1,198 research outputs found
Nucleons, Nuclear Matter and Quark Matter: A unified NJL approach
We use an effective quark model to describe both hadronic matter and
deconfined quark matter. By calculating the equations of state and the
corresponding neutron star properties, we show that the internal properties of
the nucleon have important implications for the properties of these systems.Comment: 17 pages, 5 figures, Section 5 extended, references adde
The polarized EMC effect
Conference details: Quark confinement and the hadron spectrum VII : 7th Conference on Quark Confinement and the Hadron Spectrum, QCHS7, Ponta Delgada, Açores, Portugal, 2-7 September 2006 / José Emílio F. T. Ribeiro (ed.): pp. 248-250We calculate both the spin independent and spin dependent nuclear structure functions in an effective quark theory. The nucleon is described as a composite quark-diquark state, and the nucleus is treated in the mean field approximation. We predict a sizable polarized EMC effect, which could be confirmed in future experiments.W. Bentz, I. C. Cloet, and A. W. Thoma
Spin-dependent structure functions in nuclear matter and the polarized EMC effect
An excellent description of both spin-independent and spin-dependent quark
distributions and structure functions has been obtained with a modified
Nambu-Jona-Lasinio model, which is free of unphysical thresholds for nucleon
decay into quarks - hence incorporating an important aspect of confinement. We
utilize this model to investigate nuclear medium modifications to structure
functions and find that we are readily able to reproduce both nuclear matter
saturation and the experimental F^A_2N / F_2N ratio, that is, the EMC effect.
Applying this framework to determine g^A_1p, we find that the ratio g^A_1p /
g_1p differs significantly from 1, with the quenching caused by the nuclear
medium being about twice that of the spin-independent case. This represents an
exciting result, which if confirmed experimentally, will reveal much about the
quark structure of nuclear matter.Comment: 4 pages, 4 figure
Parity-violating DIS and the flavour dependence of the EMC effect
Isospin-dependent nuclear forces play a fundamental role in nuclear
structure. In relativistic models of nuclear structure constructed at the quark
level these isovector nuclear forces affect the u and d quarks differently,
leading to non-trivial flavour dependent modifications of the nuclear parton
distributions. We explore the effect of isospin dependent forces for
parity-violating deep inelastic scattering on nuclear targets and demonstrate
that the cross-sections for nuclei with N /= Z are sensitive to the flavour
dependence of the EMC effect. Indeed, for nuclei like lead and gold we find
that these flavour dependent effects are large.Comment: 4 pages, 2 figure
Role of diquark correlations and the pion cloud in nucleon elastic form factors
Electromagnetic form factors of the nucleon in the space-like region are
investigated within the framework of a covariant and confining
Nambu-Jona-Lasinio model. The bound state amplitude of the nucleon is obtained
as the solution of a relativistic Faddeev equation, where diquark correlations
appear naturally as a consequence of the strong coupling in the colour
channel. Pion degrees of freedom are included as a perturbation
to the "quark-core" contribution obtained using the Poincar\'e covariant
Faddeev amplitude. While no model parameters are fit to form factor data,
excellent agreement is obtained with the empirical nucleon form factors
(including the magnetic moments and radii) where pion loop corrections play a
critical role for GeV. Using charge symmetry, the nucleon
form factors can be expressed as proton quark sector form factors. The latter
are studied in detail, leading, for example, to the conclusion that the
-quark sector of the Dirac form factor is much softer than the -quark
sector, a consequence of the dominance of scalar diquark correlations in the
proton wave function. On the other hand, for the proton quark sector Pauli form
factors we find that the effect of the pion cloud and axialvector diquark
correlations overcomes the effect of scalar diquark dominance, leading to a
larger -quark anomalous magnetic moment and a form factor in the -quark
sector that is slightly softer than in the -quark sector.Comment: 30 pages and 40 figure
Monte-Carlo Approach to Calculating the Fragmentation Functions in NJL-Jet Model
Recent studies of the fragmentation functions using the Nambu--Jona-Lasinio
(NJL) - Jet model have been successful in describing the quark fragmentation
functions to pions and kaons. The NJL-Jet model employs the integral equation
approach to solve for the fragmentation functions in quark-cascade description
of the hadron emission process, where one assumes that the initial quark has
infinite momentum and emits an infinite number of hadrons. Here we introduce a
Monte Carlo (MC) simulation method to solve for the fragmentation functions,,
that allows us to relax the above mentioned approximations. We demonstrate that
the results of MC simulations closely reproduce the solutions of the integral
equations in the limit where a large number of hadrons are emitted in the quark
cascade. The MC approach provides a strong foundation for the further
development of the NJL-Jet model that might include many more hadronic emission
channels with decays of the possible produced resonances, as well as inclusion
of the transverse momentum dependence (TMD), all of which are of considerable
importance to the experimental studies of the transverse structure of hadrons.Comment: 5 pages, 3 figures, Proceedings of "TROPICAL QCD II Workshop
Collins Fragmentation Function within NJL-jet Model
The NJL-jet model is extended to accommodate hadronization of a transversely
polarized quark in order to explore the Collins effect within a multihadron
emission framework. This is accomplished by calculating the polarized quark
spin flip probabilities after a pseudoscalar hadron emission and the elementary
Collins functions. The model is used to calculate the number densities of the
hadrons produced in the polarized quark's decay chain. The full Collins
fragmentation function is extracted from the sine modulation of the polarized
number densities with respect to the polar angle between the initial quark's
spin and hadron's transverse momentum. Two cases are studied here. First, a toy
model for elementary Collins function is used to study the features of the
transversely polarized quark-jet model. Second, a full model calculation of
transverse momentum dependent pion and kaon Collins functions is presented. The
remarkable feature of our model is that the 1/2 moments of the favored Collins
fragmentation functions are positive and peak at large values of z but decrease
and oscillate at small values of z. The 1/2 moments of the unfavored Collins
functions have comparable magnitude and opposite sign to the favored functions,
vanish at large z and peak at small values of z. This feature is observed for
both the toy and full models and can be attributed to the quark-jet picture of
hadronization. Moreover, the transverse momentum dependencies of the model
Collins functions differ significantly from the Gaussian form widely used in
the empirical parametrizations. Finally, a naive interpretation of the
Schafer-Teryaev sum rule is proven not to hold in our model, where the
transverse momentum conservation is explicitly enforced. This is attributed to
the sizable average transverse momentum of the remnant quark that needs to be
accounted for to satisfy the transverse momentum sum-rule.Comment: 15 pages, 22 figures. v2 - minor changes/additions to conform to the
journal published versio
Analytic approach to nuclear rotational states: The role of spin - A minimal model -
We use a simple field theory model to investigate the role of the nucleon
spin for the magnetic sum rules associated with the low-lying collective
scissors mode in deformed nuclei. Various constraints from rotational symmetry
are elucidated and discussed. We put special emphasis on the coupling of the
spin part of the M1 operator to the low lying collective modes, and investigate
how this coupling changes the sum rules.Comment: 15 pages, 4 figure
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