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     

Hard exclusive electroproduction of pions

We investigate the exclusive electroproduction of mesons from nucleons. To leading-twist, leading-order accuracy, the corresponding production amplitude can be decomposed into pseudovector and pseudoscalar parts. Both can be expressed in terms of quark double distribution functions of the nucleon. While the pseudovector contribution is connected to ordinary polarized quark distributions, the pseudoscalar part can be related to one-pion t-channel exchange. We observe that the pseudovector part of the production amplitude is important at . On the other hand, for , contributions from one-pion exchange dominate. Received: 4 January 1999 / Published online: 15 July 1999     

The theory of quark confinement

This is the second of the two last papers by V.N. Gribov concluding his 20 year long study of the problem of quark confinement in QCD. In this paper the analytic structure of quark and gluon Green functions is investigated in the framework of the theory of confinement based on the phenomenon of supercritical binding of light quarks. The problem of unitarity in a confining theory is discussed. The write-up remained unfinished and as such it is presented here. The author was planning to emphasise the link between the electroweak and strong interactions, and in particular the r?le of pions (Goldstone bosons) in confinement, to present an explicit solution for bound states, and to write down an analytic model for quark and gluon Green functions corresponding to confinement. Received: 15 February 1999 / Published online: 15 July 1999     

A dynamical mechanism for quark mixing and neutrino oscillations

We show that, if one assumes fermion generations to be given by a gauge symmetry, together with a certain Higgs mechanism for breaking it, then the known empirical features of quark and lepton mixing can be largely explained, including, in particular, the fact that the mixing (CKM) matrix element , responsible for the muon anomaly in atmospheric neutrinos, is near maximal, and much larger than its quark counterparts and , while the corner elements for both quarks () and leptons () are all very small. The mechanism also automatically gives a hierarchical fermion-mass spectrum which is intimately related to the mixing pattern. Received: 8 February 1999 / Published online: 15 July 1999     

The spin-symmetry of the quark model

Corrections to the exact heavy–quark symmetry results are expected to come from the inverse powers of the heavy-quark mass. We show, by an explicit calculation using the quark model, that the breaking of the spin symmetry is suppressed by other kinematic effects even when the quark masses are not heavy. Received: 21 January 1999 / Published online: 20 May 1999     

Polarized parton distribution in the relativistic quark exchange framework

The Spin dependent gluon and sea quark distributions of the proton and the neutron are extracted in the leading order (LO) and the next-to-leading order (NLO) QCD. The relativistic quark exchange model is used to calculate the related valence quark spin dependent structure function. The inverse Mellin transform technique is performed to evaluate the polarized x-dependent distributions of the gluon and the sea quark from the various moments of the valence quarks. It is shown that the calculated spin structure functions (SSF) of the proton and the neutron are in good agreement with the available data, such as E143, SMC, E142, E154 and Hermes experiments. A comparison is also made with the other theoretical models. Finally it is shown that the above calculated parton distributions improve the SSF of the proton and the neutron. Received: 4 January 1999 / Revised version: 12 April 1999     

Measurement of the strange quark forward-backward asymmetry around the Z^{0} peak

A precise measurement of the strange quark forward-backward asymmetry used 3.2M multihadronic events around the Z peak collected by the DELPHI experiment from 1992 to 1995. The ring imaging Cherenkov detectors in the barrel and end-cap regions identify high energy charged kaons which tag the s quark. The s quark asymmetry was measured at different centre-of-mass energies; in particular at the Z pole taking the expected d and u quark asymmetries from the Standard Model. The quark flavour fractions are assumed from the Standard Model and the fragmentation process is modelled by JETSET. From the s quark pole asymmetry the electroweak mixing angle was determined: The parity violating coupling of the s quark to the Z was determined to be: Received: 8 October 1999 / Revised version: 23 February 2000 / Published online: 18 May 2000     

Improved effective action for light quarks beyond the chiral limit

We propose an improvement of the Diakonov–Petrov effective action on the basis of the Lee–Bardeen results for the quark determinant in the instanton field. This improved effective action provides a proper account of the current quark masses, which is particularly important for strange quarks. This action is successfully tested by calculations of the quark condensate, the masses of the pseudoscalar meson octet and axial-anomaly low-energy theorems. Received: 19 October 1998 / Revised version: 17 January 1999 / Published online: 20 May 1999     

Yukawa corrections to the charged Higgs boson production in association with the top quark at hadron colliders

We calculate the Yukawa corrections of order to charged Higgs boson production in association with a top quark at the Tevatron and the LHC. The corrections are not very sensitive to the mass of the charged Higgs boson and can exceed for low values of , where the contribution of the top quark is large, and high values of where the contribution of the bottom quark becomes large. These Yukawa corrections could be significant for charged Higgs boson searches based on this production process, particularly at the LHC where the cross section is relatively large. Received: 12 October 1999 / Revised version: 3 December 1999 / Published online: 6 April 2000     

SU(N) elastic rescattering in B and D decays

The treatment of elastic final-state interactions (FSIs) under a symmetry group is presented. The proposed model is based on Watson's theorem, i.e. on symmetry properties of the -matrix and on its unitarity. This theorem provides an easy way to introduce rescattering effects by defining final-state interactions mixing matrices. A symmetry group fixes the structure of such mixing matrices, and the passage from one group to another is studied (for example, SU(2) to SU(3)). Mixings among two charmless pseudoscalar decay product states will be systematically analyzed. Finally, these mixing matrices will be used on quark diagram parametrizations of B and D decay amplitudes. This will have some important consequences on the definition of quark diagrams. It will be argued that these diagrams should not contain any FSI effects, i.e. they should be real (except for CKM factors). FSIs are then introduced at the hadronic level, by mixing basic quark diagram topologies. Received: 17 December 1998 / Revised version: 17 February 1999 / Published online: 28 September 1999     

Measurement of the gluon fragmentation function and a comparison of the scaling violation in gluon and quark jets

The fragmentation functions of quarks and gluons are measured in various three-jet topologies in Z decays from the full data set collected with the Delphi detector at the Z resonance between 1992 and 1995. The results at different values of transverse momentum-like scales are compared. A parameterization of the quark and gluon fragmentation functions at a fixed reference scale is given. The quark and gluon fragmentation functions show the predicted pattern of scaling violations. The scaling violation for quark jets as a function of a transverse momentum-like scale is in a good agreement with that observed in lower energy annihilation experiments. For gluon jets it appears to be significantly stronger. The scale dependences of the gluon and quark fragmentation functions agree with the prediction of the DGLAP evolution equations from which the colour factor ratio is measured to be: Received: 5 November 1999 / Published online: 25 February 2000     

The donnachie-landshoff pomeron and gauge invariance

We discuss a model for heavy mesons where the light quark (u or d) moves in the colour-electric field from a heavy quark (c or b) placed in the center of a bag. We calculate energy spectra for pionic and photonic transitions from excited states. The transition amplitudes and the branching ratios between electromagnetic and pionic transitions compare favorably with the limited amount of known experimental data. Received: 19 January 1999 / Published online: 18 June 1999     

Parametrization of Fully Dressed Quark Propagator

Based on an extensive study of the Dyson-Schwinger equations for a fully dressed quark propagator in the “rainbow” approximation, a parametrized form of the quark propagator is suggested. The corresponding quark self-energy Σ_f and the structure of non-local quark vacuum condensate <0|:\bar{q}(x)q(0):|0> are investigated. The algebraic form of the quark propagator proposed in this work describes a confining quark propagation, and is quite convenient to be used in any numerical calculations.     

Diquark stars with extended scalar diquarks and their stability

Within the framework of an effective -theory, an attempt is made to study diquark stars and their stability with extended scalar diquarks (ESD). In this context, an equation of state (EOS) for the ESD gas is obtained. We find the EOS for the ESD gas to be stiffer than that for a point-like diquark and/or quark gas. This EOS is then used to investigate various properties of the diquark stars. In particular, the mass and radius of the maximum mass star with ESD matter turn out to be larger than those obtained with point-like diquark and/or quark matter. However, they are in conformity with the predictions available for soliton and boson stars. The stability of ESD stars against radial oscillations is also investigated. Received: 18 May 1999 / Revised version: 19 November 1999 / Published online: 14 April 2000     

Probing a nucleon spin structure at TESLA by the real polarized gamma beam

The recent proposals concerning the usage of the real polarized gamma beam, obtained by Compton backscattering of the laser photons off the electron beams from either the linear or circular accelerators were considered. The heavy quark photoproduction process, which gives a unique opportunity to measure the polarized gluon distribution, was investigated. Received: 22 March 1999 / Revised version: 17 June 1999 / Published online: 14 October 1999     

The Q<Superscript>2</Superscript> dependence of polarized and unpolarized proton structure functions in the relativistic quark exchange framework

In this article we intend to discuss the evolution of polarized and unpolarized structure functions in the (x,Q²) plane. We analyze the proton data on the spin dependence asymmetry A₁(x,Q²), by making the dynamical assumption that at low resolution energies, the hadrons consist only of valence quarks and the scaling violation of F₂(x,Q²) at low x comes only from the gluons density. While the sea quark and the gluon distributions are calculated using the inverse Mellin technique and the various moments of the valence quarks, the valence quark distribution itself is obtained from the relativistic quark-exchange model. A comparison is made with the corresponding available experimental data. Finally in agreement with the data, it is demonstrated that there is no significant Q²-dependence of asymmetry A₁(x,Q²) for x ranging 0.014 ≤ x ≤ 0.25. Received: 11 September 1999 / Revised version: 8 December 1999     

Comment on “Understanding the scalar meson q\bar{q} nonet”

It is shown that the incomplete scalar meson nonets found by T?rnqvist [1] and T?rnqvist & Roos [2], employing a new version of the Helsinki unitarised quark model, should in fact be complete, including an as yet unconfirmed light below 1 GeV (old ) and the established (1500). A detailed comparison is presented with the predictions of the Nijmegen unitarised meson model, in which two complete scalar nonets show up below 1.5 GeV [3]. The reason for the flavour-nonet breaking found in [1,2] we argue to originate in the use of coupling constants for the three-meson vertex which are not independent of flavour. Also some statements made in [1] are critically reviewed. Received: 17 February 1999 / Published online: 15 July 1999     

R_b and R_{\ell} in MSSM without R-parity

We examined and couplings in the minimal supersymmetric model with explicit trilinear R-parity violating interactions. We found that the top quark couplings and can give sizable contributions through top quark loops. When deriving the bounds from and data, we also take into account the loop contributions of R-parity conserving interactions. The bounds from are found to be stronger than those from and serve as the hitherto strongest bounds for some couplings. Received: 14 October 1999 / Revised version: 2 May 2001 / Published online: 8 June 2001