arXiv subjects
Publications and source records attributed to R. Bijker.
We present an unquenched quark model for baryons in which the effects of quark-antiquark pairs (light flavors, u, d and s) are taken into account in an explicit form. The method is illustrated with an application to the flavor asymmetry of the nucleon sea.
The flavor content of nucleon form factors is analyzed using two different theoretical approaches. The first is based on a phenomenological two-component model in which the external photon couples to both an intrinsic three-quark structure and a meson cloud via vector-meson dominance. The flavor content of the nucleon form factors is extracted without introducing any additional parameter. A comparison with recent data from parity-violating electron scattering experiments shows a good overall agreement for the strange form factors. A more microscopic approach is that of an unquenched quark model proposed by Geiger and Isgur which is based on valence quark plus glue dominance to which quark-antiquark pairs are added in perturbation. In the original version the importance of $s \bar{s}$ loops in the proton was studied. Here we present the formalism for a new generation of unquenched quark models which, among other extensions, includes the contributions of $u \bar{u}$ and $d \bar{d}$ loops. Finally, we discuss some preliminary results in the closure limit.
We present the formalism for a new generation of unquenched quark models in which quark-antiquark pair effects are taken into account in an explicit form via a microscopic QCD-inspired quark-antiquark creation mechanism. No truncation in the sum over all the big tower of states is necessary since these states are automatically generated by means of powerful group-theoretical techniques. An important check on the formalism and the numerical results is provided by the closure limit. As an application, the effect of quark-antiquark pairs on the strange content of the proton spin, $Δs$, is discussed. The contributions of the up and down quarks, $Δu$ and $Δd$, are also calculated. This has become possible after solving the difficult problem of permutational symmetry related to quark rearrangements. Finally, we present some preliminary results in the closure limit as well as an outlook for future applications.
The strangeness content of nucleon form factors is analyzed in a two-component model with a quark-like intrinsic structure surrounded by a meson cloud. A comparison with the available experimental data from the SAMPLE, PVA4, HAPPEX and G0 collaborations shows a good overall agreement for the strange form factors.
The strange form factors of the nucleon are studied in a two-component model consisting of a three-quark intrinsic structure surrounded by a meson cloud. A comparison with the available experimental world data from the SAMPLE, PVA4, HAPPEX and G0 collaborations shows a good overall agreement. It is shown that the strangeness contribution to the electric and magnetic form factors is of the order of a few percent. In particular, the strange quark contribution to the charge radius is small (0.005 fm**2) and to the magnetic moment it is positive (0.315 n.m.).
The strange form factors of the nucleon are studied in a two-component model consisting of a three-quark intrinsic structure surrounded by a meson cloud. A comparison with the available experimental world data from the SAMPLE, PVA4, HAPPEX and G0 collaborations shows a good overall agreement. The strange magnetic moment is found to be positive, 0.315 nm.
In this contribution, I discuss the role of symmetries and algebraic methods in nuclear structure physics. In particular, I review some recent developments in nuclear supersymmetry and indicate possible applications for light nuclei in the sd- and pf-shell.
The low-energy spectra, electric quadrupole transitions and moments for the germanium isotopes are studied in the formalism of the IBM-2 with configuration mixing. The calculated observables reproduce well the available experimental information, including the newly obtained data for radioactive neutron-rich 78, 80, 82 Ge isotopes. A geometric interpretation of the model was established using a matrix formulation. The two energy surfaces determined after mixing carry information about the deformation parameters of the nucleus. Tor the even-even Ge isotopes the results are consistent with the shape transition that takes place around neutron number N=40.
Recent experimental data on space- and time-like form factors of the nucleon are analyzed in terms of a two-component model with a quark-like intrinsics structure and a meson cloud. A good overall agreement is found for all electromagnetic form factors with the exception of the neutron magnetic form factor in the time-like region.
The spectroscopic strengths of two-nucleon transfer reactions constitute a stringent test for two-nucleon correlations in the nuclear wave functions. A comparison between the recently measured 198Hg(d,alpha)196Au reaction and the predictions from the nuclear quartet supersymmetry lends further support to the validity of supersymmetry in nuclear physics.
We discuss the properties of the pentaquark in a collective stringlike model with a nonplanar configuration of the four quarks and the antiquark. In an application to the mass spectrum of exotic Theta baryons, we find that the ground state pentaquark has angular momentum and parity J(p)=1/2(-) and a small magnetic moment of 0.382 nuclear magnetons. The decay width is suppressed by the spatial overlap with the decay products.
We propose a collective stringlike model of pentaquarks with the geometry of an equilateral tetrahedron in which the four quarks are located at the four corners and the antiquark in its center. The nonplanar equilibrium configuration is a consequence of the permutation symmetry of the four quarks. In an application to the spectrum of exotic Theta baryons, we find that the ground state pentaquark has angular momentum and parity J(p)=1/2(-) and a small magnetic moment of 0.382 nuclear magnetons. The decay width is suppressed by the spatial overlap with the decay products.
We construct a complete classification of pentaquark states in terms of the spin-flavour SU(6) representations. We find that only some definite SU(3) representations are allowed, singlets, octets, decuplets, anti-decuplets, 27-plets and 35-plets. The latter three contain exotic states, which cannot be constructed from three quarks only. This complete classification scheme is general and model independent and is useful both for model builders and experimentalists. The mass spectrum is obtained from a Gursey-Radicati type mass formula, whose coefficients have been determined previously by a study of qqq baryons. The ground state pentaquark which is identified with the recently observed Theta(1540) state, is predicted to be an isosinglet anti-decuplet state. Its parity depends on the interplay between the spin-flavour and orbital contributions to the mass operator.
We show that the nuclear supersymmetry model (n-susy) in its extended version, predicts correlations in the nuclear structure matrix elements which characterize transfer reactions between nuclei that belong to the same supermultiplet. These correlations are related to the fermionic generators of the superalgebra and if verified experimentally can provide a direct test of the model.
Recent experimental data on space-like and time-like form factors of the nucleon are analyzed in terms of a two-component model with a quark-like intrinsic three-quark structure and quark-antiquark pairs.
We analyze the magnetic moment of the exotic pentaquarks of the flavor antidecuplet in the constituent quark model for the case in which the ground state is in an orbital L(p)=0(+) or a L(p)=1(-) state. We derive sum rules for the magnetic moments. The magnetic moment of the Theta(1540) is found to be 0.38, 0.09 and 1.05 mu_N for J(p)=1/2(-), 1/2(+) and 3/2(+), respectively, which is compared with the results obtained in other approaches.
We present the first extensive test of the critical point symmetry X(5) for the gamma degree of freedom, based in part on recent measurements for the gamma-band in 152 Sm. The agreement is good for some observables including the energies and most intra- and interband transitions, but there is also a serious discrepancy for one transition.
The main ideas behind nuclear supersymmetry are presented, starting from the basic concepts of symmetry and the methods of group theory in physics. We propose new, more stringent experimental tests that probe the supersymmetry classification in nuclei and point out that specific correlations should exist for particle transfer intensities among supersymmetric partners. We also discuss possible ways to generalize these ideas to cases where no dynamical symmetries are present. The combination of these theoretical and experimental studies may play a unifying role in nuclear phenomena.