Photoproduction of two neutral pions off the proton is studied using linearly polarized photons, and the polarization observables Is and Ic are measured for the first time. These two observables are ...unique to multi-meson final states; they characterize correlations between the linear photon polarization and the direction of outgoing single particles in photoproduction of three-body final states. The Is and Ic distributions suggest that, in the 1.8 to 2.0 GeV mass region, the N(1520)3/2−π intermediate state is reached with reaction dynamics consistent with a dominant JP=3/2+ wave. These data are included in the Bonn–Gatchina (BnGa) partial wave analysis which is based on a large variety of data; the analysis confirms a significant contribution from the reaction chain γp→N(1900)3/2+→N(1520)3/2−π0→pπ0π0.
The in-medium properties of the
ω
meson were investigated at the MAMI-C electron accelerator using the Crystal Ball/TAPS detector system to identify
ω
mesons through their
ω
→
π
0
γ
decay channel. ...Two solid targets (C, Nb) and a LH
2
target as a reference have been used. In the energy range 900 to 1300 MeV the
ω
lineshapes for the different nuclei are compared and a broadening of the
ω
signal for the niobium target is observed in comparison to the liquid-hydrogen target. In a second analysis the
ω
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.
We present new data on electron scattering from a range of nuclei taken in Hall C at Jefferson Lab. For heavy nuclei, we observe a rapid falloff in the cross section for $x>1$, which is sensitive to ...short range contributions to the nuclear wave-function, and in deep inelastic scattering corresponds to probing extremely high momentum quarks. This result agrees with higher energy muon scattering measurements, but is in sharp contrast to neutrino scattering measurements which suggested a dramatic enhancement in the distribution of the `super-fast' quarks probed at x>1. The falloff at x>1 is noticeably stronger in ^2H and ^3He, but nearly identical for all heavier nuclei.
Photoproduction of π0π0 and π0π± pairs from nuclei has been measured over a wide mass range (2H, 7Li, 12C, 40Ca, and natPb) for photon energies from threshold to 600 MeV. The experiments were ...performed at the MAMI accelerator in Mainz, using the Glasgow photon tagging spectrometer and a 4π electromagnetic calorimeter consisting of the Crystal Ball and TAPS detectors. A shift of the pion–pion invariant-mass spectra for heavy nuclei to small invariant masses has been observed for π0 pairs but also for the mixed-charge pairs. The precise results allow for the first time a model-independent analysis of the influence of pion final-state interactions. The corresponding effects are found to be large and must be carefully considered in the search for possible in-medium modifications of the σ-meson. Results from a transport model calculation reproduce the shape of the invariant-mass distributions for the mixed-charge pairs better than for the neutral pairs, but also for the latter differences between model results and experiment are not large, leaving not much room for σ-in-medium modification.
.
A new precise determination of the
meson mass is presented. It is based on a measurement of the threshold for the
reaction using the tagger focal-plane microscope detector at the MAMI-B facility in ...Mainz. The tagger microscope has a higher energy resolution than the standard tagging spectrometer and, hence, allowed an improvement in the accuracy compared to the previous
mass measurement at MAMI-B. Special emphasis was put on a very careful energy calibration of the electron beam and the tagging device, which reduced considerably the systematic uncertainty compared to the previous MAMI experiment. The result
MeV agrees very well with the precise values of the NA48, KLOE, CLEO and COSY-ANKE Collaborations and deviates by
from the smaller value obtained by the GEM Collaboration at COSY.
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