121408728
Behaviour of polarized ammonia in an intense electron beam
Abstract Polarized target experiments with intense particle beams suffer from the depolarization effects due to radiation damage and beam heating. The local depolarization of the target material 14 NH 3 at the electron beam spot has been investigated by hitting it with 1.2 GeV electrons. This beam has been delivered from the electron stretcher accelerator ELSA of the Bonn University. The polarized target consists of a 4 T superconducting magnet with a Helmholtz configuration and a 4 He evaporation cryostat which operates at a temperature around 1 K. The heating of the target chips by the electron beam has been measured up to an intensity of 70 nA using a specific set-up of the nuclear magnetic resonance apparatus for the polarization measurements.
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.
120449636
The new Bonn frozen spin target for experiments with real photons
At the new Bonn Electron Stretcher Accelerator ELSA scattering experiments with real photons in the single pion photoproduction γ N → π N and in the photodisintegration of the deuteron γ N pn are prepared. To determine these processes completely, it is nessesary to measure single and double polarisation observables. For these experiments a new’bonn frozen spin target’ BOFROST was developed. The target with a maximum volume of 30 cm3 is cooled in a vertical 3He/4He — dilution refrigerator. In the’frozen spin mode’ a lowest temperature of 64 mK was reached.
122999945
Pfeiffer et al. Reply
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.
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.
15260939
$\rho - \omega$ mixing and the pion electromagnetic form-factor
Abstract The suggestion of momentum dependence in the amplitude for rho-omega mixing has generated concern over related implications for vector meson dominance and the photon-rho coupling. We discuss two established and formally equivalent representations of vector meson dominance and show that in one of these such a coupling is an automatic consequence of the model. We then apply it to a calculation of the pion electromagnetic form-factor. Our analysis leads to a revised value for the on-shell rho-omega mixing amplitude of −3800 ± 370 MeV 2 .
10421165
Power counting regime of chiral extrapolation and beyond
Finite-range regularized chiral effective field theory is presented in the context of approximation schemes ubiquitous in modern lattice QCD calculations. Using FRR techniques, the power-counting regime (PCR) of chiral perturbation theory can be estimated. To fourth-order in the expansion at the 1% tolerance level, we find 0 {le} m{sub {pi}} {le} 180 MeV for the PCR, extending only a small distance beyond the physical pion mass.
120955970
Hadron structure in medium
Abstract The quark meson coupling model may be viewed as a natural generalization of QHD which takes into account the internal structure of the nucleon. In such an approach it is natural for the properties of the nucleon to change in medium. We discuss the change in the proton electric and magnetic form factors when it is bound in a specific shell model orbit. Modern quasi-elastic electron scattering experiments should be able to detect effects of the size predicted. We also examine the mean field potential felt by an ω in a finite nucleus, concluding that the ω should be bound by between 50 and 100 MeV.
11582730
Charge-symmetry breaking, rho-omega mixing, and the quark propagator
Abstract The momentum dependence of the ϱ 0 -ω mixing contribution to charge-symmetry breaking (CSB) in the nucleon-nucleon interaction is compared in a variety of models. We focus in particular on the role that the structure of the quark propagator plays in the predicted behaviour of the ϱ 0 -ω mixing amplitude. We present new results for a confining (entire) quark propagator and for typical propagators arising from explicit numerical solutions of quark Dyson-Schwinger equations. We compare these to hadronic and free quark calculations. The implications for our current understanding of CSB experiments is discussed.
25619147
Design studies of high-luminoisty ring-ring electron-ion collider at CEBAF
Experimental studies of fundamental structure of nucleons require an electron-ion collider of a center-of- mass energy up to 90 GeV at luminosity up to 1035 cm-2 s-1 with both beams polarized. A CEBAF-based collider of 9 GeV electrons/positrons and 225 GeV ions is envisioned to meet this science need and as a next step for CEBAF after the planned 12 GeV energy upgrade of the fixed target program. A ring-ring scheme of this collider developed recently takes advantage of the existing polarized electron CW beam from the CEBAF and a green-field design of an ion complex with electron cooling. We present a conceptual design and report design studies of this high-luminosity collider.