S. Baunack

Measurement of strange-quark contributions to the nucleon's form factors at Q(2) = 0.230 (GeV/c)(2).

We report on a measurement of the parity-violating asymmetry in the scattering of longitudinally polarized electrons on unpolarized protons at a Q2 of 0.230 (GeV/c)(2) and a scattering angle of theta (e) = 30 degrees - 40 degrees. Using a large acceptance fast PbF2 calorimeter with a solid angle of delta omega = 0.62 sr, the A4 experiment is the first parity violation experiment to count individual scattering events. The measured asymmetry is A(phys)=(-5.44+/-0.54(stat)+/-0.26(sys))x10(-6). The standard model expectation assuming no strangeness contributions to the vector form factors is A(0) = (-6.30+/-0.43) x 10(-6). The difference is a direct measurement of the strangeness contribution to the vector form factors of the proton. The extracted value is G(s)(E) + 0.225G(s)(M) = 0.039+/-0.034 or F(s)(1) + 0.130F(s)(2) = 0.032+/-0.028.

Evidence for Strange-Quark Contributions to the Nucleon's Form Factors at Q^2 = 0.108 (GeV/c)^2

We report on a measurement of the parity violating asymmetry in the elastic scattering of polarized electrons off unpolarized protons with the A4 apparatus at MAMI in Mainz at a four momentum transfer value of Q(2)=0.108 (GeV/c)(2) and at a forward electron scattering angle of 30 degrees p)=[-1.36+/-0.29(stat)+/-0.13(syst)]x10(-6). The expectation from the standard model assuming no strangeness contribution to the vector current is A(0)=(-2.06+/-0.14)x10(-6). We have improved the statistical accuracy by a factor of 3 as compared to our previous measurements at a higher Q2. We have extracted the strangeness contribution to the electromagnetic form factors from our data to be G(s)(E)+0.106G(s)(M)=0.071+/-0.036 at Q(2)=0.108 (GeV/c)(2). We again find the value for G(s)(E)+0.106G(s)(M) to be positive, this time at an improved significance level of two sigma.

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.

Measurement of the transverse beam spin asymmetry in elastic electron-proton scattering and the inelastic contribution to the imaginary part of the two-photon exchange amplitude

We report on a measurement of the asymmetry in the scattering of transversely polarized electrons off unpolarized protons, A( perpendicular), at two Q2 values of 0.106 and 0.230 (GeV/c)(2) and a scattering angle of 30 degrees <theta(e)<40 degrees . The measured transverse asymmetries are A( perpendicular)(Q(2)=0.106 (GeV/c)(2))=(-8.59+/-0.89(stat)+/-0.75(syst))x10(-6) and A( perpendicular)(Q(2)=0.230 (GeV/c)(2))=(-8.52+/-2.31(stat)+/-0.87(syst))x10(-6). The first errors denote the statistical error and the second the systematic uncertainties. From comparison with theoretical estimates of A( perpendicular) we conclude that piN-intermediate states give a substantial contribution to the imaginary part of the two-photon amplitude. There is no obvious reason why this should be different for the real part of the two-photon amplitude, which enters into the radiative corrections for the Rosenbluth separation measurements of the electric form factor of the proton.

Measurements and simulations of Cherenkov light in lead fluoride crystals

Abstract The anticipated use of more than 1000 lead fluoride (PbF2) crystals as a fast and compact Cherenkov calorimeter material in a parity violation experiment at MAMI stimulated the investigation of the light yield (LY) of these crystals. The number of photoelectrons (p.e.) per MeV deposited energy has been determined with a hybrid photomultiplier tube (HPMT). In response to radioactive sources a LY between 1.7 and 1.9 p.e./MeV was measured with 4% statistical and 5% systematic error. The LY optimization with appropriate wrappings and couplings was investigated by means of the HPMT. Furthermore, a fast Monte Carlo simulation based on the GEANT code was employed to calculate the characteristics of Cherenkov light in the PbF2 crystals. The computing time was reduced by a factor of 50 compared to the regular photon tracking method by implementing detection probabilities as a three-dimensional look-up table. For a single crystal a LY of 2.1 p.e./MeV was calculated. The corresponding detector response to electrons between 10 and 1000 MeV was highly linear with a variation smaller than 1%.

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.

A luminosity monitor for the A4 parity violation experiment at MAMI

A water Cherenkov luminosity monitor system with associated electronics has been developed for the A4 parity violation experiment at MAMI. The detector system measures the luminosity of the hydrogen target hit by the MAMI electron beam and monitors the stability of the liquid hydrogen target. Both are required for the precise study of the count rate asymmetries in the scattering of longitudinally polarized electrons on unpolarized protons. Any helicity correlated fluctuation of the target density leads to false asymmetries. The performance of the luminosity monitor, investigated in about 2000 h with electron beam, and the results of its application in the A4 experiment are presented.

Vertical Beam Polarization at MAMI

For the first time a vertically polarized electron beam has been used for physics experiments at MAMI in the energy range between 180 and 855 MeV. The beam-normal single-spin asymmetry An, which is a direct probe of higher-order photon exchange beyond the first Born approximation, has been measured in the reaction C12(e→,e′)C12. Vertical polarization orientation was necessary to measure this asymmetry with the existing experimental setup. In this paper we describe the procedure to orient the electron polarization vector vertically, and the concept of determining both its magnitude and orientation with the available setup. A sophisticated method has been developed to overcome the lack of a polarimeter setup sensitive to the vertical polarization component.

Measurement of the axial and the strangeness magnetic form factor of the proton with a P2 backward angle setup

The P2 experiment at the future accelerator facility MESA in Mainz aims for a precise determination of the weak charge of the proton at low momentum transfer. The experimental method is a measurement of the parity violating asymmetry in elastic electron-proton scattering at forward angle. This asymmetry is dominated by the weak charge, but also the proton structure plays a role. Here we consider a back angle measurement, which is more sensitive to the proton structure, and present its possible implications on the main P2 measurement.

Measurement of the weak charge of the carbon-12 nucleus within the P2 experiment in Mainz

The P2 experiment at the MESA facility in Mainz aims to make a high precision measurement of the weak charge of the proton, and a suitable experimental setup is currently being designed [1]. As high precision measurements of the weak charges of different particles and nuclei are differently sensitive to physics beyond the Standard Model in the form of new quantum loop correction parameters, experiments with a range of different target materials are complementary. We introduce first feasibility considerations for the determination of the Weak Mixing Angle via the scattering of electrons from carbon nuclei in a graphite target with the P2 detector setup. In order to make an estimate of the achievable precision we present analytic calculations as well as Monte-Carlo simulations. The results of Geant4 simulations of proposed experimental setups are included.