Dilepton production from ρ-mesons in a quark-gluon plasma

Assuming that ρ-mesons exist in a quark-gluon plasma at temperatures close to the QCD phase transition, we calculate the dilepton production rate from q-ˉq annihilation via a ρ-meson state using Vector Meson Dominance. The result is compared to the rates from direct q-ˉq annihilation and from π⁺-π⁻ annihilation. Furthermore we discuss the suppression of low mass dileptons if the quarks assume an effective mass in the quark-gluon plasma. Received: 8 September 1999 / Revised version: 22 October 1999     

Gauge coupling and fermion mass relations in low string scale brane models

We analyze the gauge coupling evolution in brane inspired models with U(3) x U(2) x U(1)^N symmetry at the string scale. We restrict our work to the case of brane configurations with two and three abelian factors (N = 2,3) and where only one Higgs doublet is coupled to down quarks and leptons and only one to the up quarks. We find that the correct hypercharge assignment of the standard model particles is reproduced for six viable models distinguished by different brane configurations. We investigate the third generation fermion mass relations and find that the correct low energy m_b/m_τ ratio can be obtained for b-τ Yukawa coupling equality at a string scale as low as M_S~10³ TeV. Received: 30 August 2005, Published online: 16 November 2005 PACS: 11.25.Wx, 11.25.Uv, 12.10.Kt     

Test of the flavour independence of \alpha_s using next-to-leading order calculations for heavy quarks

We present a test of the flavour independence of the strong coupling constant for charm and bottom quarks with respect to light (uds) quarks, based on a hadronic event sample obtained with the OPAL detector at LEP. Five observables related to global event shapes were used to measure in three flavour tagged samples (uds, c and b). The event shape distributions were fitted by calculations of jet production taking into account mass effects for the c and b quarks. We find: Received: 18 March 1999 / Published online: 14 October 1999     

Meson cloud and SU(3) symmetry breaking in parton distributions

We apply the meson cloud model to the calculation of non-singlet parton distributions in the nucleon sea, including the octet and the decuplet cloud baryon contributions. We give special attention to the differences between non-strange and strange sea quarks, trying to identify possible sources of SU(3) flavor breaking. An analysis in terms of the parameter is presented, and we find that the existing SU(3) flavor asymmetry in the nucleon sea can be quantitatively explained by the meson cloud. We also consider the baryon, finding similar conclusions. Received: 28 July 2000 / Published online: 23 October 2000     

Quantum chromodynamics corrections to polarised deep-inelastic electron-nucleon scattering

Quantum chromodynamics corrections to orderα _s (the running coupling constant), using the quark-parton approach are calculated for the spin-dependent structure functions in deep-inelastic polarised electron-nucleon scattering. Consequences of these corrections for the Bjorken sum rule and the asymmetry in the case of longitudinally polarised (with respect to the beam) nucleons is discussed which could provide possible tests of quantum chromodynamics. Comparison of our results with the moments of the flavour non-singlet contribution to the structure functions obtained using operator product expansion is also given. An erratum to this article is available at .     

Colour dielectric model of the proton

A model of the proton with its constituent quarks bound in a colour polarizable medium with dielectric constant varying as (a/r−b ²) from a fixed centre, is presented. The Dirac equation modified by the colour polarization is solved and the analytic expression for the wavefunction of the quarks obtained shows that quarks with higher energy lie closer to the fixed centre. The energy spectrum is equispaced without any continuum. A semiclassical approximation scheme yields closed orbits for quarks which have smaller size for higher energies and no orbits with size bigger than a certain maximum, thereby rendering the quarks permanently confined. The wavefunctions of the three quarks constituting the proton are used to calculate physical parameters of the proton such as its mass, charge radius and weak coupling constant which with suitable choice of the constantsa andb appearing in the dielectric constant agree fairly well with experimental results.     

Production of top quark pairs in association with a hard gluon to order \alpha_s^2

The production of top quark pairs accompanied by a hard gluon in annihilation is studied including next-to-leading order corrections in the strong coupling. At leading order, the fraction r of events with respect to all events is computed analytically as a function of the minimal gluon energy. Next-to-leading order results for r are given for center-of-mass energies of 0.5 and 1 TeV. We further calculate the differential distribution of r with respect to several variables, including the top quark energy and the invariant mass. We then investigate how our results depend on the choice of the renormalization scheme for the top quark mass by comparing results expressed in terms of either the pole mass or the mass. Finally we estimate the sensitivity of the fraction r on the value of the running top quark mass at a scale of 1 TeV. Received: 7 April 1999 / Published online: 12 August 1999     

The fundamental-electroweak scale hierarchy in the standard model

The multiple-point principle, according to which several vacuum states with the same energy density exist, is put forward as a fine-tuning mechanism predicting the ratio between the fundamental and electroweak scales in the Standard Model (SM). It is shown that this ratio is exponentially huge: ∼e ⁴⁰. Using renormalization group equations for the SM, we obtain the effective potential in the two-loop approximation and investigate the existence of its postulated second minimum at the fundamental scale. The investigation of the evolution of the top-quark Yukawa coupling constant in the two-loop approximation shows that, with initial values of the top-quark Yukawa coupling in the interval h(M _t )=0.95±0.03 (here, M _t is the top-quark pole mass), a second minimum of the SM effective potential can exist in the region ϕ_min2≈10¹⁶−10²² GeV. A prediction is made of the existence of a new bound state of six top quarks and six antitop quarks, formed owing to Higgs boson exchanges between pairs of quarks-antiquarks. This bound state is supposed to condense in a new phase of the SM vacuum. This gives rise to the possibility of having a phase transition between vacua with and without such a condensate. The existence of three vacuum states (new, electroweak, and fundamental) solves the hierarchy problem in the SM. The text was submitted by the authors in English.     

Determination of the b quark mass at the MZ scale with the DELPHI detector at LEP

An experimental study of the normalized three-jet rate of b quark events with respect to light quarks events (light=ℓ≡u,d,s) has been performed using the CAMBRIDGE and DURHAM jet algorithms. The data used were collected by the DELPHI experiment at LEP on the Z peak from 1994 to 2000. The results are found to agree with theoretical predictions treating mass corrections at next-to-leading order. Measurements of the b quark mass have also been performed for both the b pole mass: M_b and the b running mass: m_b(M_Z). Data are found to be better described when using the running mass. The measurement yields: for the CAMBRIDGE algorithm. This result is the most precise measurement of the b mass derived from a high energy process. When compared to other b mass determinations by experiments at lower energy scales, this value agrees with the prediction of quantum chromodynamics for the energy evolution of the running mass. The mass measurement is equivalent to a test of the flavour independence of the strong coupling constant with an accuracy of 7 ‰.     

Extended iterative scheme for QCD: three-point vertices

In the framework of a generalized iterative scheme introduced previously to account for the non-analytic coupling dependence associated with the renormalization-group invariant mass scale Λ, we establish the self-consistency equations of the extended Feynman rules (Λ-modified vertices of zeroth perturbative order) for the three-gluon vertex, the two ghost vertices, and the two vertices of massless quarks. Calculations are performed to one-loop-order, in Landau gauge, and at the lowest approximation level (r=1) of interest for QCD. We discuss the phenomenon of compensating poles inherent in these equations, by which the formalism automatically cancels unphysical poles on internal lines, and the role of composite-operator information in the form of equation-of-motion condensate conditions. The observed near decoupling of the four-gluon conditions permits a solution to the 2-and-3-point conditions within an effective one-parameter freedom. There exists a parameter range in which one solution has all vertex coefficients real, as required for a physical solution, and a narrower range in which the transverse-gluon and massless-quark propagators both exhibit complex-conjugate pole pairs. Received: 1 September 1998 / Revised version: 1 December 1998     

Parametrization of a nonlocal chiral quark model in the instantaneous three-flavor case. Basic formulas and tables

We describe the basic formulation of the parametrization scheme for the instantaneous nonlocal chiral quark model in the three-flavor case. We choose to discuss the Gaussian, Lorentzian-type, Woods-Saxon, and sharp cutoff (NJL) functional forms of the momentum dependence for the form factor of the separable interaction. The four parameters, light and strange quark masses and coupling strength (G _S) and range of the interaction (Λ), have been fixed by the same phenomenological inputs: pion and kaon masses and the pion decay constant and light quark mass in vacuum. The Woods-Saxon and Lorentzian-type form factors are suitable for an interpolation between sharp cutoff and soft momentum dependence. Results are tabulated for applications in models of hadron structure and quark matter at finite temperatures and chemical potentials, where separable models have been proven successfully. The text was submitted by the authors in English.     

Electric and weak electric dipole form factors for heavy fermions in a general two Higgs doublet model

The electric and weak electric dipole form factors for heavy fermions are calculated in the context of the most general two Higgs doublet model (2HDM). We find that a large top mass can produce a significant enhancement of the electric dipole form factor in the case of the b and c quarks. This effect can be used to distinguish between different 2HDM scenarios. Received: 30 April 1999 / Revised version: 23 Juny 1999 / Published online: 14 October 1999     

The <Emphasis Type="Italic">η</Emphasis>′ meson from lattice QCD

We study the flavour-singlet pseudoscalar mesons from first principles using lattice QCD. With N _f=2 flavours of light quarks, this is the so-called η ₂ meson and we discuss the phenomenological status of this meson. Using maximally twisted-mass lattice QCD, we extract the mass of the η ₂ meson at two values of the lattice spacing for lighter quarks than previously discussed in the literature. We are able to estimate the mass value in the limit of light quarks with their physical masses.     

η′q¯q and η′NN vertex suppression in effective theories

In an effective theory containing only quark degrees of freedom, such as the extended Nambu–Jona–Lasinio model, the influence of the axial anomaly can be incorporated by a self-interaction of the 't Hooft determinant type. I will show that despite the threshold problem related to the η′ meson this leads to a significant suppression of the η′ coupling g _η′qˉq to dynamical quarks which suggests a suppression of the η′NN vertex as compared to the ηNN vertex. Received: 8 January 1999     

Measurement of the forward backward asymmetry of c andb quarks at the Z pole using reconstructed D mesons

A measurement of the forward–backward asymmetry of and on the Z resonance is performed using about 3.5 million hadronic Z decays collected by the DELPHI detector at LEP in the years 1992 to 1995. The heavy quark is tagged by the exclusive reconstruction of several D meson decay modes. The forward–backward asymmetries for c andb quarks at the Z resonance are determined to be: The combination of these results leads to an effective electroweak mixing angle of: Received: 9 February 1999 / Revised version: 26 April 1999 / Published online: 3 August 1999     

Model for Topological Fermions

 We introduce a model designed to describe charged particles as stable topological solitons of a field with values on the internal space S ³. These solitons behave like particles with relativistic properties like Lorentz contraction and velocity dependence of mass. This mass is defined by the energy of the soliton. In this sense this model is a generalization of the Sine-Gordon model¹(We do not chase the aim to give a four-dimensional generalization of Coleman’s isomorphism between the Sine-Gordon model and the Thirring model which was shown in 2-dimensional space-time) from 1 + 1-dimensions to 3 + 1-dimensions, from S ¹ to S ³. For large distances from the centre of solitons this model tends to a dual U(1)-theory with freely propagating electromagnetic waves. Already at the classical level it describes important effects, which usually have to be explained by quantum field theory, like particle-antiparticle annihilation and the running of the coupling. Received November 30, 1999; revised June 20, 2000; accepted for publication October 2, 2000