A diquark model for baryons containing one heavy quark

We present a phenomenological ansatz for coupling a heavy quark with two light quarks to form a heavy baryon. The heavy quark is treated in the heavy mass limit, and the light quark dynamics is approximated by propagating scalar and axial vector ‘diquarks’. The resulting effective lagrangian, which incorporates heavy quark and chiral symmetry, describes interactions of heavy baryons with Goldstone bosons in the low energy region. As an application, the Isgur-Wise form factors are estimated.     

A diquark model for baryons containing one heavy quark

We present a phenomenological ansatz for coupling a heavy quark with two light quarks to form a heavy baryon. The heavy quark is treated in the heavy mass limit, and the light quark dynamics is approximated by propagating scalar and axial vector ??diquarks??. The resulting effective lagrangian, which incorporates heavy quark and chiral symmetry, describes interactions of heavy baryons with Goldstone bosons in the low energy region. As an application, the Isgur-Wise form factors are estimated.     

Experimental evidence for narrow baryons in the mass range 1.0≤M≤1.46 GeV

The reaction pp → pπ⁺ X was studied at different incident energies around T _p = 2 GeV. Narrow baryonic structures were observed in the missing mass M _X and in the invariant mass M _pπ{+}. The masses of these structures are 1004, 1044, 1094, 1136, 1173, 1249, 1277, and 1384 MeV (and possibly 1339 MeV). Some of them were also observed at the same masses in the missing-mass spectra of the dp → ppX reaction although with a weaker signature. Many checks were performed to make sure that these structures were not produced by experimental artifacts. Several narrow small-amplitude peaks, were also extracted using already published photonucleon cross-sections. The small widths of all these results, and the stability of the observed structures, regardless of the experiment, were used to conclude that they are genuine baryons and not merely the consequence of dynamical rescatterings. These baryons cannot be associated with classical q³ quark configurations. We associate them with two colored-quark cluster configurations. Received: 3 July 2002 / Accepted: 30 January 2003 / Published online: 5 June 2003     

The kaon form factor in the light-cone quark model

The electromagnetic form factor of the kaon meson is calculated in the light-cone formalism of the relativistic constituent quark model. The calculated K⁺ form factor is consistent with almost all of the available experimental data at low-energy scale, while other properties of the kaon could also be interrelated in this representation with reasonable parameters. Predictions of the form factors for the charged and neutral kaons at a higher-energy scale are also given, and we find the non-zero K⁰ form factor at Q ²≠ 0 due to the mass difference between the strange and down quarks inside K⁰. Received: 21 June 2002 / Accepted: 29 July 2002 / Published online: 3 December 2002 RID="a" ID="a"e-mail: mabq@phy.pku.edu.cn Communicated by A. Sch?fer     

Test of the heavy quark–light diquark approximation for baryons with a heavy quark

We check a commonly used approximation in which a baryon with a heavy quark is described as a heavy quark–light diquark system. The heavy quark influences the diquark internal motion reducing the average distance between the two light quarks. Besides, we show how the average distance between the heavy quark and any of the light quarks, and that between the heavy quark and the center of mass of the light diquark, are smaller than the distance between the two light quarks, which seems to contradict the heavy quark–light diquark picture. This latter result is in agreement with expectations from QCD sum rules and lattice QCD calculations. Our results also show that the diquark approximations produces larger masses than the ones obtained in a full calculation.     

Electroweak form factors of non-strange baryons

We compute electroweak form factors of the nucleon and photon transition form factors of non-strange baryon resonances up to the third resonance region in a model with instanton-induced interaction. The calculation is based on the Bethe-Salpeter equation for three light constituent quarks and is fully relativistic (U. L?ring et al., Eur. Phys. J. A 10, 309 (2001)). Static nucleon properties and photon resonance couplings are in good agreement with experiment and the Q² behaviour of the experimentally known form factors up to large momentum transfer is accounted for. Received: 4 April 2002 / Accepted: 2 May 2002     

Mass spectra of heavy baryons in the relativistic quark model

The mass spectra of the ground state and excited heavy baryons consisting of two light (u, d, s) and one heavy (c, b) quarks are calculated. The heavy-quark-light-diquark picture is used within the relativistic quark model. An overall good agreement of the obtained predictions with available experimental data is found. The text was submitted by the authors in English.     

带有两个重夸克的重子谱计算中几个困难问题的研究

由两个重夸克和一个轻夸克组成的重子可以看作是一个两体系统.它的两个重夸克组成一个玻色型的双夸克团.利用B–S方程导出了它的轻夸克和重的双夸克之间的等效相互作用势.在利用这种势计算重子质量的过程中,发现有几个困难问题需要深入探讨.它们是:(1)算符排序,(2)由非相对论展开带来的误差,(3)自旋–自旋耦合,(4)在标量双夸克组成的重子态和矢量双夸克组成的重子态之间的混合.本文详细地讨论并适当地处理了这些问题.     

Deconfinement transition: a three dimensional model

We present a three–dimensional model for quark matter with a density dependent quark–quark (confining) potential, which allows to describe a sort of deconfinement transition as the system evolves from a low density assembly of bound structures to a high density free Fermi gas of quarks. We consider different confining potentials, some of which successfully utilized in hadron spectroscopy. We find that a proper treatment of the many–body correlations induced by the medium is essential to disentangle the different nature of the two (hadronic and deconfined) phases of the system. For this purpose the ground state energy per particle and the pair correlation function are investigated. Received: 10 June 1998 / Revised version: 24 September 1998     

Radial excitations of low–lying baryons and the Z + penta–quark

Abstact: Within an extended Skyrme soliton model for baryons the interplay between the collective radial motion and the SU(3)–flavor–rotations is investigated. The coupling between these modes is mediated by flavor symmetry breaking. Collective coordinates which describe the corresponding large amplitude fluctuations are introduced and treated canonically. When diagonalizing the resulting Hamiltonian flavor symmetry breaking is fully taken into consideration. As eigenstates not only the low–lying (1/2)⁺ and (3/2)⁺ baryons but also their radial excitations are obtained and compared to the empirical data. In particular the relevance of radial excitations for the penta–quark baryon Z ⁺ (Y=2, I=0, J ^π=(1/2)⁺) is discussed. In this approach its mass is predicted to be 1.58 GeV. Furthermore the widths for various hadronic decays are estimated which, for example, yields Γ(Z ⁺→NK) ∼ 100 MeV for the only permissible decay process of the Z ⁺. Received: 20 April 1998 / Revised version: 29 May 1998     

Transport theory for a scalar quark & gluon model

A model for scalar quarks and gluons that successfully gives rise to a ln s behavior in high-energy qq scattering and which contains a non-trivial three-gluon vertex is used to study collision theory with the following aspects: i) A three-body interaction simulating QCD is present and ii) particle production and annihilation occur naturally. In this paper, the collision term in the model is examined in detail in the quasiparticle approximation. The construction of cross-sections in which self-energy terms are ordered according to a coupling constant expansion is undertaken. It is shown explicitly which terms of second order are required to obtain the scattering amplitudes that are two body in nature. Additional ordering in the number of colors shows that quark loop diagrams are suppressed and gluon production or scattering processes dominate. It is also shown that a consistent calculation of the scattering graphs at the two-loop level also simultaneously yields terms that renormalize one-loop level graphs. This can then be extended to arbitrary m↦n processes. We examine the constraint equation briefly, discussing the appearance of a width. The issue of pinch singularities is also addressed, and examples of the elimination of such singularities in equilibrium are given explicitly. Received: 7 November 2001 / Accepted: 12 June 2001     

The light-baryon spectrum in a relativistic quark model with instanton-induced quark forces

This is the second of a series of three papers treating light-baryon resonances up to 3 GeV within a relativistically covariant quark model based on the three-fermion Bethe-Salpeter equation with instantaneous two- and three-body forces. In this paper we apply the covariant Salpeter framework (which we developed in the first paper, U. L?ring, K. Kretzschmar, B.Ch. Metsch, H.R. Petry, Eur. Phys. J. A 10, 309 (2001)) to specific quark model calculations. Quark confinement is realized by a linearly rising three-body string potential with appropriate spinorial structures in Dirac space. To describe the hyperfine structure of the baryon spectrum we adopt 't Hooft's residual interaction based on QCD-instanton effects and demonstrate that the alternative one-gluon exchange is disfavored on phenomenological grounds. Our fully relativistic framework allows to investigate the effects of the full Dirac structures of residual and confinement forces on the structure of the mass spectrum. In the present paper we present a detailed analysis of the complete non-strange-baryon spectrum and show that several prominent features of the nucleon spectrum such as, e.g., the Roper resonance and approximate “parity doublets” can be uniformly explained due to a specific interplay of relativistic effects, the confinement potential and 't Hooft's force. The results for the spectrum of strange baryons will be discussed in a subsequent paper, see U. L?ring, B.Ch. Metsch, H.R. Petry, this issue, p. 447. Received: 27 March 2001 / Accepted: 17 April 2001     

Baryonic masses based on the NJL model

We employ the Nambu-Jona-Lasinio model to determine the vacuum pressure on the quarks in a baryon and hence their density inside. Then we estimate the baryonic masses by implementing the local density approximation for the mean-field quark energies obtained in a uniform and isotropic system. We obtain a fair agreement with the experimental masses. Received: 27 September 2002 / Accepted: 4 October 2002 / Published online: 4 February 2003 RID="a" ID="a"e-mail: giacosa@tphys.physik.uni-tuebingen.de Communicated by A. Molinari     

Hadronic decays of N and Δ resonances in a chiral quark model

π and η decay modes of light baryon resonances are investigated within a chiral quark model whose hyperfine interaction is based on Goldstone-boson exchange. For the decay mechanism a modified version of the ³ P ₀ model is employed. Our primary aim is to provide a further test of the recently proposed Goldstone-boson exchange constituent quark model. We compare the predictions for π and η decay widths with experiment and also with results from a traditional one-gluon exchange constituent quark model. The differences between nonrelativistic and semirelativistic versions of the constituent quark models are outlined. We also discuss the sensitivity of the results on the parameterization of the meson wave function entering the ³ P ₀ model. Received: 11 May 2001 / Accepted: 17 September 2001     

Fluctuations of the quark densities in nuclei

We study the static scalar susceptibility of the nuclear medium, i.e., the change of the quark condensate for a small modification of the quark mass. In the linear sigma model it is linked to the in-medium sigma propagator and its magnitude increases due to the mixing with the softer modes of the nucleon-hole excitations. We show that the pseudoscalar susceptibility, which is large in the vacuum, owing to the smallness of the pion mass, follows the density evolution of the quark condensate and thus decreases. At normal nuclear matter density the two susceptibilities become much closer, a partial chiral symmetry restoration effect as they become equal when the full restoration is achieved. Received: 20 July 2002 / Accepted: 14 September 2002 / Published online: 21 January 2003 RID="a" ID="a"e-mail: chanfray@ipnl.in2p3.fr Communicated by A. Molinari     

Configuration-mixed effective SU(3) symmetries

The procedure of Jarrio et al. (Nucl. Phys. A 528, 409 (1991)) for the determination of the effective SU(3) symmetry of nuclear states is extended to small deformations and to oblate nuclei. Self-consistency checks are carried out both for light and for heavy nuclei. Received: 23 April 2002 / Accepted: 25 July 2002 / Published online: 10 December 2002 RID="a" ID="a"e-mail: hess@nuclear.unam.mx Communicated by A. Molinari     

Chiral magnetism of the nucleona

We study the quark mass expansion of the magnetic moments of the nucleon in a chiral effective field theory including nucleons, pions and delta-resonances as explicit degrees of freedom. We point out that the usual power counting applied so far to this problem misses important quark mass structures generated via an intermediate isovector M1 nucleon-delta transition. We propose a modified power counting and compare the resulting chiral extrapolation function to available (quenched) lattice data. The extrapolation is found to work surprisingly well, given that the lattice data result from rather large quark masses. Our calculation raises the hope that extrapolations of lattice data utilizing chiral effective field theory might be applicable over a wider range in quark masses than previously thought, and we discuss some open questions in this context. Furthermore, we observe that within the current lattice data uncertainties the extrapolations presented here are consistent with the Padé fit ansatz introduced by the Adelaide group a few years ago. Received: 23 April 2002 / Accepted: 18 July 2002 / Published online: 17 December 2002 RID="c" ID="c"e-mail: themmert@physik.tu-muenchen.de RID="d" ID="d"e-mail: weise@ect.it Communicated by A. Sch?fer     

Charm quark energy loss in QCD matter

The energy loss of heavy quarks in a quark-gluon plasma of finite size is studied within the light-cone path integral approach. A simple analytical formulation of the radiative energy loss of heavy quarks is derived. This provides a convenient way to quantitatively estimate the quark energy loss. Our results show that if the energy of a heavy quark is much larger than its mass, the radiative energy loss approaches the radiative energy loss of light quarks.