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

Integrable systems and a moduli space for (1,6)-polarised abelian surfaces

A Hamiltonian system is a type of differential equation used in physics to describe the evolution of a mechanical system like a particle in a potential. Certain particularly well-behaved Hamiltonian systems are called integrable. For us an integrable system on C^(2n) is simply a set of n independent Poisson-commuting polynomials in 2n variables. In case the system is algebraically completely integrable the fibres of the induced map are affine parts of abelian varieties. In this thesis we study a projective model for the moduli-space of embedded (1,6)-polarised abelian surfaces first described by Gross and Popescu. We analyse its discriminant locus, the degenerations occurring, the form of the equations describing each surface and the automorphisms of this moduli space. In the last chapter we compute the cohomology of some quasi-homogeneous integrable systems on C^4.Ein Hamiltonsches System ist ein Typ von Differentialgleichung, der in der Physik benutzt wird um mechanische Systeme, wie zum Beispiel eine Punktmasse in einem Potential, zu beschreiben. Eine bestimmte Klasse Hamiltonscher Systeme, die sich besonders gut verhält, heißt integrabel. Für uns ist ein integrables System auf C^(2n) einfach eine Menge von n unabhängigen Poisson-kommutierenden Polynomen in 2n Variablen. Im Fall dass das System algebraisch vollständig integrabel ist, sind die Fasern der induzierten Abbildung affine Teile von abelschen Varietäten. In dieser Arbeit untersuchen wir ein projektives Model für den Modulraum von eingebetteten (1,6)-polarisierten abelschen Flächen, der erstmals von Gross und Popescu beschrieben wurde. Wir analysieren seine Diskriminante, die auftretenden Entartungen, die Form der Gleichungen, die jede Fläche beschreiben, und die Automorphismen dieses Modulraums. Im letzten Kapitel berechnen wir die Kohomologie einiger quasi-homogener integrabler Systeme auf C^4

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

First measurement of polarisation transfer Cx′nC^n_{x'} in deuteron photodisintegration

A first measurement of the polarisation transfer from a circularly-polarised photon to the final state neutron ($C^n_{x'}$) 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 $\sim45-120^{\circ}$. 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 $(n,p)$ charge exchange of the ejected neutrons to determine their polarisation. The new $C^n_{x'}$ 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 $C^n_{x'}$ observed in the region of the $d^*(2380)$ could not be explained by conventional models of deuteron photodisintegration

Single π0\pi^0 Production Off Neutrons Bound in Deuteron with Linearly Polarized Photons

The quasifree $\overrightarrow{\gamma} d\to\pi^0n(p)$ photon beam asymmetry, $\Sigma$, has been measured at photon energies, $E_\gamma$, 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$^\circ$. In this kinematic region, polarization observables are sensitive to contributions from the $\Delta (1232)$ and $N(1440)$ resonances. The extracted values of $\Sigma$ 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 $\Sigma$ measurements, has also been performed. In addition, isospin symmetry is examined as a way to predict $\pi^0n$ photoproduction observables, based on fits to published data in the channels $\pi^0p$, $\pi^+n$, and $\pi^-p$.Comment: 11 pages, 9 figures, 5 tables; fixed 2 glitche

Signatures of the d∗(2380)d^*(2380) hexaquark in d(γγ,pn⃗p\vec{n})

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 $E_{\gamma}\sim$~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 $d^*(2380)$ hexaquark

Spin polarizabilities of the proton by measurement of Compton double-polarization observables

The Compton double-polarization observable $\Sigma_{2z}$ has been measured for the first time in the $\Delta(1232)$ 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: $\gamma_{E1E1} = -3.18 \pm 0.52$, $\gamma_{M1M1} = 2.98 \pm 0.43$, $\gamma_{E1M2} = -0.44 \pm 0.67$ and $\gamma_{M1E2} = 1.58 \pm 0.43$, in units of $10^{-4}~\mathrm{fm}^{4}$.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