R. Frascaria

Measurement of Strange Quark Contributions to the Vector Form Factors of the Proton at Q2 = 0.22 (GeV/c)2

A new measurement of the parity violating asymmetry in elastic electron scattering on hydrogen at backward angles and at a four momentum transfer of Q;{2} = 0.22 (Ge V / c);{2} is reported here. The measured asymmetry is A_{LR} = (-17.23 +/- 0.82_{stat} +/- 0.89_{syst}) x 10;{-6}. The standard model prediction assuming no strangeness is A_{0} = (-15.87 +/- 1.22) x 10;{-6}. In combination with previous results from measurements at forward angles, it is possible to disentangle for the first time the strange form factors at this momentum transfer, G_{E};{s} = 0.050 +/- 0.038 +/- 0.019 and G_{M};{s} = -0.14 +/- 0.11 +/- 0.11.

Cross sections and asymmetries for the p(p,π0)pp reaction from threshold to 1 GeV

Abstract Total cross sections, differential cross sections and asymmetries for the reaction p( p ,π 0 )pp from threshold to 1 GeV have been measured. The outgoing π0 was detected by a 4π neutral meson spectrometer. Our data are compared with the corresponding np→NNπ± reactions. The isoscalar partial cross sections are extracted from the relevant total cross sections. They are significant throughout the whole energy range. Below the Δ-region, energy-integrated differential cross sections show a large anisotropy difference between the neutral and charged pion channels, respectively. A partial-wave fit shows that the initial T = 0 state is dominated by the 3D1 wave in this energy region. Finally, predictions of phenomenological models give reasonable good account of the measured observables but cannot reproduce the sign of the asymmetries.

GRAAL: a polarized γ-ray beam at ESRF☆

The GRAAL facility produces a highly polarized gamma-ray beam by Compton scattering of laser photons on the electrons of the European Synchrotron Radiation Facility (ESRF) at Grenoble. Preliminary results have been obtained with the LAGRANGE detector showing its excellent performances.

The analog trigger processor for the new parity violation experiment at mami

A fast analog trigger processor is being developed for the measurement of the parity violating asymmetry of the cross section of elastically scattered polarized electrons on unpolarized protons. The total event rate expected is about 100 MHz in the whole detector which consists of 1022 PbF 2 crystals. This requires fast cluster recognition, spatial and temporal pile-up rejection and fast data storage. First results of a prototype are discussed.

New Measurements of the Beam Normal Spin Asymmetries at Large Backward Angles with Hydrogen and Deuterium Targets

New measurements of the beam normal single spin asymmetry in the electron elastic and quasielastic scattering on the proton and deuteron, respectively, at large backward angles and at ⟨Q^{2}⟩=0.22  (GeV/c)^{2} and ⟨Q^{2}⟩=0.35  ( GeV/c)^{2} are reported. The experimentally observed asymmetries are compared with the theoretical calculation of Pasquini and Vanderhaeghen [Phys. Rev. C 70, 045206 (2004).PRVCAN0556-281310.1103/PhysRevC.70.045206]. The agreement of the measurements with the theoretical calculations shows a dominance of the inelastic intermediate excited states of the nucleon, πN and the Δ resonance. The measurements explore a new, important parameter region of the exchanged virtual photon virtualities.

Large anisotropy difference in the NNπ system for π° and π± channels

Cross sections and beam asymmetries for the reaction p(pπ0)pp have been measured for incident polarized proton energies from the threshold to 1 GeV, and compared with those for the charged pion channels in p(n,π±)NN reactions. The observed anisotropy difference around 540 MeV has been ascribed to the 3D1 partial wave contribution to the np initial state. (AIP)

A Monte-Carlo method to analyze the electromagnetic form factors of the nucleon

Parity violating elastic electron-nucleon scattering allows to determine the vector stangeness content of the nucleon. The final uncertainty on the strange form factors is limited, among other parameters, by the uncertainty on the electromagnetic form factors. These are usually fitted with a functional form constrained by boundary conditions at Q 2= 0 and at large Q 2. These conditions induce huge correlations between parameters which are not taken into account to full extent by purely statistical methods. We describe here a Monte-Carlo method which accounts for correlations between parameters to all orders. We also propose a method for taking into account some systematical errors induced by the dispersion of the data.

Isospin splitting of transition amplitudes from photoproduction of eta mesons on deuterium

Measurements of the total and differential cross sections for η-meson photoproduction on 1H, 2D, and 14N liquid targets from threshold to 1.2 GeV, have been taken using the tagged Bremsstrahlung photon beam produced by the electrons extracted from the ELSA storage ring at Bonn. The reaction was identified by detecting the η decay products in the neutral meson spectrometer SPES0-2π, while the recoil baryons (proton, neutron or deuteron) were detected by a variety of large angle scintillator detectors. We present here our experimental results on the Deuterium target, both the comparison of the quasi-free proton and neutron as well as results on the coherent production. Our preliminary results for the counting rates integrated on the S11 excitation show significantly smaller figures for the neutrons than for the protons. This is consistent with an Isoscalar part of the Amplitude roughly ten times smaller than the Isovector one and also explains the very low rate of coherent deuterons observed.

Photoproduction of Eta Mesons on Deuterium from Threshold to 1.2 GeV

Measurements of the total and differential cross sections for η-meson photoproduction on 1H, 2D, and 14N liquid targets from threshold to 1.2 GeV have been taken using the tagged Bremsstrahlung photon beam produced by the electrons extracted from the ELSA storage ring at Bonn. The reaction was identified by detecting the η decay products in the neutral meson spectrometer SPESO-2π, while the recoil baryons (proton, neutron, or deuteron) were detected by a variety of large angle scintillator detectors. We present here our experimental results on the deuterium target, both the comparison of the quasi-free proton and neutron as well as results on the coherent production. Our preliminary results for the counting rates integrated on the S11 excitation show significantly smaller figures for the neutrons than for the protons. This is consistent with an isoscalar part of the amplitude roughly ten times smaller than the isovector one and also explains the very low rate of coherent deuterons observed.