The neutral weak current of the nucleon

A brief review of the structure of the nucleon as determined through parity-violating electron scattering is presented. The relationship between the electromagnetic and neutral weak currents is developed in a pedagogical introduction. Recent results and present measurements are discussed.     

Single-pion production and the structure of the weak neutral current

We investigate the restrictions on the structure of the weak neutral current imposed by single pion production cross sections on single nucleons. A general vector (V), axial-vector (A) neutral current with |ΔI|?1 is assumed, where the isovector V,A neutral currents are the neutral members of the isotriplets containing the charged weak currents. From neutrino cross sections alone we derive bounds for the neutral current coupling constants. These bounds supplement the known constraints from inclusive scattering in a very useful way. More specific assumptions about the isoscalar neutral current are also considered. We discuss the resulting bounds using the existing neutrino data. Finally, it is shown that with the advent of antineutrino data for single pion production the neutral current coupling constants will be determined uniquely.     

Universality and the weak neutral current

We consider the possibility that the weak neutral current displays both muon-electron and quark-lepton universality. Such a model contains three parameters which we determine, within limits, from existing data on deep inelastic scattering of neutrinos off nucleons and elastic scattering of neutrinos off leptons. We show that such a model is consistent with other results on elastic neutrino-nucleon scattering and neutrino-induced production of single pions.     

About the isoscalar piece of the weak neutral current

The available neutral current induced one-pion production data at intermediate energy are analyzed in a model-independent way in the framework of a recent approach to the corresponding production mechanism. Clear indications about the structure of the isoscalar piece of the weak neutral current are obtained and more stringent evidence of agreement with the standard WS model is seen to emerge. For the first time the relative influence of the different amplitudes contributing to the production mechanism is analyzed, by adopting the neutral current parametrization provided by the WS model.     

The neutral weak current in muonic atoms

The measurement of parity-odd observables in muonic atoms has eluded realization for ten years. Nevertheless, determination of the coupling constants which characterize the weak neutral current of muons remains an important goal. Since the muonic atom is essentially an hydrogen-like system, the precise determination of neutral current couplings and observation of higher order weak and electromagnetic corrections may be possible. Moreover, the effects of neutral currents in atomic systems are determined by a different combination of couplings than effects measured at high energy. Finally, comparison of effects in muonic and conventional atoms would confirm the university of neutral current couplings.Current proposals to measure parity-odd observables exploit the physics of nuclei and atoms as well as of muonic atoms. This report attempts a systematic presentation of the physics required to understand the proposals and the interpretation of the measurements. The three most promising measurements in thulium, neon and boron are discussed in detail.     

An analysis of the gargamelle freon data concerning pion production by the weak neutral current

Semi-inclusive pion production of all three charge states by the weak neutral current has been analysed in the Gargamelle Freon neutrino and antineutrino experiments. Using the complete neutral current sample a κ² fit of the measured charge ratios to the hypothesis of a purely isoscalar current has a confidence level of less than 10^?4. Using a restricted pion sample, corresponding to current fragment pions, a κ² fit of the π⁺/π^- ratios in νv and $\text{ν}$ to the predictions of an isovector current has confidence level 0.017. The sign of the isovector-isoscalar interference term is found to be consistent with that predicted by the Weinberg-Salam model.     

Testing the structure of weak neutral currents by inelastic neutrino scattering from nuclei

By using selected nuclear transitions specific pieces of the weak neutral current may be greatly enhanced, leading to widely different results for different models of the weak neutral current. Predictions for low-energy inelastic neutrino scattering from ¹²C are examined within the framework of a variety of SU(2)_W × U(1) gauge theory models.     

On the structure of the hadronic neutral current

The coupling constants of the hadronic neutral current are determined independently of any gauge models. Essentially two solutions emerge from this analysis: either the isovector neutral current is predominantly vector and the isoscalar component predominantly axial-vector or vice versa.     

General bounds on the isovector coupling constants of the weak neutral current

We show that experimental data on inclusive neutrino reactions can be used to obtain general bounds on the coupling constants of the isovector part of the hadronic weak neutral current provided this isovector current is related to the charged current by isospin rotation. These bounds are free from the assumption of a specific model for the neutral current as well as any dynamical assumption on the hadronic structure functions. We derive upper bounds on the coupling constants which involve only the cross sections for isospin-averaged nucleon target as well as lower bounds which require a knowledge of the cross sections for proton and neutron separately.     

Parity conserving weak neutral current effects in fundamental bound systems

The effects induced by the Z-boson exchange in the hyperfine structure and Lamb shift of some fundamental bound systems is analyzed. The hfs of muonium appears as a promising test of the Weinberg-Salam theiry.     

Nuclear excitation by neutral weak currents

The use of neutrino excitation of nuclear levels to study weak neutral currents is examined.     

Charm production by weak neutral currents

We discuss quantitatively the production of charm in $\text{v}\text{N and}\text{v}\text{N}$ neutral current interactions, and the anomalous lepton events that follow from semi-leptonic charm decay. Diagonal neutral currents, in the Weinberg-Salam model and similar models, predict associated charm production with small cross sections: e.g. $\text{σ(v}\text{N}\text{→ vcc}{}^{\mathrm{?}}\text{X}\text{)/σ(v}\text{N}\text{→ μ}{}^{\mathrm{?}}\text{X}\text{) ? 10}{}^{\mathrm{?2}}$ at high energy. The meagre data on vN → ve⁺X are consistent with a rate of this order. Non-diagonal neutral currents, if present, could give larger cross sections via valence p → c transitions. It should be possible to distinguish diagonal from non-diagonal contributions by their x- or u-dependences, where u = x(1 ? y). We calculate the expected energy distributions of the leptons in characteristic vN → v?⁺X and vN → v?⁺?^?X charm decay events using simple models, and discuss some practical problems in neutral current measurements.     

Unique solution for the weak neutral current coupling constants in purely leptonic interactions

By combining results from the MARK-J at PETRA on Bhabha scattering, μ⁺μ^- and τ⁺τ^- production with recent world data from neutrino-electron scattering experiments, we determine unique values for the leptonic weak neutral current coupling constants g_V and g_A in the framework of electroweak models containing a single Z⁰. In contrast to previous analyses, we only use data from purely leptonic interactions, and therefore avoid the inherent uncertainties resulting from the use of hadronic targets. From the MARK-J data alone in the context of the standard SU(2) ? U (1) model of Glashow, Weinberg and Salam, we find sin²θ_W=0.24±0.11.     

Composite structure of the nucleon

The nonlinear -model with vector mesons introduced as gauge bosons and an interacting chiral quark sector depicts the nucleon as a topological soliton embedded in a chiral condensate. High-energy elastic pp and ¯pp scattering data from CERN ISR and SPS Collider appear to provide strong evidence in favor of this nucleon structure.Dedicated to Fritz Rohrlich for his lifelong devotion to physics.This work was supported in part by the U.S. Department of Energy.     

The structure of the nucleon

In this talk the current status of our knowledge of the structure of the nucleon will be reviewed as revealed in deep inelastic scattering. The current data on the structure functions of the nucleon both diffractive and non-diffractive will be discussed.     

Spin structure of the nucleon

Based on an infinite sublayer quark model, we determine the fraction of nucleon spin carried by subquarks.     

The structure of the nucleon

We assume that nucleons are made of quarks which are made of subquarks which are made of more fundamental subquarks, etc. Thus, finally, the proton and the neutron may be composed of an infinite number of pointlike quarks and antiquarks. The limit particle has quantum numbers of spinJ=1/2, isospinI=1/2, third component of isospinI ₃=1/2, and fractional electric chargeQ=(l/2)¦e¦, where ¦e¦ is the electron charge. All quantum numbers are thus just one-half and this fermion will behave as if it was lepton, since the baryon number approaches zero at an infinite sublayer level. Sum rules in lepton-nucleon scattering have been evaluated using this model. The predicted values are not incompatible with the experimental results.