JHEP11(2019)115
while the fermion transformation rule is:
δψ
µ
α(n), I
= ∂
µ
ǫ
α(n), I
−
1
4
κ h
ρσk µ
n +
1
2
γ
ρσ
ǫ
α(n), I
− nγ
β
ρσ
γ
α
ǫ
α(n−1)β, I
+ O(κ
2
).
(2.25)
This ends our brief exposition of generalized hypergravity. In the above discussion,
when the flavor index I is removed, the theory reduces to N = 1 supergravity and the
spin-5/2 Aragone-Deser hypergravity [
8] respectively for the cases of n = 0 and n = 1.
3 BRST deformation scheme
In this section we explain the BRST deformation scheme−our main analytical tool to study
the uniqueness of hypergravity. What follows is an almost verbatim repetition of the same
discussions appearing in [
22, 23]. As pointed out in [11, 12], it is possible reformulate
the classical problem of introducing consistent interactions in a gauge theory in terms of
the BRST differential and the BRST cohomology. The advantage of this cohomological
approach is that it systematizes the search for all possible consistent interactions. It also
relates the obstructions to deforming a gauge-invariant action to precise cohomological
classes of the BRST differential.
Fields and antifields. Let there be an irreducible gauge theory of a set of fields {φ
i
},
with m gauge symmetries: δ
ε
φ
i
= R
i
α
ε
α
, α = 1, 2, . . . , m. Then one introduces a ghost field
C
α
corresponding to each gauge parameter ε
α
; they have the same algebraic symmetries
but opposite Grassmann parity (ǫ). The original fields and ghosts are collectively called
fields, denoted by Φ
A
. One further introduces, for each field and ghost, an antifield Φ
∗
A
that
has the same algebraic symmetries (in the multi-index A) but opposite Grassmann parity.
Gradings. Two gradings are introduced in the algebra generated by the fields and an-
tifields: the pure ghost number (pgh) and the antighost number (agh). The former is
non-zero only for the ghost fields. For irreducible gauge theories, in particular, one has:
pgh(C
α
) = 1, while pgh(φ
i
) = 0 for any original field. On the other hand, the antighost
number is non-zero only for the antifields Φ
∗
A
, i.e., agh(Φ
∗
A
) = pgh(Φ
A
) + 1, agh(Φ
A
) =
0 = pgh(Φ
∗
A
). Another grading is the ghost number (gh), defined as gh = pgh −agh.
Antibracket. On the space of fields and antifields, one then defines an odd symplectic
structure:
(X, Y ) ≡
δ
R
X
δΦ
A
δ
L
Y
δΦ
∗
A
−
δ
R
X
δΦ
∗
A
δ
L
Y
δΦ
A
. (3.1)
This is called the antibracket that satisfies the graded Jacobi identity. It follows that
Φ
A
, Φ
∗
B
= δ
A
B
, which is real. But a field and its antifield have opposite Grassmann parity.
Therefore, if Φ
A
is purely real (imaginary), Φ
∗
B
will be purely imaginary (real).
Master action. Let S
(0)
[φ
i
] be the gauge-invariant action in terms of the original fields.
One extends it to the master action, S[Φ
A
, Φ
∗
B
], that includes terms involving ghosts and
antifields:
S[Φ
A
, Φ
∗
B
] = S
(0)
[φ
i
] + φ
∗
i
R
i
α
C
α
+ . . . . (3.2)
– 7 –