Two-photon exchange and normal spin asymmetries in the A4 experiment

The A4 Collaboration at the MAMI facility has measured, at backward angles and at Q2 = 0.23 (GeV/c)2 and Q2 = 0.35 (GeV/c)2, the asymmetry in the elastic and quasielastic scattering of normally polarized electrons on hydrogen and deuterium, respectively. Some preliminary results will be presented

Realtime calibration of the A4 electromagnetic lead fluoride calorimeter

Sufficient energy resolution is the key issue for the calorimetry in particle and nuclear physics. The calorimeter of the A4 parity violation experiment at MAMI is a segmented calorimeter where the energy of an event is determined by summing the signals of neighbouring channels. In this case the precise matching of the individual modules is crucial to obtain a good energy resolution. We have developped a calibration procedure for our total absorbing electromagnetic calorimeter which consists of 1022 lead fluoride (PbF_2) crystals. This procedure reconstructs the the single-module contributions to the events by solving a linear system of equations, involving the inversion of a 1022 x 1022-matrix. The system has shown its functionality at beam energies between 300 and 1500 MeV and represents a new and fast method to keep the calorimeter permanently in a well-calibrated state

Measurement of the parity violating asymmetry in the quasielastic electron-deuteron scattering and improved determination of the magnetic strange form factor and the isovector anapole radiative correction

A new measurement of the parity-violating asymmetry in the electron-deuteron quasielastic scattering for backward angles at ⟨Q[superscript 2]⟩=0.224 (GeV/c)[superscript 2], obtained in the A4 experiment at the Mainz Microtron accelerator (MAMI) facility, is presented. The measured asymmetry is A[subscript PV][superscript d]=(-20.11±0.87[subscript stat]±1.03[subscript sys])×10[superscript -6]. A combination of these data with the proton measurements of the parity-violating asymmetry in the A4 experiment yields a value for the effective isovector axial-vector form factor of G[subscript A][superscript e,(T=1)]=-0.19±0.43 and R[subscript A][superscript e(T=1),anap] =-0.41±0.35 for the anapole radiative correction. When combined with a reanalysis of measurements obtained in the G0 experiment at the Thomas Jefferson National Accelerator Facility, the uncertainties are further reduced to G[subscript M][superscript s]=0.17±0.11 for the magnetic strange form factors, and R[subscript A][superscript (T=1),anap]=-0.54±0.26.Deutsche Forschungsgemeinschaf

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 Q2=0.22 (GeV/c)2 and Q2=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

Potential effects of rising trpopheric concentrations of CO2 and O=3 on green-algal lichens

12 pages, figures, and tables statistics.Pormelia sulcata Taylor was used as a model to examinate the effects of elevated CO2 and/or O3 on green algal lichens. Thalli were exposed for 30 d in duplicate controlled-environment chambers to two atmospheric concentrations of CO2 ("ambient" (350umol mol-1) and "elevated" (700umol mol-1) 24 h d-1) and two O3 regimes ("non-polluted" air (CF, menos 5nmol mol-1) and polluted" air (15 nmoi mo-1 overnight rising to a midday maximum of 75 nmol mol-1)), in a factorial design. Elevated CO2 or elevated O3 depressed the light saturated rate of CO2, assmilitaion Asat) measured at ambient CO2, by 30% and 18%, respectively. However, despite this effect ultrastructure) studies revealed increased lipid storage in cells of the photobiont in response to CO2-enrichment. Simultaneous exposure to elevated O3 reduced CO2-induced lipid accumulation and reduced Asat in an additive manner. Gold-antibody labelling revealed that the decline in photosynthetic capacity induced by elevated CO2 and/or O3 was accompanied by a parallel decrease in the concentration of Rubiscoa in the algal pyrenoid (I=.0.93), interestingly, differences in the amount of Rubisco protein were not correlated with changes in pyrenoid volume. Measurements of in vivo chlorophyll-fluorescence induction kinetics showed that the decline in Asat induced by elevated CO2, and/or O2, was not associated with significant changes in the photochemical efficiency of photosystem (PS) II. Although the experimental conditions inevitably imposed some stress on the thalli, revealed as a significant decline in the efficiency of PS IIphotochemistry, and enhanced starch accumulation in the photobiont over the fornication period, the study shows that the green-algal lichen symbiosis might be influenced by future changes in atmospheric composition. Photosynthetic capacity, measured et ambient CO2, was found to be reduced after a controlled 30 d exposure to elevated CO2, and/O3 and this effect was associated with a parallel decline in the amount of Rubisco in the pyrenoid of algal choroplasts.Peer reviewe

Study of Two-Photon Exchange via the Beam Transverse Single Spin Asymmetry in Electron-Proton Elastic Scattering at Forward Angles over a Wide Energy Range

International audienceWe report on a new measurement of the beam transverse single spin asymmetry in electron-proton elastic scattering, A⊥ep, at five beam energies from 315.1 to 1508.4 MeV and at a scattering angle of 30°<θ<40°. The covered Q2 values are 0.032, 0.057, 0.082, 0.218, 0.613 (GeV/c)2. The measurement clearly indicates significant inelastic contributions to the two-photon-exchange (TPE) amplitude in the low-Q2 kinematic region. No theoretical calculation is able to reproduce our result. Comparison with a calculation based on unitarity, which only takes into account elastic and πN inelastic intermediate states, suggests that there are other inelastic intermediate states such as ππN, KΛ, and ηN. Covering a wide energy range, our new high-precision data provide a benchmark to study those intermediate states

Measurement of the parity violating asymmetry in the quasielastic electron-deuteron scattering and improved determination of the magnetic strange form factor and the isovector anapole radiative correction

International audienceA new measurement of the parity-violating asymmetry in the electron-deuteron quasielastic scattering for backward angles at ⟨Q2⟩=0.224  (GeV/c)2, obtained in the A4 experiment at the Mainz Microtron accelerator (MAMI) facility, is presented. The measured asymmetry is APVd=(-20.11±0.87stat±1.03sys)×10-6. A combination of these data with the proton measurements of the parity-violating asymmetry in the A4 experiment yields a value for the effective isovector axial-vector form factor of GAe,(T=1)=-0.19±0.43 and RA(T=1),anap=-0.41±0.35 for the anapole radiative correction. When combined with a reanalysis of measurements obtained in the G0 experiment at the Thomas Jefferson National Accelerator Facility, the uncertainties are further reduced to GMs=0.17±0.11 for the magnetic strange form factors, and RA(T=1),anap=-0.54±0.26

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

International audienceNew 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 ⟨Q2⟩=0.22  (GeV/c)2 and ⟨Q2⟩=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

Measurement of Strange Quark Contributions to the Vector Form Factors of the Proton at Q[superscript 2]=0.22 (GeV/c)[superscript 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[superscript 2]=0.22   (GeV/c)[superscript 2] is reported here. The measured asymmetry is A[subscript LR]=(-17.23±0.82stat±0.89syst)×10-6. The standard model prediction assuming no strangeness is A[subscript 0]=(-15.87±1.22)×10[superscript -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, GE[superscript s]=0.050±0.038±0.019 and GM[superscript s]=-0.14±0.11±0.11