Determination of the eta-transition form factor in the gamma p -< p eta -< p gamma e(+)e(-) reaction

The Dalitz decay eta -&gt; gamma e(+)e(-) has been measured using the combined Crystal Ball and TAPS photon detector setup at the electron accelerator MAMI-C. Compared to the most recent transition form-factor measurement in the e(+)e(-) channel, statistics have been improved by one order of magnitude. The e(+)e(-)invariant mass distribution shows a deviation from the QED prediction for a point-like particle, which can be described by a form-factor. Using the usual monopole transition form-factor parameterization, F(m(2)) = (1 - m(2)/Lambda(2))(-1), a value of Lambda(-2) = (1.92 +/- 0.35(stat) +/- 0.13(syst)) GeV(-2) has been determined. This value is in good agreement with a recent measurement of the eta Dalitz decay in the mu(+)mu(-) channel and with recent form-factor calculations. An improved value of the branching ratio BR(eta -&gt; gamma e(+)e(-)) = (6.6 +/- 0.4(stat) +/- 0.4(syst)) . 10(-3) has been determined. (C) 2011 Elsevier B.V. All rights reserved

Vergleichende Untersuchungen zwischen der konventionellen PAH- und Inulin-Clearance und der J 131-o-Hippursaeure und der Cr 51-EDTA-Clearance im Slope

SIGLEDEGerman

Measurement of the gamma p -< K-0 Sigma(+) reaction with the Crystal Ball/TAPS detectors at the Mainz Microtron

The gamma p -< K-0 Sigma(+) reaction has been measured from threshold to E-gamma = 1.45 GeV (W-CM = 1.9 GeV) using the Crystal Ball and TAPS multiphoton spectrometers together with the photon tagging facility at the Mainz Microtron MAMI. In the present experiment, this reaction was searched for in the 3 pi(0)p final state, by assuming K-S(0) -< pi(0)pi(0) and Sigma(+) -< pi(0)p. The experimental results include total and differential cross sections as well as the polarization of the recoil hyperon. The new data significantly improve empirical knowledge about the gamma p -< K-0 Sigma(+) reaction in the measured energy range. The results are compared to previous measurements and model predictions. It is demonstrated that adding the present gamma p -< K-0 Sigma(+) results to existing data allowed a better description of this reaction with various models

In-medium modifications of the omega meson near the production threshold

The in-medium properties of the omega meson were investigated at the MAMI-C electron accelerator using the Crystal Ball/TAPS detector system to identify omega mesons through their omega -< pi(0)gamma decay channel. Two solid targets (C, Nb) and a LH2 target as a reference have been used. In the energy range 900 to 1300 MeV the omega lineshapes for the different nuclei are compared and a broadening of the omega signal for the niobium target is observed in comparison to the liquid-hydrogen target. In a second analysis the omega momentum distributions for the solid targets are determined for 50 MeV/c wide bins. For both analyses a comparison to transport calculations shows that the data are consistent with an in-medium modification by collisional broadening, as observed by transparency ratio measurements

Measurement of the beam-helicity asymmetry I-circle dot in the photoproduction of pi(0)-pairs off the proton and off the neutron

Beam-helicity asymmetries have been measured at the MAMI accelerator in Mainz for the photoproduction of neutral pion pairs in the reactions (gamma) over right arrowp -< p pi(0)pi(0) and (gamma) over right arrowd -< (n)p pi(0)pi(0), (gamma) over right arrowd -< (p)n pi(0)pi(0) off free protons and off quasi-free nucleons bound in the deuteron for incident photon energies up to 1.4 GeV. Circularly polarized photons were produced from bremsstrahlung of longitudinally polarized electrons and tagged with the Glasgow magnetic spectrometer. Decay photons from the pi(0) mesons, recoil protons, and recoil neutrons were detected in the 4 pi covering electromagnetic calorimeter composed of the Crystal Ball and TAPS detectors. After kinematic reconstruction of the final state, excellent agreement was found between the results for free and quasi-free protons. This demonstrates that the free-nucleon behavior of such observables can be extracted from measurements with quasi-free nucleons, which is the only possibility for the neutron. Contrary to expectations, the measured asymmetries are very similar for reactions off protons and neutrons. The results are compared to the predictions from the Two-Pion MAID reaction model and (for the proton) also to the Bonn-Gatchina coupled channel analysis. (C) 2013 Elsevier B.V. All rights reserved