443 research outputs found
f [N pi N]: from quarks to the pion derivative coupling
We study the N pi N coupling, in the framework of a QCD-inspired confining
Nambu-Jona-Lasinio model. A simple relativistic confining and instantaneous
quark model is reviewed. The Salpeter equation for the nucleon and the boosted
pion is solved. The f [n pi n] and f[n pi Delta] couplings are calculated and
they turn out to be reasonably good. The sensibility of f[n pi n] and f[n pi
Delta] to confinement, chiral symmetry breaking and Lorentz invariance is
briefly discussed.Comment: 30 pages in LaTex RevTex, 6 postscript figure
A new Perspective on the Scalar meson Puzzle, from Spontaneous Chiral Symmetry Breaking Beyond BCS
We introduce coupled channels of Bethe-Salpeter mesons both in the boundstate
equation for mesons and in the mass gap equation for chiral symmetry.
Consistency is insured by the Ward Identities for axial currents, which
preserve the Goldstone boson nature of the pion and prevents a systematic shift
of the hadron spectrum. We study the decay of a scalar meson coupled to a pair
of pseudoscalars. We also show that coupled channels reduce the breaking of
chiral symmetry, with the same Feynman diagrams that appear in the coupling of
a scalar meson to a pair of pseudoscalar mesons. Exact calculations are
performed in a particular confining quark model, where we find that the
groundstate meson is the f_0(980) with a partial decay
width of 40MeV. We also find a 30% reduction of the chiral condensate due to
coupled channels.Comment: 17 pages, Revtex, 8 eps figures, and several eps diagrams in
equation
Vacuum replicas in QCD
The properties of the vacuum are addressed in the two- and four-dimensional
quark models for QCD. It is demonstrated that the two-dimensional QCD ('t Hooft
model) possesses only one possible vacuum state - the solution to the mass-gap
equation, which provides spontaneous breaking of the chiral symmetry (SBCS). On
the contrary, the four-dimensional theory with confinement modeled by the
linear potential supplied by the Coulomb OGE interaction, not only has the
chirally-noninvariant ground vacuum state, but it possesses an excited vacuum
replica, which also exhibits SBCS and can realize as a metastable intermediate
state of hadronic systems. We discuss the influence of the latter on physical
observables as well as on the possibility to probe the vacuum background fields
in QCD.Comment: RevTeX4, 26 pages, 8 EPS figures, extended references, corrected some
typos, to appear in Phys.Rev.
Mesons and tachyons with confinement and chiral restoration, and NA60
In this paper the spectrum of quark-antiquark systems, including light mesons
and tachyons, is studied in the true vacuum and in the chiral invariant vacuum.
The mass gap equation for the vacua and the Salpeter-RPA equation for the
mesons are solved for a simple chiral invariant and confining quark model. At
T=0 and in the true vacuum, the scalar and pseudoscalar, or the vector and
axial vector are not degenerate, and in the chiral limit, the pseudoscalar
groundstates are Goldstone bosons. At T=0 the chiral invariant vacuum is an
unstable vacuum, decaying through an infinite number of scalar and pseudoscalar
tachyons. Nevertheless the axialvector and vector remain mesons, with real
masses. To illustrate the chiral restoration, an arbitrary path between the two
vacua is also studied. Different families of light-light and heavy-light
mesons, sensitive to chiral restoration, are also studied. At higher
temperatures the potential must be suppressed, and the chiral symmetry can be
restored without tachyons, but then all mesons have small real masses.
Implications for heavy-ion collisions, in particular for the recent vector
meson spectra measured by the NA60 collaboration, are discussed.Comment: 9 pages, 5 figures, 3 table
Repulsive Core of NN S-Wave Scattering in a Quark Model with a Condensed Vacuum
We work in a chiral invariant quark model, with a condensed vacuum,
characterized by only one parameter. Bound state equations for the nucleon and
Delta are solved in order to obtain an updated value of their radii and masses.
Nucleon-nucleon S-Wave scattering is studied in the RGM framework both for
isospin T=1 and T=0. The phase shifts are calculated and an equivalent local
potential, which is consistent with K-N scattering, is derived. The result is a
reasonable microscopic short range repulsion in the nucleon-nucleon
interaction.Comment: 23 pages in latex revtex, 4 Postscript figure
Are the anti-charmed and bottomed pentaquarks molecular heptaquarks?
I study the charmed resonance D*p (3100) very recently
discovered by the H1 collaboration at Hera. An anticharmed resonance was
already predicted, in a recent publication mostly dedicated to the S=1
resonance Theta+(1540). To confirm these recent predictions, I apply the same
standard quark model with a quark-antiquark annihilation constrained by chiral
symmetry. I find that repulsion excludes the D*p (3100) as a
s-wave pentaquark. I explore the D*p (3100) as a heptaquark, equivalent to a
N-pi-D* linear molecule, with positive parity and total isospin I=0. I find
that the N-D repulsion is cancelled by the attraction existing in the N-pi and
pi-D channels. In our framework this state is harder to bind than the Theta+
described by a k-pi-N borromean bound-state, a lower binding energy is expected
in agreement with the H1 observation. Multiquark molecules N-pi-D, N-pi-B* and
N-pi-B are also predicted.Comment: 5 pages, 2 figures, RevTe
The Theta+ (1540) as a heptaquark with the overlap of a pion, a kaon and a nucleon
We study the very recently discovered Theta+ (1540) at SPring-8, at ITEP and
at CLAS-Thomas Jefferson Lab. We apply the same RGM techniques that already
explained with success the repulsive hard core of nucleon-nucleon, kaon-nucleon
exotic scattering, and the attractive hard core present in pion-nucleon and
pion-pion non-exotic scattering. We find that the K-N repulsion excludes the
Theta+ as a K-N s-wave pentaquark. We explore the Theta+ as a heptaquark,
equivalent to a N+pi+K borromean bound-state, with positive parity and total
isospin I=0. We find that the kaon-nucleon repulsion is cancelled by the
attraction existing both in the pion-nucleon and pion-kaon channels. Although
we are not yet able to bind the total three body system, we find that the
Theta^+ may still be a heptaquark state. We conclude with predictions that can
be tested experimentally.Comment: 5 pages, 5 figures, 2 tables, submitted to Phys. Rev. D, rapid
communicatio
NJL with eight quark interactions: Chiral phases at finite T
The thermodynamic potential and thermal dependence of low lying mass spectra
of scalars and pseudoscalars are evaluated in a generalized Nambu --
Jona-Lasinio model, which incorporates eight-quark interactions. These are
necessary to stabilize the scalar effective potential for the light and strange
quark flavors, which would be otherwise unbounded from below. In addition it
turns out that they are also crucial to i) lower the temperature of the chiral
transition, in conformity with lattice calculations, ii) sharpen the
temperature interval in which the crossover occurs, iii) or even allow for
first order transitions to occur with realistic quark mass values, from certain
critical values of the parameters. These are unprecedented results which cannot
be obtained within the NJL approaches restricted to quartic and six-quark
interactions.Comment: 6 pages, 3 figures, Talk presented at SCADRON 70 Workshop on Scalar
Mesons and Related Topics, Lisbon, 11-16 February 200
Assessing the degree of ecological change and baselines for reservoirs: challenges and implications for management
Tropical reservoirs are sensitive to eutrophication but long-term impacts of impoundment on their productivity and biota are poorly understood. Here, we employ a palaeolimnological approach to assess whether ecological baselines can be defined for reservoirs, and examine the challenges and management implications. We studied the environmental history of five reservoirs in Brazil with different productivities, using sediment records covering the period since reservoir construction (~ 50–90 years). Our main goals, based on the analysis of organic geochemistry (TOC, TN, TP, C:N), stable isotopes (δ13C, δ15N), and diatoms, were to reconstruct and compare the magnitude of environmental changes, to determine the conditions prior to any eutrophication and discuss the implications for reservoir management. We inferred that Ribeirão do Campo reservoir has remained oligotrophic since its construction, Itupararanga and Paineiras have both remained mesotrophic with an improvement in water quality around 1970, and Salto Grande has been eutrophic since its construction in 1949. In Rio Grande reservoir, which was originally oligotrophic, eutrophication began in the 1950s, with a slight improvement in water quality after its separation from Billings Reservoir, followed by a subsequent decline in quality since ~ 2001. We found that the studied reservoirs have unique environmental histories and there are clearly challenges associated with defining ecological baselines for reservoirs against which the extent of degradation can be assessed. Nonetheless, when the data from all reservoirs were compared, a coherent pattern in the diatom assemblages emerged, reflecting the trophic gradient. The diatom assemblages prior to enrichment were composed of two groups. The oligotrophic baseline was characterized by several benthic species with low abundances, mainly Eunotia and Brachysira, while in three reservoirs the early assemblages were characterized by planktonic taxa associated with mesotrophic conditions, namely planktonic species Aulacoseira ambigua, Aulacoseira tenella, Discostella stelligera, and Spicaticriba rudis. This work provides information on the baseline conditions, the natural variability of non-degraded reservoirs, and the degree of ecological change in degraded ecosystems. This study contributes to an improved understanding of the timing and extent of eutrophication in these systems and provides information to help better inform the management of tropical reservoirs vulnerable to anthropogenic pressures
Vorticity and magnetic shielding in a type-II superconductor
We study in detail, solving the Bogoliubov-de Gennes equations, the magnetic
field, supercurrent and order parameter profiles originated by a solenoid or
magnetic whisker inserted in a type-II superconductor. We consider solutions of
different vorticities, n, in the various cases. The results confirm the
connection between the vorticity, the internal currents and the boundstates in
a self-consistent way. The number of boundstates is given by the vorticity of
the phase of the gap function as in the case with no external solenoid. In the
limiting case of an infinitely thin solenoid, like a Dirac string, the solution
is qualitatively different. The quasiparticle spectrum and wave functions are a
function of n-n_ext, where n_ext is the vorticity of the solenoid. The flux is
in all cases determined by the vorticity of the gap function.Comment: revised version, 25 pages, LaTex, 10 figure
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