39 research outputs found
BCJ duality and double copy in the closed string sector
This paper is focused on the loop-level understanding of the
Bern-Carrasco-Johansson double copy procedure that relates the integrands of
gauge theory and gravity scattering amplitudes. At four points, the first
non-trivial example of that construction is one-loop amplitudes in N=2
super-Yang-Mills theory and the symmetric realization of N=4 matter-coupled
supergravity. Our approach is to use both field and string theory in parallel
to analyze these amplitudes. The closed string provides a natural framework to
analyze the BCJ construction, in which the left- and right-moving sectors
separately create the color and kinematics at the integrand level. At tree
level, in a five-point example, we show that the Mafra-Schlotterer-Stieberger
procedure gives a new direct proof of the color-kinematics double copy. We
outline the extension of that argument to n points. At loop level, the
field-theoretic BCJ construction of N=2 SYM amplitudes introduces new terms,
unexpected from the string theory perspective. We discuss to what extent we can
relate them to the terms coming from the interactions between left- and
right-movers in the string-theoretic gravity construction.Comment: 46 pages, 8 figures, 2 tables; v3 significantly revised published
versio
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
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
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
One-loop monodromy relations on single cuts
The discovery of colour-kinematic duality has led to significant progress in
the computation of scattering amplitudes in quantum field theories. At tree
level, the origin of the duality can be traced back to the monodromies of
open-string amplitudes. This construction has recently been extended to all
loop orders. In the present paper, we dissect some consequences of these new
monodromy relations at one loop. We use single cuts in order to relate them to
the tree-level relations. We show that there are new classes of kinematically
independent single-cut amplitudes. Then we turn to the Feynman diagrammatics of
the string-theory monodromy relations. We revisit the string-theoretic
derivation and argue that some terms, that vanish upon integration in string
and field theory, provide a characterisation of momentum-shifting ambiguities
in these representations. We observe that colour-dual representations are
compatible with this analysis
Two-loop scattering amplitudes from the Riemann sphere
Financial support from EPSRC Grant No. EP/K032208/1
during the program GTA 2016. Y. G. is supported by the
EPSRC Doctoral Prize Scheme EP/M508111/1, LJM by
the EPSRC Grant No. EP/M018911/1, and the work of P. T.
is supported by STFC Grant No. ST/L000385/1
One-loop four-graviton amplitudes in N=4 supergravity models
We evaluate in great detail one-loop four-graviton field theory amplitudes in
pure N=4 D=4 supergravity. The expressions are obtained by taking the field
theory limits of (4,0) and (2,2) space-time supersymmetric string theory
models. For each model we extract the contributions of the spin-1 and spin-2
N=4 supermultiplets running in the loop. We show that all of those
constructions lead to the same four-dimensional result for the four-graviton
amplitudes in pure supergravity even though they come from different string
theory models.Comment: 46 pages. One figure. v2: minor corrections and clarifications.
References added. v3: (2,2) analysis corrected, four-graviton amplitudes
found to be indentical in all models. Various clarifications and precisions
added. References list updated. v4: Assorted spelling and grammar
corrections. Version to be publishe
An R^4 non-renormalisation theorem in N=4 supergravity
We consider the four-graviton amplitudes in CHL constructions providing
four-dimensional N=4 models with various numbers of vector multiplets. We show
that in these models the two-loop amplitude has a prefactor of d^2R^4. This
implies a non-renormalisation theorem for the R^4 term, which forbids the
appearance of a three-loop ultraviolet divergence in four dimensions in the
four-graviton amplitude. We connect the special nature of the R^4 term to the
U(1) anomaly of pure N=4 supergravity.Comment: v2: added comments about one-loop UV divergences. Assorted stylistic
corrections. Added references. v3: Eq. III.21 corrected and assorted minor
corrections and clarifications. Version to be published. v4: minor
corrections. 18 pages. one figur
Controllable adhesion using field-activated fluids
We demonstrate that field-responsive magnetorheological fluids can be used for variable-strength controllable adhesion. The adhesive performance is measured experimentally in tensile tests (a.k.a. probe-tack experiments) in which the magnetic field is provided by a cylindrical permanent magnet. Increasing the magnetic field strength induces higher peak adhesive forces. We hypothesize that the adhesion mechanism arises from the shear resistance of a yield stress fluid in a thin gap. This hypothesis is supported by comparing the experimentally measured adhesive performance to the response predicted by a lubrication model for a non-Newtonian fluid with a field-dependent yield stress. The model predictions are in agreement with experimental data up to moderate field strengths. Above a critical magnetic field strength the model over-predicts the experimentally measured values indicating non-ideal conditions such as local fluid dewetting from the surface.U.S. Army Research Laboratory (United States. Army Research Office Contract/Grant W911NF-08-C-0055