614 research outputs found
(Non-)Abelian discrete anomalies
We derive anomaly constraints for Abelian and non-Abelian discrete symmetries
using the path integral approach. We survey anomalies of discrete symmetries in
heterotic orbifolds and find a new relation between such anomalies and the
so-called `anomalous' U(1).Comment: 32 pages, one figure; v2: matches version published in NP
A Mini-Landscape of Exact MSSM Spectra in Heterotic Orbifolds
We explore a ``fertile patch'' of the heterotic landscape based on a Z_6-II
orbifold with SO(10) and E_6 local GUT structures. We search for models
allowing for the exact MSSM spectrum. Our result is that of order 100 out of a
total 3\times 10^4 inequivalent models satisfy this requirement.Comment: 13 pages, for associated information see
http://www.th.physik.uni-bonn.de/nilles/Z6IIorbifold/, v2: matches version
published in PL
Suppressed supersymmetry breaking terms in the Higgs sector
We study the little hierarchy between mass parameters in the Higgs sector and
other SUSY breaking masses. This type of spectrum can relieve the fine-tuning
problem in the MSSM Higgs sector. Our scenario can be realized by
superconformal dynamics. The spectrum in our scenario has significant
implications in other phenomenological aspects like the relic abundance of the
lightest neutralino and relaxation of the unbounded-from-below constraints.Comment: 14 pages, late
Nanolithography and manipulation of graphene using an atomic force microscope
We use an atomic force microscope (AFM) to manipulate graphene films on a
nanoscopic length scale. By means of local anodic oxidation with an AFM we are
able to structure isolating trenches into single-layer and few-layer graphene
flakes, opening the possibility of tabletop graphene based device fabrication.
Trench sizes of less than 30 nm in width are attainable with this technique.
Besides oxidation we also show the influence of mechanical peeling and
scratching with an AFM of few layer graphene sheets placed on different
substrates.Comment: 11 pages text, 5 figure
Quark Mass Hierarchies, Flavor Mixing and Maximal CP-Violation
Flavor mixing and the quark mass spectrum are intimately related. In view of
the observed strong hierarchy of the quark and lepton masses and of the flavor
mixing angles it is argued that the description of flavor mixing must take this
into account. One particular interesting way to describe the flavor mixing
emerges, which is particularly suited for models of quark mass matrices based
on flavor symmetries. We conclude that the unitarity triangle important for
physics should be close to or identical to a rectangular triangle.
violation is maximal in this sense.Comment: 21 latex page
Explaining LSND by a decaying sterile neutrino
We propose an explanation of the LSND evidence for electron antineutrino
appearance based on neutrino decay. We introduce a heavy neutrino, which is
produced in pion and muon decays because of a small mixing with muon neutrinos,
and then decays into a scalar particle and a light neutrino, predominantly of
the electron type. We require values of few eV, being the
neutrino--scalar coupling and the heavy neutrino mass, e.g. in the
range from 1 keV to 1 MeV and . Performing a fit to
the LSND data as well as all relevant null-result experiments, we show that all
data can be explained within this decay scenario. In the minimal version of the
decay model, we predict a signal in the upcoming MiniBooNE experiment
corresponding to a transition probability of the same order as seen in LSND. In
addition, we show that extending our model to two nearly degenerate heavy
neutrinos it is possible to introduce CP violation in the decay, which can lead
to a suppression of the signal in MiniBooNE running in the neutrino mode. We
briefly discuss signals in future neutrino oscillation experiments, we show
that our scenario is compatible with bounds from laboratory experiments, and we
comment on implications in astrophysics and cosmology.Comment: 23 pages, 5 figures, minor improvements, matches published versio
Avoiding degenerate coframes in an affine gauge approach to quantum gravity
In quantum models of gravity, it is surmized that configurations with
degenerate coframes could occur during topology change of the underlying
spacetime structure. However, the coframe is not the true Yang--Mills type
gauge field of the translations, since it lacks the inhomogeneous gradient term
in the gauge transformations. By explicitly restoring this ``hidden" piece
within the framework of the affine gauge approach to gravity, one can avoid the
metric or coframe degeneracy which would otherwise interfere with the
integrations within the path integral. This is an important advantage for
quantization.Comment: 14 pages, Preprint Cologne-thp-1993-H
Molecular transport calculations with Wannier functions
We present a scheme for calculating coherent electron transport in
atomic-scale contacts. The method combines a formally exact Green's function
formalism with a mean-field description of the electronic structure based on
the Kohn-Sham scheme of density functional theory. We use an accurate
plane-wave electronic structure method to calculate the eigenstates which are
subsequently transformed into a set of localized Wannier functions (WFs). The
WFs provide a highly efficient basis set which at the same time is well suited
for analysis due to the chemical information contained in the WFs. The method
is applied to a hydrogen molecule in an infinite Pt wire and a benzene-dithiol
(BDT) molecule between Au(111) surfaces. We show that the transmission function
of BDT in a wide energy window around the Fermi level can be completely
accounted for by only two molecular orbitals.Comment: 15 pages, 12 figures, submitted to Chemical Physic
Finite SU(N)^k Unification
We consider N=1 supersymmetric gauge theories based on the group SU(N)_1 x
SU(N)_2 x ... x SU(N)_k with matter content (N,N*,1,...,1) + (1,N,N*,...,1) +
>... + (N*,1,1,...,N) as candidates for the unification symmetry of all
particles. In particular we examine to which extent such theories can become
finite and we find that a necessary condition is that there should be exactly
three families. We discuss further some phenomenological issues related to the
cases (N,k) = (3,3), (3,4), and (4,3), in an attempt to choose those theories
that can become also realistic. Thus we are naturally led to consider the
SU(3)^3 model which we first promote to an all-loop finite theory and then we
study its additional predictions concerning the top quark mass, Higgs mass and
supersymmetric spectrum.Comment: 15 page
SUSY GUT Model Building
I discuss an evolution of SUSY GUT model building, starting with the
construction of 4d GUTs, to orbifold GUTs and finally to orbifold GUTs within
the heterotic string. This evolution is an attempt to obtain realistic string
models, perhaps relevant for the LHC. This review is in memory of the sudden
loss of Julius Wess, a leader in the field, who will be sorely missed.Comment: 24 pages, 14 figures, lectures given at PiTP 2008, Institute for
Advanced Study, Princeton, to be published in the European Physical Journal
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