75 research outputs found
Photoproduction of -pairs off protons and off neutrons
Total cross sections, angular distributions, and invariant-mass distributions
have been measured for the photoproduction of pairs off free
protons and off nucleons bound in the deuteron. The experiments were performed
at the MAMI accelerator facility in Mainz using the Glasgow photon tagging
spectrometer and the Crystal Ball/TAPS detector. The accelerator delivered
electron beams of 1508 and 1557~MeV, which produced bremsstrahlung in thin
radiator foils. The tagged photon beam covered energies up to 1400~MeV. The
data from the free proton target are in good agreement with previous
measurements and were only used to test the analysis procedures. The results
for differential cross sections (angular distributions and invariant-mass
distributions) for free and quasi-free protons are almost identical in shape,
but differ in absolute magnitude up to 15\%. Thus, moderate final-state
interaction effects are present. The data for quasi-free neutrons are similar
to the proton data in the second resonance region (final state invariant masses
up to 1550~MeV), where both reactions are dominated by the
decay. At higher energies,
angular and invariant-mass distributions are different. A simple analysis of
the shapes of the invariant-mass distributions in the third resonance region is
consistent with strong contributions of an decay
for the proton, while the reaction is dominated by a sequential decay via a
intermediate state for the neutron. The data are compared to
predictions from the Two-Pion-MAID model and the Bonn-Gatchina coupled channel
analysis.Comment: accepted for publication in Eur. Phys. J.
Measurement of the beam-helicity asymmetry in the photoproduction of -pairs off protons and off neutrons
Beam-helicity asymmetries have been measured at the MAMI accelerator in Mainz
for the photoproduction of mixed-charge pion pairs in the reactions
off free protons and
and
off quasi-free nucleons bound
in the deuteron for incident photon energies up to 1.4 GeV. Circularly
polarized photons were produced from bremsstrahlung of longitudinally polarized
electrons and tagged with the Glasgow-Mainz magnetic spectrometer. The charged
pions, recoil protons, recoil neutrons, and decay photons from mesons
were detected in the 4 electromagnetic calorimeter composed of the Crystal
Ball and TAPS detectors. Using a complete kinematic reconstruction of the final
state, excellent agreement was found between the results for free and
quasi-free protons, suggesting that the quasi-free neutron results are also a
close approximation of the free-neutron asymmetries. A comparison of the
results to the predictions of the Two-Pion-MAID reaction model shows that the
reaction mechanisms are still not well understood, in particular at low
incident photon energies in the second nucleon-resonance region.Comment: accepted for publication in Eur. phys. J.
Photoproduction of π0-pairs off protons and off neutrons
Total cross sections, angular distributions, and invariant-mass distributions have been measured for the photoproduction of π0π0 pairs off free protons and off nucleons bound in the deuteron. The experiments were performed at the MAMI accelerator facility in Mainz using the Glasgow photon tagging spectrometer and the Crystal Ball/TAPS detector. The accelerator delivered electron beams of 1508 and 1557MeV, which produced bremsstrahlung in thin radiator foils. The tagged photon beam covered energies up to 1400MeV. The data from the free proton target are in good agreement with previous measurements and were only used to test the analysis procedures. The results for differential cross sections (angular distributions and invariant-mass distributions) for free and quasi-free protons are almost identical in shape, but differ in absolute magnitude up to 15%. Thus, moderate final-state interaction effects are present. The data for quasi-free neutrons are similar to the proton data in the second resonance region (final-state invariant masses up to ≈1550 MeV), where both reactions are dominated by the N(1520)3/2−→Δ(1232)3/2+π decay. At higher energies, angular and invariant-mass distributions are different. A simple analysis of the shapes of the invariant-mass distributions in the third resonance region is consistent with strong contributions of an N⋆→Nσ decay for the proton, while the reaction is dominated by a sequential decay via a Δπ intermediate state for the neutron. The data are compared to predictions from the Two-Pion-MAID model and the Bonn-Gatchina coupled-channel analysis
The isospin structure of photoproduction of pi-eta pairs from the nucleon in the threshold region
Photoproduction of -pairs from nucleons has been investigated from
threshold up to incident photon energies of ~1.4~GeV. The quasi-free
reactions , ,
, and were for
the first time measured from nucleons bound in the deuteron. The corresponding
reactions from a free-proton target were also studied to investigate
final-state interaction effects (for neutral pions the free-proton results
could be compared to previous measurements; the reaction was measured for the first time). For the
final state coherent production via the
reaction was also investigated. The experiments were performed at the tagged
photon beam of the Mainz MAMI accelerator using an almost coverage
electromagnetic calorimeter composed of the Crystal Ball and TAPS detectors.
The total cross sections for the four different final states obey the relation
as expected for a
dominant contribution from a
reaction chain,
which is also supported by the shapes of the invariant-mass distributions of
nucleon-meson and - pairs. The experimental results are compared to
the predictions from an isobar reaction model.Comment: accepted for publication in Phys. Lett.
Measurements of 12C(→γ,pp) photon asymmetries for Eγ= 200–450 MeV
The 12C (→γ ,pp) reaction has been studied in the photon energy range 200-450 MeV at the Mainz microtron MAMI-C, where linearly polarised photons were energy-tagged using the Glasgow-Mainz Tagged Photon Spectrometer and protons were detected in the Crystal Ball detector. The photon asymmetry Σ has been measured over a wider Eγ range than previous measurements. The strongest asymmetries were found at low missing energies where direct emission of nucleon pairs is expected. Cuts on the difference in azimuthal angles of the two ejected protons increased the magnitude of the observed asymmetries. At low missing energies the Σ data exhibit a strong angular dependence, similar to deuteron photodisintegration
Study of the gamma p --> pi0 pi0 p reaction with the Crystal Ball/TAPS at the Mainz
The gamma p --> pi0 pi0 p reaction has been measured from threshold to 1.4
GeV using the Crystal Ball and TAPS photon spectrometers together with the
photon tagging facility at the Mainz Microtron. The experimental results
include total and differential cross sections as well as specific angular
distributions, which were used to extract partial-wave amplitudes. In
particular, the energy region below the D13(1520) resonance was studied.Comment: 12 pages, 13 figures, corrected version, published in PR
A new determination of the eta transition form factor in the Dalitz decay eta -> e^+ e^- gamma with the Crystal Ball/TAPS detectors at the Mainz Microtron
The Dalitz decay eta -> e^+ e^- gamma has been measured in the gamma p -> eta
p reaction with the Crystal Ball and TAPS multiphoton spectrometers, together
with the photon tagging facility at the Mainz Microtron MAMI. The experimental
statistic used in this work is one order of magnitude greater than in any
previous measurement of eta -> e^+ e^- gamma. The value obtained for the slope
parameter 1/Lambda^2 of the eta transition form factor, 1/Lambda^2 = (1.95 +/-
0.15_stat +/- 0.10_syst) [1/GeV^2], is in good agreement with recent
measurements conducted in eta -> e^+ e^- gamma and eta -> mu^+ mu^- gamma
decays, as well as with recent form-factor calculations. The uncertainty
obtained in the value of 1/Lambda^2 is lower compared to results from previous
measurements of the eta -> e^+ e^- gamma decay.Comment: 11 pages, 10 figures, 1 tabl
Photoproduction of the meson off the proton near threshold
An experimental study of photoproduction on the proton was conducted
by using the Crystal Ball and TAPS multiphoton spectrometers together with the
photon tagging facility at the Mainz Microtron MAMI. The
differential cross sections are measured from threshold to the incident-photon
energy GeV ( GeV for the center-of-mass energy) with
15-MeV binning in and full production-angle coverage. The quality of
the present data near threshold gives access to a variety of interesting
physics aspects. As an example, an estimation of the scattering
length is provided.Comment: 8 pages, 5 figures. Published version; some typos were fixe
A new measurement of the rare decay eta -> pi^0 gamma gamma with the Crystal Ball/TAPS detectors at the Mainz Microtron
A new measurement of the rare, doubly radiative decay eta->pi^0 gamma gamma
was conducted with the Crystal Ball and TAPS multiphoton spectrometers together
with the photon tagging facility at the Mainz Microtron MAMI. New data on the
dependence of the partial decay width, Gamma(eta->pi^0 gamma gamma), on the
two-photon invariant mass squared, m^2(gamma gamma), as well as a new, more
precise value for the decay width, Gamma(eta->pi^0 gamma gamma) =
(0.33+/-0.03_tot) eV, are based on analysis of 1.2 x 10^3 eta->pi^0 gamma gamma
decays from a total of 6 x 10^7 eta mesons produced in the gamma p -> eta p
reaction. The present results for dGamma(eta->pi^0 gamma gamma)/dm^2(gamma
gamma) are in good agreement with previous measurements and recent theoretical
calculations for this dependence.Comment: 10 pages, 8 figures, 1 tabl
Measurement of the Slope Parameter for the decay with the Crystal Ball detector at the Mainz Microtron (MAMI-C)
The dynamics of the decay have been studied with the Crystal
Ball multiphoton spectrometer and the TAPS calorimeter. Bremsstrahlung photons
produced by the 1.5-GeV electron beam of the Mainz microtron MAMI-C and tagged
by the Glasgow photon spectrometer were used for -meson production. The
analysis of
events yields the value for the slope
parameter, which agrees with the majority of recent experimental results and
has the smallest uncertainty. The invariant-mass spectrum was
investigated for the occurrence of a cusplike structure in the vicinity of the
threshold. The observed effect is small and does not affect our
measured value for the slope parameter
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