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.     

Neutral current effects in the parity violating nuclear force

The measurements of the ΔI = 1 part of the parity violating nuclear force when combined with information on neutral current couplings from neutrino scattering and pion production experiments allow an estimate of the isoscalar admixture in the neutral current if it has a vector axial-vector structure has been shown.     

Phase space and multihadron production in annihilation processes at present energies

With the conjecture that the weak neutral current should have equal status in symmetry as the other weak and electromagnetic currents, we determine it to be a pure axial-vector. This means that the weak interaction of the neutral current is parity conserving. The fundamental interactions of leptons thereby complete the whole scheme of (1 + γ₅)/2, (1 ? γ₅)/2,1 and γ₅.     

Parity violating flavour changing neutral current and natural conservation of strangeness in vector-like theories

Two vector-like schemes of the weak interactions, based on five and seven quarks, respectively, are studied in which the suppression of |ΔS| = 1 neutral currents is natural and in which the neutral current interaction violates parity. Various aspects (parity violation, flavour changing neutral current, “$\text{ν}$ anomaly”, masses of the W and Z bosons) are investigated.     

A new way of detecting tau neutrinos

In this letter, a new kind of neutrino data analysis is proposed, which considers the special kinematic properties of charged current tau neutrino interactions which stimulate neutral currents due to hadronic decay of the tau lepton. It is shown that the differential cross section, with respect to a suitable scaling variable, of “true” neutral currents differs significantly from tau induced events. This analysis is shown to have good sensitivity for the presence of ν_gt interactions.     

Pion production by fragmentation of weak currents in Weinberg theory

The Feynman quark parton model is used to study inclusive single-pion production by the fragmentation of the neutral and charged weak currents of Weinberg theory. The structure functions for the neutral current-induced reactions are related to those for the electromagnetic and charged weak current processes. Analogues of inclusive deep-inelastic sum rules are derived. The ratios of neutral current to charged current cross sections for semi-inclusive pion production are studied. In the approximation of neglect of “core” partons, these ratios are given in terms of average pion multiplicities in the charged current-induced reaction. We finally specialise to the quark parton distributions of McElhaney and Tuan to calculate these ratios as functions of the Weinberg angle.     

A Migdal-Kadanoff renormalization-group approach for the XY spin glass

Superconducting rings with mesoscopic Josephson junctions are considered in the presence of a do voltage bias V _o and of non-classical electromagnetic fields (coherent states, squeezed states, number eigenstates etc.). Due to the quantum nature of these devices the current I is a quantum mechanical operator and therefore «I²» is in general different from «I»². Using «I ²» we define the rms current I _rms, and using the various harmonics of «I» as if they belong to a classical current we define the “classical rms current” I _{rms, class}. In the case of classical currents these two quantities are identical, but for quantum currents they are different. We study their difference for various non-classical fields, and find that in some cases this difference is large. These predictions could be observed experimentally through the electromagnetic power that these currents radiate. An experiment that confirms the I _rms and refutes the Irms, class would prove the quantum nature of these currents.     

The electron-nucleon weak neutral current and 0− -0+ nuclear isomerism

Since one photon exchange is completely forbidden in processes of the type ?0^? → ?0⁺ (? is a charged lepton) or any crossed version of such a process, weak neutral current effects are considerably enhanced. It is shown that 0̄?0⁺ nuclear isomers, if they exist, will accordingly exhibit strikingly large neutral current effects. In particular, the monoenergetic conversion electrons will have helicities h of the order of 25% which should be easily measurable. In certain favourable cases, the nuclear weak current matrix elements may be measured experimentally via the analogue β decay or electron capture so that measuring h will constitute a measurement of the electron's neutral vector current coupling constant. In any event, detection of h would prove the existence of an as yet unseen class of neutral currents.     

Unified Field Theory with EINSTEINian Photons and Heavy Bosons as Field Quants

After discussing in two previous papers [1, 2] the classical electrodynamics which corresponds to the quantum electrodynamics with two sorts of photons (photons with zero rest mass and nonvanishing rest mass), in the present paper the general field theory of a vector field A^v with two sorts if field quanta is given. It is shown that the postulate of the “unity of the four-current” determining the physical contents of this theory makes it possible to regard it as a classical ansatz of a unified theory of the electromagnetic and the weak interactions. From the “unity of the currents” results that the electrons are δ-like point-particles with a finite self-potential and finite field masses M = ε²/2 kc^?2. The COMPTON wave-length of the heavy photons k^?1 = h/mc has the meaning of an “elementary length” for the electromagnetic interactions and the rest mass m = khc^?1 of these bosons is of the order of a baryon mass.     

The possibility of new fermions with ΔI = 0 mass

In the Glashow-Weinberg-Salam model the fermions have $\text{ΔI =}\text{1}\text{2}$ masses from the breaking of the weak SU(2) gauge symmetry. In many enlarged models, such as those from grand unified and/or supersymmetric theories, there are additional fermions with undetermined ΔI = 0 (SU(2) invariant) masses. We study these new fermions. They induce flavour changing neutral currents. We show that the mixing angles of ΔI = 0 fermions of mass order M with normal $\text{ΔI =}\text{1}\text{2}$ fermions of mass order m ? M are order η or η², where η = m/M. If M > 150 δ, δ being a model-dependent mixing parameter of order a normal fermion mass, the amplitudes of all FCNC processes are below the experimental sizes and limits. Thus for δ?0.1 GeV, M can be as low as 20 GeV, close to the present experimental lower bound. δ is fixed, and less than 0.1 GeV for all relevant cases, if we assume the mass hierarchy of the known fermions is not the result of a particular choice of ΔI = 0 mass parameters. If produced ΔI = 0 mass fermions will be noticeable by the mass degeneracy within their isospin multiplets. There will be an enhanced ratio of neutral to charged decays over the normal fermions. Standard GUT predictions are changed little.     

On some tests of the basic properties of the neutral weak interaction. I. With massless neutrinos

We consider the Born approximation scattering, by electrons, of neutrinos (and antineutrinos) of the muon and of the electron types; the general Lagrangian respecting lepton locality is used. Throughout, we study only differential cross sections as the experimental observables. Some tests previously proposed for “neutrino identity” or Lorentz structure of the neutral weak interaction are reexamined. We find that, in general, these relations cannot uniquely answer the question of Lorentz structure. Similarly, in general, one cannot estblish whether the final neutral lepton is completely identical with, or “partly identical” with, or completely different from, the initial neutrino; identity allows a difference in helicity. In several experimental situations, one can exclude the possibility of a complete non-identity. In one experimental situation, assuming μe universality, one can establish that the final and the initial neutrinos are completely identical, even in their helicity. “Does an interference between neutral currents and the (V ? A) charged current exist in

e^- scattering?” Certain tests can answer this question completely if μe universality is assumed. Without μe universality, the answer is “destructive interference” if an observable (A_e) turns out to be less than 4; A_e = 4 would exclude a constructive interference. Assuming neutrino identity, time-reversal invariance for the helicity flipping types (S,P,T), and μe universality, certain simple combinations of observables were previously noted to determine the (V,A) neutral interaction couplings of the neutrino to the electron. With our general formalism, that determination is seen not to require the first two assumptions. Also, the couplings concerned are seen to be only the “diagonal” ones—which refer to that part of the final state in which the final neutrino is identical with the iniitial one. Keeping in view a recent experimental situation, the following question is answered: “When will the lack of enough events for $\text{ν}\text{e}{}^{\mathrm{?}}$ scattering (or, similarly, νe^? scattering) become a threat to lepton locality?” Some additional consequences of μe universality are noted.     

Anomalous moments of heavy charged leptons

If leptons and quarks are composite particles we expect them to have anomalous moments: an anomalous magnetic moment and an analogue of this with respect to the weak neutral current. If these moments are suppressed by chiral symmetry they will have sizeable values only for heavy fermions, e.g. for fermions of the fourth generation. The forward-backward asymmetry of e⁺e⁻ → f̄f on the Z⁰ resonance is particularly suited for detecting the anomalous “weak magnetic” moment of f if f is the charged lepton of the fourth generation. Under favourable circumstances this moment may lead to a 10% effect.     

Flavour-changing neutral currents in models with extra<Emphasis Type="Italic">Z′</Emphasis> boson

New neutral gauge bosonsZ′ are the features of many models addressing the physics beyond the standard model. Together with the existence of new neutral gauge bosons, models based on extended gauge groups (rank > 4) often predict new charged fermions also. A mixing of the known fermions with new states, with exotic weak-isospin assignments (left-handed singlets and right-handed doublets) will induce tree-level flavour-changing neutral interactions mediated byZ exchange, while if the mixing is only with new states with ordinary weak-isospin assignments, the flavour-changing neutral currents are mainly due to the exchange of the new neutral gauge bosonZ′. We review flavour-changing neutral currents in models with extraZ′ boson. Then we discuss some flavour-changing processes forbidden in the standard model and new contributions to standard model processes.     

Neutral current anomalies: Difference between dynamical scaling violations and slow rescaling

We discuss the neutral current phenomena in order to discriminate between dynamical scaling violations and heavy quark productions. both of which could explain the charged current anomalies. In the Weinberg-Salam-GIM (W-S) model, “neutral current anomalies” should appear in addition to the charged current anomalies which result from dynamical scaling violations. The “neutral current anomalies” will be observed through the final hadronic invariant mass distributions, especially in $\text{v}$ neutral current processes. The predictions of the W-S model are compared with those of heavy quark models by investigating the changes of the ratios of neutral-to charged-current cross sections for $\text{v}\text{and}\text{v}$ as functions of the $\text{v}\text{/v}$ charged current cross section ratio.     

A U(3) nonet scheme for leptons

Introducing six new heavy leptons, we construct a U(3) scheme for leptons. The leptonic U(3) is exactly parallel to the hadronic U(3). One may also introduce quark-like leptonic triplets. A “natural” value of 60° emerges for the mixing angle with m_w = 53 GeV in a theory of weak and electromagnetic interaction based on this U(3) symmetry. There are no neutral ΔS_l = 0 neutrino currents.     

A theory of flavor mixing

We investigate the origin of the Cabibbo angle and other flavor-mixing angles in an ambidextrous SU(2)_L × SU(2)_R × U(1) gauge theory of weak and electromagnetic interactions involving 2n quark flavors. We show how a discrete symmetry of the Lagrangian leads to flavor-diagonal weak currents where all flavor-mixing angles vanish. Each quark is assigned a position on the “clock of flavor” and the charged-current weak interactions are identified as nearest-flavor couplings. Soft breakdown of the discrete symmetry leads to a one-parameter family of flavor-mixing angles in the charged weak currents. All angles are expressed in terms of this parameter and quark mass ratios. The neutral currents remain flavor diagonal. Upper and lower bounds on 2n, the number of flavors, are obtained by constraining the theory to fit what is known from experiment. Departures from Cabibbo universality become intolerable if 2n < 8. It is impossible to obtain enough CP violation if 2n > 12. We conclude that the number of flavors must be 8, 10, or 12. We construct a more ambitious theory in which the Cabibbo angle and its analogs are O(α) calculable radiative corrections. For reasonable values of the quark mass ratios, we fail to obtain a Cabibbo angle of the right order of magnitude. Our theory involves an equal number of lepton flavors and quark flavors. A large number of flavors is required if neutrinos are to be sufficiently light. We explore the case of 2n = 12, and find a novel mechanism for the neutrino induction of trimuon events.     

Neutron optics and weak currents

The weak interaction can produce two small but striking effects with low energy neutrons. With a transversely polarized neutron beam the polarization rotates in passing through matter; and with an unpolarized beam a longitudinal polarization develops. These effects are of first order in the weak coupling and can only be produced by parity violating forces. With the conventional weak interaction only the nucleons in the material contribute, with “neutral currents” electrons also interact weakly. Thus, if experiments on different materials are performed, “neutral currents” may be observed by detecting an electron contribution to the effect and furthermore the isotopic structure of the weak interaction between nucleons may be determined.     

Quarks and leptons

We review the physics of quarks and leptons within the framework of gauge theories for the weak and electromagnetic interactions. The Weinberg-Salam SU(2) × U(1) theory is used as a “reference point” but models based on larger gauge groups, especially SU(2)_L × SU(2)_R × U(1), are discussed. We distinguish among thre “generations” of fundamental fermions: The first generation (e^?, ν_e, u, d), the second generation (μ^?, ν_μ, c, s) and the third generation (τ^?, ν_τ, t, b). For each generation we discuss the classification of all fermions, the charged and neutral weak currents, possible right-handed currents, parity and CP-violation, fermion masses and Cabibbo-like angles and related problems. We review theoretical ideas as well as experimental evidence, emphasizing open theoretical problems and possible experimental tests. The possibility of unifying the weak, electromagnetic and strong interactions in a grand unification scheme is reviewed. The problems and their possible solutions are presented, generation by generation, but a brief subject-index (following the table of contents) enables the interested reader to follow any specific topic throughout the three generations.     

Interference of Electromagnetic and Weak Interactions at High Energies and Neutral-Current μe Universality

A review is given on three types of experiments which recently detected the interference of electromagnetic and weak interactions at high energies in the reactions eD₂ → eX (SLAC, 1979), e⁺e⁻ → μ⁺μ⁻ (PETRA/PEP, 1981–83) and μ±C→μ±X (BCDMS, 1982). Asymmetry formulae are explicitely derived using the quark-parton model and the SU(2) × U(1) standard theory. With particular emphasis on the deep inelastic muon scattering experiment, the corresponding experiments are described and their results summarized. Combined fits to the 1983 asymmetry and νe data verify completely the muon-electron universality of the weak neutral current interaction giving for the vector and axial-vector coupling constants v_e = 0.02 ± 0.06, a_e = −0.54 ± 0.03 (electrons) and v_μ = −0.05 ± 0.16, a_μ = −0.51 ± 0.05 (muons).     

Experimental consequences and constraints for magninos

A stable weakly interacting massive particle can simultaneously solve both the solar neutrino and missing mass problems. In a previous paper we have identified this particle with a neutral lepton with mass of order 4–10 GeV and an anomalous magnetic moment of order 10⁻² (in natural units). We call this new particle a “magnino”. In one scenario, the magnino is the neutral component of an electroweak doublet. Its charged partner must have a mass only a few GeV heavier. In this paper we discuss the experimental consequences of the magnino, its charged partner and associated Higgs.