685 research outputs found
Multi-instanton and string loop corrections in toroidal orbifold models
We analyze N=2 (perturbative and non-perturbative) corrections to the
effective theory in type I orbifold models where a dual heterotic description
is available. These corrections may play an important role in phenomenological
scenarios. More precisely, we consider two particular compactifications: the
Bianchi-Sagnotti-Gimon-Polchinski orbifold and a freely-acting Z_2 x Z_2
orbifold with N=1 supersymmetry and gauge group SO(q) x SO(32-q). By exploiting
perturbative calculations of the physical gauge couplings on the heterotic
side, we obtain multi-instanton and one-loop string corrections to the K\"ahler
potential and the gauge kinetic function for these models. The non-perturbative
corrections appear as sums over relevant Hecke operators, whereas the one-loop
correction to the K\"ahler potential matches the expression proposed in [1,2].
We argue that these corrections are universal in a given class of models where
target-space modular invariance (or a subgroup of it) holds.Comment: 37 pages, 3 figure
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
Type I vacua with brane supersymmetry breaking
We show how chiral type I models whose tadpole conditions have no
supersymmetric solution can be consistently defined introducing antibranes with
non-supersymmetric world volumes. At tree level, the resulting stable non-BPS
configurations correspond to tachyon-free spectra, where supersymmetry is
broken at the string scale on some (anti)branes but is exact in the bulk, and
can be further deformed by the addition of brane-antibrane pairs of the same
type. As a result, a scalar potential is generated, that can stabilize some
radii of the compact space. This setting has the novel virtue of linking
supersymmetry breaking to the consistency requirements of an underlying
fundamental theory.Comment: 45 pages. Late
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
Partial breaking of supersymmetry, open strings and M-theory
We study total and partial supersymmetry breaking by freely acting orbifolds, or equivalently by Scherk-Schwarz compactifications, in type I string theory. In particular, we describe a four-dimensional chiral compactification with spontaneously broken N=1 supersymmetry, some models with partial and supersymmetry breaking and their heterotic and M-theory duals. A generic feature of these models is that in the gravitational sector and in the spectrum of D-branes parallel to the breaking coordinate, all mass splittings are proportional to the compactification scale, while global (extended) supersymmetry remains unbroken at tree level for the massless excitations of D-branes transverse to the breaking direction.We study total and partial supersymmetry breaking by freely acting orbifolds, or equivalently by Scherk-Schwarz compactifications, in type I string theory. In particular, we describe a four-dimensional chiral compactification with spontaneously broken N=1 supersymmetry, some models with partial and supersymmetry breaking and their heterotic and M-theory duals. A generic feature of these models is that in the gravitational sector and in the spectrum of D-branes parallel to the breaking coordinate, all mass splittings are proportional to the compactification scale, while global (extended) supersymmetry remains unbroken at tree level for the massless excitations of D-branes transverse to the breaking direction.We study total and partial supersymmetry breaking by freely acting orbifolds, or equivalently by Scherk-Schwarz compactifications, in type I string theory. In particular, we describe a four-dimensional chiral compactification with spontaneously broken N = 1 supersymmetry, some models with partial N = 4 â N = 2 and N = 4 â N = 1 supersymmetry breaking and their heterotic and M-theory duals. A generic feature of these models is that in the gravitational sector and in the spectrum of D-branes parallel to the breaking coordinate, all mass splittings are proportional to the compactification scale, while global (extended) supersymmetry remains unbroken at tree level for the massless excitations of D-branes transverse to the breaking direction
A Rationale for Long-lived Quarks and Leptons at the LHC: Low Energy Flavour Theory
In the framework of gauged flavour symmetries, new fermions in parity
symmetric representations of the standard model are generically needed for the
compensation of mixed anomalies. The key point is that their masses are also
protected by flavour symmetries and some of them are expected to lie way below
the flavour symmetry breaking scale(s), which has to occur many orders of
magnitude above the electroweak scale to be compatible with the available data
from flavour changing neutral currents and CP violation experiments. We argue
that, actually, some of these fermions would plausibly get masses within the
LHC range. If they are taken to be heavy quarks and leptons, in
(bi)-fundamental representations of the standard model symmetries, their
mixings with the light ones are strongly constrained to be very small by
electroweak precision data. The alternative chosen here is to exactly forbid
such mixings by breaking of flavour symmetries into an exact discrete symmetry,
the so-called proton-hexality, primarily suggested to avoid proton decay. As a
consequence of the large value needed for the flavour breaking scale, those
heavy particles are long-lived and rather appropriate for the current and
future searches at the LHC for quasi-stable hadrons and leptons. In fact, the
LHC experiments have already started to look for them.Comment: 10 pages, 1 figur
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
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
Non anomalous U(1)_H gauge model of flavor
A non anomalous horizontal gauge symmetry can be responsible for the
fermion mass hierarchies of the minimal supersymmetric standard model. Imposing
the consistency conditions for the absence of gauge anomalies yields the
following results: i) unification of leptons and down-type quarks Yukawa
couplings is allowed at most for two generations. ii) The term is
necessarily somewhat below the supersymmetry breaking scale. iii) The
determinant of the quark mass matrix vanishes, and there is no strong
problem. iv) The superpotential has accidental and symmetries. The
prediction allows for an unambiguous test of the model at low
energy.Comment: 5 pages, RevTex. Title changed, minor modifications. Final version to
appear in Phys. Rev.
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