46 research outputs found
Quasifree photoproduction of mesons off protons and neutrons
Differential and total cross sections for the quasifree reactions and have been determined at the
MAMI-C electron accelerator 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 and and coincident recoil nucleons were detected in a combined setup of
the Crystal Ball and the TAPS calorimeters. The -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 resonance and confirm a peak in the
neutron cross section, which is absent for the proton, at a center-of-mass
energy MeV with an intrinsic width of MeV
Quasi-free photoproduction of η-mesons off 3He nuclei
Quasi-free photoproduction of η-mesons has been measured off nucleons bound in 3He nuclei for incident photon energies from the threshold region up to 1.4 GeV. The experiment was performed at the tagged photon facility of the Mainz MAMI accelerator with an almost 4π covering electromagnetic calorimeter, combining the TAPS and Crystal Ball detectors. The η-mesons were detected in coincidence with the recoil nucleons. This allowed a comparison of the production cross section off quasi-free protons and quasi-free neutrons and a full kinematic reconstruction of the final state, eliminating effects from nuclear Fermi motion. In the S11(1535) resonance peak, the data agree with the neutron/proton cross section ratio extracted from measurements with deuteron targets. More importantly, the prominent structure observed in photoproduction off quasi-free neutrons bound in the deuteron is also clearly observed. Its parameters (width, strength) are consistent with the expectations from the deuteron results. On an absolute scale the cross sections for both quasi-free protons and neutrons are suppressed with respect to the deuteron target pointing to significant nuclear final-state interaction effects
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
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
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
Experimental study of the , , and reactions at the Mainz Microtron
This work measured for neutral kaon photoproduction
reactions from threshold up to a c.m.\ energy of 1855MeV, focussing
specifically on the , , and reactions. Our results for
are the first-ever measurements for that
reaction. These data will provide insight into the properties of
resonances and, in particular, will lead to an improved knowledge about those
states that couple only weakly to the channel. Integrated cross
sections were extracted by fitting the differential cross sections for each
reaction as a series of Legendre polynomials and our results are compared with
prior experimental results and theoretical predictions.Comment: 16 pages, 13 figures, 7 table
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
High-statistics measurement of the η→3π0 decay at the Mainz Microtron
The largest, at the moment, statistics of 7 x 10(6) eta -> 3 pi(0) decays, based on 6.2 x 10(7) eta mesons produced in the gamma p -> eta p reaction, has been accumulated by the A2 Collaboration at the MainzMicrotron, MAMI. It allowed a detailed study of the eta -> 3 pi(0) dynamics beyond its conventional parametrization with just the quadratic slope parameter a and enabled, for the first time, a measurement of the second-order term and a better understanding of the cusp structure in the neutral decay. The present data are also compared to recent theoretical calculations that predict a nonlinear dependence along the quadratic distance from the Dalitz-plot center
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