189 research outputs found
and photoproduction with fine center-of-mass energy resolution
Measurements of and cross-sections have been obtained with the photon
tagging facility and the Crystal Ball calorimeter at MAMI-C. The measurement
uses a novel meson identification technique in which the weak decay
products are characterized using the energy and timing characteristics of the
energy deposit in the calorimeter, a method that has the potential to be
applied at many other facilities. The fine center-of-mass energy ()
resolution and statistical accuracy of the new data results in a significant
impact on partial wave analyses aiming to better establish the excitation
spectrum of the nucleon. The new analyses disfavor a strong role for
quark-diquark dynamics in the nucleon.Comment: 7 pages, 8 figure
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
Determination of the eta-transition form factor in the gamma p -< p eta -< p gamma e(+)e(-) reaction
The Dalitz decay eta -> gamma e(+)e(-) has been measured using the combined Crystal Ball and TAPS photon detector setup at the electron accelerator MAMI-C. Compared to the most recent transition form-factor measurement in the e(+)e(-) channel, statistics have been improved by one order of magnitude. The e(+)e(-)invariant mass distribution shows a deviation from the QED prediction for a point-like particle, which can be described by a form-factor. Using the usual monopole transition form-factor parameterization, F(m(2)) = (1 - m(2)/Lambda(2))(-1), a value of Lambda(-2) = (1.92 +/- 0.35(stat) +/- 0.13(syst)) GeV(-2) has been determined. This value is in good agreement with a recent measurement of the eta Dalitz decay in the mu(+)mu(-) channel and with recent form-factor calculations. An improved value of the branching ratio BR(eta -> gamma e(+)e(-)) = (6.6 +/- 0.4(stat) +/- 0.4(syst)) . 10(-3) has been determined. (C) 2011 Elsevier B.V. All rights reserved
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
First measurement of the circular beam asymmetry in the gamma p --> pi0 eta p reaction
The circular photon asymmetry for pi0 eta photoproduction on the proton was
measured for the first time at the tagged photon facility of the MAMI C
accelerator using the Crystal Ball/TAPS photon spectrometer. The experimental
results are interpreted within a phenomenological isobar model that confirms
the dominant role of the Delta(1700)D33 resonance. The measured asymmetry
allows us to identify small contributions from positive-parity resonances via
interference terms with the dominant D33 amplitude.Comment: 11 pages, 3 figures, submitted to Phys.Lett.
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
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
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
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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