1,279 research outputs found
Analytic solutions for Dp branes in SFT
This is the follow-up of a previous paper [ArXiv:1105.5926], where we
calculated the energy of an analytic lump solution in SFT, representing a
D24-brane. Here we describe an analytic solution for a Dp-brane, for any p, and
compute its energy.Comment: 14 page
Scattering amplitudes in super-renormalizable gravity
We explicitly compute the tree-level on-shell four-graviton amplitudes in
four, five and six dimensions for local and weakly nonlocal gravitational
theories that are quadratic in both, the Ricci and scalar curvature with form
factors of the d'Alembertian operator inserted between. More specifically we
are interested in renormalizable, super-renormalizable or finite theories. The
scattering amplitudes for these theories turn out to be the same as the ones of
Einstein gravity regardless of the explicit form of the form factors. As a
special case the four-graviton scattering amplitudes in Weyl conformal gravity
are identically zero. Using a field redefinition, we prove that the outcome is
correct for any number of external gravitons (on-shell point functions) and
in any dimension for a large class of theories. However, when an operator
quadratic in the Riemann tensor is added in any dimension (with the exception
of the Gauss-Bonnet term in four dimensions) the result is completely altered,
and the scattering amplitudes depend on all the form factors introduced in the
action.Comment: 25 pages, 2 Figure
A fresh view on string orbifolds
In quantum field theory, an orbifold is a way to obtain a new theory from an
old one by gauging a finite global symmetry. This definition of orbifold does
not make sense for quantum gravity theories, that admit (conjecturally) no
global symmetries. In string theory, the orbifold procedure involves the
gauging of a global symmetry on the world-sheet theory describing the
fundamental string. Alternatively, it is a way to obtain a new string
background from an old one by quotienting some isometry. We propose a new
formulation of string orbifolds in terms of the group of gauge symmetries of a
given string model. In such a formulation, the `parent' and the `child'
theories correspond to different ways of breaking or gauging all potential
global symmetries of their common subsector. Through a couple of simple
examples, we describe how the higher group structure of the gauge group in the
parent theory plays a crucial role in determining the gauge group and the
twisted sector of the orbifold theory. We also discuss the dependence of this
orbifold procedure on the duality frame.Comment: 57 pages, 4 figures; v2: minor changes, references added; v3: revised
abstract and introduction, typos corrected, references added, version
accepted for publicatio
Axial gravity, massless fermions and trace anomalies
This article deals with two main topics. One is odd parity trace anomalies in
Weyl fermion theories in a 4d curved background, the second is the introduction
of axial gravity. The motivation for reconsidering the former is to clarify the
theoretical background underlying the approach and complete the calculation of
the anomaly. The reference is in particular to the difference between Weyl and
massless Majorana fermions and to the possible contributions from tadpole and
seagull terms in the Feynman diagram approach. A first, basic, result of this
paper is that a more thorough treatment, taking account of such additional
terms { and using dimensional regularization}, confirms the earlier result. The
introduction of an axial symmetric tensor besides the usual gravitational
metric is instrumental to a different derivation of the same result using Dirac
fermions, which are coupled not only to the usual metric but also to the
additional axial tensor. The action of Majorana and Weyl fermions can be
obtained in two different limits of such a general configuration. The results
obtained in this way confirm the previously obtained ones.Comment: 55 pages, comments added in section 2 and 5. Sections 6.4, 6.6, 7,
7.1, 7.2 and Appendices 5.3, 5.5 partially modifie
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