Semileptonic electroweak interactions with nucleons and nuclei

The basic formalism needed to describe parity-violating electron scattering from nucleons and nuclei is presented and the experiments anticipated in the next few years discussed. The underlying structure of the relevant leptonic and hadronic tensors is developed in some detail for both parity-conserving and -violating situations to highlight the new physics that becomes accessible in the latter case. When applying the basic formalism to specific (practical) examples, the theme of determining the strangeness content of the nucleon is stressed.Lectures given at the Indian-Summer School on Electron Scattering from Nucleous and Nuclei, Prague (The Czech Republic), September 1994.This work is supported in part by funds provided by the U. S. Department of Energy (D.O.E.) under cooperative agreement #DE-FC02-94ER40818.     

Covariant perturbation expansion in chiral theories with pions and nucleons

A covariant perturbation scheme is developed to give a coordinate independent perturbation expansion of the chiral invariant pion model with nucleons. On the mass shell the covariant approach is shown to be equivalent to the standard perturbation theory.     

Generalized local gauge symmetry and the Ward-Takahashi identities in unified field theories

We discuss the symmetry basis of unified field theories, i.e., the generalized concept of local gauge symmetry, and its physical implications. The generalized Ward-Takahashi identities and the explicit constraints among renormalization constants are derived by using the path integral in a specific model. These constraints are confirmed at the one-loop level.Work supported in part by the U.S. Energy Research and Development Administration.     

Energy spectra of cosmic pions and nucleons at the top of the atmosphere

The production spectra of cosmic pions and their primaries have been derived from the experimental muon spectrum by using the theories of Barrettet al. and Feynman. The calculated pion production spectrum takes the formI(E _π)d E _π = 0.206E _π ^{? 2.65} d E _πper cm² sec str GeV in the energy range 3<E _π<1500 GeV and the nucleon spectrum follows the relationN(E _p)d E _p = 2.7 × 10⁴ (E _p/GeV)^2.65 d E _pper m² sec str GeV in the range 5 <E _p <3000 Gev.     

The transparency of nuclei to nucleons and pions in a relativistic Glauber approximation

We present a selection of results obtained within the context of a relativistic eikonal model. First, results of relativistic Glauber calculations for the nuclear transparency extracted from photon-induced pion production are presented. Second, computed differential cross-sections for the ¹² C(p, 2p) are compared to data.     

Transverse Ward-Takahashi identities and full vertex functions in different representations of QED_3

We derive the transverse Ward-Takahashi identities(WTI) of N-dimensional quantum electrodynamics by means of the canonical quantization method and the path integration method, and subsequently attempt to prove that QED_3 is solvable based on the transverse and longitudinal WTI, indicating that the full vector and tensor vertices functions can be expressed in terms of the fermion propagators in QED_3. Further, we discuss the effect of different γ matrix representations on the full vertex function.     

Microscopic description of the absorption of bound pions

The absorption rates of bound 1s and 2p pions in some light nuclei have been investigated on the basis of a shell model picture modified by short range nucleonnucleon correlations. It turns out that the total absorption rate, the missing energy spectra and the single neutron energy spectra depend sensitively on the details of the correlation functions. The results of the present analysis indicate that one can obtain informations on the short range correlations from pion absorption experiments.     

On pole terms in the fixed angle approach to photoproduction of pions on nucleons

This note has two aims. The first one is to study the properties of well known pole contributions to photoproduction amplitudes but in the space of c.m. three-momentum squared and scattering angle. Some conclusions essential to the fixed angle approach are drawn. The second aim is to give some results of more general character. These are plots and numerical tables of pole contributions to multipole amplitudes.     

Inclusive π+ to π− ratios and quark distributions of pions and nucleons

The ratio R of inclusive $\text{π}{}^{\mathrm{+}}\text{π}{}^{\mathrm{?}}$ production at low transverse momenta is measured in π⁺ n and pn interactions at 195 GeV/c.$\text{R (}\text{1}\text{R}\text{)}$ in the proton (neutron) fragmentation region increases with Feynman-x and is smaller than R from π⁺ fragmentation. The results agree with a quark-quark scattering model originally proposed for “hard” processes and disagree with a modified version of the model which includes gluon scattering.     

Chemical potentials and high temperature phase transitions in electroweak theory

The phenomena of electroweak phase transitions in the early universe depend crucially on the inclusion of the charge corresponding to the spontaneously broken symmetry, which is usually taken as classically conserved. This paper is devoted to the study of the electroweak phase transitions at high temperatures and, correspondingly, to the controversy in the literature concerning the question of the inclusion of the weak neutral charge in the standard model, for the investigation of the phase transition ofW-boson condensation and symmetry restoration in the early universe. Several arguments, general and perturbative, are given against the introduction of the weak neutral charge conservation in the nonsymmetric phase. As a consequence of the above, theW-condensation should occur even at low lepton densities and the symmetry restoration critical curve is independent of the lepton density. The use of different gauges such as the renormalizable and the unitary ones is also discussed.

     

Electromagnetic form factors for nucleons and pions at positive and negative q 2 in the model of quark-gluon strings

The electromagnetic form factors for pions and nucleons are considered within the model of quark-gluon strings, where the momentum-transfer dependence of hadronic form factors is determined by the intercepts of the corresponding Regge trajectories and by the Sudakov form factor. Analytic expressions found for form factors in the timelike region admit an analytic continuation to the spacelike region. The resulting form factors for pions and nucleons comply well with experimental data both for positive and for negative values of the squared momentum transfer q ². It is shown that the distinctions between the absolute values of the pion and nucleon form factors F _π(q ²), G _m (q ²), and F ₂(q ²) at positive values of q ² and those at negative values of this variable are associated with the analytic properties of the double-logarithmic term in the exponent of the Sudakov form factor. The spin structure of the amplitudes for quark transitions into hadrons that is proposed in the present study makes it possible to describe fairly well available experimental data on the Pauli form factor F ₂ and on the ratio G _e /G _m .     

Strong electroweak phase transitions in the Standard Model with a singlet

It is well known that the electroweak phase transition (EWPhT) in extensions of the Standard Model with one real scalar singlet can be first-order for realistic values of the Higgs mass. We revisit this scenario with the most general renormalizable scalar potential systematically identifying all regions in parameter space that develop, due to tree-level dynamics, a potential barrier at the critical temperature that is strong enough to avoid sphaleron wash-out of the baryon asymmetry. Such strong EWPhTs allow for a simple mean-field approximation and an analytic treatment of the free-energy that leads to very good theoretical control and understanding of the different mechanisms that can make the transition strong. We identify a new realization of such mechanism, based on a flat direction developing at the critical temperature, which could operate in other models. Finally, we discuss in detail some special cases of the model performing a numerical calculation of the one-loop free-energy that improves over the mean-field approximation and confirms the analytical expectations.     

Influence of the time-step on the production of free nucleons and pions from heavy-ion collisions around 1 GeV/nucleon

By considering different values of the time-step for the potential updates in the ultra-relativistic quantum molecular dynamics(UrQMD) model, we examine its influence on observables, such as the yield and collective flow of nucleons and pions from heavyion collisions around 1 GeV/nucleon. It is found that these observables are affected to some extent by the choice of the time-step,and the impact of the time-step on the pion-related observables is more visible than that on the nucleon-related ones. However,its effect on the π-/π+yield ratio and elliptic flow difference between neutrons and protons, which have been taken as sensitive observables for probing the density-dependent nuclear symmetry energy at high densities, is fairly weak.