10,565 research outputs found
Unexpected features of e+e-->ppbar and e+e-->lambda-lambdabar cross sections near threshold
Unexpected features of the BaBar data on e+e- in baryon-antibaryon cross
sections are discussed. These data have been collected, with unprecedented
accuracy, by means of the initial state radiation technique, which is
particularly suitable in giving good acceptance and energy resolution at
threshold. A striking feature observed in the BaBar data is the non-vanishing
cross section at threshold for all these processes. This is the expectation due
to the Coulomb enhancement factor acting on a charged fermion pair. In the case
of e+e- in proton-antiproton it is found that Coulomb final state interactions
largely dominate the cross section and the form factor is |G^p(4M^2_p)|~1,
which could be a general feature for baryons. In the case of neutral baryons an
interpretation of the non-vanishing cross section at threshold is suggested,
based on quark electromagnetic interaction and taking into account the
asymmetry between attractive and repulsive Coulomb factors. Besides strange
baryon cross sections are compared to U-spin invariance predictions.Comment: 12 pages, 11 figure
Odd Parity Light Baryon Resonances
We use a consistent SU(6) extension of the meson-baryon chiral Lagrangian
within a coupled channel unitary approach in order to calculate the T-matrix
for meson-baryon scattering in s-wave. The building blocks of the scheme are
the pion and nucleon octets, the rho nonet and the Delta decuplet. We identify
poles in this unitary T-matrix and interpret them as resonances. We study here
the non exotic sectors with strangeness S=0,-1,-2,-3 and spin J=1/2, 3/2 and
5/2. Many of the poles generated can be associated with known N, Delta, Sigma,
Lambda and Xi resonances with negative parity. We show that most of the
low-lying three and four star odd parity baryon resonances with spin 1/2 and
3/2 can be related to multiplets of the spin-flavor symmetry group SU(6). This
study allows us to predict the spin-parity of the Xi(1620), Xi(1690), Xi(1950),
Xi(2250), Omega(2250) and Omega(2380) resonances, which have not been
determined experimentally yet.Comment: New appendix and references adde
On photohadronic processes in astrophysical environments
We discuss the first applications of our newly developed Monte Carlo event
generator SOPHIA to multiparticle photoproduction of relativistic protons with
thermal and power law radiation fields. The measured total cross section is
reproduced in terms of excitation and decay of baryon resonances, direct pion
production, diffractive scattering, and non-diffractive multiparticle
production. Non--diffractive multiparticle production is described using a
string fragmentation model. We demonstrate that the widely used
`--approximation' for the photoproduction cross section is reasonable
only for a restricted set of astrophysical applications. The relevance of this
result for cosmic ray propagation through the microwave background and hadronic
models of active galactic nuclei and gamma-ray bursts is briefly discussed.Comment: 9 pages including 4 embedded figures, submitted to PAS
Silicon Photo-Multiplier radiation hardness tests with a beam controlled neutron source
We report radiation hardness tests performed at the Frascati Neutron
Generator on silicon Photo-Multipliers, semiconductor photon detectors built
from a square matrix of avalanche photo-diodes on a silicon substrate. Several
samples from different manufacturers have been irradiated integrating up to
7x10^10 1-MeV-equivalent neutrons per cm^2. Detector performances have been
recorded during the neutron irradiation and a gradual deterioration of their
properties was found to happen already after an integrated fluence of the order
of 10^8 1-MeV-equivalent neutrons per cm^2.Comment: 7 pages, 6 figures, Submitted to Nucl. Inst. Meth.
The Origanum Collection of Gaetano Savi (1769-1844) in the Herbarium Horti Pisani (PI)
The Origanum Collection of Gaetano Savi (1769-1844) in the Herbarium Horti Pisani (PI). The collection of exsiccata of the genus Origanum, studied by Gaetano Savi and kept in the Herbarium Horti Pisani (PI) represents a valuable document of Savi's work as a taxonomist. It provides useful news for researchers of systematics and history of botany. The collection is enriched by some original drawings and includes the lectotypes of Origanum confertum Savi and O. fortuitum Savi
Dispersion analysis of the nucleon form factors including meson continua
Dispersion relations provide a powerful tool to analyse the electromagnetic
form factors of the nucleon for all momentum transfers. Constraints from
meson-nucleon scattering data, unitarity, and perturbative QCD can be included
in a straightforward way. In particular, we include the 2pi, rho-pi, and KKbar
continua as independent input in our analysis and provide an error band for our
results. Moreover, we discuss two different methods to include the asymptotic
constraints from perturbative QCD. We simultaneously analyze the world data for
all four form factors in both the space-like and time-like regions and
generally find good agreement with the data. We also extract the nucleon radii
and the omega-NN coupling constants. For the radii, we generally find good
agreement with other determinations with the exception of the electric charge
radius of the proton which comes out smaller. The omega-NN vector coupling
constant is determined relatively well by the fits, but for the tensor coupling
constant even the sign can not be determined.Comment: 24 pages, 9 figure
Antiproton-nucleus electromagnetic annihilation as a way to access the proton timelike form factors
Contrary to the reaction pbar + p --> e+ e- with a high momentum incident
antiproton on a free target proton at rest, in which the invariant mass M of
the (e+ e-) pair is necessarily much larger than the (pbar p) mass, in the
reaction pbar + d --> n e+ e- the value of M can take values near or below the
(pbar p) mass. In the antiproton-deuteron electromagnetic annihilation, this
allows to access the proton electromagnetic form factors in the time-like
region of q^2 near the (pbar p) threshold. We estimate the cross section
dsigma(pbar +d --> e+ e- n)/dM for an antiproton beam momentum of 1.5 GeV/c. We
find that near the (pbar p) threshold this cross section is about 1 pb/MeV. The
case of heavy nuclei target is also discussed. Elements of experimental
feasibility are presented for the process pbar + d --> n e+ e- in the context
of the Panda project.Comment: 14 pages, 11 figures. submitted to EPJ
Lectotypification of two Origanum names (Lamiaceae) described by Gaetano Savi
Origanum confertum Savi and O. fortuitum Savi are typified on herbarium specimens kept in PI. These taxa are identifiable, as heterotypic synonyms, with O. majorana L. and O. xpaniculatum W.D.J.Koch, respectively
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