37 research outputs found
Proof-of-principle of a new geometry for sampling calorimetry using inorganic scintillator plates
A novel geometry for a sampling calorimeter employing inorganic scintillators
as an active medium is presented. To overcome the mechanical challenges of
construction, an innovative light collection geometry has been pioneered, that
minimises the complexity of construction. First test results are presented,
demonstrating a successful signal extraction. The geometry consists of a
sampling calorimeter with passive absorber layers interleaved with layers of an
active medium made of inorganic scintillating crystals. Wavelength-shifting
(WLS) fibres run along the four long, chamfered edges of the stack,
transporting the light to photodetectors at the rear. To maximise the amount of
scintillation light reaching the WLS fibres, the scintillator chamfers are
depolished. It is shown herein that this concept is working for cerium fluoride
(CeF) as a scintillator. Coupled to it, several different types of
materials have been tested as WLS medium. In particular, materials that might
be sufficiently resistant to the High-Luminosity Large Hadron Collider
radiation environment, such as cerium-doped Lutetium-Yttrium Orthosilicate
(LYSO) and cerium-doped quartz, are compared to conventional plastic WLS
fibres. Finally, an outlook is presented on the possible optimisation of the
different components, and the construction and commissioning of a full
calorimeter cell prototype is presented.Comment: Submitted to Proceedings CALOR 2014, the 16th International
Conference on Calorimetry in High-Energy Physics, Giessen (Germany) 6 - 11
April 2014. To be published in Journal of Physics: Conference Series (10
pages, 15 figures
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
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 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 pairs on nuclei is similar to photoproduction on a free nucleon. This process is dominated by the partial wave with the intermediate state
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
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
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
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
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
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
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