405 research outputs found

### Highly excited negative parity baryons in the $1/N_c$ expansion

The masses of experimentally known highly excited baryons of negative parity
supposed to belong to the $[{\bf 70},\ell^-]$ multiplets ($\ell$ = 1,2,3) of
the $N = 3$ band are calculated in the $1/N_c$ expansion method to order
$1/N_c$ by using a procedure which allows to considerably reduce the number of
linearly independent operators entering the mass formula. The numerical fits to
present data show that the coefficients encoding the QCD dynamics have large,
comparable values, for the flavor and spin operators. It implies that these
operators contribute dominantly to the flavor-spin SU(6) symmetry breaking,
like for the $[{\bf 70},1^-]$ multiplet of the $N = 1$ band.Comment: 15 pages, accepted for publication in Phys.Rev.

### An explanation of the $\Delta_{5/2^{-}}(1930)$ as a $\rho\Delta$ bound state

We use the $\rho\Delta$ interaction in the hidden gauge formalism to
dynamically generate $N^{\ast}$ and $\Delta^{\ast}$ resonances. We show,
through a comparison of the results from this analysis and from a quark model
study with data, that the $\Delta_{5/2^{-}}(1930),$ $\Delta_{3/2^{-}}(1940)$
and $\Delta_{1/2^{-}}(1900)$ resonances can be assigned to $\rho\Delta$ bound
states. More precisely the $\Delta_{5/2^{-}}(1930)$ can be interpreted as a
$\rho\Delta$ bound state whereas the $\Delta_{3/2^{-}}(1940)$ and
$\Delta_{1/2^{-}}(1900)$ may contain an important $\rho\Delta$ component. This
interpretation allows for a solution of a long-standing puzzle concerning the
description of these resonances in constituent quark models. In addition we
also obtain degenerate $J^{P}=1/2^{-},3/2^{-},5/2^{-}$ $N^{*}$ states but their
assignment to experimental resonances is more uncertain.Comment: 19 pags, 8 fig

### Sum rules for charmed baryon masses

The measured masses of the three charge states of the charmed $\Sigma_c$
baryon are found to be in disagreement with a sum rule based on the quark
model, but relying on no detailed assumptions about the form of the
interaction. This poses a significant problem for the charmed baryon sector of
the quark model. Other relations among charmed baryon masses are also
discussed.Comment: 5 pages, latex, no 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

### The role of the $N^*(2080)$ resonance in the $\vec{\gamma} p \to K^+ \Lambda(1520)$ reaction

We investigate the $\Lambda(1520)$ photo-production in the $\vec{\gamma} p
\to K^+ \Lambda(1520)$ reaction within the effective Lagrangian method near
threshold. In addition to the "background" contributions from the contact,
$t-$channel $K$ exchange, and $s-$channel nucleon pole terms, which were
already considered in previous works, the contribution from the nucleon
resonance $N^*(2080)$ (spin-parity $J^P = 3/2^-$) is also considered. We show
that the inclusion of the nucleon resonance $N^*(2080)$ leads to a fairly good
description of the new LEPS differential cross section data, and that these
measurements can be used to determine some of the properties of this latter
resonance. However, serious discrepancies appear when the predictions of the
model are compared to the photon-beam asymmetry also measured by the LEPS
Collaboration.Comment: 9 pages,6 figures, 1 tabl

### On the massless contributions to the vacuum polarization of heavy quarks

Recently Groote and Pivovarov have given notice of a possible fault in the
use of sum rules involving two-point correlation functions to extract
information on heavy quark parameters, due to the presence of massless
contributions that invalidate the construction of moments of the spectral
densities. Here we show how to circumvent this problem through a new definition
of the moments, providing an infrared safe and consistent procedure.Comment: 1+9 pages, 3 figures. Discussion on QCD sum rules applications added.
Conclusions unchanged. Version to be published in Journal of Physics

### Extended Partial-Wave Analysis of piN Scattering Data

We present results from a comprehensive partial-wave analysis of pi+-p
elastic scattering and charge-exchange data, covering the region from threshold
to 2.6 GeV in the lab pion kinetic energy, employing a coupled-channel
formalism to simultaneously fit pi-p-->eta n data to 0.8 GeV. Our main result,
solution SP06, utilizes a complete set of forward and fixed-t dispersion
relation constraints applied to the piN elastic amplitude. The results of these
analyses are compared with previous solutions in terms of their resonance
spectra and preferred values for couplings and low-energy parameters.Comment: 21 pages, 11 figure

### Multichannel parametrization of \pi N scattering amplitudes and extraction of resonance parameters

We present results of a new multichannel partial-wave analysis for \pi N
scattering in the c.m. energy range 1080 to 2100 MeV. This work explicitly
includes \eta N and K \Lambda channels and the single pion photoproduction
channel. Resonance parameters were extracted by fitting partial-wave amplitudes
from all considered channels using a multichannel parametrization that is
consistent with S-matrix unitarity. The resonance parameters so obtained are
compared to predictions of quark models

### Parameterization dependence of T matrix poles and eigenphases from a fit to piN elastic scattering data

We compare fits to piN elastic scattering data, based on a Chew-Mandelstam
K-matrix formalism. Resonances, characterized by T-matrix poles, are compared
in fits generated with and without explicit Chew-Mandelstam K-matrix poles.
Diagonalization of the S matrix yields the eigenphase representation. While the
eigenphases can vary significantly for the different parameterizations, the
locations of most T-matrix poles are relatively stable.Comment: 6 pages, 3 figures, 1 tabl

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