Photon Asymmetry of Deuteron Photodisintegration Between 160 and 410 MeV

 The photon asymmetry Σ of the reaction has been measured across the photon energy range 160–410 MeV at the Mainz Microtron MAMI. Linearly polarized photons were obtained from coherent bremsstrahlung and their energies were determined with the Glasgow tagging spectrometer. The large acceptance detector DAPHNE enabled the outgoing protons to be measured over the complete azimuthal range and for polar angles from 35° to 155° in the c.m. system. The data are compared with existing results and recent theoretical calculations. Received August 22, 1998; accepted for publication February 8, 1999     

Helicity amplitudes A1/2 and A3/2 for the D13(1520) resonance obtained from the gamma-->p-->-->ppi(0) Reaction

The helicity dependence of the gamma-->p-->-->ppi(0) reaction has been measured for the first time in the photon-energy range from 550 to 790 MeV. The experiment, performed at the Mainz microtron MAMI, used a 4pi-detector system, a circularly polarized, tagged photon beam, and a longitudinally polarized frozen-spin target. These data are predominantly sensitive to the D13(1520) resonance and are used to determine its helicity amplitudes.     

Measurement of the Gerasimov-Drell-Hearn Integrand for 2H from 200 to 800 MeV

A measurement of the helicity dependence of the total inclusive photoabsorption cross section on the deuteron was carried out at MAMI (Mainz) in the energy range 200相似文献   

Measurement of the proton spin polarizabilities at MAMI

The spin polarizabilities of the nucleon are fundamental structure constants which describe the response of the nucleon spin to an incident polarized photon. The most model-independent way to measure the nucleon spin polarizabilities is the Compton scattering with polarization degrees of freedom. Three Compton scattering asymmetries on the proton were measured in the Δ(1232) region using a polarized incident photon beam and a polarized (or unpolarized) proton target at the Mainz Microtron (MAMI). These asymmetries are sensitive to values of the spin polarizabilities. Fits to asymmetry data were performed using a dispersion model calculation, and a separation of all four proton spin-polarizabilities in the multipole basis was achieved. The values of the proton spin polarizabilities are presented.     

First measurement of the helicity-dependent γ p?pη differential cross-section

The helicity dependence of the γ p↦pη reaction has been measured for the first time at a center-of-mass angle θ^* _η = 70^° in the photon energy range from 780 MeV to 790 MeV. The experiment, performed at the Mainz microtron MAMI, used a 4π-detector system, a circularly polarized, tagged photon beam, and a longitudinally polarized frozen-spin target. The helicity 3/2 cross-section is found to be small and the results for helicity 1/2 agree with predictions from the MAID analysis. Received: 19 December 2002 / Accepted: 10 March 2003 / Published online: 20 May 2003     

Neutral pion production in the threshold region Conclusive Symposium of the Collaborative Research Centre 443

We give an overview of the physics motivation and evolution of the neutral pion photoproduction measurements in the threshold region conducted in the A2 collaboration at MAMI. The latest two experiments have been performed with the almost 4π Crystal Ball detector. The first was with a linearly polarized photon beam and unpolarized liquid-hydrogen target. The data analysis is now complete and the linearly polarized beam asymmetry along with differential cross sections provide the most stringent test to date of the predictions of Chiral Perturbation Theory and its energy region of convergence. More recently a measurement was performed using both circularly polarized photons and a transversely polarized butanol frozen-spin target, with the goal of extracting both the target and beam-target asymmetries. From these we intend to extract πN scattering sensitive information for the first time in photo-pion reactions. This will be used to test isospin conservation and further test dynamics of chiral symmetry breaking in QCD as calculated at low energies by Chiral Perturbation Theory.     

Measurement of the G asymmetry for the γp↦Nπ channels in the Δ(1232) resonance region

The G asymmetry of the γp↦Nπ reaction has been measured for the first time for E_γ = 340±14 MeV. This observable, for which very scarce published data exist, plays an important role to disentangle the contributions of the various nucleon resonances. The experiment, performed at the Mainz microtron MAMI, used a 4π-detector system, a linearly polarized, tagged photon beam, and a longitudinally polarized proton target.     

Experimental check of the Gerasimov-Drell-Hearn sum rule for 1H

For the first time we checked the fundamental Gerasimov-Drell-Hearn (GDH) sum rule for the proton experimentally in the photon energy range from 0.2-2.9 GeV with the tagged photon facilities at MAMI (Mainz) and ELSA (Bonn). New data of the doubly polarized total cross section difference are presented in the energy range from 1.6 to 2.9 GeV. The contribution to the GDH integral from 0.2-2.9 GeV yields [254+/-5(stat)+/-12(syst)] microb with negative contributions in the Regge regime at photon energies above 2.1 GeV. This trend supports the validity of the GDH sum rule.     

First determination of generalized polarizabilities of the proton by a virtual compton scattering experiment

Absolute differential cross sections for the reaction ep-->epgamma have been measured at a four-momentum transfer with virtuality Q2 = 0.33 GeV2 and polarization epsilon = 0.62 in the range 33.6 to 111.5 MeV/c for the momentum of the outgoing photon in the photon-proton center of mass frame. The experiment has been performed with the high-resolution spectrometers at the Mainz Microtron MAMI. From the photon angular distributions, two structure functions which are a linear combination of the generalized polarizabilities have been determined for the first time.     

Measurement of the generalized polarizabilities of the proton in virtual Compton scattering at MAMI

Virtual Compton scattering off the proton has been studied at Q ² = 0.33 (GeV/c)² at the MAMI accelerator in Mainz (Germany). The goal of the experiment is to measure the generalized polarizabilities (GPs) of the proton using the double polarized e p→e' p'γ reaction. This paper reports the measurement of the unpolarized photon electroproduction cross-section for the new data and the extraction of two linear combinations of GPs, which are essential for the analysis of the double spin asymmetry.     

Exclusive electroproduction of K+Λ and K+Σ0 final states at Q2 = 0.030–0.055 (GeV/c)2

Cross-section measurements of the exclusive p(e, e??K ⁺)??, ?? ⁰electroproduction reactions have been performed at the Mainz Microtron MAMI in the A1 spectrometer facility using for the first time the KAOS spectrometer for kaon detection. These processes were studied in a kinematical region not covered by any previous experiment. The nucleon was probed in its third-resonance region with virtual photons of low four-momenta, Q ² = 0.030?C0.055 (GeV/c)². The MAMI data indicate a smooth transition in Q² from photoproduction to electroproduction cross-sections. Comparison with predictions of effective Lagrangian models based on the isobar approach reveal that strong longitudinal couplings of the virtual photon to the N^* resonances can be excluded from these models. Modern isobar and Regge-plus-resonance models are in agreement with the data.     

Parity-violating electron scattering at the MAMI facility in Mainz

We report here on a new measurement of the parity-violating (PV) asymmetry in the scattering of polarized electrons on unpolarized protons performed at the MAMI accelerator facility in Mainz. This experiment is the first to use counting techniques in a parity violation experiment. The kinematics of the experiment is complementary to the earlier measurements of the SAMPLE Collaboration at the MIT-Bates accelerator and the HAPPEX Collaboration at Jefferson Lab. After discussing the experimental context of the experiments, the setup at MAMI and preliminary results are presented.Received: 1 November 2002, Published online: 15 July 2003PACS: 12.15.-y Electroweak interactions - 25.30.Bf Elastic electron scatteringF.E. Maas: For the A4 Collaboration     

On photonuclear reaction mechanisms at intermediate energies

The¹²C(γ,2N) reactions have been studied using tagged photons ofE _γ = 114–600 MeV at the Mainz microtron (MAMI). The cross sections of the¹²C(γ,pn) reaction and the shape of¹²C(γ,pp) spectra compare well to model predictions over a wide range of photon energies and missing energies, suggesting that the dominant mechanisms of photonuclear reactions are understood.     

Near threshold photoproduction of η mesons from 4 He

Photoproduction of - mesons from ⁴He has been measured for the first time. The experiment was performed at the Mainz Microtron (MAMI) using the TAPS photon spectrometer and the Glasgow/Mainz tagged photon facility. - mesons were identified in coincidence with recoil nucleons. Total and differential cross sections are presented in the photon energy range between threshold and 818 MeV. The exclusive data are used to determine the ratio of the elementary production cross sections on bound neutrons and protons, C_n/C_p = 0.68 - 0.02 - 0.09. In addition, upper limits for the total coherent cross section have been derived.     

Helicity dependence of the \gamma N interaction and the GDH sum rule

First measurements of pion photoproduction using circularly polarized photons on longitudinally polarized protons were carried out at MAMI (Mainz) in the energy range -800 MeV. Results of the helicity dependence of the total inclusive photoabsorption cross-section and of the pion photoproduction channels will be presented. These data provide new input for multipole analyses and determine the main contribution to the Gerasimov-Drell-Hearn integral and the forward spin polarizability .Received: 30 September 2002, Published online: 22 October 2003PACS: 13.60.Le Meson production - 14.20.Gk Baryon resonances with S = 0P. Pedroni: For the GDH and A2 Collaborations     

Recent result from the A2 real photon facility at MAMI

The upgraded MAMI C accelerator is delivering electrons with an energy of 1558 MeV rou-tinely. The A2 collaboration is doing experiments with energy marked polarised real photons produced via "Bremsstrahlung". Recent results from the GDH-experiment using a longitudinal polarised frozen spin target in combination with the DAPHNE detector and from the Crystal Ball experiment at MAMI are presented.     

C( ,pn) photon asymmetry for Eγ=180–340 MeV

The photon asymmetry (Σ) of the ¹²C( ,pn) reaction has been measured using linearly polarised tagged photons at the Mainz microtron MAMI for E_γ=180–340 MeV. The data have been analysed in separate missing energy (E_m) regions corresponding to the ejection of nucleons from (1p)² and (1p)(1s) shells. The measured Σ values for both E_m regions are smaller in magnitude than corresponding ²H data, but the (1p)² results have a similar E_γ dependence. Calculations of direct two-nucleon emission overestimate the magnitude of Σ at all photon energies.     

Exclusive measurement of coherent η-photoproduction from the deuteron

Coherent photoproduction of η-mesons from the deuteron has been measured from threshold up to E _γ≈ 750 MeV using the photon spectrometer TAPS at the tagged photon facility at the Mainz microtron MAMI. For the first time, differential coherent cross-sections have been deduced from the coincident detection of the η-meson and the recoil deuteron. A missing-energy analysis was used for the suppression of background events so that a very clean identification of coherent η-photoproduction was achieved. The resulting cross-sections agree with previous experimental results except for angles around 90^° in the γd c.m. system where they are smaller. They are compared to various model calculations. Received: 5 April 2001 / Accepted: 3 September 2001     

Neutron detection using a crystal ball calorimeter

The program of experiments of the A2 Collaboration performed on a beam of tagged photons of the MAMI electron microtron in Mainz (Germany) includes precision measurements of the total and differential cross sections of the pion photoproduction on neutrons of a deuterium target. The determination of the detector ability to detect neutrons is undoubtedly one of the important problems of the experiment. The calorimetric system of the detector contains a segmented NaI Crystal Ball detector, which gives information about the position, energy, and detection time of neutral and charged particles in a wide angular range. In this work, we describe the measurement of the neutron detection efficiency in the energy range from 20 to 400MeV. The results are compared with BNL data obtained on a pion beam and proton target.     

Measurement of the correlation between electron spin and photon linear polarization in atomic-field bremsstrahlung

Atomic-field bremsstrahlung has been studied with a longitudinally polarized electron beam. The correlation between the initial orientation of the electron spin and the angle of photon polarization has been measured at the photon high energy tip region. In the time reversal this corresponds to a so-far unobserved phenomenon of production of longitudinally polarized electrons by photoionization of unpolarized atoms with linearly polarized photons. The results confirm the fully relativistic calculations for radiative recombination and suggest a new method for electron beam polarimetry.