2,210 research outputs found
Simplifying one-loop amplitudes in superstring theory
We show that 4-point vector boson one-loop amplitudes, computed in ref.[1] in
the RNS formalism, around vacuum configurations with open unoriented strings,
preserving at least N=1 SUSY in D=4, satisfy the correct supersymmetry Ward
identities, in that they vanish for non MHV configurations (++++) and (-+++).
In the MHV case (--++) we drastically simplify their expressions. We then study
factorisation and the limiting IR and UV behaviour and find some unexpected
results. In particular no massless poles are exposed at generic values of the
modular parameter. Relying on the supersymmetric properties of our bosonic
amplitudes, we extend them to manifestly supersymmetric super-amplitudes and
compare our results with those obtained in the D=4 hybrid formalism, pointing
out difficulties in reconciling the two approaches for contributions from N=1,2
sectors.Comment: 38 pages plus appendice
In vitro analysis of pyrogenicity and cytotoxicity profiles of flex sensors to be used to sense human joint postures
Flex sensors can be usefully adopted as mechanical-electrical transducers to measure human joint movements, since their electrical resistance varies proportionally to the angle assumed by the joint under measure. Over time, these sensors have been investigated in terms of mechanical and electrical behavior, but no reports have detailed the possibility of their adoption not just on top but under the human skin of the joint. To this aim, our work investigated in vitro the pyrogenic potential and cytotoxicity of some commercially available flex sensors as a first step toward the necessary requirements regarding their biocompatibility, to predict possible foreign body reactions when used in vivo. Results demonstrated that some specific flex sensors satisfy such requirement
The D^{2k} R^4 Invariants of N=8 Supergravity
The existence of a linearized SUSY invariant for N=8 supergravity whose
gravitational components are usually called R^4 was established long ago by
on-shell superspace arguments. Superspace and string theory methods have also
established analogous higher dimensional D^{2k} R^4 invariants. However, very
little is known about the SUSY completions of these operators which involve
other fields of the theory. In this paper we find the detailed component
expansion of the linearized R^4 invariant starting from the corresponding
superamplitude which generates all component matrix elements of the operator.
It is then quite straightforward to extend results to the entire set of D^{2k}
R^4 operators.Comment: 17 page
Simplifying instanton corrections to N=4 SYM correlators
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Stringy KLT relations, global symmetries, and E_7(7) violation
We study consequences of the Kawai-Lewellen-Tye (KLT) relations applied to
tree amplitudes in toroidal compactifications of string theory to four
dimensions. The closed string tree amplitudes with massless external states
respect a global SU(4)xSU(4) symmetry, which is enhanced to the SU(8)
R-symmetry of N=8 supergravity in the field theory limit. Our analysis focuses
on two aspects: (i) We provide a detailed account of the simplest
SU(8)-violating amplitudes. We classify these processes and derive explicit
superamplitudes for all local 5- and 6-point operators with SU(4)xSU(4)
symmetry at order alpha'^3. Their origin is the dilatonic operator exp(-6 phi)
R^4 in the closed-string effective action. (ii) We expand the 6-point closed
string tree amplitudes to order alpha'^3 and use two different methods to
isolate the SU(8)-singlet contribution from exp(-6 phi) R^4. This allows us to
extract the matrix elements of the unique SU(8)-invariant supersymmetrization
of R^4. Their single-soft scalar limits are non-vanishing. This demonstrates
that the N=8 supergravity candidate counterterm R^4 is incompatible with
continuous E_7(7) symmetry. From the soft scalar limits, we reconstruct to
quadratic order the SU(8)-invariant function of scalars that multiplies R^4,
and show that it satisfies the Laplace eigenvalue equation derived recently
from supersymmetry and duality constraints.Comment: 23 pages, published versio
Microscopic unitary description of tidal excitations in high-energy string-brane collisions
The eikonal operator was originally introduced to describe the effect of
tidal excitations on higher-genus elastic string amplitudes at high energy. In
this paper we provide a precise interpretation for this operator through the
explicit tree-level calculation of generic inelastic transitions between closed
strings as they scatter off a stack of parallel Dp-branes. We perform this
analysis both in the light-cone gauge, using the Green-Schwarz vertex, and in
the covariant formalism, using the Reggeon vertex operator. We also present a
detailed discussion of the high energy behaviour of the covariant string
amplitudes, showing how to take into account the energy factors that enhance
the contribution of the longitudinally polarized massive states in a simple
way.Comment: 58 page
Stringy instanton corrections to N=2 gauge couplings
We discuss a string model where a conformal four-dimensional N=2 gauge theory
receives corrections to its gauge kinetic functions from "stringy" instantons.
These contributions are explicitly evaluated by exploiting the localization
properties of the integral over the stringy instanton moduli space. The model
we consider corresponds to a setup with D7/D3-branes in type I' theory
compactified on T4/Z2 x T2, and possesses a perturbatively computable heterotic
dual. In the heteoric side the corrections to the quadratic gauge couplings are
provided by a 1-loop threshold computation and, under the duality map, match
precisely the first few stringy instanton effects in the type I' setup. This
agreement represents a very non-trivial test of our approach to the exotic
instanton calculus.Comment: 63 pages, 5 figures. V2: final version with minor corrections
published on JHEP05(2010)10
A simple approach to counterterms in N=8 supergravity
We present a simple systematic method to study candidate counterterms in N=8
supergravity. Complicated details of the counterterm operators are avoided
because we work with the on-shell matrix elements they produce. All n-point
matrix elements of an independent SUSY invariant operator of the form D^{2k}
R^n +... must be local and satisfy SUSY Ward identities. These are strong
constraints, and we test directly whether or not matrix elements with these
properties can be constructed. If not, then the operator does not have a
supersymmetrization, and it is excluded as a potential counterterm. For n>4, we
find that R^n, D^2 R^n, D^4 R^n, and D^6 R^n are excluded as counterterms of
MHV amplitudes, while only R^n and D^2 R^n are excluded at the NMHV level. As a
consequence, for loop order L<7, there are no independent D^{2k}R^n
counterterms with n>4. If an operator is not ruled out, our method constructs
an explicit superamplitude for its matrix elements. This is done for the 7-loop
D^4 R^6 operator at the NMHV level and in other cases. We also initiate the
study of counterterms without leading pure-graviton matrix elements, which can
occur beyond the MHV level. The landscape of excluded/allowed candidate
counterterms is summarized in a colorful chart.Comment: 25 pages, 1 figure, published versio
Populating the swampland: the case of U(1)^496 and E_8 x U(1)^248
For d=10 N=1 SUGRA coupled to d=10 N=1 SYM, anomaly cancellation places
severe constraints on the allowed gauge groups. Besides the ones known to
appear in string theory, only U(1)^496 and E_8 x U(1)^248 are allowed. There
are no known theories of quantum gravity that reduce in some limit to these two
last supergravity theories, and in this note I present some evidence that those
quantum theories might not exist. The first observation is that, upon
compactification, requring that the quantum theory possesses a moduli space
with finite volume typically implies the existence of singularities where the
4d gauge group is enhanced, but for these two theories that gauge enhancement
is problematic from the 10d point of view. I also point out that while these
four supergravity theories present repulson-type singularities, the known
mechanism that repairs those singularities for the first two - the non-Abelian
enhancon - is not available for the last two theories. In short, these two
supergravity theories might be too Abelian for their own good.Comment: 12 page
Stringy Instantons in SU(N) N=2 Non-Conformal Gauge Theories
In this paper we explicitly obtain the leading corrections to the SU(N) N=2
prepotential due to stringy instantons both in flat space-time and in the
presence of a non-trivial graviphoton background field. We show that the
stringy corrections to the prepotential are expressible in terms of the
elementary symmetric polynomials. For N>2 the theory is not conformal; we
discuss the introduction of an explicit dependence on the string scale \alpha'
in the low-energy effective action through the stringy non-perturbative sector.Comment: 22 pages, 1 figur
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