122005636
New Spin-Physics Experiments with Frozen-Spin Target at MAMI
In the physics program for the MAMI C accelerator the investigation of the spin dependent excitation spectrum of the nucleon and fundamental properties, such as the spin polarizabilities of the nucleon are an important goal. This will be tackled in the framework of the A2‐collaboration by doing double polarization experiments, using the linearly or circularly polarized energy marked photon beam in combination with a new solid state polarized target. The horizontal dilution refrigerator of the Frozen Spin Target was constructed by and is operated in close cooperation with, the Joint Institute for Nuclear Research in Dubna, Russia. The system offers both longitudinally and transversely polarized protons and deuterons. The target material, butanol and deuterated butanol, was developed and produced by the Ruhr‐University of Bochum and provides the highest degree of polarization, measured with a new NMR apparatus. The status of the experiment and preliminary results will be presented.
122902871
An internal superconducting “holding-coil” for frozen spin targets
Abstract A new concept of a small superconducting holding magnet, placed inside a polarizing refrigerator, has been developed for frozen spin targets. The superconducting wire has been wound on the inner cooling shield of the vertical dilution refrigerator of the Bonn frozen spin target. The maximum field of the magnet is 0.35 T. The total thickness of the superconducting coil consisting of the wire and the copper carrier is of the order of 500 μm. Based on this concept, a frozen spin target is under construction for the measurement of the Gerasimov-Drell-Hearn sum rule with polarized real photons at the Mainz microtron MAMI and the Bonn electron stretcher accelerator ELSA.
121573165
Polarization reversal of proton spins in a solid-state target by superradiance
The new Bonn frozen spin target developed for spin physics experiments with tagged photons from the Bonn electron stretcher accelerator ELSA has successfully been used in pion photoproduction experiments. Proton polarization values of −99% and +94% in ammonia, −96% and +90% in butanol have been achieved in a dilution refrigerator at a magnetic field of 3.53 T. At a temperature below 100 mK and a magnetic field of 0.35 T a very fast spontaneous polarization reversal within a time interval of 800 μs has been observed. This effect occurred only at negative polarization and has been identified as a collective spontaneous emission — i.e. superradiance — in the two level proton spin system. Detailed investigations have been made in order to study whether this effect is useful for a fast polarization reversal of the entire sample. Up to now the highest polarization reversal from −90% to +60% has been obtained in a target volume of 6.5 cm3.
123483800
The Gerasimov-Drell-Hearn sum rule at MAMI
Abstract.An extended experimental program to investigate the Gerasimov-Drell-Hearn (GDH) sum rule and related partial reaction cross sections on proton and neutron has been carried out by the GDH collaboration at the electron accelerators MAMI (Mainz) and ELSA (Bonn). The GDH sum rule connects the helicity-dependent photoabsorption cross section with the anomalous magnetic moment of the nucleon. The GDH collaboration has measured the total cross section of circularly polarised photons with longitudinally polarised protons at MAMI and ELSA to check this sum rule experimentally. In addition partial reaction channels like pion, double pion and eta production were determined. This provides new information on the helicity-dependent excitation spectrum of the nucleon. With the help of partial wave analyses it is possible to extract new, complementary information on the broad, overlapping resonances in this energy region. The double polarisation observable E measured in this experiment enhances the smaller multipoles via interference terms. The analysis of our data provides a new possibility to study the photon couplings to the nucleon resonances, especially above the Δ resonance where many properties of the observed states (e.g., coupling constants, branching ratios, helicity amplitudes) are only poorly known. In this paper we present several new results from our measurements on polarised proton and deuteron targets.
19511077
Narrow structure in the excitation function of η photoproduction off the neutron.
The photoproduction of η mesons off nucleons bound in 2H and 3He has been measured in coincidence with recoil protons and recoil neutrons for incident photon energies from threshold up to 1.4 GeV. The experiments were performed at the Mainz MAMI accelerator, using the Glasgow tagged photon facility. Decay photons from the η→2γ and η→3π0 decays and the recoil nucleons were detected with an almost 4π electromagnetic calorimeter combining the Crystal Ball and TAPS detectors. The data from both targets are of excellent statistical quality and show a narrow structure in the excitation function of γn→nη. The results from the two measurements are consistent, taking into account the expected effects from nuclear Fermi motion. The best estimates for position and intrinsic width of the structure are W=(1670±5)  MeV and Γ=(30±15)  MeV. For the first time precise results for the angular dependence of this structure have been extracted.
120916993
Photoproduction of positive pions from polarized protons
Abstract Target asymmetry data for single positive pion photoproduction from polarized protons have been obtained at the Bonn Electron Stretcher Accelerator ELSA. Data were taken in a wide kinematical range of E γ = 215–800 MeV and Θ CMS π = 35°–135° with the PHOENICS-detector. This detector permits a considerable kinematical overdetermination so that the nominal process could be clearly separated from the background. For the first time a combination of a frozen spin target with a tagged photon beam has been used. Butanol (C 4 H 9 OH) was used as target material with a maximum proton polarization of more than ±90%. Due to the simultaneous energy and angle coverage the systematical error in the order of ΔT sys = ±2.5% is common for all data points. The results at low photon energies show significant deviations from existing multipole analyses.
116268396
Charge Symmetry in the Parton Model
119428296
Virtual Compton scattering from the proton and the properties of nucleon excited states
We calculate the N * contributions to the generalised polarisabilities of the proton in virtual Compton scattering. The following nucleon excitations are included: N * (1535), N * (1650), N * (1520), N * (1700), Δ * (1232), Δ * (1620) and Δ * (1700). The relationship between nucleon structure parameters, N *  properties and the generalised polarisabilities of the proton is illustrated.
162341973
Ian Ellery McCarthy 1930–2005
Ian McCarthy was one of Australia's outstanding theoretical physicists. He was born in country South Australia and, after a PhD at the University of Adelaide, and periods of work in the UK and USA, he returned to South Australia where for several decades he led an outstanding research program at Flinders University. Ian's career had two major stages. In the first, he made major contributions to nuclear reaction theory, including very important insights into the physical consequences of the optical model and state-of-the-art calculations of proton knock-out from nuclei. In the second phase, he imported the concept of the knock-out reaction to atomic, molecular and solid state physics. Using the (e,2e) reaction, for which he and his colleagues developed the theoretical framework, his group made major contributions in these areas.
12042600
Quark distributions in nuclear matter and the EMC effect
Abstract Quark light cone momentum distributions in nuclear matter and the structure function of a bound nucleon are investigated in the framework of the Nambu–Jona-Lasinio model. This framework describes the nucleon as a relativistic quark–diquark state, and the nuclear matter equation of state by using the mean field approximation. The scalar and vector mean fields in the nuclear medium couple to the quarks in the nucleon and their effect on the spin independent nuclear structure function is investigated in detail. Special emphasis is placed on the important effect of the vector mean field and on a formulation which guarantees the validity of the number and momentum sum rules from the outset.
120421162
Recent Work in Three-Body Systems
In this report I shall briefly outline those contributions to Discussion Session 5 (DS5) which seem most important to me. As far as possible an attempt will also be made to place these contributions in the context of current developments in the appropriate field. Of course, it will be only too painfully obvious that given the wide range of topics included in DS5—ranging from low energy n-d scattering to hypernuclear physics—the emphasis given here is highly subjective. While I have attempted as far as possible to remove personal biases, I must apologise in advance to those whose work is undeservedly given inadequate treatment.