Form factors in relativistic quantum mechanics: constraints from space-time translations

The comparison of form factors calculated from a single-particle current in different relativistic quantum mechanic approaches evidences tremendous discrepancies. The role of constraints coming from space-time translations is considered here with this respect. It is known that invariance under these translations implies the energy-momentum conservation relation that is usually assumed to hold globally. Transformations of the current under these translations, which lead to this result, also imply constraints that have been ignored so far in relativistic quantum mechanic approaches. An implementation of these constraints is discussed in the case of a model with two scalar constituents. It amounts to incorporate selected two-body currents to all orders in the interaction. Discrepancies for form factors in different approaches can thus be removed, contributing to restore the equivalence of different approaches. Results for the standard front-form approach ($q^+=0$) are found to fulfill the constraints and are therefore unchanged. The relation with results from a dispersion-relation approach is also made.Comment: 8 pages, 5 figures; to be published in the proceedings of LC2008; Light Cone 2008. Relativistic Nuclear and Particle Physics, Mulhouse : France (2008

RQM description of PS meson form factors, constraints from space-time translations, and underlying dynamics

The role of Poincar\'e covariant space-time translations is investigated in the case of the pseudoscalar-meson charge form factors. It is shown that this role extends beyond the standard energy-momentum conservation, which is accounted for in all relativistic quantum mechanics calculations. It implies constraints that have been largely ignored until now but should be fulfilled to ensure the full Poincar\'e covariance. The violation of these constraints, which is more or less important depending on the form of relativistic quantum mechanics that is employed, points to the validity of using a single-particle current, which is generally assumed in calculations of form factors. In short, these constraints concern the relation of the momentum transferred to the constituents to the one transferred to the system. How to account for the related constraints, as well as restoring the equivalence of different relativistic quantum mechanics approaches in estimating form factors, is discussed. Some conclusions relative to the underlying dynamics are given in the pion case.Comment: 37 pages, 13 figures; figures completed for notations, revised text with better emphasis on differences with previous works; accepted for publication in EPJ

Parity violation in deuteron photo-disintegration

We analyze the energy dependence for two types of parity-non-conserving (PNC) asymmetries in the reaction $\gamma D\to np$ in the near-threshold region. The first one is the asymmetry in reaction with circularly polarized photon beam and unpolarized deuteron target. The second one corresponds to those with an unpolarized photon beam and polarized target. We find that the two asymmetries have quite different energy dependence, and their shapes are sensitive to the PNC-meson exchange coupling constants. The predictions for the future possible experiments to provide definite constraints for the PNC-coupling constants are discussed.Comment: 22 pages, 12 figures. Submitted to Phys.Rev.C 10Oct.0

Parity nonconserving cold neutron-parahydrogen interactions

Three pion dominated observables of the parity nonconserving interactions between the cold neutrons and parahydrogen are calculated. The transversely polarized neutron spin rotation, unpolarized neutron longitudinal polarization, and photon-asymmetry of the radiative polarized neutron capture are considered. For the numerical evaluation of the observables, the strong interactions are taken into account by the Reid93 potential and the parity nonconserving interactions by the DDH model along with the two-pion exchange.Comment: 17 pages, 2 figure

Parity nonconservation in deuteron photoreactions

We calculate the asymmetries in parity nonconserving deuteron photodisintegration due to circularly polarized photons gamma+d to n+p with the photon laboratory energy ranging from the threshold up to 10 MeV and the radiative capture of thermal polarized neutrons by protons n+p to gamma+d. We use the leading order electromagnetic Hamiltonian neglecting the smaller nuclear exchange currents. Comparative calculations are done by using the Reid93 and Argonne v18 potentials for the strong interaction and the DDH and FCDH "best" values for the weak couplings in a weak one-meson exchange potential. A weak NDelta transition potential is used to incorporate also the Delta(1232)-isobar excitation in the coupled-channels formalism.Comment: 14 pages, 13 figures (18 eps files), LaTeX2

Parity nonconservation effects in the photodesintegration of polarized deuterons

P-odd correlations in the deuteron photodesintegration are considered. The $\pi$-meson exchange is not operative in the case of unpolarized deuterons. For polarized deuterons a P-odd correlation due to the $\pi$-meson exchange is about $3 \times 10^{-9}$. Short-distance P-odd contributions exceed essentially than the contribution of the $\pi$-meson exchange.Comment: 12 pages, Latex, 3 figure

Parity Violation in Proton-Proton Scattering

Measurements of parity-violating longitudinal analyzing powers (normalized asymmetries) in polarized proton-proton scattering provide a unique window on the interplay between the weak and strong interactions between and within hadrons. Several new proton-proton parity violation experiments are presently either being performed or are being prepared for execution in the near future: at TRIUMF at 221 MeV and 450 MeV and at COSY (Kernforschungsanlage Juelich) at 230 MeV and near 1.3 GeV. These experiments are intended to provide stringent constraints on the set of six effective weak meson-nucleon coupling constants, which characterize the weak interaction between hadrons in the energy domain where meson exchange models provide an appropriate description. The 221 MeV is unique in that it selects a single transition amplitude (3P2-1D2) and consequently constrains the weak meson-nucleon coupling constant h_rho{pp}. The TRIUMF 221 MeV proton-proton parity violation experiment is described in some detail. A preliminary result for the longitudinal analyzing power is Az = (1.1 +/-0.4 +/-0.4) x 10^-7. Further proton-proton parity violation experiments are commented on. The anomaly at 6 GeV/c requires that a new multi-GeV proton-proton parity violation experiment be performed.Comment: 13 Pages LaTeX, 5 PostScript figures, uses espcrc1.sty. Invited talk at QULEN97, International Conference on Quark Lepton Nuclear Physics -- Nonperturbative QCD Hadron Physics & Electroweak Nuclear Processes --, Osaka, Japan May 20--23, 199

A Current Mode Detector Array for Gamma-Ray Asymmetry Measurements

We have built a CsI(Tl) gamma-ray detector array for the NPDGamma experiment to search for a small parity-violating directional asymmetry in the angular distribution of 2.2 MeV gamma-rays from the capture of polarized cold neutrons by protons with a sensitivity of several ppb. The weak pion-nucleon coupling constant can be determined from this asymmetry. The small size of the asymmetry requires a high cold neutron flux, control of systematic errors at the ppb level, and the use of current mode gamma-ray detection with vacuum photo diodes and low-noise solid-state preamplifiers. The average detector photoelectron yield was determined to be 1300 photoelectrons per MeV. The RMS width seen in the measurement is therefore dominated by the fluctuations in the number of gamma rays absorbed in the detector (counting statistics) rather than the intrinsic detector noise. The detectors were tested for noise performance, sensitivity to magnetic fields, pedestal stability and cosmic background. False asymmetries due to gain changes and electronic pickup in the detector system were measured to be consistent with zero to an accuracy of $10^{-9}$ in a few hours. We report on the design, operating criteria, and the results of measurements performed to test the detector array.Comment: 33 pages, 20 figures, 2 table

Quark and Pole Models of Nonleptonic Decays of Charmed Baryons

Quark and pole models of nonleptonic decays of charmed baryons are analysed from the point of view of their symmetry properties. The symmetry structure of the parity conserving amplitudes that corresponds to the contribution of the ground-state intermediate baryons is shown to differ from the one hitherto employed in the symmetry approach. It is pointed out that the "subtraction" of sea quark effects in hyperon decays leads to an estimate of $W$-exchange contributions in charmed baryon decays that is significantly smaller than naively expected on the basis of $SU(4)$. An $SU(2)_{W}$ constraint questioning the reliability of the factorization technique is exhibited. Finally, a successful fit to the available data is presented.Comment: 25 pages, LATEX, 1643/PH IFJ Krako

Study of relativistic bound states for scalar theories in Bethe-Salpeter and Dyson-Schwinger formalism

The Bethe-Salpeter equation for Wick-Cutkosky like models is solved in dressed ladder approximation. The bare vertex truncation of the Dyson-Schwinger equations for propagators is combined with the dressed ladder Bethe-Salpeter equation for the scalar S-wave bound state amplitudes. With the help of spectral representation the results are obtained directly in Minkowski space. We give a new analytic formula for the resulting equation simplifying the numerical treatment. The bare ladder approximation of Bethe-Salpeter equation is compared with the one with dressed ladder. The elastic electromagnetic form factors is calculated within the relativistic impulse approximation.Comment: 30 pages, 10 figures, accepted for publication in Phys. Rev.