arXiv subjects

R. Bijker

Publications and source records attributed to R. Bijker.

At least 37 records · Page 2Linked to original sources

arXiv2016

The Algebraic Cluster Model: Structure of 16O

We discuss an algebraic treatment of four-body clusters which includes both continuous and discrete symmetries. In particular, tetrahedral configurations with T(d) symmetry are analyzed with respect to the energy spectrum, transition form factors and B(EL) values. It is concluded that the low-lying spectrum of 16O can be described by four alpha-particles at the vertices of a regular tetrahedron, not as a rigid structure but rather a more floppy structure with relatively large rotation-vibration interactions and Coriolis forces.

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arXiv2016

Baryons in the unquenched quark model

In this contribution, we present the unquenched quark model as an extension of the constituent quark model that includes the effects of sea quarks via a $^{3}P_{0}$ quark-antiquark pair-creation mechanism. Particular attention is paid to the spin and flavor content of the proton, magnetic moments and $β$ decays of octet baryons.

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arXiv2016

Open flavor strong decays

In this contribution, we discuss the results of a QM calculation of the open-flavor strong decays of **** light nucleon resonances. These are the results of a recent calculation, where we used a modified $^3P_0$ model for the amplitudes and the U(7) algebraic model and the Hypercentral Quark Model to predict the baryon spectrum. The decay amplitudes are compared with the existing experimental data.

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arXiv2016

Electroproduction ratios of Baryon-Meson states and Strangeness Suppression

We describe the electroproduction ratios of baryon-meson states from nucleon, inferring from the sea quarks in the nucleon using an extension of the quark model that takes into account the sea. As a result we provide, with no adjustable parameters, the predictions of ratios of exclusive meson-baryon final states:$ΛK^+$, $Σ^{*}K$, $ΣK$, $pπ^0$, and $nπ^+$. These predictions are in agreement with the new Jlab experimental data showing that sea quarks play an important role in the electroproduction. We also predicted further ratios of exclusive reactions that can be measured and tested in future experiments. In particular, we suggested new experiments on deuterium and tritium. Such measurements can provide crucial test of different predictions concerning the structure of nucleon and its sea quarks helping to solve an outstanding problem. Finally, we computed the so called strangeness suppression factor, $λ_s$, that is the suppression of strange quark-antiquarks compared to nonstrange pairs, and we found that our finding with this simple extension of the quark model is in good agreement with the results of Jlab and CERN experiments.

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arXiv2015

Hadron Spectroscopy in the Unquenched Quark Model

We present the last applications of the unquenched quark model (UQM) to the description of flavour asymmetry and strangeness in the proton when baryon-meson components are included. In the meson sector, the UQM is used to calculate the charmonium and bottomonium spectra with self-energy corrections due to the coupling to the meson-meson continuum.

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arXiv2015

The unquenched QM

In this contribution, we briefly analyze the formalism of the unquenched quark model (UQM) and its application to the calculation of hadron spectra with self-energy corrections, due to the coupling to the meson-meson continuum. In the UQM, the effects of $q \bar q$ sea pairs are introduced explicitly into the quark model through a QCD-inspired $^{3}P_0$ pair-creation mechanism. The UQM formalism can be extended to include also the effects of hybrid mesons, i.e. hybrid loops. The main applications to spectroscopy and decays are analyzed.

hep-ph↗

arXiv2014

Evidence for Triangular D_3h Symmetry in 12C

We report a measurement of a new high spin Jp = 5- state at 22.4(0.2) MeV in 12C which fits very well to the predicted (ground state) rotational band of an oblate equilateral triangular spinning top with a D_3h symmetry characterized by the sequence 0+, 2+, 3-, 4+/-, 5- with almost degenerate 4+ and 4- (parity doublet) states. Such a D_3h symmetry was observed in triatomic molecules and it is observed here for the first time in nuclear physics. We discuss a classification of other rotation-vibration bands in 12C such as the (0+) Hoyle band and the (1-) bending mode band and suggest measurements in search of the predicted ("missing") states that may shed new light on clustering in 12C and light nuclei. In particular the observation (or non-observation) of the predicted ("missing") states in the Hoyle band will allow us to conclude the geometrical arrangement of the three alpha-particle composing the Hoyle state at 7.654 MeV in 12C.

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arXiv2014

Evidence for tetrahedral symmetry in 16O

We derive the rotation-vibration spectrum of a 4-alpha configuration with tetrahedral symmetry, T_d, and show evidence for the occurrence of this symmetry in the low-lying spectrum of 16O. All vibrational states with A, E and F symmetry appear to have been observed, as well as the rotational bands with L(P)=0(+), 3(-), 4(+), 6(+) on the A states, and part of the rotational bands built on the E, F states. We derive analytic expressions for the form factors and B(EL) values of the ground state rotational band and show that the measured values support the tetrahedral symmetry of this band.

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arXiv2009

Unquenched quark model for baryons: magnetic moments, spins and orbital angular momenta

We present an unquenched quark model for baryons in which the effects of the quark-antiquark pairs (up, down and strange) are taken into account in an explicit form via a microscopic, QCD-inspired, quark-antiquark creation mechanism. In the present approach, the contribution of the quark-antiquark pairs can be studied for any inital baryon and for any flavor of the pairs. It is shown that, while the inclusion of quark-antiquark pairs does not affect the baryon magnetic moments, it leads to a sizeable contribution of the orbital angular momentum to the spin of the proton and the Lambda hyperon.

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arXiv2009

Eigenvalues correlations and the distribution of ground state angular momenta for random many-body quantum systems

The observed preponderance of ground states with angular momentum L=0 in many-body quantum systems with random two-body interactions is analyzed in terms of correlation coefficients (covariances) among different eigenstates. It is shown that the geometric analysis of Chau {\it et al.} can be interpreted in terms of correlations (covariances) between energy eigenvalues thus providing an entirely statistical explanation of the distribution of ground state angular momenta of randomly interacting quantum systems which, in principle, is valid for both fermionic and bosonic systems. The method is illustrated for the interacting boson model.

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arXiv2009

New developments in nuclear supersymmetry

We discuss several new developments in nuclear supersymmetry, in particular the identification of a new supersymmetric quartet of nuclei in the A~190 mass region, consisting of the 192,193 Os and 193,194 Ir nuclei, and a study of correlations between different transfer reactions.

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arXiv2009

New supersymmetric quartet of nuclei in the A=190 mass region

We present evidence for a new supersymmetric quartet in the A=190 region of the nuclear mass table. New experimental information on transfer and neutron capture reactions to the odd-odd nucleaus 194 Ir strongly suggests the existence of a new supersymmetric quartet, consisting of the 192,193 Os and 193,194 Ir nuclei. We make explicit predictions for the odd-neutron nucleus 193 Os, and suggest that its spectroscopic properties be measured in dedicated experiments.

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arXiv2008

Flavor content of the nucleon in an unquenched quark model

We discuss the flavor content of the nucleon in an unquenched quark model in which the effects of quark-antiquark pairs (up, down and strange) are taken into account in an explicit form. It is shown that the inclusion of quark-antiquark pairs leads to an excess of anti-d over anti-u quarks in the proton and to a large contribution of orbital angular momentum to the spin of the proton.

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arXiv2008

Flavor asymmetry of the nucleon

The flavor asymmetry of the nucleon sea is discussed in an unquenched quark model for baryons in which the effects of quark-antiquark pairs (u anti-u, d anti-d and s anti-s) are taken into account in an explicit form. The inclusion of q anti-q pairs leads automatically to an excess of anti-d over anti-u quarks in the proton, in agreement with experimental data.

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arXiv2008

New supersymmetric quartet of nuclei: 192Os, 193Os, 193Ir, 194Ir

We present evidence of the existence of a new supersymmetric quartet of nuclei in the A=190 mass region. The analysis is based on new experimental information on the odd-odd nucleus 194Ir from transfer and capture reactions. The new data allow the identification of a new supersymmetric quartet, consisting of the 192,193Os and 193,194Ir nuclei. We make explicit predictions fo r193Os, and suggest that its spectroscopic properties be measured in dedicated experiments. Finally, we study correlations between different transfer reactions.

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arXiv2008

Flavor asymmetry of the nucleon sea in an unquenched quark model

We discuss the flavor asymmetry of the nucleon in an unquenched quark model for baryons in which the effects of quark-antiquark pairs (u anti-u, d anti-d and s anti-s) are taken into account in an explicit form. It is shown that the inclusion of q anti-q pairs leads to an excess of anti-d over anti-u quarks in the proton.

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arXiv2008

Recent developments in the constituent quark model including quark-antiquark pairs

We present the formalism for a new generation of unquenched quark models for baryons in which the effects of quark-antiquark pairs (u anti-u, d anti-d and s anti-s) are taken into account in an explicit form via a microscopic, QCD-inspired, quark-antiquark creation mechaniscm. The present approach is an extension of the flux-tube breaking model of Geiger and Isgur in which now the contributions of quark-antiquark pairs an be studied for any initial baryon and for any flavor of the q anti-q pair. It is shown that the inclusion of q anti-q paris leads to a large contribution of orbital angular momentum to the proton spin.

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arXiv2008

Two-component model for the axial form factor of the nucleon

The axial form factor of the nucleon is studied in a two-component model consisting of a three-quark intrinsic structure surrounded by a meson cloud. The experimental data in the space-like region are well reproduced with a minimal number of parameters. The results are similar to those obtained from a dipole fit up to 1 GeV$^2$, but they surprisingly show a zero between 2 and 3 GeV$^2$. Such behavior, predicted by the present model completely disagrees with the dipole parametrization. The theoretical expression for the form factor is then analytically extrapolated to the time-like region, giving the first predictions in this regime. This kinematical region is especially interesting for present and future colliders.

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