T. Hammel

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 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%.

PICOSECOND POLARIZED ELECTRON BUNCHES FROM A STRAINED LAYER GAASP PHOTOCATHODE

Abstract At the Mainz electron accelerator MAMI a facility has been set up that allows the measurement of the bunchlength, the energy spread and the averaged and phase resolved polarization of picosecond electron bunches from a source of polarized electrons for MAMI. Electron bunches of typically 10 to 30 ps duration are generated by illuminating a strained layer GaAsP photocathode with femtosecond laser pulses.

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.

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.

Selfstarting modelocked Ti:sapphire laser at a repetition rate of 1.039 GHz

Abstract At the Mainz electron accelerator MAMI a selfstarting Kerr lens modelocked Ti:sapphire pulselaser with a repetition rate of 1.039 GHz was designed and tested. A laser of this type will be used at the source of polarized electrons to increase the lifetime of the photocathodes. The compact lasersystem, which contains an etalon for the compensation of group velocity dispersion, emits light pulses with a pulse length of 210 ps FWHM at an average output power of 520 mW.

Emission of polarized ps-electron bunches from III–V semiconductor cathodes

At the pulsed electron gun testfacility at MAMI we have measured electron bunches from GaAs samples with different layer thicknesses. We observe an increasing dc-polarization with decreasing layer thickness. From the pulse measurements this behaviour may be explained in terms of bunchlength and spin relaxation time.