Physics Letters B 790 (2019) 389–395
Contents lists available at ScienceDirect
Physics Letters B
www.elsevier.com/locate/physletb
Spin projection operators and higher-spin Cotton tensors in three
dimensions
Evgeny I. Buchbinder, Sergei M. Kuzenko
∗
, James La Fontaine, Michael Ponds
Department of Physics M013, The University of Western Australia, 35 Stirling Highway, Crawley, W.A. 6009, Australia
a r t i c l e i n f o a b s t r a c t
Article history:
Received
10 January 2019
Accepted
18 January 2019
Available
online 24 January 2019
Editor:
M. Cveti
ˇ
c
We elaborate on the spin projection operators in three dimensions and use them to derive a new
representation for the linearised higher-spin Cotton tensors.
© 2019 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license
(http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP
3
.
1. Introduction
In four dimensions (4D), there exists a remarkably simple ex-
pression
for the linearised higher-spin Weyl tensors in terms of
gauge prepotentials h
α
1
...α
s
˙
α
1
...
˙
α
s
= h
(α
1
...α
s
)(
˙
α
1
...
˙
α
s
)
, see e.g. [1,2]
and
section 6.9 of [3]. In the case of an integer spin s ≥1, it reads
C
α
1
...α
2s
= ∂
(α
1
˙
β
1
...∂
α
s
˙
β
s
h
α
s+1
...α
2s
)
˙
β
1
...
˙
β
s
. (1.1)
For a half-integer spin s +
1
2
≥
3
2
, with s = 1, 2, ..., we have
C
α
1
...α
2s+1
= ∂
(α
1
˙
β
1
...∂
α
s
˙
β
s
ψ
α
s+1
...α
2s+1
)
˙
β
1
...
˙
β
s
. (1.2)
It follows from (1.1) and (1.2) that the higher-spin Weyl tensors are
invariant under gauge transformations of the form
δ
ζ
h
α
1
...α
s
˙
α
1
...
˙
α
s
= ∂
(α
1
(
˙
α
1
ζ
α
2
...α
s
)
˙
α
2
...
˙
α
s
)
, (1.3)
δ
ξ
ψ
α
1
...α
s+1
˙
α
1
...
˙
α
s
= ∂
(α
1
(
˙
α
1
ξ
α
2
...α
s+1
)
˙
α
2
...
˙
α
s
)
. (1.4)
It should be remarked that there are two ways for the bosonic
gauge field h
α(s)
˙
α(s)
= h
α
1
...α
s
˙
α
1
...
˙
α
s
to occur in higher-spin gauge
theories.
1
Firstly, h
α(s)
˙
α(s)
is one of the two gauge prepotentials
{h
α(s)
˙
α(s)
, h
α(s−2)
˙
α(s−2)
} in the Fronsdal massless integer-spin mod-
els
2
[4,5](see section 6.9 of [3]for a review). Secondly, it is the
gauge field in the Fradkin–Tseytlin conformal integer-spin theo-
ries [6].
In the former theories, C
α(2s)
and its conjugate are the
*
Corresponding author.
E-mail
addresses: evgeny.buchbinder@uwa.edu.au (E.I. Buchbinder),
sergei.kuzenko@uwa.edu.au (S.M. Kuzenko), 21319182@student.uwa.edu.au
(J. La Fontaine),
michael.ponds@research.uwa.edu.au (M. Ponds).
1
The story with the fermionic gauge field ψ
α(s+1)
˙
α(s)
is analogous.
2
The compensator h
α(s−2)
˙
α(s−2)
transforms under (1.3)by the rule
δ
ζ
h
α(s−2)
˙
α(s−2)
∝ ∂
β
˙
β
ζ
βα(s−2)
˙
β
˙
α(s−2)
.
only gauge invariant field strengths which survive on the mass
shell. In the latter theories, the gauge-invariant action may be for-
mulated
in terms of C
α(2s)
and its conjugate [1,2].
In
three dimensions, the Weyl tensor vanishes identically and
all information about the conformal geometry of spacetime is en-
coded
in the Cotton tensor. Spacetime is conformally flat if and
only if the Cotton tensor vanishes [7](see [8]for a modern
proof). Linearised higher-spin extensions of the Cotton tensor in
Minkowski space were constructed in [9] and [10]in the bosonic
and fermionic cases, respectively. In terms of a gauge prepotential
h
α
1
...α
n
=h
(α
1
...α
n
)
, with n > 1, the linearised Cotton tensor is given
by the expression [10]
C
α(n)
(h) =
1
2
n−1
n/2
j=0
n
2 j
+1
×2
j
∂
(α
1
β
1
...∂
α
n−2 j−1
β
n−2 j−1
h
α
n−2 j
...α
n
)β
1
...β
n−2 j−1
,
(1.5)
where x denotes the floor function and returns the integer part
of a real number x ≥ 0. The fundamental properties of C
α(n)
are
the following:
(i) C
α(n)
is invariant under gauge transformations of the form
δ
ζ
h
α(n)
= ∂
(α
1
α
2
ζ
α
3
...α
n
)
=⇒ δ
ζ
C
α(n)
= 0 ; (1.6)
(ii) C
α(n)
is divergenceless
∂
βγ
C
βγα
1
...α
n−2
=0 . (1.7)
Unlike the 4D relations (1.1)and (1.2), the expression for C
α(n)
given by (1.5)is not illuminating. It is not obvious from (1.5) that
https://doi.org/10.1016/j.physletb.2019.01.040
0370-2693/
© 2019 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by
SCOAP
3
.