913 research outputs found
On tadpoles and vacuum redefinitions in String Theory
Tadpoles accompany, in one form or another, all attempts to realize
supersymmetry breaking in String Theory, making the present constructions at
best incomplete. Whereas these tadpoles are typically large, a closer look at
the problem from a perturbative viewpoint has the potential of illuminating at
least some of its qualitative features in String Theory. A possible scheme to
this effect was proposed long ago by Fischler and Susskind, but incorporating
background redefinitions in string amplitudes in a systematic fashion has long
proved very difficult. In the first part of this paper, drawing from field
theory examples, we thus begin to explore what one can learn by working
perturbatively in a ``wrong'' vacuum. While unnatural in Field Theory, this
procedure presents evident advantages in String Theory, whose definition in
curved backgrounds is mostly beyond reach at the present time. At the field
theory level, we also identify and characterize some special choices of vacua
where tadpole resummations terminate after a few contributions. In the second
part we present a notable example where vacuum redefinitions can be dealt with
to some extent at the full string level, providing some evidence for a new link
between IIB and 0B orientifolds. We finally show that NS-NS tadpoles do not
manifest themselves to lowest order in certain classes of string constructions
with broken supersymmetry and parallel branes, including brane-antibrane pairs
and brane supersymmetry breaking models, that therefore have UV finite
threshold corrections at one loop.Comment: 51 pages, LaTeX, 7 eps figures. Typos corrected, refs added. Final
version to appear in Nucl. Phys. B. Thanks to W. Mueck for very interesting
correspondence. v3 was accidentally in draft forma
Stringy Instanton Effects in Models with Rigid Magnetised D-branes
We compute instantonic effects in globally consistent T^6/Z2xZ2 orientifold
models with discrete torsion and magnetised D-branes. We consider fractional
branes and instantons wrapping the same rigid cycles. We clarify and analyse in
detail the low-energy effective action on D-branes in these models. We provide
explicit examples where instantons induce linear terms in the charged fields,
or non-perturbative mass terms are generated. We also find examples where the
gauge theory on fractional branes has conformal symmetry at one-loop, broken by
instantonic mass terms at a hierarchically small energy scale.Comment: 60 pages. Refs added. Typos corrected in some eqs. Modified comments
in subsection 4.
Testing extra dimensions with boundaries using Newton's law modifications
Extra dimensions with boundaries are often used in the literature, to provide
phenomenological models that mimic the standard model. In this context, we
explore possible modifications to Newton's law due to the existence of an
extra-dimensional space, at the boundary of which the gravitational field obeys
Dirichlet, Neumann or mixed boundary conditions. We focus on two types of extra
space, namely, the disk and the interval. As we prove, in order to have a
consistent Newton's law modification (i.e., of the Yukawa-type), some of the
extra-dimensional spaces that have been used in the literature, must be ruled
out.Comment: Published version, title changed, 6 figure
Appeal No. 0888: Edco Drilling and Producing, v. Division of Oil & Gas Resources Management
Chief\u27s Order 2014-254 (Hedrick #1A Well
Gauge vs. Gravity mediation in models with anomalous U(1)'s
In an attempt to implement gauge mediation in string theory, we study string
effective supergravity models of supersymmetry breaking, containing anomalous
gauge factors. We discuss subtleties related to gauge invariance and the
stabilization of the Green-Schwarz moduli, which set non-trivial constraints on
the transmission of supersymmetry breaking to MSSM via gauge interactions.
Given those constraints, it is difficult to obtain the dominance of gauge
mediation over gravity mediation. Furthermore, generically the gauge
contributions to soft terms contain additional non-standard terms coming from
D-term contributions. Motivated by this, we study the phenomenology of recently
proposed hybrid models, where gravity and gauge mediations compete at the GUT
scale, and show that such a scenario can respect WMAP constraints and would be
easily testable at LHC.Comment: 40 pages, 5 figure
Magnetized Type I Orbifolds in Four Dimensions
I review the basic features of four dimensional Z_2 x Z_2 (shift)
orientifolds with internal magnetic fields, describing two examples with N=1
supersymmetry. As in the corresponding six-dimensional examples, D9-branes
magnetized along four internal directions can mimic D5-branes, even in presence
of multiplets of image branes localized on different fixed tori. Chiral
low-energy spectra can be obtained if the model also contains D5-branes
parallel to the magnetized directions.Comment: 4 pages, LATEX; misprints correcte
Non-tachyonic Scherk-Schwarz compactifications, cosmology and moduli stabilization
It is well-known that Scherk-Schwarz compactifications in string theory have
a tachyon in the closed string spectrum appearing for a critical value of a
compact radius. The tachyon can be removed by an appropriate orientifold
projection in type II strings, giving rise to tachyon-free compactifications.
We present explicit examples of this type in various dimensions, including six
and four-dimensional chiral examples, with softly broken supersymmetry in the
closed sector and non-BPS configurations in the open sector. These vacua are
interesting frameworks for studying various cosmological issues. We discuss
four-dimensional cosmological solutions and moduli stabilization triggered by
nonperturbative effects like gaugino condensation on D-branes and fluxes.Comment: 36 pages, LaTeX; added reference
Moduli stabilization with Fayet-Iliopoulos uplift
In the recent years, phenomenological models of moduli stabilization were
proposed, where the dynamics of the stabilization is essentially
supersymmetric, whereas an O'Rafearthaigh supersymmetry breaking sector is
responsible for the "uplift" of the cosmological constant to zero. We
investigate the case where the uplift is provided by a Fayet-Iliopoulos sector.
We find that in this case the modulus contribution to supersymmetry breaking is
larger than in the previous models. A first consequence of this class of
constructions is for gauginos, which are heavier compared to previous models.
In some of our explicit examples, due to a non-standard gauge-mediation type
negative contribution to scalars masses, the whole superpartner spectrum can be
efficiently compressed at low-energy. This provides an original phenomenology
testable at the LHC, in particular sleptons are generically heavier than the
squarks.Comment: 29 pages, 2 figure
Beyond MFV in family symmetry theories of fermion masses
Minimal Flavour Violation (MFV) postulates that the only source of flavour
changing neutral currents and CP violation, as in the Standard Model, is the
CKM matrix. However it does not address the origin of fermion masses and mixing
and models that do usually have a structure that goes well beyond the MFV
framework. In this paper we compare the MFV predictions with those obtained in
models based on spontaneously broken (horizontal) family symmetries, both
Abelian and non-Abelian. The generic suppression of flavour changing processes
in these models turns out to be weaker than in the MFV hypothesis. Despite
this, in the supersymmetric case, the suppression may still be consistent with
a solution to the hierarchy problem, with masses of superpartners below 1 TeV.
A comparison of FCNC and CP violation in processes involving a variety of
different family quantum numbers should be able to distinguish between various
family symmetry models and models satisfying the MFV hypothesis.Comment: 34 pages, no figure
F-term uplifting via consistent D-terms
The issue of fine-tuning necessary to achieve satisfactory degree of
hierarchy between moduli masses, the gravitino mass and the scale of the
cosmological constant has been revisited in the context of supergravities with
consistent D-terms. We have studied (extended) racetrack models where
supersymmetry breaking and moduli stabilisation cannot be separated from each
other. We show that even in such cases the realistic hierarchy can be achieved
on the expense of a single fine-tuning. The presence of two condensates changes
the role of the constant term in the superpotential, W_0, and solutions with
small vacuum energy and large gravitino mass can be found even for very small
values of W_0. Models where D-terms are allowed to vanish at finite vevs of
moduli fields - denoted `cancellable' D-terms - and the ones where D-terms may
vanish only at infinite vevs of some moduli - denoted `non-cancellable' -
differ markedly in their properties. It turns out that the tuning with respect
to the Planck scale required in the case of cancellable D-terms is much weaker
than in the case of non-cancellable ones. We have shown that, against
intuition, a vanishing D-term can trigger F-term uplifting of the vacuum energy
due to the stringent constraint it imposes on vacuum expectation values of
charged fields. Finally we note that our models only rely on two dimensionful
parameters: M_P and W_0.Comment: 10 pages, 2 figures, plain Latex, references adde
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