5,748 research outputs found

### Higgs Searches and Prospects from LEP2

The status of the search at LEP2 for the Higgs in the Standard Model (SM) and
in the minimal supersymmetric extension of the Standard Model MSSM) is
reviewed. A preliminary lower limit of 95.5/c^2 at 95% C.L. on the SM Higgs is
obtained after a preliminary analysis of the data collected at sqrt(s)= 189
GeV. For standard choices of MSSM parameter sets, the search for the neutral
Higgs bosons h and A leads to preliminary 95% C.L. exclusion lower limits of
83.5GeV/c^2 and 84.5 GeV/c^2, respectively.Comment: 6 page

### Orbital ordering, Jahn-Teller distortion, and resonant x-ray scattering in KCuF3

The orbital, lattice, and spin ordering phenomena in KCuF3 are investigated
by means of LDA+U calculations, based on ab-initio pseudopotentials.We examine
the Cu-3d orbital ordering and the associated Jahn-Teller distortion in several
different spin-ordered structures of KCuF3. The ground state is correctly
predicted to be an A-type antiferromagnetic structure, and the calculated
Jahn-Teller distortion agrees also well with experiment. Concerning the orbital
ordering, we find that even for a highly ionic compound such as KCuF3, the
orbital-order parameter is significantly reduced with respect to its nominal
value due to Cu(3d)--F(2p) hybridization. We also calculate the Cu K-edge
resonant x-ray scattering spectra for Bragg reflections associated with orbital
order. Consistent with previous studies, we find that the resonant signal is
dominated by the structural anisotropy in the distribution of the F neighbors
of the resonant Cu atom, and that the Cu-3d orbital ordering has only a minor
influence on the spectra. Our LDA+U results, however, also indicate that a
change in the magnetic structure has a small influence on the Jahn-Teller
distortion, and hence on the resonant spectrum, in the conventional
(room-temperature) crystallographic structure of KCuF3. This may indicate that
the large change observed experimentally in the resonant signal near the N\'eel
temperature is related to a low-temperature structural transformation in KCuF3.Comment: 11 pages, 9 figure

### Parton distributions in a constituent quark scenario

A simple picture of the constituent quark as a composite system of point-like
partons is used to construct the unpolarized and polarized parton distributions
by a convolution between constituent quark momentum distributions and
constituent quark structure functions. We achieve good agreement with
experiments in the unpolarized, as well as, in the polarized case. When our
results are compared with similar calculations using non-composite constituent
quarks, the accord with the experiments of the present scheme is impressive. We
conclude that DIS data are consistent with a low energy scenario dominated by
composite constituents of the nucleon.Comment: 4 pages; latex using espcrc1.sty; 4 postscript figures; Invited talk
at the Workshop ``Nucleon '99'', Frascati; Italy 7-9 June 1999. Submitted to
Nuc. Phys.

### GRSV parton densities revisited

An updated next-to-leading order (NLO) QCD analysis of all presently
available longitudinally polarized deep-inelastic scattering (DIS) data is
presented in the framework of the radiative parton model.Comment: 3 pages, LaTeX, 2 eps figures, uses amssymb, npb (included), and
epsfig styles; Contribution to the Proceedings of the 7th International
Workshop on `Deep Inelastic Scattering and QCD (DIS99)' [Nucl. Phys. B (Proc.
Suppl.)], Zeuthen, Germany, April 199

### Optimizing spread dynamics on graphs by message passing

Cascade processes are responsible for many important phenomena in natural and
social sciences. Simple models of irreversible dynamics on graphs, in which
nodes activate depending on the state of their neighbors, have been
successfully applied to describe cascades in a large variety of contexts. Over
the last decades, many efforts have been devoted to understand the typical
behaviour of the cascades arising from initial conditions extracted at random
from some given ensemble. However, the problem of optimizing the trajectory of
the system, i.e. of identifying appropriate initial conditions to maximize (or
minimize) the final number of active nodes, is still considered to be
practically intractable, with the only exception of models that satisfy a sort
of diminishing returns property called submodularity. Submodular models can be
approximately solved by means of greedy strategies, but by definition they lack
cooperative characteristics which are fundamental in many real systems. Here we
introduce an efficient algorithm based on statistical physics for the
optimization of trajectories in cascade processes on graphs. We show that for a
wide class of irreversible dynamics, even in the absence of submodularity, the
spread optimization problem can be solved efficiently on large networks.
Analytic and algorithmic results on random graphs are complemented by the
solution of the spread maximization problem on a real-world network (the
Epinions consumer reviews network).Comment: Replacement for "The Spread Optimization Problem

### Discrete Flavor Symmetries and Models of Neutrino Mixing

We review the application of non abelian discrete groups to the theory of
neutrino masses and mixing, which is strongly suggested by the agreement of the
Tri-Bimaximal mixing pattern with experiment. After summarizing the motivation
and the formalism, we discuss specific models, based on A4, S4 and other finite
groups, and their phenomenological implications, including lepton flavor
violating processes, leptogenesis and the extension to quarks. In alternative
to Tri-Bimaximal mixing the application of discrete flavor symmetries to
quark-lepton complementarity and Bimaximal Mixing is also considered.Comment: 54 pages, 3 figures, minor changes in the text and references adde

### Neutrino Masses and Mixing, Quark-lepton Symmetry and Strong Right-handed Neutrino Hierarchy

Assuming the same form of all mass matrices as motivated by quark-lepton
symmetry, we discuss conditions under which bi-large mixing in the lepton
sector can be obtained with a minimal amount of fine tuning requirements for
possible models. We assume hierarchical mass matrices, dominated by the 3-3
element, with off-diagonal elements much smaller than the larger neighboring
diagonal element. Characteristic features of this scenario are strong hierarchy
in masses of right-handed neutrinos, and comparable contributions of both
lighter right-handed neutrinos to the resulting left-handed neutrino Majorana
mass matrix. Due to obvious quark lepton symmetry, this approach can be
embedded into grand unified theories. The mass of the lightest neutrino does
not depend on details of a model in the leading order. The right-handed
neutrino scale can be identified with the GUT scale in which case the mass of
the lightest neutrino is given as (m_{top}^2/M_{GUT}) |U_{\tau 1}|^2.Comment: 7 page

### Probing the Majorana nature of the neutrino with neutrinoless double beta decay

Neutrinoless double beta decay (NDBD) is the only experiment that could probe
the Majorana nature of the neutrino. Here we study the theoretical implications
of NDBD for models yielding tri-bimaximal lepton mixing like A4 and S4.Comment: Talk given at TAUP09, July 1-5, 2009 (Roma).The proceeding will be
published in Journal of Physics, Conference Series (Editors: E. Coccia, L.
Pandola, N. Fornengo, R. Aloisio

### Higgs-Flavor Groups, Naturalness, and Dark Matter

In the absence of low-energy supersymmetry, a multiplicity of weak-scale
Higgs doublets would require additional fine-tunings unless they formed an
irreducible multiplet of a non-abelian symmetry. Remnants of such symmetry
typically render some Higgs fields stable, giving several dark matter particles
of various masses. The non-abelian symmetry also typically gives simple,
testable mass relations.Comment: Some comments added after Eqs. (2) and (12

### Large Transverse Momentum Jet Production and DIS Distributions of the Proton

We have calculated the single jet inclusive cross section as measured at
Fermilab in next-to-leading order QCD using recent parton distributions of the
CTEQ collaboration. We studied the scheme dependence of the jet cross section
by employing the \overline{\mbox{MS}} and DIS factorization schemes
consistently. For $E_T > 200$ GeV, we find that the cross section in the DIS
scheme is larger than in the \overline{\mbox{MS}} scheme yielding a
satisfactory description of the CDF data over the whole $E_T$ range in the DIS
scheme.Comment: 7 pages, latex, 2 figures include

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