57 research outputs found
Non-commutative D- and M-brane Bound States
We analyze certain brane bound states in M-theory and their descendants in type IIA string theory, all involving 3-form or 2-form background fluxes. Among them are configurations which represent NCYM, NCOS and ODp-theories in the scaling limit of OM-theory. In particular, we show how the conditions for the embedding to preserve supersymmetry are modified by the presence of the flux and discuss their relations for the various different bound states. Via the formalism of geometric quantization such a deformation of a supersymmetric cycle is related to a non-commutativity of its coordinates. We also study possible non-commutative deformations of the Seiberg-Witten curve of N=2 supersymmetric gauge theories due to non-trivial H-flux
Electron-positron pair creation in a vacuum by an electromagnetic field in 3+1 and lower dimensions
We calculate the probability of electron-positron pair creation in vacuum in
3+1 dimensions by an external electromagnetic field composed of a constant
uniform electric field and a constant uniform magnetic field, both of arbitrary
magnitudes and directions. The same problem is also studied in 2+1 and 1+1
dimensions in appropriate external fields and similar results are obtained.Comment: REVTeX, 10 pages, no figure, a brief note and some more references
added in the proo
Triviality and the (Supersymmetric) See-Saw
For the D=5 Majorana neutrino mass operator to have a see-saw ultraviolet
completion that is viable up to the Planck scale, the see-saw scale is bounded
above due to triviality limits on the see-saw couplings. For supersymmetric
see-saw models, with realistic neutrino mass textures, we compare constraints
on the see-saw scale from triviality bounds, with those arising from
experimental limits on induced charged-lepton flavour violation, for both the
CMSSM and for models with split supersymmetry.Comment: 27 pages, 7 figures, references adde
Anomaly Cancellation in Supergravity with Fayet-Iliopoulos Couplings
We review and clarify the cancellation conditions for gauge anomalies which
occur when N=1, D=4 supergravity is coupled to a Kahler non-linear sigma-model
with gauged isometries and Fayet-Iliopoulos couplings. For a flat sigma-model
target space and vanishing Fayet-Iliopoulos couplings, consistency requires
just the conventional anomaly cancellation conditions. A consistent model with
non-vanishing Fayet-Iliopoulos couplings is unlikely unless the Green-Schwarz
mechanism is used. In this case the U(1) gauge boson becomes massive and the
D-term potential receives corrections. A Green-Schwarz mechanism can remove
both the abelian and certain non-abelian anomalies in models with a gauge
non-invariant Kahler potential.Comment: 27 page
Supersymmetry Breaking and Moduli Stabilization with Anomalous U(1) Gauge Symmetry
We examine the effects of anomalous U(1)_A gauge symmetry on soft
supersymmetry breaking terms while incorporating the stabilization of the
modulus-axion multiplet responsible for the Green-Schwarz (GS) anomaly
cancellation mechanism. In case of the KKLT stabilization of the GS modulus,
soft terms are determined by the GS modulus mediation, the anomaly mediation
and the U(1)_A mediation which are generically comparable to each other,
thereby yielding the mirage mediation pattern of superparticle masses at low
energy scale. Independently of the mechanism of moduli stabilization and
supersymmetry breaking, the U(1)_A D-term potential can not be an uplifting
potential for de Sitter vacuum when the gravitino mass is smaller than the
Planck scale by many orders of magnitude. We also discuss some features of the
supersymmetry breaking by red-shifted anti-brane which is a key element of the
KKLT moduli stabilization.Comment: 32 pages; references are adde
A Supersymmetric Stueckelberg U(1) Extension of the MSSM
A Stueckelberg extension of the MSSM with only one abelian vector and one
chiral superfield as an alternative to an abelian extension with Higgs scalars
is presented. The bosonic sector contains a new gauge boson Z' which is a sharp
resonance, and a new CP-even scalar, which combines with the MSSM Higgs bosons
to produce three neutral CP-even massive states. The neutral fermionic sector
has two additional fermions which mix with the four MSSM neutralinos to produce
an extended 6x6 neutralino mass matrix. For the case when the LSP is composed
mostly of the Stueckelberg fermions, the LSP of the MSSM will be unstable,
which leads to exotic decays of sparticles with many leptons in final states.
Prospects for supersymmetry searches and for dark matter are discussed.Comment: 10 page
Un-oriented Quiver Theories for Majorana Neutrons
In the context of un-oriented open string theories, we identify quivers
whereby a Majorana mass for the neutron is indirectly generated by exotic
instantons. We discuss two classes of (Susy) Standard Model like quivers,
depending on the embedding of SU(2)_W in the Chan-Paton group. In both cases,
the main mechanism involves a vector-like pair mixing through a
non-perturbative mass term. We also discuss possible relations between the
phenomenology of Neutron-Antineutron oscillations and LHC physics in these
models. In particular, a vector-like pair of color-triplet scalars or
color-triplet fermions could be directly detected at LHC, compatibly with
n-\bar{n} limits. Finally we briefly comment on Pati-Salam extensions of our
models.Comment: More comments on phenomenology and fluxes, Re-discussion of
SM-quivers compatible with n-cycles conditions Version accepted by JHE
Two-Loop Diagrammatics in a Self-Dual Background
Diagrammatic rules are developed for simplifying two-loop QED diagrams with
propagators in a constant self-dual background field. This diagrammatic
analysis, using dimensional regularization, is used to explain how the fully
renormalized two-loop Euler-Heisenberg effective Lagrangian for QED in a
self-dual background field is naturally expressed in terms of one-loop
diagrams. The connection between the two-loop and one-loop vacuum diagrams in a
background field parallels a corresponding connection for free vacuum diagrams,
without a background field, which can be derived by simple algebraic
manipulations. It also mirrors similar behavior recently found for two-loop
amplitudes in N=4 SUSY Yang-Mills theory.Comment: 16 pp, Latex, Axodra
R-parity Conservation via the Stueckelberg Mechanism: LHC and Dark Matter Signals
We investigate the connection between the conservation of R-parity in
supersymmetry and the Stueckelberg mechanism for the mass generation of the B-L
vector gauge boson. It is shown that with universal boundary conditions for
soft terms of sfermions in each family at the high scale and with the
Stueckelberg mechanism for generating mass for the B-L gauge boson present in
the theory, electric charge conservation guarantees the conservation of
R-parity in the minimal B-L extended supersymmetric standard model. We also
discuss non-minimal extensions. This includes extensions where the gauge
symmetries arise with an additional U(1)_{B-L} x U(1)_X, where U(1)_X is a
hidden sector gauge group. In this case the presence of the additional U(1)_X
allows for a Z' gauge boson mass with B-L interactions to lie in the sub-TeV
region overcoming the multi-TeV LEP constraints. The possible tests of the
models at colliders and in dark matter experiments are analyzed including
signals of a low mass Z' resonance and the production of spin zero bosons and
their decays into two photons. In this model two types of dark matter
candidates emerge which are Majorana and Dirac particles. Predictions are made
for a possible simultaneous observation of new physics events in dark matter
experiments and at the LHC.Comment: 38 pages, 7 fig
Intersecting Brane Worlds -- A Path to the Standard Model ?
In this review we describe the general geometrical framework of brane world
constructions in orientifolds of type IIA string theory with D6-branes wrapping
3-cycles in a Calabi-Yau 3-fold. These branes generically intersect in points
on the internal space, and the patterns of intersections govern the chiral
fermion spectra. We discuss how the open string spectra in intersecting brane
models are constructed, how the Standard Model can be embedded, and also how
supersymmetry can be realized in this class of string vacua. After the general
considerations we specialize the discussion to the case of orbifold backgrounds
with intersecting D6-branes and to the quintic Calabi-Yau manifold. Then, we
discuss parts of the effective action of intersecting brane world models.
Specifically we compute from the Born-Infeld action of the wrapped D-branes the
tree-level, D-term scalar potential, which is important for the stability of
the considered backgrounds as well as for questions related to supersymmetry
breaking. Second, we review the recent computation concerning of gauge coupling
unification and also of one-loop gauge threshold corrections in intersecting
brane world models. Finally we also discuss some aspects of proton decay in
intersecting brane world models.Comment: 31 pages, To appear in the proceedings of the RTN-workshop ``The
quantum structure of spacetime and the geometric nature of fundamental
interactions'', September 2003 in Copenhagen, revised version contains new
refs and one corrected equatio
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