B. M. K. Nefkens

K{sup -}p{yields}{pi}{sup 0}{pi}{sup 0}{sigma}{sup 0} at p{sub K{sup -}}=514-750 MeV/c and comparison with other {pi}{sup 0}{pi}{sup 0} production

Reaction K{sup -}p{yields}{pi}{sup 0}{pi}{sup 0}{sigma}{sup 0} was measured with the Crystal Ball multiphoton spectrometer at eight K{sup -} momenta from 514 to 750 MeV/c. Dynamics and the energy dependence of the reaction are shown by Dalitz plots, invariant mass spectra, production angular distributions, and total cross sections. The dipion invariant-mass spectra for K{sup -}p{yields}{pi}{sup 0}{pi}{sup 0}{sigma}{sup 0} show an enhancement in the low mass region; the opposite is the case for the K{sup -}p{yields}{pi}{sup 0}{pi}{sup 0}{lambda} and {pi}{sup -}p{yields}{pi}{sup 0}{pi}{sup 0}n reactions. There is no direct evidence for a f{sub 0}(600)-meson contribution to {pi}{sup 0}{pi}{sup 0} production. Everywhere {sigma}{sub t}(K{sup -}p{yields}{pi}{sup 0}{pi}{sup 0}{sigma}{sup 0})<<{sigma}{sub t}(K{sup -}p{yields}{pi}{sup 0}{pi}{sup 0}{lambda})

Narrow structure in the excitation function of η photoproduction off the neutron

The photoproduction of η mesons off nucleons bound in 2H and 3He has been measured in coincidence with recoil protons and recoil neutrons for incident photon energies from threshold up to 1.4 GeV. The experiments were performed at the Mainz MAMI accelerator, using the Glasgow tagged photon facility. Decay photons from the η→2γ and η→3π0 decays and the recoil nucleons were detected with an almost 4π electromagnetic calorimeter combining the Crystal Ball and TAPS detectors. The data from both targets are of excellent statistical quality and show a narrow structure in the excitation function of γn→nη. The results from the two measurements are consistent, taking into account the expected effects from nuclear Fermi motion. The best estimates for position and intrinsic width of the structure are W=(1670±5)  MeV and Γ=(30±15)  MeV. For the first time precise results for the angular dependence of this structure have been extracted.

Quasifree photoproduction of η mesons off protons and neutrons

Differential and total cross sections for the quasifree reactions γp→ηp and γn→ηn have been determined at the MAMI-C electron accelerator by using a liquid deuterium target. Photons were produced via bremsstrahlung from the 1.5 GeV incident electron beam and energy tagged with the Glasgow photon tagger. Decay photons of the neutral decay modes η→2γ and η→3π0→6γ and coincident recoil nucleons were detected in a combined setup of the Crystal Ball and the TAPS calorimeters. The η-production cross sections were measured in coincidence with recoil protons, recoil neutrons, and in an inclusive mode without a condition on recoil nucleons, which allowed a check of the internal consistency of the data. The effects from nuclear Fermi motion were removed by a kinematic reconstruction of the final-state invariant mass and possible nuclear effects on the quasifree cross section were investigated by a comparison of free and quasifree-proton data. The results, which represent a significant improvement in statistical quality compared to previous measurements, agree with the known neutron-to-proton cross-section ratio in the peak of the S11(1535) resonance and confirm a peak in the neutron cross section, which is absent for the proton, at a center-of-mass energy W=(1670±5) MeV with an intrinsic width of Γ≈30 MeV.

The New Determination of the ηN S-wave Scattering Length From a Three-Channel, Multi-Resonance Amplitude Analysis

A three-channel, multi-resonance, unitary model developed in 1995 is used to determine the πN → ηN and ηN → ηN amplitudes by re-analyzing the available data on πN elastic scattering and the weighted data for the πN → ηN total and differential cross sections. The input πN elastic scattering amplitude in the S11 channel has been improved, following suggestions of G. Hohler. Our new result of ηN scattering length, aηN = (0.717±0.030)+i (0.263±0.025) fm, suggests that the ηd system is unbound or loosely bound.

TEST OF CHARGE SYMMETRY IN PI+ AND PI- ELASTIC SCATTERING ON TRITIUM AND HE-3

Elastic scattering of 180-MeV ..pi../sup +/ and ..pi../sup -/ on /sup 3/He and tritium has been measured from theta/sub ..pi../(c.m.) = 44/sup 0/ to 96/sup 0/. The superratio Requivalent(dsigma(..pi../sup +/+/sup 3/H)/dsigma(..pi../sup -/+/sup 3/He))/(dsigma(..pi../sup +/+/sup 3/He) dsigma(..pi../sup -/+/sup 3/H)) is found to vary with angle reaching a maximum of 1.31 +- 0.09 at theta/sub ..pi../ = 65/sup 0/. The direct Coulomb effects do not account for the observed differences of R from 1.0 which is the expected value on the basis of charge symmetry.

Radiative pi(0) photoproduction on protons in the Delta(+)(1232) region

The reaction γp → pπ0γ′ has been measured with the Crystal Ball/TAPS detectors using the energy-tagged photon beam at the electron accelerator facility MAMI-B. Energy and angular differential cross-sections for the emitted photon γ′ and angular differential cross-sections for the π0 have been determined with high statistics in the energy range of the Δ+ (1232)-resonance. Cross-sections and the ratio of the cross-section to the nonradiative process γp → pπ0 are compared to theoretical reaction models, having the anomalous magnetic moment κΔ as free parameter. As the shape of the experimental distributions is not reproduced in detail by the model calculations, currently no extraction of κΔ is feasible.

Incoherent neutral pion photoproduction on C-12

We present the first detailed measurement of incoherent photoproduction of neutral pions to a discrete state of a residual nucleus. The 12C(gamma,pi(0))(12)C*(4.4 MeV) reaction has been studied with the Glasgow photon tagger at MAMI employing a new technique which uses the large solid angle Crystal Ball detector both as a pi(0) spectrometer and to detect decay photons from the excited residual nucleus. The technique has potential applications to a broad range of future nuclear measurements with the Crystal Ball and similar detector systems elsewhere. Such data are sensitive to the propagation of the Delta in the nuclear medium and will give the first information on matter transition form factors from measurements with an electromagnetic probe. The incoherent cross sections are compared to two theoretical predictions including a Delta-hole model.

Reaction K- p ---> pi0 pi0 lambda from pK- = 514-MeV/c to 750-MeV/c

Reaction

Test of charge symmetry in. pi. /sup +/ and. pi. /sup -/ elastic scattering on tritium and /sup 3/He

{K}^{\ensuremath{-}}p\ensuremath{\rightarrow}{\ensuremath{\pi}}^{0}{\ensuremath{\pi}}^{0}\ensuremath{\Lambda}

Measurement of the slope parameter {alpha} for the {eta}{yields}3{pi}{sup 0} decay with the Crystal Ball detector at the Mainz Microtron (MAMI-C)

was measured at eight incident