Nucleon charge form factors and chiral quark models

Calculations of the proton and neutron charge form factors G_E^p,n(q²) are presented, based on chiral bag as well as confining Dirac potential models with chiral pion-quark coupling. Special emphasis is placed on a detailed treatment of the charged pion cloud contribution to the nucleon current. The role of a finite extension of the pion-quark vertex in truncating the summation over intermediate quark bag states is studied. Quark core radii (including recoil corrections) are constrained by a simultaneous calculation of the nucleon axial form factor. The proton charge form factor is well reproduced for $\text{|q}{}^2\text{|}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{? 0.7}\text{GeV}$ with quark core rms radii between 0.5–0.6 fm. About $\text{1}\text{3}$ of the proton charge is carried by the pion cloud in this model. The neutron charge form factor is obtained with the correct sign and overall q² dependence but needs further refinements, probably at the level of the isoscalar form factor.     

Experimental results on one-pion neutral current reaction in all channels induced by antineutrinos at CERN PS

Single pion production, in antineutrino-nucleon interactions via the neutral current, has been studied in the bubble chamber Gargamelle, filled with propane. Complete analysis of the secondaries, including π^0,s, has allowed a detailed study of all channels. After background and nuclear corrections, results are obtained which can be interpreted in terms of isospin structure of the neutral weak current and of the Salam-Weinberg angle. Agreement is found with the currently accepted value of sin²θ_W and the data suggest an important contribution of $\text{I =}\text{3}\text{2}$ final states.     

Flavor changing Z0 decay and the top quark

The flavor changing neutral current decay of the Z⁰ boson into charge $\text{2}\text{3}$ quarks in the standard three generation SU(2)_L × U(1) theory of electroweak interactions has been studied. This process occurs first at one-loop order, where it has been calculated without approximation. The possibility of producing the as yet undiscovered top quark by this decay has been considered. The branching ratios are extremely small, independent of the top quark mass and plausible quark mixing matrices if there are three generations, making it unlikely that the top quark will be produced by this mechanism. However, a massive fourth bottom quark could increase the rates.     

Asymptotic behavior of the pion electromagnetic form factors in Bethe-Salpeter model

Asymptotic behavior of the pion form factors has been discussed treating pion as a bound state of the elementary nucleon and antinucleon system. The matrix-element of the electromagnetic current is written in terms of the Bethe-Salpeter amplitude for the nucleon-antinucleon bound system. Using suitable approximations the Bethe-Salpeter equation for nucleon-antinucleon bound state with nucleons as Dirac spinors has been solved and the corresponding off-shell pion form factors are determined. The form factors are strongly interaction dependent. For coupling $\text{g}{}^2\text{16π}{}^2\text{=}\text{15}\text{4}$ the form factors vanish asymptotically like t^?1 and for $\text{14}\text{4}\text{<}\text{g}{}^2\text{16π}{}^2\text{<}\text{15}\text{4}$ the form factors still vanish but less rapidly than t^?1.     

Parity nonconserving nucleon-nucleon scattering (one-boson-exchange) in the weinberg-salam model

In the present paper we discuss the implications of the Weinberg-Salam model of weak interactions for parity non-conserving nucleon-nucleon scattering. The one-boson-exchange nucleon-nucleon potentials are determined by the use of the factorization method and current algebra. The effect of the neutral hadronic currents is to enhance both the pion contribution $\text{(Δ}\text{I}\text{?}\text{=1)}$ and the vector meson contribution (ΔI=0, (2))). The Weinberg-Salam model is consistent with the recent experiment on ¹⁹F and leads to a sizable measurable polarization effect in the transition 0^-, 0(1.08 MeV) to the ground state of ¹⁸F, namely, P ～ 5 × 10^?3.     

A model for chiral symmetry breaking in QCD

A recently proposed model for dynamical breaking of chiral symmetry in QCD is extended and developed for the calculation of pion and chiral symmetry breaking parameters. The pion is explicitly realized as a massless Goldstone boson and as a bound state of the constituent quarks. We compute, in the limit of exact chiral symmetry, M_Q, the constituent quark mass $\text{?}{}_{\mathrm{\pi }}$ the pion decay coupling, $\text{〈}\text{u}\text{u〉}$, the constituent quark loop density, μ_π²/m_q, the ratio of the Goldstone boson mass squared to the bare quark mass, and 〈r²〉_π, the pion electromagnetic charge radius squared.     

Study of inclusive neutral current interactions of neutrinos and antineutrinos

We report on results from a study of hadron-energy distributions for ν and $\text{ν}$ inclusive neutral current interactions. There is no significant variation of the neutral to charged current total cross-section ratios R_ν and $\text{R}\text{ν}$ with neutrino energy. The space-time structure of neutral currents is dominated by V?A, with a significant admixture of V+A. The Weinberg-Salam model is in agreement with all data if sin²θ_w=0.24±0.02.     

Pion photoproduction and inelastic scattering in A=12 nuclei: Transitions to the lowest isovector 1+ and 2+ levels

The continuity equation for the nuclear electric charge and convection current $\text{? ·}\text{j}\text{?}{}_{\mathrm{N}}\text{=}\text{?α}\text{ρ}\text{?}{}_{\mathrm{N}}\text{ρt}$ has been used in an analysis of nuclear transition densities in ¹²C. The results differ considerably from the former derivations. The standard M1 and newly-calculated E2 nuclear structure input is applied to estimate the pionic cross sections. A consistent improvement in several instances over the earlier results has been achieved; the calculations agree nicely with data both in the (3, 3) resonance region and at lower pion energies.     

A study of second-order interference for pions produced in various hadronic interactions

Second-order interference of pion pairs of equal charge is observed in good statistics samples of 16 GeV/cπ⁺p and K^?p interactions (2 × 10⁶ and 10⁶ pion pair combinations, respectively) and in a smaller sample of $\text{p}\text{p}$ annihilations at rest. The effective radius R of the pion source is found to be approximately 1.4 fm for all three types of interaction. The interference does not appear to be “total”, about 60–80% of the like pairs interfering. It is shown that the numerical results for the radius R, for the depth cτ of the source “photosphere” and for the fraction λ of the interfering pairs are sensitive to any dynamical correlation in the background, and hence depend on the background chosen. It is argued that further refinements in the theoretical formulation of the phenomenon and in the experimental approach are needed for truly quantitative results.     

Double-charge-exchange and inelastic scattering in π−+3He

The reactions π^?+³He→⁺+3n and π^?+³He→^?+³He were studied to investigate the $\text{T=}\text{3}\text{2}$ three-nucleon system. The differential cross sections were measured at scattering angles from 20 to 40 degrees. The secondary pion was momentum analyzed in a magnetostrictive-readout wire-chamber spectrometer. The double-charge-exchange reaction yielded a secondary pion energy distribution, the features of which can be due to either a $\text{T=}\text{3}\text{2}$ three-nucleon resonance or a resonace of the nucleons in the ³He nucleus. The inelasticc scaterring reaction yielded a secondary pion energy distribution peaked near threshold, consistent with resonances in both the $\text{T=}\text{3}\text{2}$ and $\text{T=}\text{1}\text{2}$ three-nucleon systems.     

Neutrino-excitation of baryon resonances and single pion production

This is an attempt to describe all existing data on neutrino production of single pions in the resonance region up to W = 2 GeV in terms of the relativistic quark model of Feynman, Kislinger and Ravndal (FKR). We considered single pion production to be mediated by all interfering resonances below 2 GeV. A simple noninterfering, nonresonant background of isospin $\text{1}\text{2}$ was added. It improved agreement with experiment, particularly in the ratio of isospin amplitudes in charged current reactions, at the expense of one additional constant. All total cross sections, cross section ratios and W-distributions are well reproduced at low and high energies, with charged and neutral currents (supposing the Salam-Weinberg theory with sin²$\text{θ}{}_{\mathrm{w}}\text{≈}\text{1}\text{4}$ to be correct), and for neutrinos and antineutrinos, giving predictions where data are lacking. New predictions have been made for complex angular distributions in $\text{N}$π channels exhibiting strong interference between neighbouring resonances. These are sensitive (for 1.1 GeV ? W ? 1.5 GeV) to the sign of the Roper resonance P₁₁(1450) which is controversial in photoproduction experiments.     

Origin of the N1 (1150) enhancement and the d1(2200) effect

Using pion exchange and Δ(1236) excitation inside the deuteron, the N¹(1150) enhancement with isospin $\text{I =}\text{1}\text{2}$, seen in evidence in dp coherent interactions, is explained. Good agreement is obtained, without any arbitrary normalization, between the Frascati-Caen-Saclay data and the predictions of this model. Its leading features and applications to other experiments are given.     

Charge density and single-particle structure of 58Ni

The charge distribution of ⁵⁸Ni has been calculated by summing the squares of the proton wave functions in a one-body non-local potential whose parameters were adjusted to give the experimental centroid energies. The squared wave functions were weighted by the occupation numbers. A definite procedure to solve the problems connected with the state dependence of such a potential has been developed. The calculated ⁵⁸Ni charge density depends essentially on the ld state centroid energy and on the 1f_{$\text{7}\text{2}$} occupation number. These have been determined by a fit to the experimental charge density.     

Non-linear density dependence of the Δ-self-energy in nuclear matter and its implications in pionic atoms

The $\text{Δ(}\text{3}\text{2}\text{,}\text{3}\text{2}$) self-energy in a nuclear medium is shown to be highly non-linear in density, when proper care is taken of the virtual meson propagation in the medium and retardation effects. As a consequence the p-wave absorptive potential for pionic atoms diverges appreciably from the standard ρ² form. A fit to the existing data on pionic atoms is carried out with the new functional of the density and turns out to be as good as those with the ρ² functional. The successful fits with such different density functionals are due to a very narrow range of nuclear effective densities felt by the pion in the observed pionic atoms. The influence of these effects in related problems is discussed along with the suggestion to widen the range of nuclear densities felt by the pions by looking at other nuclear phenomena.     

Pion exchange and the universal impact parameter hypothesis

In order to describe pion exchange reactions a modification to the $\text{“b-}\text{universality}\text{”}$ hypothesis is proposed which simply relates the impact parameter profiles of helicity amplitudes for different values of net helicity-flip. This proposal is shown to give an excellent quantitative fit to the π^?p → ?⁰n and π^?p → f⁰ data at 17.2 Ge V/c. It also provides a natural explanation for the presence and size of the necessary absorptive cut corrections.     

A non-relativistic operator convenient for analysis of pion photoproduction on nuclei in the Δ(1236) region

In view of its application to the analysis of medium energy photonuclear reactions we have obtained an operator valid, to order $\text{(}\text{p}\text{m}\text{)}{}^2$, in any frame of reference. We have started from a model where the pion photoproduction on a nucleon is described by the Born terms and the s-channel Δ(1236) formation amplitude and we have deduced the non-relativistic limit of the corresponding matrix elements. Both pseudo-scalar and pseudo-vector pion-nucleon couplings are compared. The Δ(1236) parameters are chosen so as to lead to a good account of pion-nucleon reaction cross sections. The agreement with the charged photoproduction data is very good from threshold through the Δ(1236) energy region, whereas the addition of ω⁰ exchange in the t-channel is necessary to reproduce the neutral pion photoproduction cross sections. As an example this operator is used to derive the cross section of the γD → ppπ^? reaction in the spectator nucleon model when the undiscernability of the two outgoing protons is taken into account.     

Test of amplitude signs in pion photoproduction predicted by the transformation from current to constituent quarks

The transformation from current to constituent quark basis states is discussed as it applies to relating amplitudes for photon-nucleon decays of baryon resonances. The predictions for the relative signs of pion photoproduction amplitudes through baryon resonances in the $\text{70}\text{L = 1}$ and $\text{56}\text{L = 2}$ multiplets are presented and compared with experiment. Theory and experiment are found to be completely, with the pion-nucleom decay amplitudes of resonances in the $\text{70}\text{L = 1}$ having the signs characteristic of the $\text{3,}\text{3}\text{)?(}\text{3}\text{, 3)}$ rather than (8, 1)?(1, 8) term in the transformation axial-vector charge.     

Measurement of the neutral to charged current cross section ratio in neutrino and antineutrino interactions

We report on the analysis of inclusive neutral current events produced in neutrino and antineutrino narrow band beams. We find for incident neutrino energies in the range 12–200 GeV and for hadron energies above 12 GeV a neutral to charged current cross-section ratio of R_v = 0.293 ± 0.010 for incident neutrinos, and $\text{R}{}_{\mathrm{<mtext>v</mtext>}}\text{= 0.35 ± 0.03}$ for antineutrinos. These ratios are consistent with the Weinberg-Salam model, with sin²θ_w = 0.24 ± 0.02.     

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.