103 research outputs found
Neutron skin of Pb from Coherent Pion Photoproduction
Information on the size and shape of the neutron skin on Pb has been
extracted from coherent pion photoproduction cross sections measured using the
Crystal Ball together with the Glasgow tagger at the MAMI electron beam
facility. On exploitation of an interpolated fit of a theoretical model to the
measured cross sections the half-height radius and diffuseness of the neutron
distribution are found to be 6.70 fm and 0.55 fm respectively, corresponding to a neutron
skin thickness =0.15 fm.
The results give the first successful extraction of a neutron skin with an
electromagnetic probe and indicate the skin of Pb has a halo character.
The measurement provides valuable new constraints on both the structure of
nuclei and the equation of state for neutron-rich matter.Comment: 4 figures 5 pages. Version submitted to journal. Includes additional
studies of systematic effects in the extracted diffuseness, which led to a
small increase in the quoted systematic error. These additional studies are
discussed in the revised manuscript. Also includes minor editorial
improvements to the tex
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
Determination of the Dalitz plot parameter alpha for the decay eta->3pi^0 with the Crystal Ball at MAMI-B
A precise measurement of the Dalitz plot parameter, alpha, for the eta->3pi^0
decay is presented. The experiment was performed with the Crystal Ball and TAPS
large acceptance photon detectors at the tagged photon beam facility of the
MAMI-B electron accelerator in Mainz. High statistics of 1.8*10^6 eta->3pi^0
events were obtained, giving the result alpha = -0.032 +/- 0.002(stat) +/-
0.002(syst).Comment: 9 pages, 6 figures, published in the online-first section of EPJ A,
included changes referees asked for, added DO
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
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 double-polarized compton scattering asymmetries and extraction of the proton spin polarizabilities
The spin polarizabilities of the nucleon describe how the spin of the nucleon responds to an incident polarized photon. The most model-independent way to extract the nucleon spin polarizabilities is through polarized Compton scattering. Double-polarized Compton scattering asymmetries on the proton were measured in the Δ(1232) region using circularly polarized incident photons and a transversely polarized proton target at the Mainz Microtron. Fits to asymmetry data were performed using a dispersion model calculation and a baryon chiral perturbation theory calculation, and a separation of all four proton spin polarizabilities in the multipole basis was achieved. The analysis based on a dispersion model calculation yields γE1E1=−3.5±1.2, γM1M1=3.16±0.85, γE1M2=−0.7±1.2, and γM1E2=1.99±0.29, in units of 10−4  fm4
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
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