V. Kunde

The three-spectrometer facility at the Mainz microtron MAMI

Abstract A set-up of three high-resolution magnetic spectrometers, for simplicity named A, B and C, has been built as the central facility for the precise determination of double and triple coincidence cross sections of hadron knock-out and meson production through the scattering of electrons at the Mainz microtron MAMI. The spectrometers A and C with point-to-point optics in the dispersive plane and parallel-to-point optics in the non-dispersive plane have a solid angle of 28 msr and a momentum acceptance of 20 and 25%, respectively. They each consist of a quadrupole, a sextupole and two dipole magnets, reaching maximum momenta of 735 and 550 MeV/c, respectively. The spectrometer B has a solid angle of 5.6 msr and a somewhat reduced momentum acceptance of 15%, but it reaches a maximum momentum larger than that of the MAMI electron beam (855 MeV/c). It consists of a single-clamshell dipole magnet with point-to-point optics in both planes. Each spectrometer is equipped with a position-sensitive detector system consisting of four planes of vertical drift chambers, two planes of plastic scintillators and a threshold gas Cherenkov detector. The operational experiences demonstrate that all three spectrometers exceed the specifications. Selected results of double (e, e′ x ) and triple (e, e′ x 1 x 2 ) coincidence experiments, x 1 and x 2 standing for charged hadrons, are presented, which demonstrate the performance of the whole set-up.

Precise measurements of the neutron magnetic form-factor

Abstract The neutron magnetic form factor Gmn has been determined via a measurement of the ratio of cross sections D(e,e′n) and D(e,e′p). The absolute detection efficiency of the neutron detector was measured with high accuracy using tagged neutrons produced from H(n,p)n elastic scattering by means of a high intensity neutron beam. This approach minimizes the model dependence and improves upon the weakest points of previous experiments. Data in the range q2=0.2–0.8 (GeV/c)2 with uncertainties of

A measurement of the axial form factor of the nucleon by the p(e,e′π+)n reaction at

Abstract The reaction p(e,e′ π + )n was measured at the Mainz Microtron MAMI at an invariant mass of W=1125 MeV and four-momentum transfers of Q 2 =0.117, 0.195 and 0.273 ( GeV/c ) 2 . For each value of Q 2 , a Rosenbluth separation of the transverse and longitudinal cross sections was performed. An effective Lagrangian model was used to extract the `axial mass from experimental data. We find a value of M A =(1.077±0.039) GeV which is (0.051±0.044) GeV larger than the axial mass known from neutrino scattering experiments. This is consistent with recent calculations in chiral perturbation theory.

Investigation of short-range nucleon-nucleon correlations using the reaction 12C( e,e′ pp) in close to 4π geometry

Abstract Two-proton correlations were studied in close to 4 π geometry using the reaction 12 C (e,e′pp) . The beam energy was 705xa0MeV, the energy transfer 225xa0MeV, and the three-momentum transfer 412xa0MeV/c. The electrons were observed in a large-acceptance magnetic spectrometer in coincidence with protons observed in a BGO crystal ball. Missing energy and momentum, relative momentum and angular distributions were derived and compared with a factorized two-nucleon emission model. Soft-core correlation functions are favoured by the data.

Large recoil momenta in the D(e,e′p)n reaction

Abstract The D(e,e′p)n reaction cross section has been measured for recoil momenta ranging from 17 MeV/c up to 950 MeV/c at momentum transfers between 600 MeV/c and 700 MeV/c. At recoil momenta above 400 MeV/c, the gross features of the cross section are only reproduced if virtual nucleon excitations are included in the calculations.

High-momentum components in the 1p orbitals of 16O

Abstract We have investigated that part of the proton momentum distribution of 16O that is linked through the (e, e′p) reaction to low-lying discrete states in 15N. The MAMI facility with its new generation of 100%-duty-cycle electron accelerator has allowed to cover in this experiment the previously not accessible momentum range of 265

Pion electroproduction in the 3He(e,é π+)3H reaction at intermediate energies

Abstract The differential coincident pion electroproduction cross section in the 3 He(e,e π + ) 3 H reaction has been measured with high resolution with the three-spectrometer set-up at the Mainz Microtron (MAMI) electron accelerator. Measurements were performed at the four incident energies E 0 = 855, 675, 600, and 555 MeV at fixed four-momentum transfer Q 2 = 0.045 GeV 2 , with the pions detected in parallel kinematics. This enables a separation of the measured cross section into the transverse and longitudinal structure functions by means of the Rosenbluth method. The experimental data are compared to model calculations, in which the elementary pion production amplitude includes the standard Born-amplitudes and also delta and higher resonance terms. Three-body Fadeev wave functions are used and the final state interaction of the outgoing pion is taken into account. The calculation describes the data only after medium modifications of the delta and of the pion are included.

Precise pion electroproduction in thep(e, e′ π+)n reaction at W=1125 MeV

The reactione+p →> e+π++n at c.m. energyW=1125MeV and momentum transfer Q2=0.117GeV2/c2 has been measured. The transverse and longitudinal structure functions have been separated by varying the polarization of the virtual photon (Rosenbluth plot) with a 3 to 4% error. In addition the longitudinal-transverse interference term has been determined measuring the right-left asymmetry with an accuracy of 3%. The experimental data are compared to model calculations, and the sensitivity of the results to the axial and pion formfactors is discussed.

fLTresponse function of2H(e,e'p)natQ2=0.33(GeV/c)2

The interference response function f_LT (R_LT) of the D(e,ep)n reaction has been determined at squared four-momentum transfer Q^2 = 0.33 (GeV/c)^2 and for missing momenta up to p_miss= 0.29 (GeV/c). The results have been compared to calculations that reproduce f_LT quite well but overestimate the cross sections by 10 - 20% for missing momenta between 0.1 (GeV/c) and 0.2 (GeV/c) .

No evidence for medium effects in the12C(e, e'p)11Bg.s. reaction

The12C(e, ep)11Bg.s reaction has been measured in parallel and anti-parallel kinematics over apm range of −120<pm<120 MeV/c at a centralTp of 90 MeV. In contrast to previous measurements at NIKHEF, both kinematics can be described without enhancement of the transverse form factor of the bound proton (η=0.98(5)). The spectroscopic factor for this transition as determined with the new Mainz data (Sα=1.54(5)) is 22% smaller than the NIKHEF value.