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A. Moalem

Publications and source records attributed to A. Moalem.

15 recordsLinked to original sources
arXiv2002

A Chiral Effective Field Theory and Radiative Decays of Mesons

An extended $U(3)_L\bigotimes U(3)_R$ chiral effective field theory which includes pseudoscalar and vector meson nonets as dynamic variables is presented. The theory combines a hidden symmetry approach with a general procedure of including the $η'$ meson into chiral theory, and accounts for $direct$ and $indirect$ symmetry breaking effects via a mechanism based on the quark mass matrix. The theory is applied to anomalous radiative decays using particle mixing schemes, corresponding to different symmetry breaking assumptions and uniquely determined by the lagarngian presumed. Radiative decays of light flavor mesons are best explained within the framework of a one mixing angle scheme and provide evidence for $SU(3)_F$ and nonet symmetry breaking.

hep-ph↗
arXiv2002

Effective Chiral Theory for Pseudoscalar and Vector Mesons

We consider the vector meson mixing scheme and mass splitting within the framework of an extended $U(3)_L\bigotimes U(3)_R$ chiral effective field theory based on the hidden local symmetry approach where, the pseudoscalar and vector meson nonets play the role of dynamic variables. Unlike other variants of this model, we show that the diagonalization of the vector meson mass matrix and the assumption that its eigenvalues are identical with the physical meson masses, determines the mixing scheme as well as the model free parameters. We show that VMD can be derived from our lagrangian and that for electromagnetic processes at low momenta the VMD model is a good first approximation. The model reproduces nicely the radii of the charged pions and kaons.

hep-ph↗
arXiv2001

Pseudoscalar Meson Mixing in Effective Field Theory

We show that for any effective field theory of colorless meson fields, the mixing schemes of particle states and decay constants are not only related but also determined exclusively by the kinetic and mass Lagrangian densities. In the general case, these are bilinear in terms of the intrinsic fields and involve non-diagonal kinetic and mass matrices. By applying three consecutive steps this Lagrangian can be reduced into the standard quadratic form in terms of the physical fields. These steps are : (i) the diagonalization of the kinetic matrix, (ii) rescaling of the fields, and (iii) the diagonalization of the mass matrix. In case, where the dimensions of the non-diagonal kinetic and mass sub-matrices are respectively, $k\times k$ and $n\times n$, this procedure leads to mixing schemes which involve $[k(k-1)/2] + [n(n-1)/2]$ angles and $k$ field rescaling parameters. This observation holds true irrespective with the type of particle interactions presumed. The commonly used mixing schemes, correspond to a proper choice of the kinetic and mass matrices, and are derived as special cases. In particular, $η$-$η'$ mixing, requires one angle, if and only if, the kinetic term with the intrinsic fields has a quadratic form.

hep-ph↗
arXiv2000

On convergence of the ChPT HFF expansion for one loop contribution to meson production in NN collisions

We consider the application of heavy fermion formalism based chiral perturbation theory to meson production in nucleon-nucleon collisions. It is shown that for one loop contributions the heavy fermion formalism expansion corrections for the nucleon propagator produce infinite series of correction terms which are of the same momentum power order. This destroys the one-to-one correspondence between the perturbative and small momentum expansion and thus negates the application of any finite order heavy fermion formalism chiral perturbation theory to the $NN\to NN π$ reactions.

hep-ph↗
arXiv2000

Effective Chiral Theory for Radiative Decays of Mesons

An extended $U(3)_L\bigotimes U(3)_R$ chiral theory which includes pseudoscalar and vector meson nonets as dynamic degrees of freedom is presented. We combine a hidden symmetry approach with a general procedure of including the $η'$ meson into chiral theory. The $U(3)_L\bigotimes U(3)_R$ and the SU(3) symmetries are broken by the mechanism based on quark mass matrix. Meson radiative decay widths are parameterized in terms of a single and real $η-η'$ mixing angle $θ_P$, a U(3)_V symmetry breaking scale parameter c_W, and the radiative decay constants $F_π, F_K, F_η, F_{η'}$, for the $π, η$, K, $η'$ mesons, respectively. Taking $F_π= 93 MeV$, a global fit to decay width data yields F_K/F_π= 1.16 \pm 0.11, F_η/F_π= 1.14\pm 0.04, F_{η'}/F_π= 1.09 \pm 0.04, c_W = -0.19 \pm0.03, θ_P =-(15.4 \pm 1.8)^o.

nucl-th↗
arXiv2000

Near-threshold eta production in the p d -> p d eta reaction

The total cross section of the p d -> p d eta reaction has been measured at two energies near threshold by detecting the final proton and deuteron in a magneti spectrometer. The values are somewhat larger than expected on the basis of two simple theoretical estimates.

nucl-ex↗
arXiv1999

On convergence of the HFF expoansion for meson production in NN collisions

We consider the application of heavy fermion formalism based ChPT to meson production in nucleon-nucleon collisions. It is shown that to each lower chiral order irreducible diagram there corresponds an infinite sequence of loop diagrams which are of the same momentum power order. This destroys the one-to-one correspondence between the loop and small momentum expansion and thus rules out the application of any finite order HFF ChPT yo the NN-> NNpion reactions.

nucl-th↗
arXiv1998

S-wave $π^0$ Production in pp Collision in a Covariant OBE Model

The total cross section for the pp -> pp pi^0 reaction at energies close to threshold is calculated using a covariant one-boson-exchange model, where a boson B created on one of the incoming protons is converted into a neutral pion on the second. The amplitudes for the conversion processes, B N -> N pi^0, are taken to be the sum of s, u and t-channel pole terms. The main contributions to the primary production amplitude is due to an effective isoscalar sigma meson pole in a t-channel, which is enhanced strongly due to offshellness. With this contribution included the model reproduces, both the scale and energy dependence of the cross section.

nucl-th↗
arXiv1998

Is The Heavy Fermion Formalism Applicable to Meson Production Process?

We consider tree diagram contributions to neutral pion production in pp ->pp pi0 using chiral perturbation theory in the relativistic and extremely non-relativistic limits. In marked difference with the results from heavy fermion formalism, the impulse and s-wave rescattering terms in the relativistic limit have equal signs and therefore add constructively, giving rise to a substantial contribution to the cross section. We argue that the power series expansion of the nucleon propagator is on the border of its convergence circle. Consequently, a finite order heavy fermion formalism does not predict nucleon pole terms correctly and therefore can not be applied to meson production.

nucl-th↗
arXiv1998

On The pp->ppη(eta') Reactions Near Threshold

The production rate for η' in pp ->pp η' at rest is calculated in a covariant one boson exchange model, previously applied to study pi^0 and ηproduction in NN collisions. The transition amplitudes for the elementary BN -> eta' N processes with B being the meson exchanged (B = pi, sigma, eta, rho, omega and delta) are taken to be the sum of s and u channels with a nucleon in the intermediate states, and a delta meson pole in a t-channel. The couplings of the eta' to hadrons are a factor 0.437 weaker than the respective eta-hadron couplings, as suggested by a quark model and a singlet-octetmixing angle theta = -23 degree. The model reproduces near threshold cross sections for the quasielastic processes pi- p ->n eta (eta') and pp -> pp eta (eta') reactions.

nucl-th↗
arXiv1998

$χ$PT Calculations with two-pion Loops for S-wave $π^0$ Production in pp Collision

The total cross section for the $pp \to pp π^0$ reaction at energies close to threshold is calculated within the frame of a chiral perturbation theory, taking into account tree and one loop diagrams up to chiral order D=2. Two-pion loop contributions dominate $π^0$ production at threshold. The calculated cross section reproduces data, both scale and energy dependence, fairly well.

nucl-th↗
arXiv1997

A Covariant OBE Model for $η$ Production in NN Collisions

A relativistic covariant one boson exchange model, previously applied to describe elastic nucleon-nucleon scattering, is extended to study $η$ production in NN collisions. The transition amplitude for the elementary BN->$η$N process with B being the meson exchanged (B=$π$, $|sigma$,$η$, $ρ$, $ω$ and $δ$) are taken to be the sum of four terms corresponding to s and u-channels with a nucleon or a nucleon isobar N*(1535MeV) in the intermediate states. Taking the relative phases of the various exchange amplitudes to be +1, the model reproduces the cross sections for the $NN\to Xη$ reactions in a consistent manner. In the limit where all $η$'s are produced via N^* excitations, interference terms between the overall contributions from the exchange of pseudoscalart and scalar mesons with that of vector mesons cancel out. Consequently, much of the ambiguities in the model predictions due to unknown relative phases of different vector pseudoscalar exchanges are strongly reduced.

hep-ph↗
arXiv1996

S-wave $π^0$ production In pp Collision In A Covariant OBE Model

The total cross section for the pp to pp $π^0$ reaction at energies close to threshold is calculated in a covariant one-boson-exchange model. The amplitudes for the elementary BN to N$π^0$ processes are taken to be the sum of s, u and t pole terms. The main contributions to the primary productionamplitude is due to a sigma meson exchange. Both the scale and energy dependence of the cross section are perfectly reproduced.

nucl-th↗
arXiv1995

$FSI$ Corrections For Near Threshold Meson Production In Nucleon-Nucleon Collisions

A procedure is proposed which accounts for final state interaction corrections for near threshold meson production in nucleon-nucleon scattering. In analogy with the Watson-Migdal approximation, it is shown that in the limit of extremely strong final state effects, the amplitude factorizes into a primary production amplitude and an elastic scattering amplitude describing a $3 \to 3$ transition. This amplitude determines the energy dependence of the reaction cross section near the reaction threshold almost solely. The approximation proposed satisfies the Fermi-Watson theorem and the coherence formalism. Application of this procedure to meson production in nucleon-nucleon scattering shows that, while corrections due to the meson-nucleon interaction are small for s-wave pion production, they are crucial for reproducing the energy dependence of the $η$ production cross section.

hep-ph↗