44 research outputs found
Characterisation and calibration of a scintillating fibre detector with > 4000 multi-anode photomultiplier channels
In the Kaos spectrometer at the Mainz Microtron a high-resolution coordinate
detector for high-energy particles is operated. It consists of scintillating
fibres with diameters of 4000 multi-anode
photomultiplier channels. It is one of the most modern focal-plane detectors
for magnetic spectrometers world-wide. To correct variations in the detection
efficiency, caused by the different gains and the different optical
transmittances, a fully automated off-line calibration procedure has been
developed. The process includes the positioning of a radioisotope source
alongside the detector plane and the automated acquisition and analysis of the
detector signals. It was possible to characterise and calibrate each individual
fibre channel with a low degree of human interaction.Comment: Nucl. Instrum. Meth. A (2012
Helicity-dependent cross sections and double-polarization observable E in η photoproduction from quasifree protons and neutrons
Precise helicity-dependent cross sections and the double-polarization observable E were measured for η
photoproduction from quasifree protons and neutrons bound in the deuteron. The η → 2γ and η → 3π0 → 6γ
decay modes were used to optimize the statistical quality of the data and to estimate systematic uncertainties. The
measurement used the A2 detector setup at the tagged photon beam of the electron accelerator MAMI in Mainz.
A longitudinally polarized deuterated butanol target was used in combination with a circularly polarized photon
beam from bremsstrahlung of a longitudinally polarized electron beam. The reaction products were detected with
the electromagnetic calorimeters Crystal Ball and TAPS, which covered 98% of the full solid angle. The results
show that the narrow structure observed earlier in the unpolarized excitation function of η photoproduction off
the neutron appears only in reactions with antiparallel photon and nucleon spin (σ1/2). It is absent for reactions
with parallel spin orientation (σ3/2) and thus very probably related to partial waves with total spin 1/2. The
behavior of the angular distributions of the helicity-dependent cross sections was analyzed by fitting them with Legendre polynomials. The results are in good agreement with a model from the Bonn-Gatchina group, which
uses an interference of P11 and S11 partial waves to explain the narrow structure
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
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
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
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
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
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