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.
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.
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.
122999945
Pfeiffer et al. Reply
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.
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.
15721650
CVC in Particle Physics
We review the hypothesis of the conserved vector current (CVC) within the Standard Model. In addition to the classic tests, such as pion beta decay and neutrino scattering, we mention recent tests involving LEP data. As well as providing a clear indication that the isovector current is not conserved, rho-omega mixing offers a fascinating opportunity to study CP violation at B-factories and we outline these ideas. Finally we briefly touch on a new approach to mass generation in the Standard Model which, for example, leads to the up-down mass difference which breaks CVC.
119039781
Convergence of chiral effective field theory
Abstract We formulate the expansion for the mass of the nucleon as a function of pion mass within chiral perturbation theory using a number of different ultra-violet regularisation schemes; including dimensional regularisation and various finite-ranged regulators. Leading and next-to-leading order non-analytic contributions are included through the standard one-loop Feynman graphs. In addition to the physical nucleon mass, the expansion is constrained by recent, extremely accurate, lattice QCD data obtained with two flavors of dynamical quarks. The extent to which different regulators can describe the chiral expansion is examined, while varying the range of quark mass over which the expansions are matched. Renormalised chiral expansion parameters are recovered from each regularisation prescription and compared. We find that the finite-range regulators produce consistent, model-independent results over a wide range of quark mass sufficient to solve the chiral extrapolation problem in lattice QCD.
93360192
Resonant Phase Shift from Finite Volume Interactions
A scattering process can be modelled by a finite volume lattice with appropriate boundary conditions. Such lattices have intermediate states with discretized energy spectrum. With Luscher’s formula we can obtain the phase shifts of these states. We constructed a three dimensional, non-relativistic model for the scattering of two scalar mesons on a lattice. The resonant energy spectra at different lattice sizes were obtained. Using Luscher’s formula, the corresponded phase shifts were calculated. Then, we compared the infinite volume phase shifts with the finite volume phase shifts acquired by Luscher’s formula.
18936688
Mapping the x dependence of the axial anomaly in polarised deep inelastic scattering
Abstract We discuss the role of the U(1) axial anomaly in the spin structure functions of the nucleon, with particular emphasis on how one might determine its x dependence in present and future deep inelastic scattering experiments. We focus on the C -odd spin structure function g 3 and also the deuteron structure function g 1 d
122270810
CHARGE SYMMETRY IN PION DEUTERON ELASTIC SCATTERING