75 research outputs found
Suppressing Proton Decay in Theories with Localised Fermions
We calculate the contribution to the proton decay amplitude from Kaluza-Klein
lepto-quarks in theories with extra dimensions, localised fermions and gauge
fields which propagate in the bulk. Such models naturally occur within the
context of M-theory. In SU(5) models we show that carefully including all such
modes gives a distinctive pattern of decays through various channels including
a strong suppression of decays into neutrinos or right handed positrons. By
contrast there is no such suppression for SO(10).Comment: 19 page
De Sitter from T-branes
Hidden sector D7-branes with non-zero gauge flux are a generic feature of
type IIB compactifications. A non-vanishing Fayet-Iliopoulos term induced by
non-zero gauge flux leads to a T-brane configuration. Expanding the D7-brane
action around this T-brane background in the presence of three-form
supersymmetry breaking fluxes, we obtain a positive definite contribution to
the moduli scalar potential which can be used as an uplifting source for de
Sitter vacua. In this way we provide a higher-dimensional understanding of
known 4D mechanisms of de Sitter uplifting based on hidden sector F-terms which
are non-zero because of D-term stabilisation.Comment: 23 pages, 2 figure
Phenomenology from the Landscape of String Vacua
This article is the author's PhD thesis. After a review of string vacua
obtained through compactification (with and wothout fluxes), it presents and
describes various aspects of the Landscape of string vacua. At first it gives
an introduction and an overview of the statistical study of the set of four
dimensional string vacua, giving the detailed study of one corner of this set
(G2-holonomy compactifications of M-theory). Then it presents the ten
dimensional approach to string vacua, concentrating on the ten dimensional
description of the Type IIA flux vacua. Finally it gives two examples of models
having some interesting and characteristic phenomenological features, and that
belong to two different corners of the Landscape: warped compactifications of
Type IIB String Theory and M-theory compactifications on G2-holonomy manifolds.Comment: 170 pages; Ph.D. thesis (Advisor: Bobby S. Acharya); added reference
Moduli Stabilisation with Nilpotent Goldstino: Vacuum Structure and SUSY Breaking
We study the effective field theory of KKLT and LVS moduli stabilisation
scenarios coupled to an anti-D3-brane at the tip of a warped throat. We
describe the presence of the anti-brane in terms of a nilpotent goldstino
superfield in a supersymmetric effective field theory. The introduction of this
superfield produces a term that can lead to a de Sitter minimum. We fix the
Kaehler moduli dependence of the nilpotent field couplings by matching this
term with the anti-D3-brane uplifting contribution. The main result of this
paper is the computation, within this EFT, of the soft supersymmetry breaking
terms in both KKLT and LVS for matter living on D3-brane (leaving the D7-brane
analysis to an appendix). A handful of distinct phenomenological scenarios
emerge that could have low energy implications, most of them having a split
spectrum of soft masses. Some cosmological and phenomenological properties of
these models are discussed. We also check that the attraction between the
D3-brane and the anti-D3-brane does not affect the leading contribution to the
soft masses and does not destabilise the system.Comment: 26+14 pages, 1 figure; references and clarifications added, typos
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T-branes through 3d mirror symmetry
T-branes are exotic bound states of D-branes, characterized by mutually
non-commuting vacuum expectation values for the worldvolume scalars. The
M/F-theory geometry lifting D6/D7-brane configurations is blind to the T-brane
data. In this paper, we make this data manifest, by probing the geometry with
an M2-brane. We find that the effect of a T-brane is to deform the membrane
worldvolume superpotential with monopole operators, which partially break the
three-dimensional flavor symmetry, and reduce supersymmetry from N=4 to N=2.
Our main tool is 3d mirror symmetry. Through this language, a very concrete
framework is developed for understanding T-branes in M-theory. This leads us to
uncover a new class of N=2 quiver gauge theories, whose Higgs branches mimic
those of membranes at ADE singularities, but whose Coulomb branches differ from
their N=4 counterparts.Comment: 36 page
N=2 Heterotic-Type II duality and bundle moduli
Heterotic string compactifications on a surface depend on
a choice of hyperk\"ahler metric, anti-self-dual gauge connection and
Kalb-Ramond flux, parametrized by hypermultiplet scalars. The metric on
hypermultiplet moduli space is in principle computable within the
superconformal field theory on the heterotic string worldsheet, although little
is known about it in practice. Using duality with type II strings compactified
on a Calabi-Yau threefold, we predict the form of the quaternion-K\"ahler
metric on hypermultiplet moduli space when is elliptically
fibered, in the limit of a large fiber and even larger base. The result is in
general agreement with expectations from Kaluza-Klein reduction, in particular
the metric has a two-stage fibration structure, where the -field moduli are
fibered over bundle and metric moduli, while bundle moduli are themselves
fibered over metric moduli. A more precise match must await a detailed analysis
of -corrected ten-dimensional supergravity.Comment: 20 page
A database of Calabi-Yau orientifolds and the size of D3-tadpoles
The classification of 4D reflexive polytopes by Kreuzer and Skarke allows for a systematic construction of Calabi-Yau hypersurfaces as fine, regular, star triangulations (FRSTs). Until now, the vastness of this geometric landscape remains largely unexplored. In this paper, we construct Calabi-Yau orientifolds from holomorphic reflection involutions of such hypersurfaces with Hodge numbers h(1,1) <= 12. In particular, we compute orientifold configurations for all favourable FRSTs for h(1,1) <= 7, while randomly sampling triangulations for each pair of Hodge numbers up to h(1,1) = 12. We find explicit string compactifications on these orientifolded Calabi-Yaus for which the D3-charge contribution coming from Op-planes grows linearly with the number of complex structure and Kahler moduli. We further consider non-local D7-tadpole cancellation through Whitney branes. We argue that this leads to a significant enhancement of the total D3-tadpole as compared to conventional SO(8) stacks with (4 + 4) D7-branes on top of 07-planes. In particular, before turning-on worldvolume fluxes, we find that the largest D3-tadpole in this class occurs for Calabi-Yau threefolds with (h(+)(1,1), h(-)(1,2) ) = (11, 491) with D3-brane charges vertical bar Q(D3)vertical bar = 504 for the local D7 case and vertical bar Q(D3)vertical bar = 6, 664 for the non-local Whitney branes case, which appears to be large enough to cancel tadpoles and allow fluxes to stabilise all complex structure moduli. Our data is publicly available under the following link https://github.com/AndreasSchachner/CY_Orientifold_database
Higgs branches of 5d rank-zero theories from geometry
Abstract: We study the Higgs branches of five-dimensional N = 1 rank-zero theories obtained
from M-theory on two classes non-toric non-compact Calabi-Yau threefolds: Reid’s
pagodas, and Laufer’s examples. Our approach consists in reducing to IIA with D6-branes
and O6-planes, and computing the open-string spectra giving rise to hypermultiplets. Starting
with the seven-dimensional worldvolume theories, we switch on T-brane backgrounds
to give rise to bound states with angles. We observe that the resulting partially Higgsed
5d theories have discrete gauge groups, from which we readily deduce the geometry of the
Higgs branches as orbifolds of quaternionic varieties
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