118 research outputs found
Soft sub-leading divergences in Yang-Mills amplitudes
In this short note I show that the soft limit for colour-ordered tree-level
Yang-Mills amplitudes contains a sub-leading divergent term analogous to terms
found recently by Cachazo and Strominger for tree-level gravity amplitudes
Infrared behaviour of the one-loop scattering equations and supergravity integrands
The recently introduced ambitwistor string led to a striking proposal for
one-loop maximal supergravity amplitudes, localised on the solutions of the
ambitwistor one-loop scattering equations. However, these amplitudes have not
yet been explicitly analysed due to the apparent complexity of the equations
that determine the localisation. In this paper we propose an analytic solution
to the four-point one-loop scattering equations in the infrared (IR) regime of
the amplitude. Using this solution, we compute the ambitwistor integrand and
demonstrate that it correctly reproduces the four-graviton integrand, in the IR
regime. This solution qualitatively extends to n points. To conclude, we
explain that the ambitwistor one-loop scattering equations actually correspond
to the standard Gross & Mende saddle point.Comment: 23 pages, 5 figure
Windings of twisted strings
Twistor string models have been known for more than a decade now but have
come back under the spotlight recently with the advent of the scattering
equation formalism which has greatly generalized the scope of these models. A
striking ubiquitous feature of these models has always been that, contrary to
usual string theory, they do not admit vibrational modes and thus describe only
conventional field theory. In this paper we report on the surprising discovery
of a whole new sector of one of these theories which we call "twisted strings,"
when spacetime has compact directions. We find that the spectrum is enhanced
from a finite number of states to an infinite number of interacting higher spin
massive states. We describe both bosonic and world sheet supersymmetric models,
their spectra and scattering amplitudes. These models have distinctive features
of both string and field theory, for example they are invariant under stringy
T-duality but have the high energy behavior typical of field theory. Therefore
they describe a new kind of field theories in target space, sitting on their
own halfway between string and field theory.Comment: 6 pages. v2 : a few clarifications and references added. v3 :
published PRD versio
On the null origin of the ambitwistor string
In this paper we present the null string origin of the ambitwistor string.
Classically, the null string is the tensionless limit of string theory, and so
too is the Ambitwistor string. Both have as constraint algebra the Galilean
Conformal Algebra in two dimensions. But something interesting happens in the
quantum theory since there is an ambiguity in quantizing the null string. We
show that, given a particular choice of quantization scheme and a particular
gauge, the null string coincides with the ambitwistor string both classically
and quantum mechanically. We also show that the same holds for the spinning
versions of the null string and Ambitwistor string. With these results we
clarify the relationship between the Ambitwistor string, the null string, the
usual string and the Hohm-Siegel-Zwiebach theory.Comment: 22 pages, 1 figur
Notes on the pure spinor superparticle
The pure spinor superparticle has been shown to describe linearized
supergravity in a manifestly covariant way. A number of authors have
proposed that its correlation functions be used to compute amplitudes. The use
of the scalar structure of the eleven-dimensional pure spinor top cohomology
introduces a natural measure for computing such correlation functions. This
prescription requires the construction of ghost number one and zero vertex
operators. In these notes, we construct explicitly a ghost number one vertex
operator but show the incompatibiliy of a ghost number zero vertex operator
satisfying a standard descent equation for supergravity.Comment: 13 page
New Ambitwistor String Theories
We describe new ambitwistor string theories that give rise to the recent
amplitude formulae for Einstein-Yang-Mills, (Dirac)-Born-Infeld, Galileons and
others introduced by Cachazo, He and Yuan. In the case of the
Einstein-Yang-Mills amplitudes, an important role is played by a novel
worldsheet conformal field theory that provides the appropriate colour factors
precisely without the spurious multitrace terms of earlier models that had to
be ignored by hand. This is needed to obtain the correct multitrace terms that
arise when Yang-Mills is coupled to gravity.Comment: 34 pages, 2 figures, 5 tables. v2: minor changes, published versio
Celestial Amplitudes as AdS-Witten Diagrams
Both celestial and momentum space amplitudes in four dimensions are beset by
divergences resulting from spacetime translation and sometimes scale
invariance. In this paper we consider a (linearized) marginal deformation of
the celestial CFT for Yang-Mills theory which preserves 2D conformal invariance
but breaks both spacetime translation and scale invariance and involves a
chirally coupled massive scalar. The resulting celestial amplitudes are
completely finite (apart from the usual soft and collinear divergences) and
take the canonical CFT form. Moreover, we show they can be simply rewritten in
terms of AdS-Witten contact diagrams which evaluate to the well-known
-functions, thereby forging a direct connection between flat and AdS
holography.Comment: 16 pages, 3 figure
The complex null string, Galilean conformal algebra and scattering equations
The scattering equation formalism for scattering amplitudes, and its stringy
incarnation, the ambitwistor string, remains a mysterious construction. In this
paper, we pursue the study a gauged-unfixed version of the ambitwistor string
known as the null string. We explore the following three aspects in detail; its
complexification, gauge fixing, and amplitudes. We first study the
complexification of the string; the associated symmetries and moduli, and
connection to the ambitwistor string. We then look in more details at the
leftover symmetry algebra of the string, called Galilean conformal algebra; we
study its local and global action and gauge-fixing. We finish by presenting an
operator formalism, that we use to compute tree-level scattering amplitudes
based on the scattering equations and a one-loop partition function. These
results hopefully will open the way to understand conceptual questions related
to the loop expansion in these twistor-like string models
Recommended from our members
Worldsheet methods for perturbative quantum field theory
This thesis is divided into two parts. The first part concerns the study of the ambitwistor string and the scattering equations, while the second concerns the interplay of the symmetries of the asymptotic null boundary of Minkowski space, called \scri, and scattering amplitudes.
The first part begins with a review of the CHY formulas for scattering amplitudes, the scattering equations and the ambitwistor string including its pure spinor version. Next are the results of this thesis concerning these topics, they are: generalizing the ambitwistor model to higher genus surfaces; calculating the one-loop NS-NS scattering amplitudes and studying their modular and factorization properties; deriving the one-loop scattering equations and analyzing their factorization; showing that, in the case of the four graviton amplitude, the ambitwistor amplitude gives the expected kinematical prefactor; matching this amplitude to the field theory expectation in a particular kinematical regime; solving the one loop scattering equations in this kinematical regime; a conjecture for the IR behaviour of the one-loop ambitwistor integrand; computing the four graviton, two-loop amplitude using pure spinors; showing that this two-loop amplitude has the correct kinematical prefactor and factorizes as expected for a field theory amplitude; generalizing the ambitwistor string to curved backgrounds; obtaining the field equations for type II supergravity as anomaly cancellation on the worldsheet; generalizing the scattering equations for curved backgrounds.
The second part begins with a review of the definition of the null asymptotic boundary of four dimensional Minkowski space, its symmetry algebra, and their relation to soft particles in the S-matrix. Next are the results of this thesis concerning these topics, they are: constructing two models consisting of maps from a worldsheet to \scri, one containing the spectrum of supergravity, and the other the spectrum of super Yang-Mills; showing how certain correlators in these theories calculate the tree-level S-matrix of sugra and sYM respectively; defining worldsheet charges which encode the action of the appropriate asymptotic symmetry algebra and showing that their Ward-identities recover the soft graviton, and soft gluon factors; defining worldsheet charges for proposed extensions of these symmetry algebras and showing that their Ward-identities give the subleading soft graviton and subleading soft gluon factors.Cambridge Commonwealth, European and International Trus
- …