Determination of the Dalitz plot parameter alpha for the decay eta->3pi^0 with the Crystal Ball at MAMI-B

A precise measurement of the Dalitz plot parameter, alpha, for the eta->3pi^0 decay is presented. The experiment was performed with the Crystal Ball and TAPS large acceptance photon detectors at the tagged photon beam facility of the MAMI-B electron accelerator in Mainz. High statistics of 1.8*10^6 eta->3pi^0 events were obtained, giving the result alpha = -0.032 +/- 0.002(stat) +/- 0.002(syst).Comment: 9 pages, 6 figures, published in the online-first section of EPJ A, included changes referees asked for, added DO

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

First measurement of the circular beam asymmetry in the gamma p --> pi0 eta p reaction

The circular photon asymmetry for pi0 eta photoproduction on the proton was measured for the first time at the tagged photon facility of the MAMI C accelerator using the Crystal Ball/TAPS photon spectrometer. The experimental results are interpreted within a phenomenological isobar model that confirms the dominant role of the Delta(1700)D33 resonance. The measured asymmetry allows us to identify small contributions from positive-parity resonances via interference terms with the dominant D33 amplitude.Comment: 11 pages, 3 figures, submitted to Phys.Lett.

Study of the helicity dependence of single pion photoproduction on the deuteron

Der erste experimentelle Test der GDH-Summenregel für das Proton wurde 1998 am MAMI-Beschleuniger der Universität Mainz durchgeführt. Ferner wurde ein Pilot-Experiment mit einem polarisierten Deuteron-Target vorgenommen. Dieselbe Kollaboration führte 2003 ein auf das Deuteron ausgerichtetes Experiment durch mit der Absicht, die GDH-Summenregel für das Neutron zu untersuchen. Der in beiden Experimenten verwendete Aufbau erlaubt nicht nur die Messung des totalen Wirkungsquerschnitts, sondern auch gleichzeitig einzelne Teilreaktionen zu studieren. In dieser Arbeit werden die Daten des Deuteron-Pilot-Experiments von 1998 analysiert. Desweiteren wird eine Studie der Helizitätsabhängigkeit der differenziellen Wirkungsquerschnitte für drei Pion- Photoproduktionskanäle des Deuterons in der oberen Hälfte der Delta-Resonanz präsentiert. Diese Ergebnisse werden mit einem theoretischen Modell verglichen. Dabei wurde eine hinreichend gute Übereinstimmung für die unpolarisierten Reaktionen gefunden, während für die polarisierten Kanäle kleinere Diskrepanzen beobachtet wurden. Der Targetpolarisationsgrad ist einer der relevanten Parameter, der für eine absolute Normalisierung der Wirkungsquerschnitte notwendig ist. Die Analyse dieses Parameters für die 2003er Daten wird in der vorliegenden Arbeit vorgestellt. Zur Zeit ist in Mainz ein Frozen-Spin-Target im Bau. Es wird als Target für polarisierte Protonen oder polarisierte Deuteronen für zukünftige Experimente mit dem Crystal Ball zur Verfügung stehen. Die Vorbereitungen der verschiedenen Subsysteme dieses Aufbaus stellten einen wichtigen Teil dieser Arbeit dar. Die fundamentalen Grundlagen der Methode und deren technische Umsetzung, sowie der momentane Status der Aufbauarbeiten am Target werden im Detail präsentiert.The first experimental test of the GDH sum rule on the proton was carried out in 1998 at the MAMI accelerator facility in Mainz University. A pilot experiment with a polarized deuteron target was also performed. In 2003 the same collaboration carried out a dedicated experiment on the deuteron with the aim to investigate the GDH sum rule on the neutron. The setup used in both experiments allows not only to measure total cross sections, but also to study partial reaction channels. In this thesis, data from the 1998 pilot experiment on the deuteron are analyzed. A study of the helicity dependence of the differential cross section on three single pion photoproduction channels on the deuteron in the upper half of the Delta-resonance will be presented. These results are compared with a theoretical model. A reasonably good agreement was obtained for unpolar- ized reactions, while some small discrepancies are observed in the polarized channels. The target degree of polarization is one of the relevant parameters re- quired for an absolute normalization of the cross section. The analysis of this parameter for the 2003 data is given in this work. A Frozen Spin Target is under construction in Mainz. It will be used with the Crystal Ball detector and it will provide a polarized proton and polarized deuteron target for future experiments. The preparation of the different subsystems needed for the setup was an important part of this thesis. The basic principles of this technique and the actual status of the target will be presented in some detail

Measurement of the helicity-dependent total cross-section for the gamma n -> p pi(-)pi(0) reaction

The helicity dependence of the total cross-section for the gamma n -&gt; p pi(-)pi(0) reaction has been measured for the first time at incident photon energies from 450 to 800MeV. The measurement was performed with the large-acceptance detector DAPHNE at the tagged photon beam facility of the MAMI accelerator in Mainz. Both the measured unpolarized and the helicity-dependent observables are not well described by the existing theoretical models

Helicity dependence of the γd → πNN reactions in the Δ-resonance region

The helicity dependence of the differential cross-section for the gamma d -> pi NN reactions has been measured for the first time in the Delta-resonance region. The measurement was performed with the large-acceptance detector DAPHNE at the tagged photon beam facility of the MAMI accelerator in Mainz. The data show that the main reaction mechanisms for the pi(+/-) NN channels are the quasi-free N pi processes on one bound nucleon with nuclear dynamics playing a minor role. On the contrary, for the pi(0)np channel nuclear mechanisms involving the reabsorption of the photoproduced pi(0) by the np pair have to be taken into account to reproduce the experimental data

First measurement of the helicity dependence for the gamma p -&gt; p pi(+)pi(-) reaction

The helicity dependence of the total cross-section and the invariant-mass distributions in the (p pi(+/-)) and (pi(+) pi(-)) final states for the gamma p -> p pi(+)pi(-) reaction have been measured for the first time at incident photon energies from 400 to 800MeV. The measurement was performed with the large-acceptance detector DAPHNE at the tagged photon beam facility of the MAMI accelerator in Mainz. Although this channel is found to be predominantly excited by the intermediate production of a Delta pi state, both the measured unpolarized and the helicity-dependent observables are generally not well described by the existing theoretical models