571 research outputs found
Evidence for an Excited Hyperon State in pp -> p K^+ Y^{0*}
Indications for the production of a neutral excited hyperon in the reaction
pp -> p K^+ Y^{0*} are observed in an experiment performed with the ANKE
spectrometer at COSY-J\"ulich at a beam momentum of 3.65 GeV/c. Two final
states were investigated simultaneously, viz. Y^{0*} -> pi^+X^- and pi^-X^+,
and consistent results were obtained in spite of the quite different
experimental conditions. The parameters of the hyperon state are M(Y^{0*})=
(1480 +/- 15) MeV/c^2 and Gamma(Y^{0*})= (60 +/- 15) MeV/c^2. The production
cross section is of the order of few hundred nanobarns. Since the isospin of
the Y^{0*} has not been determined here, it could either be an observation of
the Sigma(1480), a one-star resonance of the PDG tables, or alternatively a
Lambda hyperon. Relativistic quark models for the baryon spectrum do not
predict any excited hyperon in this mass range and so the Y^{0*} may be of
exotic nature.Comment: 4 pages, 3 figures, accepted for publication in Phys.Rev.Let
Lineshape of the Lambda(1405) Hyperon Measured Through its Sigma0 pion0 Decay
The pp -> p K+ Y0 reaction has been studied for hyperon masses m(Y0)<1540
MeV/c2 at COSY-Juelich by using a 3.65 GeV/c circulating proton beam incident
on an internal hydrogen target. Final states comprising two protons, one
positively charged kaon and one negatively charged pion have been identified
with the ANKE spectrometer. Such configurations are sensitive to the production
of the ground state Lambda and Sigma0 hyperons as well as the Sigma0(1385) and
Lambda(1405) resonances. Applying invariant- and missing-mass techniques, the
two overlapping excited states could be well separated, though with limited
statistics. The shape and position of the Lambda(1405) distribution,
reconstructed cleanly in the Sigma0 pion0 channel, are similar to those found
from other decay modes and there is no obvious mass shift. This finding
constitutes a challenging test for models that predict Lambda(1405) to be a
two-state resonance.Comment: 10 pages, 4 figures, accepted for publication in Phys. Lett.
New Way to Produce Dense Double-Antikaonic Dibaryon System, \bar{K}\bar{K} NN, through Lambda(1405)-Doorway Sticking in p+p Collisions
A recent successful observation of a dense and deeply bound \bar{K} nuclear
system, K^-pp, in the p + p \rightarrow K^+ + K^-pp reaction in a DISTO
experiment indicates that the double-\bar{K} dibaryon, K^-K^-pp, which was
predicted to be a dense nuclear system, can also be formed in p+p collisions.
We find theoretically that the K^- -K^- repulsion plays no significant role in
reducing the density and binding energy of K^-K^-pp and that, when two
\Lambda(1405) resonances are produced simultaneously in a short-range p+p
collision, they act as doorways to copious formation of K^-K^-pp, if and only
if K^-K^-pp is a dense object, as predicted.Comment: 8 pages, 9 figures, Accepted Apr. 19, 201
Single-pole nature of Lambda (1405) and structure of K-pp
We have studied the structure of K- pp by solving this system in a
variational treatment, starting from ansatz that Lambda(1405) is a K-p
quasi-bound state, Lambda* with mass 1405 MeV/c2. The structure of K-pp reveals
a molecular feature, namely, the K- in an "atomic center", Lambda*, plays a key
role in producing strong covalent bonding with the other proton. Deeply bound
Kbar nuclear systems are formed by this "super-strong" nuclear force due to
migrating real bosons, Kbar, a la Heitler-London-Heisenberg, which
overcompensates the stiff nuclear incompressibility. Theoretical background of
the Lambda (1405) ansatz is discussed in connection with the double-pole
picture of Lambda (1405) based on chiral SU(3) dynamics. Detailed analysis
reveals single-pole nature of the observable Lambda (1405). There are two kinds
of Sigma pi invariant masses experimentally observable, the usual T22 invariant
mass and the conversion T21 invariant mass. It is of vital importance to
determine whether the Lambda* mass is 1405 MeV or 1420 MeV. The T21 invariant
mass from K- absorption at rest in deuteron can provide decisive information
about this Lambda* mass problem.Comment: 8 pages, 4 figures, Proc. Hyp-
Antikaon interactions with nucleons and nuclei
This report summarizes our understanding of antikaon-nucleon interactions and
reviews the present theoretical situation in the quest for quasibound
antikaon-nuclear systems.Comment: 8 pages, 5 figures, Proceedings Int. Conf. on Hypernuclear and
Strange Particle Physics, Sept. 14-18, 2009, Tokai, Japa
Aspects of Strangeness -1 Meson-Baryon Scattering
We consider meson-baryon interactions in S-wave with strangeness -1. This is
a sector populated by plenty of resonances interacting in several two-body
coupled channels. We consider a large set of experimental data, where the
recent experiments are remarkably accurate. This requires a sound theoretical
description to account for all the data and we employ Unitary Chiral
Perturbation Theory up to and including O(p^2). The spectroscopy of our
solutions is studied within this approach, discussing the rise from the pole
content of two \Lambda(1405) resonances and of the \Lambda(1670),
\Lambda(1800), \Sigma(1480), \Sigma(1620) and \Sigma(1750). We finally argue
about our preferred fit.Comment: 6 figures, 3 figures, talk given in the IVth International Conference
on Quarks and Nuclear Physics (QNP06), Madrid June 5th and 10th, 2006. One
reference is update
New insights into antikaon-nucleon scattering and the structure of the Lambda(1405)
We perform a combined analysis of antikaon-nucleon scattering cross sections
and the recent SIDDHARTA kaonic hydrogen data in the framework of a
coupled-channel Bethe-Salpeter approach at next-to-leading order in the chiral
expansion of the effective potential. We find a precise description of the
antikaon-proton scattering amplitudes and are able to extract accurate values
of the scattering lengths, a0=-1.81^+0.30_-0.28 + i 0.92^+0.29_-0.23 fm,
a1=+0.48^+0.12_-0.11 + i 0.87^+0.26_-0.20 fm. We also discuss the two-pole
structure of the Lambda(1405).Comment: 7 pages, 4 figure
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