432 research outputs found
D3 Brane Action and Fermion Zero Modes in Presence of Background Flux
We derive the fermion bilinear terms in the world volume action for a D3
brane in the presence of background flux. In six-dimensional compactifications
non-perturbative corrections to the superpotential can arise from an Euclidean
D3-brane instanton wrapping a divisor in the internal space. The bilinear terms
give rise to fermion masses and are important in determining these corrections.
We find that the three-form flux generically breaks a U(1) subgroup of the
structure group of the normal bundle of the divisor. In an example of
compactification on T^6/Z_2, six of the sixteen zero modes originally present
are lifted by the flux.Comment: Important factor of ``i'' was overlooked in Euclidean continuation of
WZ term. This changes the count of zero-modes in the T^6/Z_2 example. Main
result stays unchanged. We thank Bergshoeff, Kallosh, Kashani-Poor, Sorokin
and Tomasiello for pointing this ou
D-terms and D-strings in open string models
We study the Fayet-Iliopoulos (FI) D-terms on D-branes in type II Calabi-Yau
backgrounds. We provide a simple worldsheet proof of the fact that, at tree
level, these terms only couple to scalars in closed string hypermultiplets. At
the one-loop level, the D-terms get corrections only if the gauge group has an
anomalous spectrum, with the anomaly cancelled by a Green-Schwarz mechanism. We
study the local type IIA model of D6-branes at SU(3) angles and show that, as
in field theory, the one-loop correction suffers from a quadratic divergence in
the open string channel. By studying the closed string channel, we show that
this divergence is related to a closed string tadpole, and is cancelled when
the tadpole is cancelled. Next, we study the cosmic strings that arise in the
supersymmetric phases of these systems in light of recent work of Dvali et. al.
In the type IIA intersecting D6-brane examples, we identify the D-term strings
as D4-branes ending on the D6-branes. Finally, we use N=1 dualities to relate
these results to previous work on the FI D-term of heterotic strings.Comment: 29 pages, 5 figures; v2: improved referencin
Twisting K3 x T^2 Orbifolds
We construct a class of geometric twists of Calabi-Yau manifolds of
Voisin-Borcea type (K3 x T^2)/Z_2 and study the superpotential in a type IIA
orientifold based on this geometry. The twists modify the direct product by
fibering the K3 over T^2 while preserving the Z_2 involution. As an important
application, the Voisin-Borcea class contains T^6/(Z_2 x Z_2), the usual
setting for intersecting D6 brane model building. Past work in this context
considered only those twists inherited from T^6, but our work extends these
twists to a subset of the blow-up modes. Our work naturally generalizes to
arbitrary K3 fibered Calabi-Yau manifolds and to nongeometric constructions.Comment: 57 pages, 4 figures; uses harvmac.tex, amssym.tex; v3: minor
corrections, references adde
High Dose Methylprednisolone for Veno-Occlusive Disease of the Liver in Pediatric Bone Marrow Transplant Recipients
Gravitational Smearing of Minimal Supersymmetric Unification Predictions
A short and mean paper.Comment: 10 pages total + 1 postscript figure (included), revised: all lines
are TRULY < 70 characters long (try it!); LBL-32905, UCB-PTH-92/3
Axions In String Theory
In the context of string theory, axions appear to provide the most plausible
solution of the strong CP problem. However, as has been known for a long time,
in many string-based models, the axion coupling parameter F_a is several orders
of magnitude higher than the standard cosmological bounds. We re-examine this
problem in a variety of models, showing that F_a is close to the GUT scale or
above in many models that have GUT-like phenomenology, as well as some that do
not. On the other hand, in some models with Standard Model gauge fields
supported on vanishing cycles, it is possible for F_a to be well below the GUT
scale.Comment: 62 pages, v2; references, acknowledgements and minor corrections
adde
The Taming of Closed Time-like Curves
We consider a orbifold, where acts by time and space
reversal, also known as the embedding space of the elliptic de Sitter space.
The background has two potentially dangerous problems: time-nonorientability
and the existence of closed time-like curves. We first show that closed causal
curves disappear after a proper definition of the time function. We then
consider the one-loop vacuum expectation value of the stress tensor. A naive
QFT analysis yields a divergent result. We then analyze the stress tensor in
bosonic string theory, and find the same result as if the target space would be
just the Minkowski space , suggesting a zero result for the
superstring. This leads us to propose a proper reformulation of QFT, and
recalculate the stress tensor. We find almost the same result as in Minkowski
space, except for a potential divergence at the initial time slice of the
orbifold, analogous to a spacelike Big Bang singularity. Finally, we argue that
it is possible to define local S-matrices, even if the spacetime is globally
time-nonorientable.Comment: 37 pages, LaTeX2e, uses amssymb, amsmath and epsf macros, 8 eps and 3
ps figures; (v2): Two additional comments + one reference added; (v3):
corrections in discussion of CTCs + some clarification
A Complete Supersymmetric SO(10) Model
A complete supersymmetric SO(10) model is constructed, which is the most
general consistent with certain , discrete, and flavor symmetries.
The desired vacuum of the theory has vevs which lie in particular directions of
group space. This leads to both doublet triplet splitting and to the generation
of just four operators for charged fermion masses. The model illustrates how
many features of superunification become related in the context of a complete
theory. The features discussed here include: the weak mixing angle prediction,
the doublet-triplet splitting problem, proton decay, the generation of the
parameter, neutrino masses and the generation of the operators which lead
to charged fermion mass predictions.Comment: 18 page
Half-BPS cosmic string in N=2 supergravity in the presence of a dilaton
We construct new half-BPS cosmic string solutions in D=4 N=2 supergravity
compatible with a consistent truncation to N=1 supergravity where they describe
D-term cosmic strings. The constant Fayet-Iliopoulos term in the N=1 D-term is
not put in by hand but is geometrically engineered by a gauging in the mother
N=2 supergravity theory. The coupling of the N=2 vector multiplets is
characterized by a cubic prepotential admitting an axion-dilaton field, a
common property of many compactifications of string theory. The axion-dilaton
field survives the truncation to N=1 supergravity. On the string configuration
the BPS equations constrain the dilaton to be an arbitrary constant. All the
cosmic string solutions with different values of the dilaton have the same
energy per unit length but different lenght scales.Comment: 52 pages; typos correcte
Natural Theories of Ultra-Low Mass PNGB's: Axions and Quintessence
We consider the Wilson Line PNGB which arises in a U(1)^N gauge theory,
abstracted from a latticized, periodically compactified extra dimension U(1).
Planck scale breaking of the PNGB's global symmetry is suppressed, providing
natural candidates for the axion and quintessence. We construct an explicit
model in which the axion may be viewed as the 5th component of the U(1)_Y gauge
field in a 1+4 latticized periodically compactified extra dimension. We also
construct a quintessence PNGB model where the ultra-low mass arises from
Planck-scale suppressed physics itself.Comment: 20 pages, fixed typo and reference
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