29 research outputs found
First measurement of polarisation transfer in deuteron photodisintegration
A first measurement of the polarisation transfer from a circularly-polarised
photon to the final state neutron () in deuterium photodisintegration
has been carried out. This quantity is determined over the photon energy range
370~--~700~MeV and for neutron centre-of-mass breakup angles
. The polarisation of the final state neutrons was
determined by an ancillary large-acceptance nucleon polarimeter, surrounding a
cryogenic liquid deuterium target within the Crystal Ball detector at MAMI. The
polarimeter characterised charge exchange of the ejected neutrons to
determine their polarisation. The new data are also compared to a
theoretical model based on nucleonic and nucleon resonance degrees of freedom
constrained by the current world-database of deuterium photodisintegration
measurements. Structures in observed in the region of the
could not be explained by conventional models of deuteron
photodisintegration
Evaluation of the E2/M1 ratio in the transition from the reaction
A new data set for the helicity-dependent differential cross section of the
single-meson photoproduction reaction was obtained for
the photon energy interval 150-400 MeV. The experiment was performed at the A2
tagged photon facility of the Mainz Microtron MAMI using a circularly polarized
photon beam and a longitudinally polarized proton target. The reaction products
were detected with the large acceptance Crystal Ball/TAPS calorimeter covering
97\% of the full solid angle. These new results, obtained with a fine energy
and polar angle binning, greatly increase both the existing quantity and
quality of the data available for this observable. A moment analysis, based on
a finite expansion in Legendre polynomials, was applied to these data by using
a bootstrap-based fitting method to correctly account for their systematic
uncertainties. From the resulting decomposition of the differential cross
sections, the ratio for the transition was
determined to be . Combining this value with previous results also allowed us to
evaluate the most precise available estimate of the ratio to be used
for all further reference and model comparisons.Comment: 16 pages, 14 figure
Single Production Off Neutrons Bound in Deuteron with Linearly Polarized Photons
The quasifree photon beam asymmetry,
, has been measured at photon energies, , from 390 to 610
MeV, corresponding to center of mass energy from 1.271 to 1.424 GeV, for the
first time. The data were collected in the A2 hall of the MAMI electron beam
facility with the Crystal Ball and TAPS calorimeters covering pion
center-of-mass angles from 49 to 148. In this kinematic region,
polarization observables are sensitive to contributions from the and resonances. The extracted values of have been
compared to predictions based on partial-wave analyses (PWAs) of the existing
pion photoproduction database. Our comparison includes the SAID, MAID, and
Bonn-Gatchina analyses; while a revised SAID fit, including the new
measurements, has also been performed. In addition, isospin symmetry is
examined as a way to predict photoproduction observables, based on
fits to published data in the channels , , and .Comment: 11 pages, 9 figures, 5 tables; fixed 2 glitche
Signatures of the hexaquark in d(,)
We report a measurement of the spin polarisation of the recoiling neutron in deuterium photodisintegration, utilising a new large acceptance polarimeter within the Crystal Ball at MAMI. The measured photon energy range of 300~--~700~MeV provides the first measurement of recoil neutron polarisation at photon energies where the quark substructure of the deuteron plays a role, thereby providing important new constraints on photodisintegration mechanisms. A very high neutron polarisation in a narrow structure centred around ~570~MeV is observed, which is inconsistent with current theoretical predictions employing nucleon resonance degrees of freedom. A Legendre polynomial decomposition suggests this behaviour could be related to the excitation of the hexaquark
Spin polarizabilities of the proton by measurement of Compton double-polarization observables
The Compton double-polarization observable has been measured
for the first time in the resonance region using a circularly
polarized photon beam incident on a longitudinally polarized target at the
Mainz Microtron. This paper reports these results, together with the
model-dependent extraction of four proton spin polarizabilities from fits to
additional asymmetry data using dispersion relation and chiral perturbation
theory calculations, with the former resulting in: , , and
, in units of .Comment: 7 pages, 5 figures, 2 tables, accepted for publication in PR
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
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
Helicity-dependent cross sections for the photoproduction of pairs from nucleons
The double-polarization observable and helicity-dependent cross sections
, have been measured for the photoproduction of
pairs off quasi-free protons and neutrons at the Mainz MAMI accelerator
with the Crystal Ball/TAPS setup. A circularly polarized photon beam was
produced by bremsstrahlung from longitudinally polarized electrons and impinged
on a longitudinally polarized deuterated butanol target. The reaction products
were detected with an almost covering calorimeter. The results reveal
for the first time the helicity- and isospin-dependent structure of the reaction. They are compared to predictions from
reaction models in view of nucleon resonance contributions and also to a refit
of one model that predicted results for the proton and for the neutron target.
The comparison of the prediction and the refit demonstrate the large impact of
the new data.Comment: Submitted to Phys. Rev. Let
Measurement of the beam-helicity asymmetry in photoproduction of π0η pairs on carbon, aluminum, and lead
The beam-helicity asymmetry was measured, for the first time, in photoproduction of
π0η pairs on carbon, aluminum, and lead, with the A2 experimental setup at MAMI. The results are compared to an earlier measurement on a free proton and to the corresponding theoretical calculations. The Mainz model is used to predict the beam-helicity asymmetry for the nuclear targets. The present results indicate that the photoproduction mechanism for π0η pairs on nuclei is similar to photoproduction on a free nucleon. This process is dominated by the D33 partial wave with the ηΔ(1232) intermediate state
Deuteron photodisintegration by polarized photons in the region of the d⁎ (2380)
We report the first large-acceptance measurement of the beam–spin asymmetry for deuteron photodisintegration (γ→d→pn) in the photon energy range 420<E γ<620MeV. The measurement provides important new constraints on the mechanisms of photodisintegration above the Δ resonance and on the photocoupling of the recently discovered d ⁎(2380) hexaquark