Transverse spin asymmetries in \(\mathbf{\pi}\) and \(\mathbf{\eta}\) photoproduction
Transverse target (T) and beam-target (F) asymmetries for the \(\gamma p\to\pi^0 p\) and \(\gamma p\to\eta p\) reactions were measured with the Crystal Ball detector at MAMI during the first half of the funding period.The data are fully analysed and first results are already published. 3 additional publications are under preparation. In these measurements a spin-polarised nucleon target (P ≈ 70%) was used for more than 2000h per year without any failure. A second data set with improved trigger and DAQ will be measured in 2015.
Using all MAMI results for \(d_\sigma /d_{\Omega,\Sigma,T~and~F}\), a model independent extraction of all relevant s- and p-wave multipole amplitudes close to the energy threshold of the \(\gamma p\to\pi^0 p\) reaction will be possible for the first time.
Helicity dependence of \(\pi^0\)photoproduction
Data with longitudinal target polarisation were measured in 2013/14. These data will allow us to extract the spin observables E and G in \(\pi^0\)production. The data analysis has started. First preliminary results are expected in 2015. Further measurements with improved trigger and DAQ are foreseen in 2015.
Photoproduction of meson pairs
The \(\gamma N\to\pi^0\pi^0 N\) and \(\gamma N\to\pi^0\eta N\) reaction provide a complementary access to nucleon and \(/Delta\) excitations via sequential decays like \(\Delta^\ast\to\Delta(1232)\eta\to N\pi^0\eta\). The study of these reactions does not require additional beamtime as they can be measured in parallel with single\(\pi\) and \(\eta\) production. First results on transverse asymmetries for the \(\gamma p\to\pi^0\eta p\) reactions are submitted for publication in PRC.
High precision measurement of the \(\mathbf{\gamma p\to\pi^0 p}\) cross section
The cross section of the \(\gamma p\to\pi^0 p\) was measured from threshold up to 1500 MeV with unprecedented precision. A publication is under preparation.
Photoproduction of neutrons
In addition to measurements with the polarised proton (butanol) target, data for quasifree \(\pi^0\) and \(\eta\) production from polarised deuterons (D-butanol) were obtained. The data are presently analysed in the group of Prof. B. Krusche in Basel. A discussion about strategies to extract neutron amplitudes from these measurements has started.
Development of an internal polarising coil
A solenoid (10-layer 0.2mm Nb-Ti, 2 layer correction coils, homogeneity of 10−3 at 2.3T) was constructed an tested at 4.2K. Field and homogeneity are sufficient to polarise protons (butanol) in-situ during data taking. Further studies to increase the homogeneity are planned in the framework of the proposed EU program CRYPTA (CRYogenic Polarised Target Applications).
Upgrade of the A2 DAQ and trigger system
In 2012/13 the DAQ and trigger system of the Crystal Ball experiment was significantly improved with main contributions from the projects S3 and M2. Old NIM and CAMAC components were replaced. The trigger logic is now fully implemented in FPGA-based VME modules. The readout hard- and software of the forward spectrometer (TAPS) was completely exchanged. In 2012 the typical DAQ rate was 900Hz (60% lifetime) with in- stabilities due to the coupling of different readout schemes for TAPS and the CB. In 2013 we had a stable operation at 3-4kHz (60% lifetime) with significantly improved trigger capabilities (cluster algorithm for multiplicity triggers, inclusion of TAPS and the PID, coplanarity).
Development of strategies and software for a model independent PWA
The main goal of the project is the reliable extraction of single-energy partial wave amplitudes for \(\pi\) and \(\eta\) photoproduction with a minimum number of model constraints. Methods and tools to extract partial wave amplitudes from spin-dependent cross sections have been developed and tested with pseudo-data. Preliminary results with experimental data have been obtained and presented at conferences.
