Physics Letters B 746 (2015) 293–299
Contents lists available at ScienceDirect
Physics Letters B
www.elsevier.com/locate/physletb
Local contributions to factorized soft graviton theorems at loop level
Johannes Broedel
a
, Marius de Leeuw
a,b
, Jan Plefka
a,c
, Matteo Rosso
a,∗
a
Institut für Theoretische Physik, Eidgenössische Technische Hochschule Zürich, Wolfgang-Pauli-Strasse 27, 8093 Zürich, Switzerland
b
Niels Bohr Institute, Copenhagen University, Blegdamsvej 17, 2100 Copenhagen Ø, Denmark
c
Institut für Physik und IRIS Adlershof, Humboldt-Universität zu Berlin, Zum Großen Windkanal 6, 12489 Berlin, Germany
a r t i c l e i n f o a b s t r a c t
Article history:
Received
20 February 2015
Received
in revised form 7 May 2015
Accepted
8 May 2015
Available
online 14 May 2015
Editor:
L. Alvarez-Gaumé
Keywords:
Scattering
amplitudes
Graviton
amplitudes
Gluon
amplitudes
Soft
radiation
We analyze the low-energy behavior of scattering amplitudes involving gravitons at loop level in four
dimensions. The single-graviton soft limit is controlled by soft operators which have been argued to
separate into a factorized piece and a non-factorizing infrared divergent contribution. In this note we
show that the soft operators responsible for the factorized contributions are strongly constrained by
gauge and Poincaré invariance under the assumption of a local structure. We show that the leading
and subleading orders in the soft-momentum expansion cannot receive radiative corrections. The first
radiative correction occurs for the sub-subleading soft graviton operator and is one-loop exact. It depends
on only two undetermined coefficients which should reflect the field content of the theory under
consideration.
© 2015 The Authors. 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
Recently the low energy behavior of graviton scattering am-
plitudes
with a single soft graviton momentum has been related
[1] to Ward identities of the extended Bondi, van der Burg, Met-
zner
and Sachs (BMS) symmetry [2]. The role of BMS symmetry as
a potential hidden symmetry of the quantum gravity S-matrix in
asymptotically flat four-dimensional space-time has triggered con-
siderable
interest.
In
the soft limit the (n + 1)-point scattering amplitude is dom-
inated
by a soft pole as was shown by Weinberg about fifty years
ago [3]. The soft amplitude factorizes on the pole into a universal
soft function and the remaining hard graviton n-point amplitude.
This property holds in any spacetime dimension.
Revived
by the work of Cachazo and Strominger [4] the uni-
versal
factorization has been shown to extend to sub- and sub-
subleading
order in the soft-momentum expansion [5]. In distinc-
tion
to the leading Weinberg pole, which is a function of the soft
and hard momenta and the soft graviton polarization, the sub-
and
sub-subleading soft behavior of the (n +1)-point graviton am-
plitude
can be expressed in terms of differential operators in the
*
Corresponding author.
E-mail
addresses: jbroedel@itp.phys.ethz.ch (J. Broedel), deleeuwm@nbi.ku.dk
(M. de Leeuw),
plefka@physik.hu-berlin.de (J. Plefka), mrosso@itp.phys.ethz.ch
(M. Rosso).
hard momenta and hard graviton polarizations acting on the hard
n-point amplitude.
Gluon
amplitudes exhibit a similar universal leading and sub-
leading
soft behavior at tree level. Known as Low’s theorem [6],
the form of the soft operators was recently recast into the lan-
guage
of modern on-shell techniques [7]. A discussion of the soft
behavior of quark-gluon tree-level amplitudes was provided in
Ref. [8]. The soft limit of open-string tree amplitudes was stud-
ied
in Refs. [9,10], where further corrections to the field-theory
soft theorems have been excluded. Interestingly, the subleading
soft theorems for gravitons and gluons may also be derived from
the soft limit of vertex operators in a dual ambitwistor string the-
ory [11].
In
a series of recent papers [12,13] the existence of subleading
soft-graviton and soft-gluon theorems was shown to be a conse-
quence
of on-shell gauge invariance and Feynman diagrammatic
reasoning. Simultaneously, an alternative point of view was put
forward by the present authors in Ref. [14]: The tree-level soft-
graviton
and soft-gluon expressions are composed from a highly
restricted class of operators compatible with on-shell gauge in-
variance,
factorization and Poincaré symmetry. This result could be
established without referring to underlying Feynman diagrammat-
ics
at all. For the graviton case, this reasoning supplements the
established sub- and sub-subleading soft operators with one fur-
ther
candidate each.
Given
these results it is natural to ask whether the soft-
graviton
theorems receive corrections at loop level [15,16]. In four-
dimensional
gravity the dimension of the coupling constant κ
http://dx.doi.org/10.1016/j.physletb.2015.05.018
0370-2693/
© 2015 The Authors. 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
.