[Show abstract][Hide abstract]ABSTRACT: The strange-quark vector current form factors of the nucleon are analyzed within the framework of dispersion relations. Particular attention is paid to contributions made by $K\bar{K}$ intermediate states to the form factor spectral functions. It is shown that, when the $K\bar{K}\to N\bar{N}$ amplitude is evaluated in the Born approximation, the $K\bar{K}$ contributions are identical to those arising from a one-loop calculation and entail a serious violation of unitarity. The mean square strangeness radius and magnetic moment are evaluated by imposing unitarity bounds on the kaon-nucleon partial wave amplitudes. The impact of including the kaon's form factor in the dispersion integrals is also evaluated. Comment: Revtex, 35 pages, 11 Postscript figures, uses epsf.sty, rotate.sty
[Show abstract][Hide abstract]ABSTRACT: The strange-quark vector current form factors of the nucleon are
analyzed within the framework of dispersion relations. Particular
attention is paid to contributions made by KK¯ intermediate states
to the form factor spectral functions. It is shown that, when the
KK¯-->NN¯ amplitude is evaluated in the Born approximation,
the KK¯ contributions are identical to those arising from a
one-loop calculation and entail a serious violation of unitarity. The
mean square strangeness radius and magnetic moment are evaluated by
imposing unitarity bounds on the kaon-nucleon partial wave amplitudes.
The impact of including the kaon's strangeness vector current form
factor in the dispersion integrals is also evaluated.
[Show abstract][Hide abstract]ABSTRACT: The use of spin observables to study the semi-leptonic and non-leptonic weak interaction in atoms and nuclei is surveyed. In particular, the use of semi-leptonic neutral current scattering and atomic parity violation to search for physics beyond the Standard Model is reviewed. The status of nuclear parity violation as a probe of the weak N-N interaction is surveyed. Possible atomic and nuclear signatures of parity conserving, time-reversal violating interactions are also discussed.
[Show abstract][Hide abstract]ABSTRACT: The nucleon's strange-quark vector current form factors are studied from the perspective of chiral symmetry. It is argued that chiral perturbation theory cannot yield a prediction for the strangeness radius and magnetic moment. Arrival at definite predictions requires the introduction of additional, model-dependent assumptions which go beyond the framework of chiral perturbation theory. A variety of such model predictions is surveyed, and the credibility of each is evaluated. The most plausible prediction appears in a model where the unknown chiral counterterms are identified with $t$-channel vector meson exchange amplitudes. The corresponding prediction for the mean square Dirac strangeness radius is $\langle r_s^2\rangle = 0.24$ fm$^2$, which would be observable in up-coming semileptonic determinations of the nucleon's strangeness form factors. Comment: LaTex 31 pages, four figures available from authors.
[Show abstract][Hide abstract]ABSTRACT: The extraction of the nucleon's strangeness axial charge, Delta_s, from inclusive, quasielastic neutral current neutrino cross sections is studied within the framework of the plane-wave impulse approximation. We find that the value of Delta_s can depend significantly on the choice of nuclear model used in analyzing the quasielastic cross section. This model-dependence may be reduced by one order of magnitude when Delta_s is extracted from the ratio of total proton to neutron yields. We apply this analysis to the interpretation of low-energy neutrino cross sections and arrive at a nuclear theory uncertainty of plus/minus 0.03 on the value of Delta_s expected to be determined from the ratio of proton and neutron yields measured by the LSND collaboration. This error compares favorably with estimates of the SU(3)-breaking uncertainty in the value of Delta_s extracted from inclusive, polarized deep-inelastic structure function measurements. We also point out several general features of the quasielastic neutral current neutrino cross section and compare them with the analogous features in inclusive, quasielastic electron scattering.
[Show abstract][Hide abstract]ABSTRACT: The strange-quark vector current $\rho$-to-$\pi$ meson transition form factor is computed at one-loop order using strange meson intermediate states. A comparison is made with a $\phi$-meson dominance model estimate. We find that one-loop contributions are comparable in magnitude to those predicted by $\phi$-meson dominance. It is possible that the one-loop contribution can make the matrix element as large as those of the electromagnetic current mediating vector meson radiative decays. However, due to the quadratic dependence of the one-loop results on the hadronic form factor cut-off mass, a large uncertainty in the estimate of the loops is unavoidable. These results indicate that non-nucleonic strange quarks could contribute appreciably in moderate-$|Q^2|$ parity-violating electron-nucleus scattering measurements aimed at probing the strange-quark content of the nucleon.
[Show abstract][Hide abstract]ABSTRACT: We deduce constraints on time-reversal-noninvariant (TRNI), parity-conserving (PC) hadronic interactions from nucleon, nuclear, and atomic electric dipole moment (EDM) limits. Such interactions generate EDM's through weak radiative corrections. We consider long-range mechanisms, i.e., those mediated by meson exchanges in contrast with short-range two-loop mechanisms. We find that the ratio of typical TRNI, PC nuclear matrix elements to those of the strong interaction are $\lesssim${}${10}^{\mathrm{$-${}}5}$, a limit about 2 orders of magntiude more stringent than those from direct detailed balance studies of such interactions. This corresponds to a bound of $\Vert${}g${\mathrm{\ifmmode\bar\else\textasciimacron\fi{}}}_{\mathrm{$\rho${}}}$$\Vert${}$\lesssim${}${10}^{\mathrm{$-${}}3}$, where g${\mathrm{\ifmmode\bar\else\textasciimacron\fi{}}}_{\mathrm{$\rho${}}}$ is a TRNI PC $\rho${}NN coupling.
Article · Oct 1994 · Physical review D: Particles and fields
[Show abstract][Hide abstract]ABSTRACT: Meson-exchange current (MEC) contributions to the parity-violating (PV) asymmetry for elastic scattering of polarized electrons from $^4$He are calculated over a range of momentum transfer using Monte Carlo methods and a variational $^4$He ground state wavefunction. The results indicate that MEC's generate a negligible contribution to the asymmetry at low-$|\qv|$, where a determination of the nucleon's mean square strangeness radius could be carried out at CEBAF. At larger values of momentum transfer -- beyond the first diffraction minimum -- two-body corrections from the $\rho$-$\pi$ \lq\lq strangeness charge" operator enter the asymmetry at a potentially observable level, even in the limit of vanishing strange-quark matrix elements of the nucleon. For purposes of constraining the nucleon's strangeness electric form factor, theoretical uncertainties associated with these MEC contributions do not appear to impose serious limitations. Comment: 32 TEX pages and 7 figures (not included, available from authors upon request), CEBAF Preprint #TH-94-10
[Show abstract][Hide abstract]ABSTRACT: Meson-exchange current contributions of the strangeness radius of 4He are computed in the one-boson exchange approximation. It is found that these contributions introduce a ≳ 10% correction to the one-body contribution. They should not, therefore, hamper the extraction of the nucleon strangeness radius from the parity-violating electron-4He asymmetry.
[Show abstract][Hide abstract]ABSTRACT: The present status and future prospects of intermediate-energy semileptonic neutral current studies are reviewed. Possibilities for using parity-violating electron scattering from nucleons and nuclei to study hadron structure and nuclear dynamics are emphasized, with particular attention paid to probes of strangeness content in the nucleon. Connections are drawn between such studies and tests of the electroweak gauge theory using electron or neutrino scattering. Outstanding theoretical issues in the interpretation of semileptonic neutral current measurements are highlighted and the prospects for undertaking parity-violating electron or neutrino scattering experiments in the near future are surveyed.
[Show abstract][Hide abstract]ABSTRACT: Intrinsic strangeness contributions to lowenergy strange quark matrix elements of the nucleon are modelled using kaon loops and meson-nucleon vertex functions taken from nucleon-nucleon and nucleon-hyperon scattering. A comparison with pion loop contributions to the nucleon electromagnetic (EM) form factors indicates the presence of significantSU(3)-breaking in the mean-square charge radii. As a numerical consequence, the kaon loop contribution to the mean square Dirac strangeness radius is significantly smaller than could be observed with parity-violating elastic\(\vec e\)
p experiments planned for CEBAF, while the contribution to the Sachs radius is large enough to be observed with PV electron scattering from (0+, 0) nuclei. Kaon loops generate a strange magnetic moment of the same scale as the isoscalar EM magnetic moment — a scale large enough to be observed at CEBAF — and a strange axial vector form factor having roughly one-third of the magnitude extracted from\(vp/\bar vp\) elastic scattering. In the chiral limit, the loop contribution to the fraction of the nucleon's scalar density arising from strange quarks has roughly the same magnitude as the value extracted from analyses of∑
πN
. The importance of satisfying the Ward-Takahashi Identity, not obeyed by previous calculations, is also illustrated, and the sensitivity of results to input parameters is analyzed.
[Show abstract][Hide abstract]ABSTRACT: The theoretical aspects of an exciting new class of scattering experiments that use polarized electrons are discussed. These experiments will probe the weak neutral current of nucleons and nuclei and will form an important component of the future experimental program at CEBAF facility. (AIP)
[Show abstract][Hide abstract]ABSTRACT: Contributions from physics beyond the Standard Model, strange quarks in the nucleon, and nuclear structure effects to the left-right asymmetry measured in parity-violating (PV) electron scattering from $\nuc{12}{C}$ and the proton are discussed. It is shown how lack of knowledge of the distribution of strange quarks in the nucleon, as well as theoretical uncertainties associated with higher-order dispersion amplitudes and nuclear isospin-mixing, enter the extraction of new limits on the electroweak parameters $S$ and $T$ from these PV observables. It is found that a series of elastic PV electron scattering measurements using $^{4}$He could significantly constrain the $s$-quark electric form factor if other theoretical issues are resolved. Such constraints would reduce the associated form factor uncertainty in the carbon and proton asymmetries below a level needed to permit extraction of interesting low-energy constraints on $S$ and $T$ from these observables. For comparison, the much smaller scale of $s$-quark contributions to the weak charge measured in atomic PV is quantified. It is likely that only in the case of heavy muonic atoms could nucleon strangeness enter the weak charge at an observable level. Comment: 29 pages, MIT preprint 2149, CEBAF preprint TH-92-29
[Show abstract][Hide abstract]ABSTRACT: The theoretical interpretation of several prospective intermediate-energy parity-violating (p.v.) electron-scattering experiments is discussed. Particular attention is paid to the interplay of various sources of theoretical hadronic-physics uncertainties. It is argued that these uncertainties may render high-precision, p.v. electron scattering less suitable for standard-model tests than atomic parity-violation experiments. However, p.v. electron scattering offers a unique window on various aspects of nucleon and nullear structure, such as the presense of strange quarks in the nucleon. In particular, it is found that a series of measurements on the protoon could provide interesting constraints on the two strangeness vector-current form factors, although not to the degree needed to permit a 1% extraction of the weak mixing angle. The prospects for further tightening these constraints with elastic scattering from the (Jπ, T) = (o+, 0) targets and from the deuteron are evaluated in detail. Associated, unresolved theoretical issues are highlighted, and the relationship between theoretical interpretability and experimental doability is analyzed. These considerations are applied to three recent proposals for experiments at CEBAF. A treatment of quasielastic p.v. electron scattering is also given, including a reanalysis of the recent Mainz 9Be(, e′) measurement to account for radiative-correction uncertainties in the quoted value of the weak mixing angle. For each case considered, the kinematic conditions are identified for which interpretability of a prospective experiment might be optimized.
[Show abstract][Hide abstract]ABSTRACT: Parity-violating quasielastic electron scattering from nuclei is studied within the context of the relativistic Fermi gas model. Three issues are discussed: (i) the merits of such studies for obtaining new information about single-nucleon form factors, especially the roles played by the axial-vector and strangeness form factors, (ii) the degree to which the parity-violating asymmetry is sensitive to specifics of the nuclear model employed, and to a lesser extent (iii) the suitability of using quasielastic scattering from nuclei to test the standard model of the electroweak interaction. It is found that improved limits on the isovector axial form factor could be obtained from a backward angle, moderate momentum transfer measurement, while an experiment performed at forward angles and higher momentum transfer is sensitive to the strangeness electric form factor at a potentially significant level. In addition, it is argued that quasielastic parity-violating scattering is less suitable for high-precision standard model tests than are experiments performed in other sectors, but may provide an interesting new window on nuclear many-body processes.
[Show abstract][Hide abstract]ABSTRACT: The properties of the neutrino charge radius (NCR) and anapole moments (AM's) of elementary fermions, nucleons, and nuclei are discussed. The dependence of these off-shell electromagnetic couplings on the weak gauge parameter is explicitly demonstrated by a calculation performed in the Rxi gauge. The gauge dependence of the AM's and NCR implies that they cannot be observed in isolation from other second-order, electroweak effects. It is shown, however, that the AM's of various hadronic systems having an SU(2)L quantum number TL3=0 can be considered ``observables'' in certain formal, though unphysical, limits. It is argued that, apart from these special limits, the AM is a physically meaningful entity only for heavy and/or nearly degenerate nuclei.
Article · Jun 1991 · Physical review D: Particles and fields
[Show abstract][Hide abstract]ABSTRACT: We calculate the two-loop anomalous dimension and the first-order coefficient function of weak-interaction currents in the heavy-quark effective field theory. Together with the two-loop QCD β-function, these quantities form the complete sub-leading logarithmic corrections to the mass-factorization of weak form factors.
[Show abstract][Hide abstract]ABSTRACT: We calculate the leading electroweak corrections to parity-violating, leptonic and semi-leptonic neutral current amplitudes in the framework of the standard model and estimate additional contributions to lepton-nucleon amplitudes from mesonic intermediate states. We find that second order electroweak effects generate large (⩾(5–10)%) corrections and that theoretical uncertainties in corrections to semi-leptonic amplitudes can have the same scale as the corrections themselves. Implications of these results for prospective low- and medium-energy parity-violating lepton scattering experiments are discussed.
[Show abstract][Hide abstract]ABSTRACT: The leading T-conserving P-nonconserving (PNC) electromagnetic coupling to the nucleon or nucleus is known as the anapole moment. We evaluate the pion-cloud contribution to the nucleon anapole moment, and the enhancements in nuclei associated with meson-exchange currents and with the mixing of the nuclear ground state with opposite-parity excited states. We find that the anapole moment becomes the dominant PNC spin-dependent coupling in heavy nuclei.