Vanishing Property of BRST Cohomology for Modiﬁed Highest Weight Modules

: We construct certain modiﬁed highest weight modules which are called quasi highest weight modules in this paper. Using the quasi highest weight modules, we introduce a new category of modules over an afﬁne Lie superalgebra which contains projective covers. We also prove that both these projective covers and the quasi highest weight modules satisfy the vanishing property of BRST cohomology.


Introduction
Let g be a simple finite-dimensional complex Lie superalgebra and f be its any even nilpotent element. Then, we can construct an associated algebra denoted by W(g, f ) through the cohomology of Becchi-Rouet-Strora-Tyutin (shortly BRST) complex (see [1]). We call this associated algebra a W-algebra. W-algebras appeared around 80's in the study of rational conformal field theories and can be considered as a generalization of vertex algebras [2][3][4][5].
Let g be the affinization of g, and fix its positive root system ∆ + in the root system ∆ of g. Let h be the associated Cartan subalgebra of g (see Section 2.2 for the details). Then, we obtain the full subcategory O k of the category of left g-modules with level k whose objects satisfy the following conditions (see [6]) : The set of weights of V is contained in n i=1 λ i − Z ≥0 ∆ + for some finite subset {λ 1 , . . . , λ n } of h * , where Z ≥0 ∆ + is the Z ≥0 -span of ∆ + .
Through the theory of BRST cohomology, one can construct a functor H from the category O k to the category of W(g, f )-modules. More explicitly, the W(g, f )-module corresponding to a g-module M is the cohomology H(M) of the BRST complex associated to M (see [7][8][9]). This functor was studied in [1,[10][11][12] in order to compute the characters of W(g, f )-modules. In addition, it is known that the vanishing property of BRST cohomology is satisfied in the category O k (see [13][14][15]). Namely, for any object of O k its BRST cohomology is vanished except for the degree 0. In [16], this vanishing property of BRST cohomology was extended to a certain larger category containing O k .
One of the main purposes of this article was to search for another category of g-modules which satisfies the vanishing property of BRST cohomology. For this purpose, we shall construct a new category Q k of modules over an affine Lie superalgebra based on certain modified highest weight modules. We shall call these modified highest weight modules the quasi highest weight modules in this paper. The quasi highest weight modules are motivated from the the generalized Verma modules introduced in [17] (Section 6) . We are particularly interested in a certain specific quasi highest weight modules P λ (see Section 3.2), and we shall prove that they satisfy the vanishing property of BRST cohomology (see Theorem 5).
However, we should point out that it is necessary to know if the category Q k contains projective covers satisfying the vanishing property of BRST cohomology in order to prove the vanishing property of BRST cohomology for any object in Q k . For this reason, we are also interested in constructing projective covers in Q k which satisfy the vanishing property of BRST cohomology. In fact we shall prove that there exist projective covers in Q k , and they yield the composition series whose successive factors are equal to the quasi highest weight modules P λ (see Theorem 4). In addition, it turns out that these composition series give rise to the vanishing property of BRST cohomology (see Theorem 6).
In the forthcoming article, we shall show how the projective covers constructed in this article can be used to prove the vanishing property of BRST cohomology for any object of Q k .

Setting-Up
Assume that g is a simple finite-dimensional complex Lie superalgebra with a nondegenerate even supersymmetric bilinear form ( | ). Let {e, x, f } be an sl 2 -triple of even elements of g normalized as [e f ] = x, [x e] = e, [x f ] = − f . Then, we obtain the following properties from the representation theory of sl 2 : (P1) There exists the eigenspace decomposition g = j∈ 1 2 Z g j with respect to the action of adx.
(P2) ad f : yields a vector space isomorphism. We should notice that the element f gives rise to a skew-supersymmetric even bilinear form , on g 1 2 defined by the formula a, b = ( f |[a b]). In addition, we obtain from (P2) that , is a nondegenerate bilinear form on g 1 2 .
Write g f for the centralizer of f in g. In other words, g f = {x ∈ g|[ f x] = 0}. Then, it follows from the representation theory of sl 2 that g f = ⊕ j∈ 1 [18]).
Consider a Cartan subalgebra h ⊂ g 0 of g containing x. Let ∆ ⊂ h * be the set of roots of g. Fix the root vector u α ∈ g α satisfying (u α |u −α ) = 1 for each α ∈ ∆. It is well-known that each root space g α is one-dimensional except for the case of type A(1, 1) (see [19]). To avoid this exceptional case, we shall always assume that g is a simple basic Lie superalgebra different from the type A(1, 1) in the remaining part of this paper.

Affine Lie Superalgebras
[ Recall that the bilinear form ( | ) is extended from g to g by the rules 1.
In the remaining part of this paper, we shall fix the triangular decomposition We also write ∆, ∆ + and ∆ − for the set of roots, positive roots and negative roots of g, respectively. In addition, we denote by h * k the set λ ∈ h * |λ(K) = k for a complex number k.

New Category
We first introduce a new triangular decomposition of g. Definition 1.

1.
The quasi triangular decomposition of g is A simultaneous eigenvector of h that is annihilated by n 0,− ⊕ g >0 ⊕ g > is called a quasi highest weight vector of g.

3.
A U( g)-module generated by a single quasi highest weight vector is called a quasi highest weight U( g)-module, where U( g) denotes the universal enveloping algebra of g.
Example 1. Let Cv be the 1-dimensional U(g)-module generated by a single vector v such that v is a simultaneous eigenvector of h and n 0,− ⊕ g >0 acts trivially. Set K acts as scalar k on Cv.
Then, we see that V is the quasi highest weight U( g)-module with quasi highest weight vector 1 ⊗ v. We also point out that V is an example of the generalized Verma modules defined in [17] (Section 6). Now, we introduce new categories of U( g)-modules containing quasi highest weight modules.

Definition 2.
The category Q k is the full subcategory of the category of the left U( g)-modules at level k which has objects V satisfying the following conditions: There exists a finite subset {µ 1 , . . . , µ n } of h * k such that Definition 3. Let Q k (Λ) be the full subcategory of Q k whose objects are those modules V of Q k satisfying

Definition 4.
Let FQ k (Λ) be the full subcategory of Q k (Λ) whose objects are finitely generated U( g)-modules.
Proof. (1) is immediate from the definition of P 1 . In oder to prove (2), assume that α + β ∈ P 2 . Then, we have α + β ∈ P 1 since Let us now consider the decomposition where U( g > ⊕ g >0 ) is graded by declaring that each monomial x j 1 x j 2 · · · x j n x j i ∈ g α j i for α j i ∈ ∆ + − ∆ 0,+ is of degree α j 1 + · · · + α j n .

Lemma 2. Let V be an object of
where C µ is the 1-dimensional h-module with basis 1 whose action is h · 1 = µ(h)1.
We first should check that Φ(A) is an U h -module homomorphism. In fact, for h ∈ h we get This implies that Φ is well-defined. We now prove that Ψ is well-defined. We first show that x · y · f (1) is independent of the choice of y ∈ U( g ⊕ g >0 ). Let y = y + u for u ∈ α∈P 2 U( g > ⊕ g >0 ) α . Since V ∈ Obj(Q k (Λ)), we should have for f ∈ Hom U( h) C µ , V and α ∈ P 2 . This yields that u · f (1) = 0, and Ψ( f )(x ⊗ [y]) is independent of the choice of y. In addition, Thus, one has Ψ( f ) ∈ Hom U( g) (P(µ), V) and hence Ψ is well-defined. Finally, we see that .
This shows that Φ and Ψ are inverse of one another. The result now follows.
The following result shows that P(µ) is a projective object in the category FQ k (Λ).
It is obvious that we can find f ∈ Hom g C µ , N making the diagram (5) commutes. So, by Lemma 2 the homomorphism f gives rise to the associated homomorphism Ψ( f ) ∈ Hom g (P(µ), N). Moreover, for x ⊗ [y] ∈ P(µ) we see that The result now follows.
In the following theorem, we prove that the category FQ k (Λ) contains enough projective objects.
Proof. Let {v 1 , · · · , v n } be a set of generators of V consisting of weight vectors, say v i ∈ V µ i . By Lemma 2, we obtain The result now follows.

Composition Series
For λ ∈ h * k , we define an 1-dimensional U g > ⊕ g >0 ⊕ n 0,− ⊕ h -module structure on C λ with basis 1 as follows: Notice that P λ is a free U( g < ⊕ g <0 ⊕ n 0,+ )-module of rank 1 with basis 1 ⊗ 1. We also point out that P λ is a quasi highest weight U( g)-module with quasi highest weight vector 1 ⊗ 1.
In the following theorem, we construct an analogue of a Verma composition series for the U( g)-module P(µ) defined in (3).
Proof. By (2), the weights of W(µ) are of the form µ + α k for α k ∈ P 1 . Choose a basis {w 1 , · · · , w n } of W(µ) so that w i ∈ W(µ) µ+α i for α i ∈ P 1 . We arrange w i so that α r ≥ α s implies r ≥ s, and define W j = i≥j Cw i . Then, we have Notice that U g > ⊕ g >0 ⊕ n 0,− ⊕ h W j ⊂ W j+1 since n 0,− acts trivially on W(µ) and g β w j ∈ W(µ) µ+α j +β for all β ∈ ∆ + − ∆ 0,+ . This implies that each W j is a Then, we obtain from (7) that Notice that M j M j+1 is a free U( g < ⊕ g <0 ⊕ n 0,+ )-module of rank 1 with basis 1 ⊗ w j , and hence M j M j+1 is isomorphic to the quasi highest weight module P λ j , where λ j = µ + α j .
Define deg1 = 0, degψ α (n) = −1 and degψ α (n) = 1 for α ∈ ∆ >0 and n ∈ Z. Then, one obtain the induced decomposition of F(Lg >0 ): Recall that g 1 2 is a superspace with the nondegenerate skew-supersymmetric bilinear form , (see Section 2.1). Denote by g ne ⊗ C t, t −1 , and let CL Lg ne 1 2 be the Clifford superalgebra associated with Lg ne 1 2 . Then, the superalgebra CL Lg ne 1 2 is generated by Φ α (n) α ∈ ∆ 1 2 , n ∈ Z with the relation Write {u α } α∈∆ 1 2 for the dual basis of {u α } α∈∆ 1 2 with respect to , ; that is u α , u β = δ α,β , and denote by Φ α the corresponding dual basis of g ne 1 2 associated with u α . It is immedi- Let F ne ( f ) be the irreducible representation of CL Lg ne 1 2 generated by the vector 1 with the property Φ α (n)1 = 0 for α ∈ ∆ 1 2 and n ≥ 0. For an object V Q k , set where where p(α) denotes the parity of u α . Notice that the operator d is an odd operator, and dC i (V) ⊂ C i+1 (V). In addition, we obtain [d d] = 2d 2 = 0 from Theorem 2.1 in [17]. Thus (C(V), d) becomes a cohomology complex. The cohomology H i (V) := H i (C(V), d) for i ∈ Z. (11) given by the complex (C(V), d) is called the cohomology of the BRST complex of the quantized Drinfeld-Sokolov reduction.

Main Results
We first prove the vanishing property of BRST cohomology for the quasi highest weight module P λ defined in (6).
Recall from (6) that P λ is a free U( g < ⊕ g <0 ⊕ n 0,+ )-module of rank 1. Thus, due to the isomorphism 1 ⊗ ω 0 defined in (12), P λ is isomorphic to the Verma module M(λ) with highest weight λ. Since the boundary operator d in (10) commutes with the isomorphism 1 ⊗ ω 0 , we have H i (P λ ) = H i (M(λ)) for all i. The result now follows from Theorem 6.3.1 in [14].

Conflicts of Interest:
The authors declare no conflict of interest.