5,124 research outputs found
Charm lifetime
A review of the charmed meson and baryon lifetimes is presented. Our
knowledge of charmed particle lifetimes has greatly improved over the past two
years, a crucial r\^ole having been played by the E687 experiment at Fermilab,
which has almost quadrupled the samples of mesons. The lifetime ratios
and are now known with an
accuracy of 1.7\% and 3.7\% respectively. In the baryon sector the statistics
is still limited, but the experimental results on , and
exhibit a clear pattern of lifetime hierarchy, as expected from
simple theoretical arguments. The first measurement of from
E687 is also presented to complete the charmed baryon lifetime picture. The
more accurate experimental scenario can provide information on non-perturbative
QCD effects and the hadronic matrix elements.Comment: 10 pages, latex, 3 figures. Talk presented at the 6th. International
Symposium on Heavy Flavour Physics (Pisa, June 1995
PAMELA Measurements of Cosmic-ray Proton and Helium Spectra
Protons and helium nuclei are the most abundant components of the cosmic
radiation. Precise measurements of their fluxes are needed to understand the
acceleration and subsequent propagation of cosmic rays in the Galaxy. We report
precision measurements of the proton and helium spectra in the rigidity range 1
GV-1.2 TV performed by the satellite-borne experiment PAMELA. We find that the
spectral shapes of these two species are different and cannot be well described
by a single power law. These data challenge the current paradigm of cosmic-ray
acceleration in supernova remnants followed by diffusive propagation in the
Galaxy. More complex processes of acceleration and propagation of cosmic rays
are required to explain the spectral structures observed in our data.Comment: 13 pages, 4 figures, link to SOM (with tables) in the references.
This manuscript has been accepted for publication in Science. This version
has not undergone final editing. Please refer to the complete version of
record at http://www.sciencemag.org/ [www.sciencemag.org
A new measurement of the antiproton-to-proton flux ratio up to 100 GeV in the cosmic radiation
A new measurement of the cosmic ray antiproton-to-proton flux ratio between 1
and 100 GeV is presented. The results were obtained with the PAMELA experiment,
which was launched into low-earth orbit on-board the Resurs-DK1 satellite on
June 15th 2006. During 500 days of data collection a total of about 1000
antiprotons have been identified, including 100 above an energy of 20 GeV. The
high-energy results are a ten-fold improvement in statistics with respect to
all previously published data. The data follow the trend expected from
secondary production calculations and significantly constrain contributions
from exotic sources, e.g. dark matter particle annihilations.Comment: 10 pages, 4 figures, 1 tabl
The cosmic-ray electron flux measured by the PAMELA experiment between 1 and 625 GeV
Precision measurements of the electron component in the cosmic radiation
provide important information about the origin and propagation of cosmic rays
in the Galaxy. Here we present new results regarding negatively charged
electrons between 1 and 625 GeV performed by the satellite-borne experiment
PAMELA. This is the first time that cosmic-ray electrons have been identified
above 50 GeV. The electron spectrum can be described with a single power law
energy dependence with spectral index -3.18 +- 0.05 above the energy region
influenced by the solar wind (> 30 GeV). No significant spectral features are
observed and the data can be interpreted in terms of conventional diffusive
propagation models. However, the data are also consistent with models including
new cosmic-ray sources that could explain the rise in the positron fraction.Comment: 11 pages, 3 figures, accepted for publication in PR
Phase Separation in Electronic Models for Manganites
The Kondo lattice Hamiltonian with ferromagnetic Hund's coupling as a model
for manganites is investigated. The classical limit for the spin of the
(localized) electrons is analyzed on lattices of dimension 1,2,3 and
using several numerical methods. The phase diagram at low temperature
is presented. A regime is identified where phase separation occurs between hole
undoped antiferromagnetic and hole-rich ferromagnetic regions. Experimental
consequences of this novel regime are discussed. Regions of incommensurate spin
correlations have also been found. Estimations of the critical temperature in
3D are compatible with experiments.Comment: Accepted in Phys. Rev. Letter
Diagonalization in Reduced Hilbert Spaces using a Systematically Improved Basis: Application to Spin Dynamics in Lightly Doped Ladders
A method is proposed to improve the accuracy of approximate techniques for
strongly correlated electrons that use reduced Hilbert spaces. As a first step,
the method involves a change of basis that incorporates exactly part of the
short distance interactions. The Hamiltonian is rewritten in new variables that
better represent the physics of the problem under study. A Hilbert space
expansion performed in the new basis follows. The method is successfully tested
using both the Heisenberg model and the model with holes on 2-leg ladders
and chains, including estimations for ground state energies, static
correlations, and spectra of excited states. An important feature of this
technique is its ability to calculate dynamical responses on clusters larger
than those that can be studied using Exact Diagonalization. The method is
applied to the analysis of the dynamical spin structure factor on
clusters with sites and 0 and 2 holes. Our results confirm
previous studies (M. Troyer, H. Tsunetsugu, and T. M. Rice, Phys. Rev. ,
251 (1996)) which suggested that the state of the lowest energy in the spin-1
2-holes subspace corresponds to the bound state of a hole pair and a
spin-triplet. Implications of this result for neutron scattering experiments
both on ladders and planes are discussed.Comment: 9 pages, 8 figures, Revtex + psfig; changed conten
Charge and spin inhomogeneous phases in the Ferromagnetic Kondo Lattice Model
We study numerically the one-dimensional ferromagnetic Kondo lattice. This
model is widely used to describe nickel and manganese perovskites. Due to the
competition between double and super-exchange, we find a region where the
formation of magnetic polarons induces a charge-ordered state. This ordering is
present even in the absence of any inter-site Coulomb repulsion. There is an
insulating gap associated to the charge structure formation. We also study the
insulator-metal transition induced by a magnetic field which removes
simultaneously both charge and spin ordering.Comment: 7 pages, 11 figure
PAMELA results on the cosmic-ray antiproton flux from 60 MeV to 180 GeV in kinetic energy
The satellite-borne experiment PAMELA has been used to make a new measurement
of the cosmic-ray antiproton flux and the antiproton-to-proton flux ratio which
extends previously published measurements down to 60 MeV and up to 180 GeV in
kinetic energy. During 850 days of data acquisition approximately 1500
antiprotons were observed. The measurements are consistent with purely
secondary production of antiprotons in the galaxy. More precise secondary
production models are required for a complete interpretation of the results.Comment: 11 pages, 3 figures, 1 table. Accepted for publication in Physical
Review Letter
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