83 research outputs found
Fine-Tuning Constraints on Supergravity Models
We discuss fine-tuning constraints on supergravity models. The tightest
constraints come from the experimental mass limits on two key particles: the
lightest CP even Higgs boson and the gluino. We also include the lightest
chargino which is relevant when universal gaugino masses are assumed. For each
of these particles we show how fine-tuning increases with the experimental mass
limit, for four types of supergravity model: minimal supergravity, no-scale
supergravity (relaxing the universal gaugino mass assumption), D-brane models
and anomaly mediated supersymmetry breaking models. Among these models, the
D-brane model is less fine tuned.The experimental propects for an early
discovery of Higgs and supersymmetry at LEP and the Tevatron are discussed in
this framework.Comment: 17 pages, Latex, including 5 eps figure
Naturalness Implications of LEP Results
We analyse the fine-tuning constraints arising from absence of superpartners
at LEP, without strong universality assumptions. We show that such constraints
do not imply that charginos or neutralinos should have been seen at LEP,
contrary to the usual arguments. They do however imply relatively light gluinos
(m_{\tilde g} \lsim 350 GeV) and/or a relation between the soft-breaking
SU(3) gaugino mass and Higgs soft mass . The LEP limit on the Higgs
mass is significant, especially at low , and we investigate to what
extent this provides evidence for both a lighter gluino and correlations
between soft masses.Comment: 22 pages, Latex, including 2 eps figure
Quartic Anomalous Couplings in Colliders
We study the production of gauge boson pairs at the next generation of linear
colliders operating in the mode. The processes (, , or and or )
can give valuable information on possible deviations of the quartic vector
boson couplings from the Standard Model predictions. We establish the range of
the new couplings that can be explored in these colliders based on a
effect in the total cross section. We also present several kinematical
distributions of the final state particles that could manifest the underlying
new dynamics. Our results show that an collider can extend
considerably the bounds on anomalous interactions coming from oblique radiative
corrections and from direct searches in colliders.Comment: 21 pages and 7 figures appended as a postscript file (uuencoded),
MAD/PH/764, IFT-P.031/93, IFUSP-P 1047 (uses REVTeX3
Brane Mediated Supersymmetry Breaking
We propose a mechanism for mediating supersymmetry breaking in Type I string
constructions. The basic set-up consists of a system of three D-branes: two
parallel D-branes, a matter D-brane and a source D-brane, with supersymmetry
breaking communicated via a third D-brane, the mediating D-brane, which
intersects both of the parallel D-branes. We discuss an example in which the
first and second family matter fields correspond to open strings living on the
intersection of the matter D-brane and mediating D-brane, while the gauge
fields, Higgs doublets and third family matter fields correspond to open
strings living on the mediating D-brane. As in gaugino mediated models, the
gauginos and Higgs doublets receive direct soft masses from the source brane,
and flavour-changing neutral currents are naturally suppressed since the first
and second family squarks and sleptons receive suppressed soft masses. However,
unlike the gaugino mediated model, the third family squarks and sleptons
receive unsuppressed soft masses, resulting in a very distinctive spectrum with
heavier stops, sbottoms and staus.Comment: Version to appear in Nucl.Phys.B. 28 pages, Late
CP and Lepton-Number Violation in GUT Neutrino Models with Abelian Flavour Symmetries
We study the possible magnitudes of CP and lepton-number-violating quantities
in specific GUT models of massive neutrinos with different Abelian flavour
groups, taking into account experimental constraints and requiring successful
leptogenesis. We discuss SU(5) and flipped SU(5) models that are consistent
with the present data on neutrino mixing and upper limits on the violations of
charged-lepton flavours and explore their predictions for the CP-violating
oscillation and Majorana phases. In particular, we discuss string-derived
flipped SU(5) models with selection rules that modify the GUT structure and
provide additional constraints on the operators, which are able to account for
the magnitudes of some of the coefficients that are often set as arbitrary
parameters in generic Abelian models.Comment: 30 pages, 6 figure
A Model of Fermion Masses and Flavor Mixings with Family Symmetry
The family symmetry is proposed to solve flavor problems
about fermion masses and flavor mixings. It's breaking is implemented by some
flavon fields at the high-energy scale. In addition a discrete group is
introduced to generate tiny neutrino masses, which is broken by a real singlet
scalar field at the middle-energy scale. The low-energy effective theory is
elegantly obtained after all of super-heavy fermions are integrated out and
decoupling. All the fermion mass matrices are regularly characterized by four
fundamental matrices and thirteen parameters. The model can perfectly fit and
account for all the current experimental data about the fermion masses and
flavor mixings, in particular, it finely predicts the first generation quark
masses and the values of and in neutrino
physics. All of the results are promising to be tested in the future
experiments.Comment: 14 pages, 1 figure, to make a few of corrections to the old version.
arXiv admin note: substantial text overlap with arXiv:1011.457
Non-universal gaugino masses: a signal-based analysis for the Large Hadron Collider
We discuss the signals at the Large Hadron Collider (LHC) for scenarios with
non-universal gaugino masses in supersymmetric (SUSY) theories. We perform a
multichannel analysis, and consider the ratios of event rates in different
channels such as , - and -
, as well as and final states
together with . Low-energy SUSY spectra corresponding to
high-scale gaugino non-universality arising from different breaking schemes of
SU(5) as well as SO(10) Grand Unified (GUT) SUSY models are considered, with
both degenerate low-energy sfermion masses and those arising from a
supergravity scenario. We present the numerical predictions over a wide range
of the parameter space using the event generator {\tt Pythia}, specifying the
event selection criteria and pointing out regions where signals are likely to
be beset with backgrounds. Certain broad features emerge from the study, which
may be useful in identifying the signatures of different GUT breaking schemes
and distinguishing them from a situation with a universal gaugino mass at high
scale. The absolute values of the predicted event rates for different scenarios
are presented together with the various event ratios, so that these can also be
used whenever necessary.Comment: 54 pages, 18 figure
An observation of spin-valve effects in a semiconductor field effect transistor: a novel spintronic device
We present the first spintronic semiconductor field effect transistor.
The injector and collector contacts of this device were made from magnetic
permalloy thin films with different coercive fields so that they could be
magnetized either parallel or antiparallel to each other in different applied
magnetic fields. The conducting medium was a two dimensional electron gas
(2DEG) formed in an AlSb/InAs quantum well.
Data from this device suggest that its resistance is controlled by two
different types of spin-valve effect: the first occurring at the
ferromagnet-2DEG interfaces; and the second occuring in direct propagation
between contacts.Comment: 4 pages, 2 figure
Spin-dependent thermoelectric transport coefficients in near-perfect quantum wires
Thermoelectric transport coefficients are determined for semiconductor
quantum wires with weak thickness fluctuations. Such systems exhibit anomalies
in conductance near 1/4 and 3/4 of 2e^2/h on the rising edge to the first
conductance plateau, explained by singlet and triplet resonances of conducting
electrons with a single weakly bound electron in the wire [T. Rejec, A. Ramsak,
and J.H. Jefferson, Phys. Rev. B 62, 12985 (2000)]. We extend this work to
study the Seebeck thermopower coefficient and linear thermal conductance within
the framework of the Landauer-Buettiker formalism, which also exhibit anomalous
structures. These features are generic and robust, surviving to temperatures of
a few degrees. It is shown quantitatively how at elevated temperatures thermal
conductance progressively deviates from the Wiedemann-Franz law.Comment: To appear in Phys. Rev. B 2002; 3 figure
On low temperature kinetic theory; spin diffusion, Bose Einstein condensates, anyons
The paper considers some typical problems for kinetic models evolving through
pair-collisions at temperatures not far from absolute zero, which illustrate
specific quantum behaviours. Based on these examples, a number of differences
between quantum and classical Boltzmann theory is then discussed in more
general terms.Comment: 25 pages, minor updates of previous versio
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