Non-perturbative determination of the running coupling constant in quenched SU (2)

Through a finite-size renormalization group technique we calculate the running coupling constant for quenched SU(2) with a few percent error over a range of energy varying by a factor thirty. The definition is based on ratio of correlations of Polyakov loops with twisted boundary conditions. The extrapolation to the continuum limit is governed by corrections due to lattice artifacts which appear to be rather smooth and proportional to the square of the lattice spacing.     

Exotic mesons and the hadronic energy momentum tensor

It is argued that SU(3)-singlet exotic (i.e. built of two or more quark-antiquark pairs) tensor mesons make large contributions to matrix elements of the energy-momentum tensor. An apparent inconsistency between meson dominance models and exchange degeneracy is thereby removed. New predictions about exotic meson couplings follw.     

Density and momentum dependence of the coupling strengths for scalar and vector fields

The collision term has been deduced based on the effective Lagrangian in addition to the mean-field. We propose the momentum- and density-dependent coupling strengths for scalar and vector fields. The saturation properties of nuclear matter, the experimental data of the energy-dependent optical potential and the mean free path in the nuclei can be reproduced simultaneously. The in-mediumn–n cross section has also been calculated for different densities and energies, our results are consistent with Dirac-Brueckner-Hartree-Fock calculations.Supported by National Natural Science Foundation of China     

On transverse momentum event–by–event fluctuations in string hadronic models

Transverse momentum event–by–event fluctuations are studied within the string–hadronic model of high energy nuclear collisions, LUCIAE. Data on non–statistical fluctuations in p+p interactions are reproduced. Fluctuations of similar magnitude are predicted for nucleus–nucleus collisions, in contradiction to the preliminary NA49 results. The introduction of a string clustering mechanism (Firecracker Model) leads to a further, significant increase of fluctuations for nucleus–nucleus collisions. Secondary hadronic interactions, as implemented in LUCIAE, cause only a small reduction of fluctuations. Received: 23 October 1998 / Published online: 11 March 1999     

Medium dependence of the bag constant in the quark-meson coupling model

Possible variations of the quark-meson coupling (QMC) model are examined in which the bag constant decreases in the nuclear medium. The reduction is supposed to depend on either the mean scalar field or the effective mass of the nucleon. It is shown that the electric and magnetic radii of the bound nucleon are almost linearly correlated with the bag constant. Using the fact that the size of the bound nucleon inside a nucleus is strongly constrained by y-scaling data in quasi-elastic, electron-nucleus scattering, we set a limit for the reduction allowed in the bag constant for these two models. The present study implies that the bag constant can decrease up to 10–17% at average nuclear density, depending on the details of the model.     

Measurements of the strong coupling constant and the QCD colour factors using four-jet observables from hadronic Z decays

Data from annihilation into hadrons, taken with the ALEPH detector at the Z resonance, are analyzed. The four-jet rate is studied as a function of the resolution parameter and compared to next-to-leading order calculations combined with resummation of large logarithms. Angular correlations in four-jet events are measured and compared to next-to-leading order QCD predictions. With these observables two different measurements are performed. In a first analysis the strong coupling constant is measured from the four-jet rate yielding . In a second measurement the strong coupling constant and the QCD colour factors are determined simultaneously from a fit to the four-jet rate and the four-jet angular correlations, giving in good agreement with the expectation from QCD. Received: 14 May 2002 / Published online: 15 January 2003     

Bounds on the coupling constant in two dimensional boson models

We consider two dimensional boson field theories with an interaction potentialV (). We show how to define a cut-off, renormalised Hamiltonian for a certain class of non-polynomialV (), which are defined via an integral transform. We formulate precisely a variational argument devised by Coleman, obtaining a constraint on the coupling constant of the theory with generalV (), and illustrate the argument with several examples.     

The transverse momentum dependence of quark fragmentation functions from cascade models

A covariant generalization of the one-dimensional cascade model for quark fragmentation functions is presented, so as to include the transverse momentum behaviour and the possibility to produce different particles at different vertices along the chain. In the scaling limit the exact solution is given, if the primordial function is of the type αz ^α?1·T(p _T ). For the more general case of factorizing primordial functions an analytic expression for the seagull effect is derived, which turns out to be independent of the functionT(p _T ).     

Transverse momentum of muon pairs produced in hadronic collisions

The observed p_⊥ distributions of large mass muon pairs produced in proton-nucleus collisions are quantitatively explained in QCD by also explicitly taking into account the intrinsic wave function p_⊥ spread of partons inside the hadrons. The experimental data indicate 〈p_⊥〉 ～ 500–600MeV for each parton.     

On the scale dependence of the dimension of hadronic matter

We advance the notion of a dynamical, scale-dependent Hausdorff dimension for hadronic matter. The physical motivations behind such a concept are explained. For very small scales, we relate this dimension to the anomalous dimension of the product of two momentum densities. For hadrons, we rely on the precocious onset of Bjorken scaling in e-p inelastic scattering and a heuristic parton model to obtain the Hausdorff dimensional profile for the proton d(q²) as a function of the probing photon momentum q². We find that d reaches a maximum around d = 1 for a range of low q² values. This feature is indicative of a string-like structure of the hadrons at low momentum transfers.     

An inequality onS wave bound states,with correct coupling constant dependence

We prove that the number ofS wave bound states in a spherically symmetric potentialgV(r) is less than 1 $$g^{1/2} \left[ {\int\limits_0^\infty {r^2 V^ - (r)dr} \int\limits_0^\infty {V^ - (r)dr} } \right]^{1/4}$$ whereV ^? is the attractive part of the potential, in units where ?²/2M=1.     

Oxygen dependence of the fermi-level and electron-phonon coupling constant in YBa2Cu3Ox films

We present a detailed investigation of the dynamics of laser-excited carriers in YBa₂Cu₃O_x thin films with various oxygen contents x. We observe a Fermi level shift due to oxygen reduction and determine the energy gap between the Fermi level and the upper Hubbard band. The electron-phonon coupling constant is determined as a function of transition temperature.     

The vacuum energy forP(ø)2: Infinite volume limit and coupling constant dependence

LetE(g) be the vacuum energy for theP(ø)₂ Hamiltonian with space cutoffg(x)0 and coupling constant 0. For suitable families of cutoffsg1, the vacuum energy per unit volume converges; i.e., –E(g)/g(x)dx (). We obtain bounds on the dependence of () for large and small . These lead to estimates forE(g) as a functional ofg that permit a weakening of the standard regularity conditions forg. Typical of such estimates is the linear lower bound, –E(g)const g(x)² dx, valid for allg0 provided thatP is normalized so thatP(0)=0. Finally we show that the perturbation series for () is asymptotic to second order.Research partially supported by AFOSR under Contract No. F 44620-71-C-0108.Postal address after September 30, 1972: via A. Falcone 70, 80127, Napoli, Italy.     

Embedding of non-simple Lie groups,coupling constant relations and non-uniqueness of models of unification

A general derivation of the coupling constant relations which result on embedding a non-simple group like SU _L (2) ⇔ U(1) in a larger simple group (or graded Lie group) is given. It is shown that such relations depend only on the requirement (i) that the multiplet of vector fields form an irreducible representation of the unifying algebra and (ii) the transformation properties of the fermions under SU _L (2). This point is illustrated in two ways, one by constructing two different unification groups containing the same fermions and therefore have same Weinberg angle; the other by putting different SU _L (2) structures on the same fermions and consequently have different Weinberg angles. In particular the value sin²ϑ=3/8 is characteristic of the sequential doublet models or models which invoke a large number of additional leptons likeE ₆, while addition of extra charged fermion singlets can reduce the value of sin² ϑ to 1/4. We point out that at the present time the models of grand unification are far from unique.     

Gluon-exchange currents: The nucleon axial-vector coupling constant and magnetic moments in bag models

Possible numerical importance of the two-body currents arising from exchange of one hard gluon between two constituent quarks is investigated by studying the evaluation of the nucleon axial-vector coupling constantg _A and magnetic moments μ _p ^tot , μ _n ^tot in the context of the MIT bag model. The double counting problem is resolved only to the extent that, in accord with the standard wisdom, hard gluons have nothing to do with the formation of bags (quark confinement) and so are not included in bag-model wave functions. For reasonable choices of the parameters, we find that (1) the contribution tog _A arising from one-gluon-exchange currents (OGEC) could account for the discrepancy between the observed value and the one-body bag-model prediction, and that (2) whereas the OGEC contribution to μ _n ^tot is negligible, the contribution to μ _p ^tot could be important but does not allow further understanding of the observed μ _p /μ _n ratio.     

On the momentum dependence ofJ/ψ suppression

The formation of deconfined matter in high energy collisions is expected to result in reducedJ/ψ production. The finite space-time extension of any quark-gluon plasma thus produced leads to a rather characteristic dependence ofJ/ψ suppression on the longitudinal and transverse momentum of theJ/ψ. We study in particular the longitudinal momentum dependence and discuss its implications for nuclear collision experiments.