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我们得出了新的振幅关系,揭示了在任意时空维度中各种理论中隐藏的统一性。 我们的结果依赖于一组Lorentz不变微分算子,该算子将物理树级散射幅度转化为新的。 通过变换耦合到dilaton和两种形式的重力振幅,我们生成了爱因斯坦-杨-米尔斯理论,狄拉克-博恩-内菲尔德理论,特殊的伽利略,非线性sigma模型和双联标量理论的所有振幅。 mut变还与弦理论及其变体中的振幅有关。 作为必然结果,胶子和引力子散射的著名方面,如颜色运动学对偶性,KLT关系和CHY构造,都是the变振幅的遗传特征。 We变重铸了Adler零,这是Weinberg软定理的琐碎结果,并暗示了某些标量理论的新的次导软定理。
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JHEP02(2018)095
Published for SISSA by Springer
Received: June 5, 2017
Revised: November 14, 2017
Accepted: February 8, 2018
Published: February 15, 2018
Unifying relations for scattering amplitudes
Clifford Cheung,
a
Chia-Hsien Shen
b
and Congkao Wen
a,b
a
Walter Burke Institute for Theoretical Physics, California Institute of Technology,
Pasadena, CA 91125, U.S.A.
b
Mani L. Bhaumik Institute for Theoretical Physics,
Department of Physics and Astronomy, UCLA,
Los Angeles, CA 90095, U.S.A.
E-mail: clifford.cheung@caltech.edu, chshen@caltech.edu,
cwen@caltech.edu
Abstract: We derive new amplitudes relations revealing a hidden unity among a wide-
ranging variety of theories in arbitrary spacetime dimensions. Our results rely on a set of
Lorentz invariant differential operators which transmute physical tree-level scattering am-
plitudes into new ones. By transmuting the amplitudes of gravity coupled to a dilaton and
two-form, we generate all the amplitudes of Einstein-Yang-Mills theory, Dirac-Born-Infield
theory, special Galileon, nonlinear sigma model, and biadjoint scalar theory. Transmuta-
tion also relates amplitudes in string theory and its variants. As a corollary, celebrated
aspects of gluon and graviton scattering like color-kinematics duality, the KLT relations,
and the CHY construction are inherited traits of the transmuted amplitudes. Transmu-
tation recasts the Adler zero as a trivial consequence of the Weinberg soft theorem and
implies new subleading soft theorems for certain scalar theories.
Keywords: Effective Field Theories, Scattering Amplitudes
ArXiv ePrint: 1705.03025
Open Access,
c
The Authors.
Article funded by SCOAP
3
.
https://doi.org/10.1007/JHEP02(2018)095
JHEP02(2018)095
Contents
1 Introduction and summary 1
2 Transmutation 4
2.1 Physical constraints 4
2.2 Transmutation operators 5
3 Unified web of theories 8
3.1 Single trace amplitudes 9
3.2 Multiple trace amplitudes 10
3.3 Summary of unifying relations 11
3.4 Ultraviolet completion 11
4 Examples 13
4.1 Three-particle amplitudes 13
4.2 Four-particle amplitudes 13
4.3 Gluon and graviton amplitudes 15
5 Proof by induction 16
5.1 On-shell constructibility 17
5.2 Factorization 17
5.3 Double copy construction 20
6 Infrared structure 21
6.1 Soft theorems at leading order 22
6.2 Soft theorems at subleading order 23
7 Outlook and future directions 24
1 Introduction and summary
The modern S-matrix program has exposed marvelous structures long hidden in plain sight
within gauge theory and gravity. The fact that these theories are endowed with exceptional
properties is perhaps unsurprising given that their form is uniquely dictated by locality and
gauge invariance. Nevertheless, in recent years it has become abundantly clear that many
of these structures are actually commonplace. For example, the Kawai-Lewellen-Tye (KLT)
relations [1], Bern-Carrasco-Johansson (BCJ) color-kinematics duality [2], and Cachazo-He-
Yuan (CHY) construction [3–5] all apply across a tremendous range of theories, including
Einstein-Yang-Mills (EYM) theory, Dirac-Born-Infeld (DBI) theory, the special Galileon
(SG), the nonlinear sigma model (NLSM), and the biadjoint scalar (BS) theory. A natural
– 1 –
JHEP02(2018)095
question then arises: why these theories? Imbued with symmetries of disparate origin and
character, these theories do not obviously conform to any cohesive organizing principle that
would place them on equal footing in the eyes of the S-matrix.
In this paper, we offer an explanation for the peculiar universality of these structures.
Our results follow from a set of simple unifying relations which “transmute” the tree-level
scattering amplitudes of certain theories into those of others. By transmuting the S-matrix
of the mother of all theories — gravity coupled to a dilaton and two-form, i.e., “extended
gravity” — we beget the S-matrices of all the theories previously mentioned, thus revealing
their many shared traits as congenital.
Transmutation is by design independent of the particular form in which an ampli-
tude happens to be represented. To achieve this, we study amplitudes as formal functions
of Lorentz invariant products of the external on-shell kinematic data: e
i
e
j
, p
i
e
j
, and p
i
p
j
where e
i
and p
i
denotes polarization and momentum of particle i, respectively, and i 6= j.
1,2
In this language, it is straightforward to construct a basis of gauge invariant operators which
preserve on-shell kinematics. Perhaps unsurprisingly, these operators automatically trans-
mute gauge invariant scattering amplitudes into new ones. Our roster of transmutation
operators is summarized as follows:
• The trace operator T
ij
= ∂
e
i
e
j
reduces the spin of particles i and j by one unit and
places them within a new color trace structure. This operator transmutes gravitons
into photons, gluons into biadjoint scalars, and BI photons into DBI scalars.
• The insertion operator T
ijk
= ∂
p
i
e
j
− ∂
p
k
e
j
reduces the spin of particle j by one unit
and inserts it between particles i and k within a color trace structure. This operator
transmutes gravitons into gluons, gluons into biadjoint scalars, and BI photons into
pions, at the level of color-ordered amplitudes of the resulting theories, which we will
properly define in section 3.
• The longitudinal operator L
i
=
P
j
p
i
p
j
∂
p
j
e
i
reduces the spin of particle i by one
unit while converting it to a longitudinal mode. Necessarily applied to all particles
at once, this operator transmutes gravitons into BI photons, gluons into pions, and
BI photons into SG scalars.
These transmutation operators form the building blocks of the unified web of theories
depicted in figure 1. Theory space is partitioned into three triangular regions corresponding
to the nearest of kin to extended gravity, BI, and YM. The corners of the web denote all of
the theories previously discussed, while the edge and bulk regions denote hybrid theories.
Remarkably, these triangular regions are not independent — due to KLT [1] and BCJ [2],
extended gravity can be recast as the double copy of YM, and BI as the product of YM
1
So as not to complicate our notation we omit the usual “·” denoting Lorentz-invariant contractions.
2
In the extended gravity, all the degrees of freedom can be incorporated into a single polarization tensor
e
µν
. It can written as a product of two copies of YM polarizations, e
µν
= e
µ
˜e
ν
. The usual pure graviton
corresponds to the symmetric and traceless component of e
µν
. Since the transmutation only acts on one of
the copies, we focus on the un-tilded copy without loss of generality.
– 2 –
JHEP02(2018)095
T
ij
L
EM
T
ij
T
ijk
L
DBI
T
ij
YMS
T
ijk
BS
L
YM
T
ijk
BI
NLSM
SG
G
Figure 1. Diagram depicting the unified web of theories. The corners represent extended gravity
(G), Einstein-Maxwell (EM) theory, Yang-Mills (YM) theory, Born-Infeld (BI) theory, Dirac-Born-
Infeld (DBI) scalar theory, nonlinear sigma model (NLSM), special Galileon (SG), Yang-Mills scalar
(YMS) theory, and biadjoint (BS) theory. The arrows correspond to the transmutation operators,
T
ij
, T
ijk
, and L. The shaded regions and edges correspond to hybrid theories.
and the NLSM. Hence, one can actually construct the entire web purely from the middle
triangle corresponding to YM and its descendants.
Armed with the notion of transmutation, it is then straightforward to import familiar
structures in gauge theory and gravity into their descendants. For example, transmutation
trivially explains the near-universal applicability of the KLT, BCJ, and CHY constructions.
Likewise, the Weinberg soft theorems transmute into the Adler zero conditions on pions
and Galileons, as first observed in [6]. At subleading order, we can also derive new soft
theorems in the spirit of [7].
The remainder of this paper is organized as follows. In section 2, we systematically
construct a basis of transmutation operators. We define the notion of color-ordering which
is crucial for insertion and discuss the web of theories related by transmutation in section 3.
Examples are illustrated in section 4. A proof by induction of our claims is presented in
section 5, followed by a discussion of the infrared structure in section 6. Finally, we conclude
with a discussion of outlook and future directions in section 7.
– 3 –
JHEP02(2018)095
2 Transmutation
2.1 Physical constraints
In an abstract sense, the purpose of a transmutation operator T is to convert a gauge
invariant object A into a new gauge invariant object T · A. At the very minimum, T
should conform to the following baseline physical criteria:
i) T preserves on-shell kinematics.
ii) T preserves gauge invariance.
Let us consider each of these conditions in turn.
On-shell kinematics. A physical scattering amplitude of massless particles is well-
defined on the support of the on-shell conditions, p
i
p
i
= p
i
e
i
= 0, and momentum con-
servation,
P
i
p
i
= 0. Representations of the amplitude that differ by terms which vanish
on-shell are physically equivalent. It is then crucial that our construction be agnostic to
such differences.
To preserve the on-shell conditions, we simply define the physical scattering amplitude
A to be a function of p
i
p
j
, p
i
e
j
, and e
i
e
j
for i 6= j. For momentum conservation, we define
the total momentum operator
P
v
≡
X
i
p
i
v , (2.1)
where i runs over all external legs and v labels any momentum or polarization vector. Due
to the implicit momentum-conserving delta function in A, we have that
P
v
· A = 0 . (2.2)
To ensure that T conserves momentum it is then sufficient to require that
[ P
v
, T ] · A = 0 , (2.3)
so P
v
· T · A = T · P
v
· A = 0, i.e. the transmuted amplitude also conserves momentum.
This does not trivially hold for differential operators involving p
i
p
j
and p
i
e
j
as we will see
explicitly in next section. Since [ P
v
, P
w
] = 0, this operator itself conserves momentum.
Gauge invariance. A physical scattering amplitude should also be gauge invariant. To
incorporate this constraint we define a differential operator corresponding to the Ward
identity on particle i,
W
i
≡
X
v
p
i
v ∂
ve
i
. (2.4)
Here v runs over all momentum and polarization vectors in the amplitude. Any gauge
invariant amplitude is annihilated by this operator,
W
i
· A = 0 . (2.5)
– 4 –
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