1,326 research outputs found
Catching the phantom: the MSSM on the Z6-orientifold
These lecture notes give a short introduction of the derivation of the
supersymmetric standard model on the Z6-orientifold as published in
hep-th/0404055. Untwisted and twisted cycles are constructed and one specific
model is discussed in more detail.Comment: To appear in the proceedings of RTN Workshop on the Quantum Structure
of Space-time and the Geometric Nature of Fundamental Interactions and EXT
Workshop on Fundamental Interactions and the Structure of Spacetime,
Kolymbari, Crete, Greece, 5-10 Sep 200
Standard Model statistics of a Type II orientifold
We analyse four-dimensional, supersymmetric intersecting D-brane models in a
toroidal orientifold background from a statistical perspective. The
distribution and correlation of observables, like gauge groups and couplings,
are discussed. We focus on models with a Standard Model-like gauge sector,
derive frequency distributions for their occurence and analyse the properties
of the hidden sector.Comment: 9 pages, 9 figures; v2: typos corrected, one reference added;
Contribution to the proceedings of the RTN workshop "Constituents,
Fundamental Forces and Symmetries of the Universe", Corfu, Greece, 20-26 Sep
200
Statistical Predictions From Anarchic Field Theory Landscapes
Consistent coupling of effective field theories with a quantum theory of
gravity appears to require bounds on the the rank of the gauge group and the
amount of matter. We consider landscapes of field theories subject to such to
boundedness constraints. We argue that appropriately "coarse-grained" aspects
of the randomly chosen field theory in such landscapes, such as the fraction of
gauge groups with ranks in a given range, can be statistically predictable. To
illustrate our point we show how the uniform measures on simple classes of N=1
quiver gauge theories localize in the vicinity of theories with certain typical
structures. Generically, this approach would predict a high energy theory with
very many gauge factors, with the high rank factors largely decoupled from the
low rank factors if we require asymptotic freedom for the latter.Comment: 22 pages, 5 figure
Orientational and phase-coexistence behaviour of hard rod-sphere mixtures
Results are presented from Monte Carlo simulations of bulk mixtures of Hard Gaussian Overlap particles with an aspect ratio of 3:1 and hard spheres with diameters equal to the breadths of the rods. For sphere number-concentrations of 50% and lower, compression of the isotropic fluid results in formation of a homogeneous (i.e. compositionally mixed) nematic phase. The volume fraction of this isotropic-nematic transition is found to increase approximately linearly with sphere concentration. On compression to higher volume fractions, however, this homogeneous nematic phase separates out into coexisting nematic and isotropic phases.</p
Statistics in the Landscape of Intersecting Brane Models
An approach towards a statistical survey of four dimensional supersymmetric
vacua in the string theory landscape is described and illustrated with three
examples of ensembles of intersecting D-brane models. The question whether it
is conceivable to make predictions based on statistical distributions is
discussed. Especially interesting in this context are possible correlations
between low energy observables. As an example we look at correlations between
properties of the gauge sector of intersecting D-brane models and Gepner model
constructions.Comment: Submitted for the SUSY07 proceedings, 4 pages, 2 figure
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
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
Progress in D-brane model building
The state of the art in D-brane model building is briefly reviewed, focusing
on recent achievements in the construction of D=4 N = 1 type II string vacua
with semi-realistic gauge sectors. Such progress relies on a better
understanding of the spectrum of BPS D-branes, the effective field theory
obtained from them and the explicit construction of vacua. We first consider
D-branes in standard Calabi-Yau compactifications, and then the more involved
case of compactifications with fluxes. We discuss how the non-trivial interplay
between D-branes and fluxes modifies the previous model-building rules, as well
as provides new possibilities to connect string theory to particle physics.Comment: Using w-art.cls, 27 pages, 6 figures. Based on a talk given at the
RTN `Constituents, Fundamental Forces and Symmetries of the Universe'
Workshop and Midterm meeting in Napoli, October 2006. v2: typos corrected and
references adde
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
Exophobic Quasi-Realistic Heterotic String Vacua
We demonstrate the existence of heterotic-string vacua that are free of
massless exotic fields. The need to break the non-Abelian GUT symmetries in k=1
heterotic-string models by Wilson lines, while preserving the GUT embedding of
the weak-hypercharge and the GUT prediction sin^2\theta_w(M(GUT))=3/8,
necessarily implies that the models contain states with fractional electric
charge. Such states are severely restricted by observations, and must be
confined or sufficiently massive and diluted. We construct the first
quasi-realistic heterotic-string models in which the exotic states do not
appear in the massless spectrum, and only exist, as they must, in the massive
spectrum. The SO(10) GUT symmetry is broken to the Pati-Salam subgroup. Our PS
heterotic-string models contain adequate Higgs representations to break the GUT
and electroweak symmetry, as well as colour Higgs triplets that can be used for
the missing partner mechanism. By statistically sampling the space of
Pati-Salam vacua we demonstrate the abundance of quasi--realistic three
generation models that are completely free of massless exotics, rendering it
plausible that obtaining realistic Yukawa couplings may be possible in this
space of models.Comment: 14 pages. Standard LaTex. 1 figure. 2 tables. Typos correcte
- …