On photoexcitation of baryon antidecuplet

We show that the photoexcitation of the nonexotic members baryon antidecuplet, suggested by the soliton classification of low-lying baryons, is strongly suppressed on the proton target. The process occurs mostly on the neutron target. This qualitative prediction can be useful in identifying the nonexotic members of the antidecuplet in the known baryon spectrum. We also analyze the interrelation between photocouplings of various baryon multiplets in the soliton picture and in the nonrelativistic quark model.Received: 9 April 2003, Published online: 18 November 2003PACS: 13.30.-a Decays of baryons - 13.60.-r Photon and charged-lepton interactions with hadrons - 14.20.-c Baryons (including antiparticles)     

Relativistic study of nucleon electroweak properties in a constituent-quark model

We discuss the predictions of the Goldstone-boson-exchange constituent-quark model for the proton and neutron electric and magnetic form factors as well as the nucleon axial and induced pseudoscalar form factors. The results are calculated in a covariant framework using the point-form approach to relativistic quantum mechanics. The only input into the calculations is the nucleon wave function from the constituent-quark model. A remarkably consistent picture, with all aspects of the electroweak nucleon structure close to existing experimental data, is obtained.Received: 30 September 2002, Published online: 22 October 2003PACS: 12.39.-x Phenomenological quark models - 13.66.-a Lepton-lepton interactions - 14.20.Dh Protons and neutrons     

Electromagnetic properties of baryons in a relativistic quark model

Results on electromagnetic form factors of the nucleon and photon transition form factors of non-strange baryon resonances calculated in a relativistically covariant quark model based on the Bethe-Salpeter equation are presented. The relevance of the instanton-induced quark interaction on these properties is discussed.Received: 30 September 2002, Published online: 22 October 2003PACS: 11.10.St Bound and unstable states; Bethe-Salpeter equations - 12.39.Ki Relativistic quark model - 12.40.Yx Hadron mass models and calculations - 13.40.Gp Electromagnetic form factors     

Extraction of the pion-nucleon sigma-term from the spectrum of exotic baryons

The pion-nucleon sigma-term is extracted on the basis of the soliton picture of the nucleon from the mass spectrum of usual and the recently observed exotic baryons, assuming that they have positive parity. The value found is consistent with that inferred by means of conventional methods from pion-nucleon scattering data. The study can also be considered as a phenomenological consistency check of the soliton picture of baryons.Received: 6 February 2004, Revised: 3 May 2004, Published online: 12 October 2004PACS: 12.39.Ki Relativistic quark model - 12.38.Lg Other nonperturbative calculations - 14.20.-c Baryons (including antiparticles)     

A coherent interpretation of the form factors of the nucleon in terms of a pion cloud and constituent quarks

The recent measurements of the electric form factor of the neutron suggest that its shape may be interpreted as a smooth broad distribution with a bump at 0.3 (GeV/c)² superimposed. As a consequence the corresponding charge distribution in the Breit frame shows a negative charge extending as far out as 2 fm. It is natural to identify this charge with the pion cloud. This realisation is then used to reanalyse all old and new data of the electric and magnetic from factors of the proton and the neutron by a phenomenological fit and by a fit based on the constituent quark model. It is shown that it is possible to fit all form factors coherently with both ansaetzen and that they all show the signal of the pion cloud.Received: 6 March 2003, Published online: 17 June 2003PACS: 14.20.Dh Protons and neutrons - 13.40.Gp Electromagnetic form factors - 21.10.Ft Charge distribution     

Weak form factors for semileptonic octet baryon decays in the chiral quark model

We study the weak vector and axial-vector form factors of first- and second-class currents for the semileptonic octet baryon decays in the spirit of the chiral quark model. Our results for the weak magnetism form factors are consistent with the conserved vector current (CVC) results. The induced pseudotensor form factors, which are highly model dependent, are small. The overall performance of the chiral quark model is quite good and in general agreement with existing experimental data. Received: 9 March 1998 / Published online: 12 August 1998     

Quark-diquark approximation of the three-quark structure of baryons in the quark confinement model

Octet (1/2⁺) and decuplet (3/2⁺) baryons as relativistic three-quark states are investigated using the quark confinement model (QCM), the relativistic quark model, based on some assumptions about hadronization and quark confinement. The quark-diquark approximation of the three-quark, structure of baryons is proposed. In the framework of this approach, the main low-energy characteristics of baryons, such as magnetic moments, electromagnetic radii and form factors, the ratio of axial and vector constants in semileptonic baryon decays, strong form factors and decay widths, are described. The obtained results agree with experimental data.     

A Diquark-Quark Model with Its Use in Nucleon Form Factors

The nucleon electromagnetic form factors are investigated within a simple diquark-quark model using the light-front formalism. In this model, baryon is described as a bound state of one quark and one clustering diquark.The calculational results are compared with the experimental ones. We also regard the quarks in a baryon as pointlike constituent quarks.     

A Diquark-Quark Model with Its Use in Nucleon Form Factorst

The nucleon electromagnetic form factors are investigated within a simple diquark-quark model using the light-front formalism. In this model, baryon is described as a bound state of one quark and one clustering diquark. The calculational results are compared with the experimental ones. We also regard the quarks in a baryon as pointlike constituent quarks.     

How well do we know the electromagnetic form factors of the proton?

Recent measurements of recoil polarization in elastic scattering have been used to extract the ratio of the electric to the magnetic proton form factors. These results disagree with Rosenbluth extractions from cross-section measurements, indicating either an inconsistency between the two techniques, or a problem with either the polarization transfer or cross-section measurements. To obtain precise knowledge of the proton form factors, we must first understand the source of this discrepancy.Received: 1 November 2002, Published online: 15 July 2003PACS: 25.30.Bf Elastic electron scattering - 13.40.Gp Electromagnetic form factors - 14.20.Dh Protons and neutrons     

Weak magnetism in chiral quark models

We discuss symmetry breaking in the weak magnetism form factors for the semileptonic octet baryon decays. In the chiral quark model, the symmetry breaking can be accounted for in the masses and the quark spin polarizations can take on more general values due to Goldstone boson depolarization. Here we clarify some features of the chiral quark model prediction for the weak magnetism and compare to the corresponding result of the chiral quark soliton model. Received: 29 June 1999 / Revised version: 15 September 1999 / Published online: 8 December 1999     

Magnetic moments and structure of the nuclei3H,3He and of baryons

From a solution of the problem of magnetic moments of the nuclei³H and³He, two properties are obtained: - These nuclei have mixed orbital ground states. - These states are not charge symmetric. The first property is expected to hold also for baryons in the quark model, on account of recently measured magnetic moments. Supporting evidence and implications for baryon structure are discussed.     

Recent results from the SAMPLE experiment

The SAMPLE experiment at MIT-Bates provides measurements of parity-violating electron scattering at backward angles and low momentum transfer. These measurements yield unique information on the contribution of strange quarks to the magnetic moment of the proton and also electroweak corrections such as the anapole moment. Recent results, some possible interpretations, and outstanding issues for the future are discussed.Received: 1 November 2002, Published online: 15 July 2003PACS: 13.60.Fz Elastic and Compton scattering - 11.30.Er Charge conjugation, parity, time reversal, and other discrete symmetries - 13.40.Gp Electromagnetic form factors - 14.20.Dh Protons and neutronsR.D. McKeown: Representing the SAMPLE Collaboration     

A chiral bag model with a soft surface : structure and solutions of the Fuzzy and the modified Fuzzy Bag Model

In the present work we propose a new bag model for hadrons, called the modified fuzzy bag model (MFBM). The distinguishing feature of this model is the suppression of the pion field, as it enters the bag, by means of a scalar potential for the pions, while still preserving chiral symmetry. The mechanism of pion suppression in the MFBM is similar to the mechanism of quark suppression in the fuzzy bag model (FBM). The standard chiral transformation for the pion field suffers a natural alteration in the MFBM, and as a result the model is chiral invariant. We present also a discussion of the FBM and study, in the quark sector, the implications of the soft surface of the bag on the expectation value of the mass operator. In the pion-quark sector, we study the effects of the suppression of the pion field on the form factor for the pion-nucleon interaction, on the pion-nucleon coupling constant and on the nucleon axial charge . Calculations of the pion-nucleon form factor exhibit, in particular, an improvement over previous results. The pionic axial current induces, in the MFBM, a nonvanishing and orientation dependent contribution to axial charge. An analysis of the asymptotic behaviour of the axial charge shows that the role of the surface is to increase the difference of the contributions associated to different orientations. Received: 10 March 1997 / Revised version: 14 October 1997     

Spin Structure of Baryons in Orbiting Valence Quark Model

The quark model with orbital motion of the valence quarks is constructed to reproduce the spin structure of baryons. The relations between the spin-averaged sum rules and baryon magnetic moments found in the previous works do not remain, unless the small orbital magnetic moments are neglected. In particular, when the orbital motion of the valence quarks leads to the small contribution of quark orbit-spin to baryon magnetic moments, the sum rules for polarized nucleon are in agreement with the recent experiment.     

Strange Quark Matter and Strangelets in a Strong Magnetic Field

We study the stability properties of magnetized strange quark matter and strangelets under a strong magnetic field in the MIT bag model. The free energy per baryon of strange quark matter feels a great influence from the magnetic field. At the field strength about 10¹⁷G, the magnetized strange quark matter becomes more stable. Considering the finite size effect, the magnetic influence on strangelets becomes complicated. For a given magnetic field, there exists a critical baryon number, below which the magnetized strangelets have lower energy than the non-magnetized strangelets. For the field strength of 5× 10¹⁷G, the critical baryon number is A_c ～ 100. Generally, the critical baryon number increases with the decreasing external magnetic field. When the field strength is smaller than 10¹⁷G, the critical baryon number goes up to A_c～ 10⁵. The stable radius, electric charge, and quark flavor fractions of magnetized strangelets are shown.     

Effects of perturbative exchanges in a QCD string model

The QCD string model for baryons derived by Yu. A. Simonov and used for the calculation of baryon magnetic moments in a previous paper is extended to include also perturbative gluon and meson exchanges. The mass spectrum of the baryon multiplet is studied. For the meson interaction, either pseudoscalar or pseudovector coupling is used. Predictions are compared with the experimental data. Besides these exchanges, the influence of excited quark orbitals on the baryon ground state are considered by performing a multichannel calculation. The nucleon-Δ splitting increases due to the mixing of higher quark states, while the baryon magnetic momenta decrease. The multichannel calculation with perturbative exchanges is shown to yield reasonable magnetic moments, while the mass spectrum is close to experiment.     

Phase transition between three- and two-flavor QCD?

We explore the possibility that QCD may undergo a phase transition as a function of the strange quark mass. This would hint towards models with ”spontaneous color symmetry breaking” in the vacuum. For two light quark flavors we classify possible colored quark-antiquark, diquark and gluon condensates that are compatible with a spectrum of integer charged states and conserved isospin and baryon number. The ”quark mass phase transition” would be linked to an unusual realization of baryon number in QCD₂ and could be tested in lattice simulations. We emphasize, however, that at the present stage the Higgs picture of the vacuum cannot predict a quark mass phase transition - a smooth crossover remains as a realistic alternative. Implications of the Higgs picture for the high-density phase transition in QCD₂ suggest that this transition is characterized by the spontaneous breaking of isospin for nuclear and quark matter. Received: 12 March 2001 / Revised version: 2 April 2003 / Published online: 2 June 2003 RID="a" ID="a" e-mail: C.Wetterich@thphys.uni-heidelberg.de     

Weak production of charmed baryon resonances

We evaluate weak neutrino production cross sections for 7 conjectured charmed baryon resonances in the four flavour quark model. Effects resulting from the mass difference of uncharmed and charmed quarks are explicitly taken into account. The quark model results are only taken at q²=0 to determine the q² = 0 values of the invariant transition form factors. These are then continued to q ≠ 0 by suitably chosen form factors. Our numerical results are compared with the results of other calculations of weak charm production.     

Spectroscopy of the Ωccb baryon in the hypercentral constituent quark model

We extract the mass spectrum of the triply heavy baryon Ω_ccb using the hypercentral constituent quark model. The first order correction is also added to the potential term of the Hamiltonian. The radial and orbital excited state masses are determined, and the Regge trajectories and magnetic moments for this baryon are also given.