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M. Nielsen

Publications and source records attributed to M. Nielsen.

122 records · Page 7Linked to original sources
arXiv1999

$g_{NKΛ}$ and $g_{NKΣ}$ from QCD Sum Rules in the $γ_5 σ_{μν}$ structure

The NKY coupling constant for $Y = Λ$ and $Σ$ is evaluated in a QCD sum rule calculation. We study the Borel sum rule for the three point function of one pseudoscalar one nucleon and one hyperon current up to order six in the operator product expansion. The Borel transform is performed with respect to the nucleon and hyperon momenta, which are taken to be equal. We discuss and extend the result of a previous analysis in the $\rlap{/}{q}iγ_5$ structure and compare it with the result obtained with the use of the $γ_5 σ_{μν}$ structure. We find that the effect of pole-continuum transitions is very important in the $\rlap{/}{q}iγ_5$ structure and that it changes completely results obtained in previous analysis.

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arXiv1998

QCD Sum Rules Calculation of Heavy $Λ$ Semileptonic Decay

We set up sum rules for heavy lambda decays in a full QCD calculation which in the heavy quark mass limit incorporates the symmetries of heavy quark effective theory. For the semileptonic $\La_c$ decay we obtain a reasonable agreement with experiment. For the $\La_b$ semileptonic decay we find at the zero recoil point a violation of the heavy quark symmetry of about 20%.

hep-ph↗
arXiv1998

Alternative Linear Chiral Models for Nuclear Matter

The equation of state of a family of alternative linear chiral models in the mean field approximation is discussed. We investigate the analogy between some of these models with current models in the literature, and we show that it is possible to reproduce very well the saturation properties of nuclear matter.

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arXiv1998

Quark-meson coupling model with constituent quarks: Exchange and pionic effects

The binding energy of nuclear matter including exchange and pionic effects is calculated in a quark-meson coupling model with massive constituent quarks. As in the case with elementary nucleons in QHD, exchange effects are repulsive. However, the coupling of the mesons directly to the quarks in the nucleons introduces a new effect on the exchange energies that provides an extra repulsive contribution to the binding energy. Pionic effects are not small. Implications of such effects on observables are discussed.

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arXiv1998

$g_{NDΛ_{c}}$ from QCD Sum Rules

The $NDΛ_c$ coupling constant is evaluated in a full QCD sum rule calculation. We study the Borel sum rule for the three point function of one pseudoscalar one nucleon and one $Λ_c$ current up to order seven in the operator product expansion. The Borel transform is performed with respect to the nucleon and $Λ_c$ momenta, which are taken to be equal, whereas the momentum $q^2$ of the pseudoscalar vertex is taken to be zero. This coupling constant is relevant in the meson cloud description of the nucleon which has been recently used to explain exotic events observed by the H1 and ZEUS Collaborations at HERA.

hep-ph↗
arXiv1998

K^* Mesons and Nucleon Strangeness

We study contributions to the nucleon strange quark vector current form factors from intermediate states containing K^* mesons. We show how these contributions may be comparable in magnitude to those made by K mesons, using methods complementary to those employed in quark model studies. We also analyze the degree of theoretical uncertainty associated with K^* contributions.

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arXiv1997

Optimized delta expansion for relativistic nuclear models

The optimized $δ$-expansion is a nonperturbative approach for field theoretic models which combines the techniques of perturbation theory and the variational principle. This technique is discussed in the $λϕ^4$ model and then implemented in the Walecka model for the equation of state of nuclear matter. The results obtained with the $δ$ expansion are compared with those obtained with the traditional mean field, relativistic Hartree and Hartree-Fock approximations.

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arXiv1996

Virtual Meson Cloud of the Nucleon and Intrinsic Strangeness and Charm

We have applied the Meson Cloud Model (MCM) to calculate the charm and strange antiquark distribution in the nucleon. The resulting distribution, in the case of charm, is very similar to the intrinsic charm momentum distribution in the nucleon. This seems to corroborate the hypothesis that the intrinsic charm is in the cloud and, at the same time, explains why other calculations with the MCM involving strange quark distributions fail in reproducing the low x region data. From the intrinsic strange distribution in the nucleon we have extracted the strangeness radius of the nucleon, which is in agreement with other meson cloud calculations.

hep-ph↗
arXiv1996

A Relativistic Thomas-Fermi Description of Collective Modes in Droplets of Nuclear Matter

Isoscalar collective modes in a relativistic meson-nucleon system are investigated in the framework of the time-dependent Thomas-Fermi method. The energies of the collective modes are determined by solving consistently the dispersion relations and the boundary conditions. The energy weighted sum rule satisfied by the model allows the identification of the giant ressonances. The percentage of the energy weighted sum rule exhausted by the collective modes is in agreement with experimental data, but the energies come too high.

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arXiv1995

Can $Ω^- \to Ξ^- γ$ be sensitive to new physics beyond the Standard Model ?

We investigate the impact of new physics beyond the Standard Model to the $s \rightarrow d γ$ process, which is responsible for the short-distance contribution to the radiative decay $\Otogamma$. We study three representative extensions of the Standard Model, namely a one-family technicolor model, a two Higgs doublet model and a model containing scalar leptoquarks. When constraints arising from the observed $b \to s γ$ transition and the upper limit on $D^0 - \bar{D}^0$ mixing are taken into account, we find no significant contributions of new physics to the $s \to d γ$ process.

hep-ph↗
arXiv1995

Studying medium effects with the optimized $\bdelta$ expansion

The possibility of using the optimized $δ$ expansion for studying medium effects on hadronic properties in quark or nuclear matter is investigated. The $δ$ expansion is employed to study density effects with two commonly used models in hadron and nuclear physics, the Nambu--Jona-Lasinio model for the dynamical chiral symmetry breaking and the Walecka model for the equation of state of nuclear matter. The results obtained with the $δ$ expansion are compared to those obtained with the traditional Hartree-Fock approximation. Perspectives for using the $δ$ expansion in other field theoretic models in hadron and nuclear physics are discussed.

hep-ph↗
arXiv1994

Calculation of $ $ from QCD sum rule and the neutron-proton mass difference

The proton matrix element of the isovector-scalar density, $ /2M_p$, is calculated by evaluating the nucleon current correlation function in an external isovector-scalar field using the QCD sum-rule method. In addition to the usual chiral and gluon condensates of the QCD vacuum, the response of the chiral condensates to the external isovector field enters the calculation. The latter is determined by two independent methods. One relates it to the difference between the up and down quark chiral condensates and the other uses the chiral perturbation theory. To first order in the quark mass difference $δm=m_d-m_u$, the non-electromagnetic part of the neutron-proton mass difference is given by the product $δm /2M_p$; the resulting value is in reasonable agreement with the experimental value.

hep-ph↗
arXiv1993

Ghost Poles in the Nucleon Propagator: Vertex Corrections and Form Factors

Vertex corrections are taken into account in the Schwinger-Dyson equation for the nucleon propagator in a relativistic field theory of fermions and mesons. The usual Hartree-Fock approximation for the nucleon propagator is known to produce the appearance of complex (ghost) poles which violate basic theorems of quantum field theory. In a theory with vector mesons there are vertex corrections that produce a strongly damped vertex function in the ultraviolet. One set of such corrections is known as the Sudakov form factor in quantum electrodynamics. When the Sudakov form factor generated by massive neutral vector mesons is included in the Hartree-Fock approximation to the Schwinger-Dyson equation for the nucleon propagator, the ghost poles disappear and consistency with basic requirements of quantum field theory is recovered.

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