N AGT relation, surface operators have been studied in several papers [9,14–18,1]. When viewed from the six-dimensional perspective there are two ways a surface operator can arise [1]: either from a 4d defect wrapping the 2d Riemann surface, or as a 2d defect intersecting the 2d Riemann surface at a point. The second class of surface operators can be described in the dual 2d CFT by inserting a certain degenerate field operator localised at a point. Such surface operators were first considered in [9] and have been further studied in [14–18]. For the first class of surface operators it was recently proposed [1] that the effect of wrapping the 4d defect around the Riemann surface is to modify the 2d CFT to another 2d CFT. For the SU(2) quiver gauge theories it was argued that the surface operator insertion modifies the dual Liouville theory to a theory with (untwisted) affine sl(2) symmetry. Conformal blocks in this theory should therefore be related to instanton partition functions in SU(2) quiver gauge theories in the presence of a surface operator [1] that arises from a 4d defect. Itwasfurtherrealisedin[1]thatthetechnologytocomputesuchinstantonpartition functions already exists in the mathematics literature [19–21]. Using these results several checks of the proposed relation were performed. In this paper we extend the proposal in [1] to a relation between conformal blocks in theories with affine sl(N) symmetry and instanton partition functions in conformal = 2 SU(N) quiver gauge theories in the presence of a surface operator arising from N 2 a 4d defect. In other words, we argue that the effect of the 4d defect is to replace the A Toda field theory and its associated -algebra symmetry by a theory with N−1 N W affine sl(N) symmetry. We perform several checks of the proposed relation and also extend it to non-conformal = 2 SU(N) theories. N In the next section we review some facts about instanton counting in SU(N) quiver gauge theories in the presence of a surface operator, and in section 3 we review the proposal in [1] and perform some additional tests using a different perturbative scheme compared to the one in [1] which allows us to sum up certain infinite sets of terms. For the rank one case we also discuss the relation to the surface operator arising from a degenerate field insertion in the Liouville theory. Then in section 4 we propose a relationbetweenconformalblocksinatheorywithaffinesl(N)symmetryandinstanton partition functions in SU(N) quiver gauge theories with a surface operator insertion. The extension to non-conformal theories is discussed in section 5. In the appendix some technical details are collected. Note added: After this work was finished [22] appeared. This paper has some overlap with our results, but only considers the case of SU(2). 2 Surface operators and instanton counting A surface operator in a four-dimensional gauge theory is a certain object supported on a two-dimensional submanifold of spacetime. One way to define a surface operator is by specifying the (singular) behaviour of the gauge field (and scalars, if present) near the submanifold where the surface operator is supported. An extensive study of surface operators in the context of the = 4 SU(N) gauge theories (in a flat N spacetime) was carried out in[12]. There it was found that the possible types of surface operators supported on an R2 submanifold are in one-to-one correspondence with the so called Levi subgroups (whose classificiation in turn is in one-to-one correspondence with the various (non-trivial) ways of embedding SU(2) inside SU(N), or equivalently the number of possible ways of breaking SU(N) to a U(1)ℓ−1 ℓ SU(N ) (proper) i=1 i subgroup). Concretely this means that for every (non-trivial) partition N = N +...+ 1 Q N there is a possible surface operator. In this paper we study surface operators1 in 4d ℓ SU(N) theories with = 2 supersymmetry; such surface operators are also classified N by the Levi subgroups. For = 2 theories a surface operator depends on a certain N number of continuous complex parameters, one for each of the abelian U(1) factors in the Levi subgroup (unbroken group)2. In [1] the following terminology was used: a full surface operator corresponds to the breaking of SU(N) to U(1)N−1 and depends on N 1 continuous parameters (this − is the maximal number of parameters possible), whereas a simple surface operator cor- responds to the breaking of SU(N) to SU(N 1) U(1) and depends on one parameter. − × A surface operator with a given Levi type of singularity can be realised both by 4d or by 2d defects, in the 6d language. In particular there will be full surface operators coming from 2d and 4d defects as well as simple surface operators coming from 2d 1Throughout we assume that the surface operator is supported on an R2 submanifold. 2For =4 theories the surface operators depend on four real parameters for each U(1) factor. N 3 and 4d defects. Different realisations are not supposed to give rise to the same surface operator, however one may speculate that the instanton partition function may not be sensitive to the difference. We will explore this possibility in section 3.4. In this paper surface operators that arise (in the 6d language) from 4d defects will always be full surface operators, whereas the surface operators that arise from 2d defects are simple surface operators. Sometimes, for convenience we will refer to the two classes just asfull andsimple surface operators, respectively. Butthe reader should keep in mind that there are in general two realisations for each Levi type of singularity. 2.1 SU(N) instanton counting in the presence of a simple surface operator A natural question to address is how the instanton partition function in an = 2 N gauge theory [23] (which is valid in the absence of surface operators) changes when a surface operator is present. In [9] it was conjectured that a simple surface operator in a (mass-deformed) con- formal SU(2) theory has a dual description in the Liouville theory in terms of the insertion of a certain degenerate field. It was shown that in a semi-classical limit this implies that the effect of the simple surface operator in the gauge theory can be com- puted from the Seiberg-Witten data, i.e. the curve and the differential. In a further development [15] it was shown how to go beyond the semi-classical analysis performed in [9] in an order-by-order (“B-model”) expansion (this method also works for the cases where several simple surface operators are present). In [15] it was also shown that by combining the conjectures in [5] and [9] (using also a result in [24]) one can obtain (conjectural) closed expressions for the gauge theory instanton partition function in SU(N) theories when simple surface operators are present (this method also works for the non-conformal cases). When lifted to 5d these instanton partition functions have a natural (“A-model”) topological string interpretation. As emphasized by Gukov, in the topological string language a simple surface operator corresponds to a toric brane. Computing topological string partition functions with toric brane insertions leads to agreement [15,16] with what one obtains fromthecombinationoftheconjecturesin[5]and[9]. Inparticular, in[16]itwasargued that in the topological string language this type of conjectured duality corresponds to a geometric transition (see also [18]). For an arbitrary surface operator, generic features of the instanton expansion were discussed in [9]. For a full surface operator one can obtain exact results as we discuss next. 2.2 SU(N) instanton counting in the presence of a full surface operator In a recent paper [1] Alday and Tachikawa proposed that the formalism needed to determine the instanton partition function in the presence of a full surface operator in an SU(N) theory has already been developed in the mathematical literature [19– 21]. (Strictly speaking, it is not completely obvious that the problem solved by the mathematicians is really equivalent to the physics problem, but this is believed to be the case.) 4 Before we describe this construction it is convenient to first briefly recapitulate some relevant facts about the partition function, Z, in an = 2 SU(N) quiver gauge N theory (without surface operators). The partition function contains all information about the low-energy effective action and contains both perturbative (classical and one loop levels only) as well as instanton contributions; in other words Z = Z Z . (2.1) pert inst The Nekrasov instanton partition function Z is obtained from certain (regularised) inst integrals over the moduli space of instantons (first studied in [25]). The regularisation involves two deformationparameters, ǫ andǫ , that ensure that these integrals localise 1 2 to isolated fixed points and can be explicitly evaluated in closed form [23]. The fixed points are labelled by a vector of Young tableaux, λ = (λ1,...,λN) [23], and the resulting instanton partition function takes the form Z = Z (λ)yk, (2.2) inst k λ X where the sum is over all vectors of Young tableaux λ, and the instanton number k = λ is equal to the sum of the boxes in all the λi. | | In general, a succinct way to summarise the result is in terms of a certain character. The character encodes thecontribution to theinstanton partitionfunction fromagiven fixed point and takes the general form χ = ( )ewi . (2.3) ± i X Thecontributiontotheinstantonpartitionfunctionfromthegivenfixedpoint(denoted Z (λ) above) is given by the product over the weights w where the weights coming k i from terms in (2.3) with a minus sign contribute in the denominator and those arising from terms with a plus sign contribute in the numerator. A basic building block is the character for a hypermultiplet of mass m transforming in the bifundamental representation of SU(N) SU(N), which is of the general form × χ (a,a˜,λ,ξ,m). (2.4) bif (The precise form can be found in [26], but will not be needed in this paper.) In the expression (2.4), a = (a ,...,a ) are the Coulomb moduli of the first SU(N) factor 1 N in the gauge group and λ = (λ1,...,λN) is a vector of Young tableaux referring to the same SU(N) factor; ξ = (ξ1,...,ξN) is a vector of Young tableaux referring to the second SU(N) factor and a˜ = (a˜ ,...,a˜ ) are the associated Coulomb moduli. Since 1 N we want the gauge group to be SU(N) we need to impose (by hand) the restriction a = 0 (and similarly for the a˜ ’s). i i i From the expression (2.4) one can obtain the character for other representations of P interest such as the character for N hypermultiplets transforming in the fundamental representation of the first (or second) SU(N) factor, which are arise from χ (a,λ,µ˜) = χ (a,µ˜,λ, ,0), Nfunds bif ∅ χ (a˜,ξ,µ) = χ (µ,a˜, ,ξ,0), (2.5) Nfunds bif ∅ 5 where µ = (µ ,...,µ ) and µ˜ = (µ˜ ,...,µ˜ ) denote the masses of the fundamentals 1 N 1 N without any restriction on µ and µ˜ , and transform under a U(N) flavour sym- i i i i metry. (Alternatively, one can decompose µ into a part transforming under an SU(N) P P flavour symmetry plus anadditional mass parameter transforming under a U(1) flavour symmetry.) The character for a matter multiplet of mass m transforming in the adjoint repre- sentation of SU(N) is given by χ (a,λ,m) = χ (a,a,λ,λ,m), (2.6) adj bif and finally the character of the gauge vector multiplet of SU(N) is obtained via χ (a,λ) = χ (a,a,λ,λ,0). (2.7) vec bif − Just as in the absence of surface operators, the instanton partition function in an SU(N) theory with a full surface operator involves a sum over a certain N-dimensional vector of Young tableaux λ = (λ1,...,λN) where each λi denotes a Young tableau, or equivalently, a partition3, i.e. λi λi . 1 ≥ 2··· It turns out to be very convenient to view the partitions as having a periodicity, λi λi+N. Similarly, the Coulomb moduli are assumed to have the same property: ≡ a a . The character for a bifundamental multiplet can then be written [21,1] i i+N ≡ χbif(a,a˜,λ,ξ,m) = e−m N eak−a˜k−ℓ′eǫ2(⌊ℓ′N−k⌋−⌊−Nk⌋) ξℓk′−ℓe′ǫ1s k=1ℓ′≥1 s=1 XX X e−m N eak−ℓ+1−a˜k−ℓ′eǫ2(⌊ℓ′N−k⌋−⌊ℓ−Nk−1⌋)(eǫ1ξℓk′−ℓ′ 1) λℓk−ℓ+1eǫ1(s−λℓk−ℓ+1) − − k=1 ℓ≥1 ℓ′≥1 s=1 XXX X + e−m N eak−ℓ+1−a˜k−ℓ′+1eǫ2(⌊ℓ′−Nk−1⌋−⌊ℓ−Nk−1⌋)(eǫ1ξℓk′−ℓ′+1 1) λℓk−ℓ+1eǫ1(s−λℓk−ℓ+1) − k=1 ℓ≥1 ℓ′≥1 s=1 XXX X N λℓk−ℓ+1 + e−m eak−ℓ+1−a˜keǫ2(⌊−Nk⌋−⌊ℓ−Nk−1⌋) eǫ1(s−λℓk−ℓ+1) (2.8) k=1 ℓ≥1 s=1 XX X where x denotes the largest integer smaller than or equal to x. ⌊ ⌋ From the result (2.8) one can obtain the character for N hypermultiplets trans- forming in the fundamental representation of the first gauge group by setting ξj = ∅ for all j, cf. (2.5). Similarly, for N hypers in the fundamental representation of the second factor one sets λi = , cf. (2.5). (The masses of the fundamentals are assumed ∅ to have the same periodicity as the Coulomb moduli and the partitions, i.e. µ = µ i i+N etc.) The character for a massive matter multiplet transforming in the adjoint can also easily be obtained, cf. (2.6). Finally, the contribution from a gauge vector multiplet is obtained by setting ξ = λ and m = 0, cf. (2.7). 3In contrast to [21] we label the components, λi, of λi starting from j =1 rather than j =0. j 6 From these building blocks the instanton partition function for an SU(N) quiver gauge theory with bifundamental and fundamental matter multiplets in the presence of a full surface operator can be determined. For a gauge group with a single SU(N) factor the result is of the form Z = Z (λ) yki, (2.9) inst k1,...,kN i λ i X Y where the instanton numbers k are given by [21,1] i k = λi−j+1, (2.10) i j j≥1 X and the variables y (defined for i = 1,...,N and not assumed to be periodic in i) i correspond to the N 1 (holomorphic) parameters of the full surface operator together − with the usual instanton expansion parameter. In the general case of a quiver gauge group with several SU factors, there is a set of y and k for each factor, thus a full i i surface operator corresponds to breaking the complete gauge group to U(1)r where r is the sum of the ranks of all factors of the quiver gauge group. Next we consider in more detail three examples with a single SU(N) factor: the pure SU(N) theory, as well as two superconformal theories, the = 2∗ theory (i.e. the N theory withanadjointmatter multiplet), andthetheorywith N = 2N (i.e. 2N matter f multiplets in the fundamental representation). First we consider the terms with only one k non-zero. In this case, one easily sees i from (2.10) that only λi can be non-zero and furthermore can have boxes only in the first column, i.e. only λi is = 0 . This is because a non-zero λj with j = i inevitably 1 6 6 makes at least one k with j = i non-zero, and the same is true for a non-zero λi with j 6 j j 2. With only λi non-zero and composed of only one column of height n λi, there ≥ ≡ 1 is only one contribution at each order in the instanton expansion. From (2.8) we find that for the = 2∗ SU(N) theory the character corresponding to the yn term in the N i instanton expansion becomes n (e−m 1)(eai+1−ai +1) eǫ1s (i N 1) − ≤ − s=1 X n (e−m 1)(eai+1−ai+ǫ2 +1) eǫ1s (i = N) (2.11) − s=1 X (for the pure SU(N) theory the result is the same but the terms involving e−m are absent), whereas for the SU(N) theory with N = 2N one finds f n ( eai+1−ai +eµi+1−ai +eai−µ˜i−ǫ1n 1) eǫ1s (i N 1) − − ≤ − s=1 X s ( eai+1−ai+ǫ2 +eµi+1−ai+ǫ2 +eai−µ˜i−ǫ1n 1) eǫ1s (i = N) (2.12) − − n=1 X 7 These results lead to the following terms in the instanton partition function for the pure SU(N) theory ∞ n 1 y (0,i) i Z = . (2.13) inst (ai+1 ai + ǫ2 i +1) n! (ǫ )2 n=1 ǫ1 − ǫ1 ǫ1⌊N⌋ n (cid:18) 1 (cid:19) X Similarly, for the = 2∗ SU(N) theory one gets N ∞ (ai+1 ai + ǫ2 i +1 m) (1 m) Z(0,i) = ǫ1 − ǫ1 ǫ1⌊N⌋ − ǫ1 n − ǫ1 n (y )n, (2.14) inst (ai+1 ai + ǫ2 i +1) n! i n=1 ǫ1 − ǫ1 ǫ1⌊N⌋ n X whereas for the SU(N) theory with N = 2N the result is f ∞ (µi+1 ai + ǫ2 i +1) (µ˜i ai) Z(0,i) = ǫ1 − ǫ1 ǫ1⌊N⌋ n ǫ1 − ǫ1 n ( y )n. (2.15) inst (ai+1 ai + ǫ2 i +1) n! − i n=1 ǫ1 − ǫ1 ǫ1⌊N⌋ n X (0,i) In the latter two cases, Z is a hypergeometric function of the form F (A,B;C;y ). inst 2 1 i It is also possible to write down corrections to the above results. One natural class of corrections involve terms of the form yny with i = j. Terms of this type get i j 6 contributions from at most two types of Young tableaux at each order. One always gets a contribution when λi has only one column with n boxes and λj contains only one box, with all other λk empty. In addition, there are two special cases. First, when j = i + 1 one gets a contribution when λi has n boxes in the first column and one box in the second column with all other λk empty. Second, when i = j +1 one gets a contribution when λi has n 1 boxes in the first column and λi−1 has one box in both − the first and second columns, with all other λk empty. As the resulting formulæ are somewhat lengthy they have been relegated to the appendix, cf. (A.3), (A.4). Because of the presence of in the above formulæ, the terms involving y are N ⌊·⌋ treated differently compared to the terms involving only the other y . We will see i in later sections that this result is reflected in the affine conformal blocks where the worldsheet coordinate z is on a different footing compared to the isospin x variables. i The terms in the instanton partition function that are independent of y form an N important subsector that was studied in [20]. Such terms have k = 0, which by (2.10) N implies that λj = 0. Thus only a finite number of components of each λj can N−j+1 be non-zero. In this case the character (2.8) can be simplified. One finds after some algebra that ′ N−1k+1 k ξkj−j′+1−λjk−j+2 χ (a,a˜,λ,ξ,m) = e−m eaj−a˜j′ eǫ1s bif |kN=0 k=1 j=1 j′=1 s=1 XXX X ′ N−1 k k ξkj−j′+1−λjk−j+1 e−m eaj−a˜j′ eǫ1s, (2.16) − k=1 j=1 j′=1 s=1 XXX X which agrees with proposition 5.22 in [20] (after some changes in notation). An impor- tant thing to note is that the y -independent terms only depend on ǫ and not on ǫ , N 1 2 8 which is similar to the setting in [27] (see also [28,29]). It was shown in [20] that the instanton partition function for the = 2∗ theory with k = 0 is (up to a prefactor) N N an eigenfunction of the quantum trigonometric Calogero-Sutherland model. Connec- tions between eigenfunctions of quantum integrable systems and instanton partition functions in the presence of surface operators have also been studied in [30,1,17]. In particular, in [1] (see also [31]) it was argued that the instanton partition function for the = 2∗ theory in the critical limit ǫ 0 is an eigenfunction of the quantum 2 N → elliptic Calogero-Moser model. This result is more directly related to the setup in [27]. Whereas instanton partition functions built from the character (2.8) are intimately connected with the affine sl(N) algebra the results in [20] are based on the ordinary sl(N) algebra. We will see in later sections that this fact has a natural explanation since the part of the affine conformal blocks independent of the worldsheet coordinate z is constructed from descendants that only involve the zero-modes of the affine current, which span the ordinary sl(N) Lie algebra. It is also possible to consider quivers with more that one SU(N) factor. Here we consider one of the simplest such models, the superconformal SU(N) SU(N) model × with onematter multiplet of mass mtransforming inthebifundamental representation, N multiplets with masses µ transforming in the fundamental representation of the i first SU(N) factor and N multiplets with masses µ˜ transforming in the fundamental i representation of the second SU(N) factor. ˜ The simplest class of terms are the ones with k = n and k = p (which arise i j when only λi = n and ξj = p are non-zero). For terms of this type we find that the 1 1 contribution to the instanton partition function is given by ∞ ∞ (µ˜i ai) (a˜i ai+m) (µj+1 a˜j+ǫ2 j +1) (aj+1 a˜j+ǫ2 j +1 m) ǫ1−ǫ1 n ǫ1−ǫ1 ǫ1 n ǫ1 −ǫ1 ǫ1⌊N⌋ p ǫ1 −ǫ1 ǫ1⌊N⌋ −ǫ1 p (ai+1 ai + ǫ2 i +1) n!(a˜j+1 a˜j + ǫ2 j +1) p! Xn=0Xp=0 ǫ1 − ǫ1 ǫ1⌊N⌋ n ǫ1 − ǫ1 ǫ1⌊N⌋ p (a˜i ai p+ m) δij (a˜j aj+1 ǫ2 j + m) δi,j+1 ǫ1 − ǫ1 − ǫ1 n ǫ1 − ǫ1 − ǫ1⌊N⌋ ǫ1 n yny˜p.(2.17) ×" (ǫa˜1i − aǫ1i + ǫm1)n # "(a˜ǫ1j − ajǫ+11 − ǫǫ21⌊Nj ⌋−p+ ǫm1)n# i j It is convenient to change notation for the masses µ˜ µ ǫ i µ µ˜ ǫ i i i+1 2 i+1 i 2 + +1, 1. (2.18) ǫ → ǫ ǫ ⌊N⌋ ǫ → ǫ − ǫ ⌊N⌋− 1 1 1 1 1 1 Using this notation the above expression becomes ∞ ∞ (µi+1 ai+ǫ2 i +1) (a˜i ai+m) (aj+1 a˜j+ǫ2 j +1 m) (µ˜j a˜j) ǫ1 −ǫ1 ǫ1⌊N⌋ n ǫ1−ǫ1 ǫ1 n ǫ1 −ǫ1 ǫ1⌊N⌋ −ǫ1 p ǫ1−ǫ1 p (ai+1 ai + ǫ2 i +1) n!(a˜j+1 a˜j + ǫ2 j +1) p! Xn=0Xp=0 ǫ1 − ǫ1 ǫ1⌊N⌋ n ǫ1 − ǫ1 ǫ1⌊N⌋ p (a˜i ai p+ m) δij (a˜j aj+1 ǫ2 j + m) δi,j+1 ǫ1 − ǫ1 − ǫ1 n ǫ1 − ǫ1 − ǫ1⌊N⌋ ǫ1 n yny˜p.(2.19) ×" (ǫa˜1i − aǫ1i + ǫm1)n # "(a˜ǫ1j − ajǫ+11 − ǫǫ21⌊Nj ⌋−p+ ǫm1)n# i j In this form it is easy to see that the terms with p = 0 or n = 0 reduce to (2.15) with (a ,µ ,µ˜ ) = (a ,µ ,a˜ +m) and (a ,µ ,µ˜ ) = (a˜ ,a m,µ˜ ), respectively. i i i i i i i i i i i i − 9 3 Affine sl(2) and surface operators in SU(2) gauge theories In [1] it was argued that the instanton partition function in an SU(2) quiver gauge theory with a full surface operator insertion is equal to a modified version of an affine sl(2) conformal block. In this section we review and check this proposal, showing how the analytical results of the previous section can be reproduced from affine conformal blocks. We consider the four- and five-point conformal blocks on the sphere and the one-point conformal block on the torus. These are associated to the SU(2) theory with four flavours, the SU(2) SU(2) quiver with a bifundamental hypermultiplet and × two flavours in each SU(2) factor, and the = 2∗ SU(2) gauge theory which has one N adjoint hypermultiplet. In order to fix our conventions, we start by reviewing some basic facts about the affine sl(2) Lie algebra. Thecommutationrelationsthatdefinetheuntwistedaffinesl(2)Liealgebra(usually (1) denoted sl(2) or A ) are given by 1 k [J0,J0b] = nδ , [J0,J±] = J± , [J+,J−] = 2J0 +knδ . (3.1) n m 2 n+m,0 n m ± n+m n m n+m n+m,0 Primary states with respect to this algebra satisfy J0 j = j j and are annihilated by 0| i | i J− j = J0 j = J+ j = 0 (n = 0,1,2,...), (3.2) 1+n| i 1+n| i n | i which implies that j J+ = j J0 = j J− = 0 (n = 0, 1, 2,...). (3.3) h | −1+n h | −1+n h | n − − We denote the corresponding primary field V (x,z), where x is an isospin variable and j z is the worldsheet coordinate. The action of the generators on a primary field can be expressed in terms of differential operators: [JA,V (x,z)] = znDAV (x,z), (3.4) n j j where D+ = 2jx x2∂ , D0 = x∂ +j, D− = ∂ , (3.5) x x x − − which satisfy4 [D0,D±] = D±, [D+,D−] = 2D0. (3.6) ∓ − The descendants of a primary state, j , are denoted n,A;j , where h | h | n,A;j = j JA1 JAℓ , (3.7) h | h | n1 ··· nℓ and we define the level n = n and charge Υ = A . For later reference, we recall i i i i that for the affine sl(2) algebra the matrix of inner products of descendants (usually P P called the Gram or Shapovalov matrix) satisfies Xn,A;n′,A′(j) = n,A;j n′,A′;j δn,n′δΥ,Υ′, (3.8) h | i ∝ i.e. it is a block-diagonal matrix where each block contains only descendants with given values for the level n and charge Υ. 4Since [JA,[JB,V ]] = zn+mDBDAV , consistency of (3.4) implies that [[JA,JB],V (x,z)] = n m j j n m j zn+m[DA,DB]V (x,z). j − 10