96,194 research outputs found
Preheating After Modular Inflation
We study (p)reheating in modular (closed string) inflationary scenarios, with
a special emphasis on Kahler moduli/Roulette models. It is usually assumed that
reheating in such models occurs through perturbative decays. However, we find
that there are very strong non-perturbative preheating decay channels related
to the particular shape of the inflaton potential (which is highly nonlinear
and has a very steep minimum). Preheating after modular inflation, proceeding
through a combination of tachyonic instability and broad-band parametric
resonance, is perhaps the most violent example of preheating after inflation
known in the literature. Further, we consider the subsequent transfer of energy
to the standard model sector in scenarios where the standard model particles
are confined to a D7-brane wrapping the inflationary blow-up cycle of the
compactification manifold or, more interestingly, a non-inflationary blow up
cycle. We explicitly identify the decay channels of the inflaton in these two
scenarios. We also consider the case where the inflationary cycle shrinks to
the string scale at the end of inflation; here a field theoretical treatment of
reheating is insufficient and one must turn instead to a stringy description.
We estimate the decay rate of the inflaton and the reheat temperature for
various scenarios.Comment: 34 pages, 10 figures. Accepted for publication in JCA
De Sitter String Vacua from Dilaton-dependent Non-perturbative Effects
We consider a novel scenario for modulus stabilisation in IIB string
compactifications in which the Kahler moduli are stabilised by a general set-up
with two kinds of non-perturbative effects: (i) standard Kahler
moduli-dependent non-perturbative effects from gaugino condensation on
D7-branes or E3-instantons wrapping four-cycles in the geometric regime; (ii)
dilaton-dependent non-perturbative effects from gaugino condensation on
space-time filling D3-branes or E(-1)-instantons at singularities. For the
LARGE Volume Scenario (LVS), the new dilaton-dependent non-perturbative effects
provide a positive definite contribution to the scalar potential that can be
arbitrarily tuned from fluxes to give rise to de Sitter vacua. Contrary to anti
D3-branes at warped throats, this term arises from a manifestly supersymmetric
effective action. In this new scenario the "uplifting" term comes from F-terms
of blow-up modes resolving the singularity of the non-perturbative quiver. We
discuss phenomenological and cosmological implications of this mechanism. This
set-up also allows a realisation of the LVS for manifolds with zero or positive
Euler number.Comment: 22 pages + two appendices, typos correcte
The Statistics of Supersymmetric D-brane Models
We investigate the statistics of the phenomenologically important D-brane
sector of string compactifications. In particular for the class of intersecting
D-brane models, we generalise methods known from number theory to determine the
asymptotic statistical distribution of solutions to the tadpole cancellation
conditions. Our approach allows us to compute the statistical distribution of
gauge theoretic observables like the rank of the gauge group, the number of
chiral generations or the probability of an SU(N) gauge factor. Concretely, we
study the statistics of intersecting branes on T^2 and T^4/Z_2 and T^6/Z_2 x
Z_2 orientifolds. Intriguingly, we find a statistical correlation between the
rank of the gauge group and the number of chiral generations. Finally, we
combine the statistics of the gauge theory sector with the statistics of the
flux sector and study how distributions of gauge theoretic quantities are
affected.Comment: 62 pages, 31 figures, harvmac; v3: sections 3.2. + 3.7. added, figs.
7,28,29 added, figs. 24,25,26 corrected, refs. added, typos correcte
Gauge sector statistics of intersecting D-brane models
In this article, which is based on the first part of my PhD thesis, I review
the statistics of the open string sector in T^6/(Z_2xZ_2) orientifold
compactifications of the type IIA string. After an introduction to the
orientifold setup, I discuss the two different techniques that have been
developed to analyse the gauge sector statistics, using either a saddle point
approximation or a direct computer based method. The two approaches are
explained and compared by means of eight- and six-dimensional toy models. In
the four-dimensional case the results are presented in detail. Special emphasis
is put on models containing phenomenologically interesting gauge groups and
chiral matter, in particular those containing a standard model or SU(5) part.Comment: 51 pages, 29 figures; v2: ref. added, version to appear in Fortsch.
Phys; v3: ref. adde
Chiral D-brane Models with Frozen Open String Moduli
Most intersecting D-brane vacua in the literature contain additional massless
adjoint fields in their low energy spectrum. The existence of these additional
fields make it difficult to obtain negative beta functions and, eventually,
asymptotic freedom. We address this important issue for N=1 intersecting
D-brane models, rephrasing the problems in terms of (open string) moduli
stabilization. In particular, we consider a Z2 x Z2 orientifold construction
where D6-branes wrap rigid 3-cycles and such extra adjoint fields do not arise.
We derive the model building rules and consistency conditions for intersecting
branes in this background, and provide N=1 chiral vacua free of adjoint fields.
More precisely, we construct a Pati-Salam-like model whose SU(4) gauge group is
asymptotically free. We also comment on the application of these results for
obtaining gaugino condensation in chiral D-brane models. Finally, we embed our
constructions in the framework of flux compactification, and construct new
classes of N=1 and N=0 chiral flux vacua.Comment: 55 pages, 4 figures. Bibtex forma
New Grand Unified Models with Intersecting D6-branes, Neutrino Masses, and Flipped SU(5)
We construct new supersymmetric SU(5) Grand Unified Models based on Z4 x Z2
orientifolds with intersecting D6-branes. Unlike constructions based on Z2 x Z2
orientifolds, the orbifold images of the three-cycles wrapped by D6-branes
correspond to new configurations and thus allow for models in which, in
addition to the chiral sector in 10 and 5-bar representations of SU(5), only,
there can be new sectors with (15 + 15-bar) and (10 + 10-bar) vector-pairs. We
construct an example of such a globally consistent, supersymmetric model with
four-families, two Standard Model Higgs pair-candidates and the gauge symmetry
U(5) x U(1) x Sp(4). In a N = 2 sector, there are 5 x (15 + 15-bar) and 1 x (10
+ 10-bar) vector pairs, while another N = 1 sector contains one vector-pair of
15-plets. The N = 2 vector pairs can obtain a large mass dynamically by
parallel D6-brane splitting in a particu- lar two-torus. The 15-vector-pairs
provide, after symmetry breaking to the Standard Model (via parallel D-brane
splitting), triplet pair candidates which can in principle play a role in
generating Majorana- type masses for left-handed neutrinos, though the
necessary Yukawa couplings are absent in the specific construction. Similarly,
the 10- vector-pairs can play the role of Higgs fields of the flipped SU(5),
though again there are phenomenological difficulties for the specific
construction.Comment: 38 pages, 2 figures Minor corrections, references adde
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