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A. J. Buchmann

Publications and source records attributed to A. J. Buchmann.

At least 37 records · Page 2Linked to original sources

Moments of the neutron charge form factor and the N->Delta quadrupole transition

Recent data allow a new parametrization of the neutron charge form factor G_En. A parameter-free quark model relation between G_En and the N->Delta quadrupole form factor is used to predict the latter from the G_En data. In particular, the neutron charge radius is related to N->Delta quadrupole moment, while the fourth moment of G_En connects to the N->Delta quadrupole transition radius. From the latter we derive an experimental value for the charge radius of the light constituent quarks. Finally, the C2/M1 ratio in pion electroproduction is predicted from the elastic neutron form factor data.

hep-ph↗

Intrinsic quadrupole moment of the nucleon

We address the question of the intrinsic quadrupole moment Q_0 of the nucleon in various models. All models give a positive intrinsic quadrupole moment for the proton. This corresponds to a prolate deformation. We also calculate the intrinsic quadrupole moment of the Delta(1232). All our models lead to a negative intrinsic quadrupole moment of the Delta corresponding to an oblate deformation.

hep-ph↗

Pion-Baryon Couplings

We have extended and applied a general QCD parameterization method to the emission of pions from baryons. We use it to calculate the strength and sign of the coupling of pions to the octet and decuplet of baryons. Certain relations between octet and decuplet couplings are pointed out.

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N->Delta Quadrupole Transition in the Constituent Quark Model

Information on the intrinsic deformation of the proton can be obtained by studying the electromagnetic p-->Delta+ quadrupole transition. Recent experiments have shown that the electric quadrupole (E2) strength in gamma p --> Delta+ is about 10 times larger than predicted by the simple quark model using only one-body currents. Our analysis provides evidence for the dominance of exchange currents in the N-->Delta quadrupole transition and identifies the physical mechanism leading to the observed E2 strength.

hep-ph↗

Exchange currents in octet hyperon charge radii

Octet hyperon charge radii are calculated in a chiral constituent quark model including electromagnetic exchange currents between quarks. In impulse approximation one observes a decrease of the hyperon charge radii with increasing strangeness. This effect is reduced by exchange currents. Due to exchange currents, the charge radius of the negatively charged hyperons are close to the proton charge radius.

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Radiative decays of decuplet hyperons

We calculate the radiative decay widths of decuplet hyperons in a chiral constituent quark model including electromagnetic exchange currents between quarks. Exchange currents contribute significantly to the E2 transition amplitude, while they largely cancel for the M1 transition amplitude. Strangeness suppression of the radiative hyperon decays is found to be weakened by exchange currents. Differences and similarities between our results and other recent model predictions are discussed.

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The d'-dibaryon in a colored cluster model

The mass and wave function of a six-quark system with quantum numbers J^P=0^-, T=0, called d', are calculated. We use a colored diquark-tetraquark cluster model for the six-quark wave function. A constituent quark model Hamiltonian with a two-body confinement potential, and residual one-gluon, one-pion, and one-sigma exchange interactions is used. The complications due to the quark exchange interactions between tetraquark and diquark clusters (Pauli principle) are taken into account within the framework of the Resonating Group Method. The calculated d' mass is some 350 MeV above the empirical value if the same two-body confinement strength as in the nucleon and Delta is used. This paper also examines the validity of the usual assumption of a universal two-quark confinement strength. We propose that the effective two-body confinement strength in an exotic six-quark system, such as the d', could be weaker than in a single baryon. The weaker confinement hypothesis leads to a d' mass of M=2092 MeV and a d' radius of r=1.53 fm.

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Exchange Currents in Photoproduction of Baryon Resonances

We calculate photoexcitation amplitudes for several nucleon and delta resonances. We use a chiral quark model including two-body exchange currents. The two-body currents give important contributions. For the delta (1232) and the D13 (1520) we observe that the individual exchange current contributions considerably cancel each other while in the case of the Roper resonance and the S11 (1535) we get a reinforcement of the two-body amplitudes. In comparison with present experimental data, we obtain both for the S11 (1535) and for the Roper resonance an improvement with respect to the impulse approximation.

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Moscow-type NN-potentials and three-nucleon bound states

A detailed description of Moscow-type (M-type) potential models for the NN interaction is given. The microscopic foundation of these models, which appear as a consequence of the composite quark structure of nucleons, is discussed. M-type models are shown to arise naturally in a coupled channel approach when compound or bag-like six-quark states, strongly coupled to the NN channel, are eliminated from the complete multiquark wave function. The role of the deep-lying bound states that appear in these models is elucidated. By introducing additional conditions of orthogonality to these compound six-quark states, a continuous series of almost on-shell equivalent nonlocal interaction models, characterized by a strong reduction or full absence of a local repulsive core (M-type models), is generated. The predictions of these interaction models for 3N systems are analyzed in detail. It is shown that M-type models give, under certain conditions, a stronger binding of the 3N system than the original phase-equivalent model with nodeless wave functions. An analysis of the 3N system with the new versions of the Moscow NN potential describing also the higher even partial waves is presented. Large deviations from conventional NN force models are found for the momentum distribution in the high momentum region. In particular, the Coulomb displacement energy for nuclei ^3He - ^3H displays a promising agreement with experiment when the ^3H binding energy is extrapolated to the experimental value.

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The interaction between H-dibaryons

We investigate the interaction between H-dibaryons employing a quark cluster model with a one-gluon-exchange potential and an effective meson exchange potential (EMEP). A deeply-bound state of two H-dibaryons due to the medium range attraction of the EMEP is obtained. The bound H--H system has a size of about 0.8 $\sim$ 0.9 fm because of the short-range repulsion generated by the color-magnetic interaction and the Pauli principle.

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Dibaryons in nuclear matter

The possibility for occurrence of a Bose condensate of dibaryons in nuclear matter is investigated within the framework of the Walecka model in the mean-field approximation. Constraints for the omega - and sigma - meson coupling constants with dibaryons following from the requirement of stability of dibaryon matter against compression are derived and the effect of sigma - and pi - meson exchange current contributions to the sigma - dibaryon coupling constant is discussed. The mean-field solutions of the model are constructed. The effective nucleon mass vanishes when the density of dibaryons approaches a critical value about 0.15/fm^3. The Green's functions of the equilibrium binary mixture of nucleons and dibaryons are constructed by solving the Gorkov-Dyson system of equations in the no-loop approximation. We find that when the square of the sound velocity is positive, the dispersion laws for all elementary excitations of the system are real functions. This indicates stability of the ground state of the heterophase nucleon-dibaryon mixture. In the model considered, production of dibaryons becomes energetically favorable at higher densities as compared to estimates based on a model of non-interacting nucleons and dibaryons.

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Nuclear matter with a Bose condensate of dibaryons in relativistic Hartree approximation

The Green's functions are constructed and a one-loop calculation is given for the scalar and vector densities and for the equation of state of nuclear matter with a Bose condensate of dibaryons. This is the lowest approximation in the loop expansion of quantum hadrodynamics, sufficient to account for the presence of dibaryons not in the condensate in the heterophase nucleon-dibaryon matter. It leads to a finite effective nucleon mass and remains consistent with increasing the density.

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The d'-dibaryon in a colored cluster model

We calculate the mass and structure of a J^P=0^-, T=0 six-quark system using a colored diquark-tetraquark cluster wave function and a nonrelativistic quark model Hamiltonian. The calculated mass is some 350 MeV above the empirical value if the same confinement strength as in the nucleon is used. If the effective two-body confinement strength is weaker in a compound six-quark system than in a single baryon, as expected from a simple harmonic oscillator model, one obtains M_d' = 2092 MeV close to experiment.

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Pionic decay of a possible d'-dibaryon and the short-range NN interaction

We study the pionic decay of a possible dibaryon d' --> N N πin the microscopic quark shell model. The initial d' dibaryon wave function (J^P=0^-, T=0) consists of one 1\hbarωsix-quark shell-model s^5p[51]_X configuration. The most important final six-quark configurations s^6[6]_X, s^4p^2[42]_X and (s^4p^2-s^52s)[6]_X are properly projected onto the NN channel. The final state NN interaction is investigated by means of two phase-equivalent - but off-shell different - potential models. We demonstrate that the decay width Γ_d' depends strongly on the short-range behavior of the NN wave function. In addition, the width Γ_d' is very sensitive to the mass and size of the d' dibaryon. For dibaryon masses slightly above the experimentally suggested value M_d'=2.065 GeV, we obtain a pionic decay width of Γ_d' = 0.18 - 0.32 MeV close to the experimental value Γ_d' = 0.5 MeV.

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Constraints to Coupling Constants of the Omega- and Sigma-Mesons with Dibaryons

The effect of narrow dibaryon resonances to nuclear matter and structure of neutron stars is investigated in the mean-field theory (MFT) and in the relativistic Hartree approximation (RHA). The existence of massive neutron stars imposes constraints to the coupling constants of the omega- and sigma-mesons with dibaryons. We conclude that the experimental candidates to dibaryons d1(1920) and d'(2060) if exist form in nuclear matter a Bose condensate stable against compression. This proves stability of the ground state for nuclear matter with a Bose condensate of the light dibaryons.

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Nuclear Matter with a Bose Condensate of Dibaryons in Relativistic Mean-Field Theory

If sufficiently light dibaryon resonances exist, a Bose condensate of dibaryons can occur in nuclear matter before the quark-hadron phase transition. Within a relativistic mean-field model we show that heterophase nuclear-dibaryon matter is for a wide set of parameters energetically more favorable than normal nuclear matter. Production of dibaryons is, however, relatively suppressed as compared to estimates based on the model of non-interacting nucleons and dibaryons.

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Electromagnetic Properties of the Delta(1232)

We calculate the electromagnetic moments and radii of the Delta(1232) in the nonrelativistic quark model, including two-body exchange currents. We show that two-body exchange currents lead to nonvanishing Delta and N-->Delta transition quadrupole moments even if the wave functions have no D-state admixture. The usual explanation based on the single-quark transition model involves D-state admixtures but no exchange currents. We derive a parameter- free relation between the N-->Delta transition quadrupole moment and the neutron charge radius: Q(N-->Delta) = r^2(neutron)/sqrt(2). Furthermore, we calculate the M1 and E2 amplitudes for the process photon + N -->Delta. We find that the E2 amplitude receives sizeable contributions from exchange currents. These are more important than the ones which result from D-state admixtures due to tensor forces between quarks if a reasonable quark core radius of about 0.6 fm is used. We obtain a ratio of E2/M1=-3.5%.

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Dibaryon Condensate in Nuclear Matter and Neutron Stars: Exact Analysis in One-Dimensional Models

We investigate dense nuclear matter with a dibaryon Bose-Einstein condensate as a possible intermediate state before the quark-gluon phase transition. An exact analysis of this state of matter is presented in a one-dimensional model. The analysis is based on a reduction of the quantization rules for the N-body problem to N coupled algebraic transcendental equations. We observe that when the Fermi momentum approaches the resonance momentum, the one-particle distribution function increases near the Fermi surface. When the Fermi momentum is increased beyond the resonance momentum, the equation of state becomes softer. The observed behavior can be interpreted in terms of formation of a Bose-Einstein condensate of two-fermion resonances (dibaryons). In cold nuclear matter, it should occur if 2(m_N + epsilon_F) is greater or equal to m_D, where m_N and m_D are respectively the nucleon and dibaryon masses and epsilon_F is the nucleon Fermi energy.

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