Perturbative QCD and weak asymmetries in lepton pair production

The interference of electromagnetic and weak production mechanisms for lepton pair production may give rise to several effects which violate parity and charge symmetry. These effects are generally of the order of 1% for dilepton masses of 10 GeV. The theoretical calculations presented here show that experimental studies of these asymmetries may be useful. In particular, measurements of these asymmetries in collisions of pions with polarized protons may lead to a greatly enhanced understanding of the polarization distribution of quarks in a polarized proton. The polarization structure of the d quark is shown to be of special interest. Measurement of the parity-violating asymmetries in proton-polarized proton collisions may prove to be a sensitive probe of the flavor symmetry of the proton antiquark sea. Analysis of the parity conserving charge asymmetry (which is predicted to occur in collisions of unpolarized hadrons) allows a unique further test of the Drell-Yan model for lepton pair production, as well as of our understanding of weak interactions.The above asymmetries mentioned are computed within the framework of the parton model. The focus of this work is on the asymmetries calculated with the inclusion of first-order perturbative QCD effects. The asymmetries are calculated in a differential form for values of the dilepton transverse momentum large compared with the typical (～1 GeV) scale of nonperturbative effects, and also in a form in which this transverse momentum has been integrated over. Corrections to the parton model results, which can be large, show rather different structure for the various asymmetries.A parity-violating asymmetry which may occur in collisions of unpolarized hadrons is also discussed. This asymmetry is very sensitive to the nonperturbative structure of hadrons, and is estimated to be approximately 0.01% for dilepton masses of 10 GeV.     

THE QUARK CLUSTER MODEL OF NUCLEAR FORCE:(Ⅱ) QUARK CONFINEMENT POTEPNTIAL AND COLOR VAN DER WAALS FORCE

The color-analog Van der Waals force between two hadrons is studied by the coupling channel resonating group method in the framework of the Gaussian-type quark confinement potential. The problem of the boundary values for the two channel coupling differential equations is changed to the problem of the initial values. The equations are solved numerically by the Gill method. The calculated results show that there is no color Van der Waals force between hadrons in the confinement potential model. This indicates that the confinement potential model not only can describe the internal structure of hadrons but also can be used to calculate the hadron-hadron interactions.     

Interactions of heavy stable hadronising particles

In this article, we study the interactions of stable, hadronising new states, arising in certain extensions of the standard model. A simple model, originally intended for stable gluino hadrons, is developed to describe the nuclear interactions of hadrons containing any new colour triplet or octet stable parton. Hadron mass spectra, nuclear scattering cross sections and interaction processes are discussed. Furthermore, an implementation of the interactions of heavy hadrons in GEANT 3 is presented, signatures are studied, and a few remarks about possible detection with the ATLAS experiment are given.Received: 2 April 2004, Published online: 6 August 2004     

Lepton-hadron relation and the electromagnetic form factor of deuteron

The electromagnetic form factor of deuteron is calculated here on the basis of a model of hadrons which incorporates a particular lepton-hadron relation. In view of this model, the pionic electromagnetic form factor is found to be the main contributing factor and when double scattering effect as envisaged by Glauber is taken into account, the results are found to be in nice agreement with experiments.     

Electromagnetic corrections in hadronic processes

In many applications of chiral perturbation theory, one has to purify physical matrix elements from electromagnetic effects. On the other hand, the splitting of the Hamiltonian into a strong and an electromagnetic part cannot be performed in a unique manner, because photon loops generate ultraviolet divergences. In the present article, we propose a convention for disentangling the two effects: one matches the parameters of two theories - with and without electromagnetic interactions - at a given scale , referred to as the matching scale. This method enables one to analyse the separation of strong and electromagnetic contributions in a transparent manner. We first study in a Yukawa-type model the dependence of strong and electromagnetic contributions on the matching scale. In a second step, we investigate this splitting in the linear sigma model at one-loop order, and consider in some detail the construction of the corresponding low-energy effective Lagrangian, which exactly implements the splitting of electromagnetic and strong interactions carried out in the underlying theory. We expect these model studies to be useful in the interpretation of the standard low-energy effective theory of hadrons, leptons and photons.Received: 28 August 2003, Published online: 20 November 2003     

A phenomenological quark matter equation of state

Both the MIT bag model and potential models are able to reproduce static properties of hadrons (e.g. their mass spectrum) with reasonable accuracy. However, while the extrapolation of the MIT bag model from hadrons to dense matter is rather straightforward, it is not the same for potential models. To deal with this problem, in this paper, the methods of relativistic quantum many-body theory are applied to the study of quark matter interacting through an instantaneous potential at zero temperature. It is shown that under some conditions, the quark plasma undergoes a first order transition from a massive state at low density to a gas of particles of decreasing mass at high density—as one expects from perturbative QCD. In addition, immediately after the transition, the quarks are in a collective bound state, which might perhaps be interpreted as the fact that they just start to leave the inside of hadrons.     

Inclusive production of charged hadrons and {\rm K}^0_{\rm S} mesons in photon–photon collisions

The production of charged hadrons and mesons in the collisions of quasi-real photons has been measured using the OPAL detector at LEP. The data were taken at centre-of-mass energies of 161 and 172 GeV. The differential cross-sections as a function of the transverse momentum and the pseudorapidity of the charged hadrons and mesons have been compared to the leading order Monte Carlo simulations of PHOJET and PYTHIA and to perturbative next-to-leading order (NLO) QCD calculations. The distributions have been measured in the range GeV of the hadronic invariant mass W. By comparing the transverse momentum distribution of charged hadrons measured in interactions with -proton and meson-proton data we find evidence for hard photon interactions in addition to the purely hadronic photon interactions. Received: 4 June 1998 / Published online: 6 November 1998     

Static spherically symmetric anisotropic fluid distribution in presence of electromagnetic field

Einstein’s field equations of general relativity corresponding to the anisotropic (principal stresses unequal) static fluid sphere in presence of electromagnetic field have been solved exactly. The integration constants are determined by matching the obtained solution with the Reissner-Nordström solution over the boundary. It has been found that the flaid model has non-negative expression for mass density and pressure. The mass density and stresses are everywhere regular and monotonically decreasing functions of the radial coordinate.     

Predicted new class of hadrons and the average parton charge

A semiempirical mass formula of a particular parton model, in which it is assumed that partons are highly charged, requires the existence of a new class of hadrons, at least one of which must be absolutely stable and whose masses are of the order of a few GeV. Several experiments (with accelerators and cosmic rays) appear to support the notions that partons are highly charged and that a new class of hadrons exists. The recently reported particle of Niu et al., with an apparent lifetime ～10^?13 sec, may be a member of the new predicted class.     

Irreducible representations of a parafield and the connection of the parafield with usual fields

All irreducible separable representations of the non-relativistic para-Fermi field of order 3 in the configuration space are considered. The existence of many different irreducible representations of the parafield permits us to interpret the excited states of this field as the states of particles with internal degrees of freedom. These states can be labelled by the Young patterns and the eigenvalues of internal quantum number like baryonic and hypercharges. The parafield theory is shown to be equivalent to the theory of three kinds of ordinary ferminons, like quarks, and one of them, ‘strange’, can be distinguished from the other two by means of its interaction, not only statistically but also dynamically. Thus the parafield theory is shown to be equivalent to some model of the physicalSU (3) symmetry of hadrons when the strong and medium-strong interactions could be switched on but the electromagnetic and weak interactions should be switched off.     

Restoring universality

In models of weak and electromagnetic interactions with six or more quarks, universality and, in some cases, the V-A structure of leptonic and semi-leptonic interactions have to be put in by hand. We show how both properties remerge naturally if replacing the usual SU(2) weak isospin group by a large SU(4) group, which also lends itself quite naturally to grand unifications (including strong interactions). The model needs eight quarks and two charged heavy leptons, and features, nesides universality and V-A, non-equality of neutral current scattering of ν's on leptons and on hadrons, and very reasonable relations for (bare) fermion masses.     

Chiral-invariant phase space model

The masses of the SU(3)×SU(6) hadrons are calculated in the chiral-invariant phase space (CHIPS) model as a sum of the mean energies of the quarks at a constant temperature T _c with the color-magnetic splitting and the color-electric shift. The masses of hadrons are parametrized by four constants: T _c, m_s, E _CE and A _CM. With the same number of parameters the CHIPS model fits the masses of hadrons better than the classic bag model. The small mass of the d-quark ( m _d = 2.7MeV) is used to prove that the isotopic shifts of hadrons can be explained by the mass difference between the d- and u-quarks. The dibaryon mass is estimated in CHIPS to be 200MeV higher than in the bag model. The prediction for the mass of the α^* cluster is about the same in both models. It is close to 4 ^. m _Δ. Received: 12 December 2001 / Accepted: 23 May 2002     

Photoproduction and electroproduction

Various aspects of the electromagnetic interactions of hadrons (strongly interacting particles) are reviewed in these lectures. After a discussion of the properties of the electromagnetic current of the hadrons and the idea of vector meson dominance, the general features of photoproduction cross sections are presented and compared with the very similar behavior of purely strong interaction processes. Given this close similarity in behavior, particular photon induced reactions are then considered in some detail, illustrating the application of theoretical ideas used in treating both strong and electromagnetic processes. The particular subjects discussed in some detail are photoproduction of pions at low energy and partial wave analysis, Compton scattering, vector meson photoproduction and tests of the vector dominance model, and photoproduction of charged pions at high energy. In the second half of the lectures inelastic electron-nucleon scattering is the principal topic. After a presentation of the kinematics and structure functions of the nucleon, the principal results of the inelastic electron scattering experiments are given together with the evidence for the remarkable scaling behavior of the structure functions. This leads to a presentation of the parton model of point constituents of the nucleon and the interpretation of the experimental results in terms of the properties of the constituent partons. A different view of inelastic electron scattering in terms of strong interaction ideas is presented in the last two lectures. These concern the high energy behavior of the scattering and the application of duality concepts which tie the behavior of nucleon resonance electroproduction to the behavior of the deep inelastic scattering.     

Electromagnetic backscattering from one-dimensional drifting fractal sea surface I: Wave-current coupled model

To study the electromagnetic backscattering from a one-dimensional drifting fractal sea surface, a fractal sea surface wave-current model is derived, based on the mechanism of wave-current interactions. The numerical results show the effect of the ocean current on the wave. Wave amplitude decreases, wavelength and kurtosis of wave height increase, spectrum intensity decreases and shifts towards lower frequencies when the current occurs parallel to the direction of the ocean wave. By comparison, wave amplitude increases, wavelength and kurtosis of wave height decrease, spectrum intensity increases and shifts towards higher frequencies if the current is in the opposite direction to the direction of ocean wave. The wave-current interaction effect of the ocean current is much stronger than that of the nonlinear wave-wave interaction. The kurtosis of the nonlinear fractal ocean surface is larger than that of linear fractal ocean surface. The effect of the current on skewness of the probability distribution function is negligible. Therefore, the ocean wave spectrum is notably changed by the surface current and the change should be detectable in the electromagnetic backscattering signal.     

Deep inelastic lepton-nucleon scattering without point-like constituents

Neutrino- and antineutrino-nucleon scattering into hadrons is analyzed and related to electron-nucleon scattering according to a scaling model which has been recently proposed. The electromagnetic and weak structure functions are determined and explicitly given in terms of the πN total cross sections. Predictions are made in good agreement with experiments.     

Hadronization from color interactions

A quark coalescence model, based on semi-relativistic molecular dynamics with color interactions among quarks, is presented and applied to pp collisions. A phenomenological potential with two tunable parameters is introduced to describe the color interactions between quarks and antiquarks. The interactions drive the process of hadronization that finally results in different color neutral clusters, which can be identified as hadrons based on some criteria. A Monte Carlo generator PYTHIA is used to generate quarks in the initial state of hadronization, and different values of tunable parameters are used to study the final state distributions and correlations. Baryon-to-meson ratio, transverse momentum spectra, pseudorapidity distributions and forward-backward multiplicity correlations of hadrons produced in the hadronization process, obtained from this model with different parameters, are compared with those from PYTHIA.     

A space-time description of quarks and hadrons

A more concrete formulation of the previously developed massive quark model (MQM) is given by describing hadrons as finite space-time domains (bags) in which simple quark systems can be found. A well-defined and appealing picture of hadrons and their interactions emerges.     

Multiplicity distributions in a thermodynamical model of hadron production ine + e − collisions

Predictions of a thermodynamical model of hadron production for multiplicity distribution ine ⁺ e ^? annihilations at LEP and PEP-PETRA centre of mass energies are shown. The production process is described as a two-step process in which primary hadrons emitted from the thermal source decay into final observable particles. The final charged tracks multiplicity distributions turn out to be of Negative Binomial type and are in quite good agreement with experimental observations. The average number of clans calculated from fitted Negative Binomial coincides with the average number of primary hadrons predicted by the thermodynamical model, suggesting that clans should be identified with primary hadrons.