126224714
Double polarized measurements with frozen spin target at MAMI
The A2 Collaboration at the Mainz Microtron MAMI measures photon absorption cross sections using circularly and linearly polarized 'Bremsstrahlung' photons up to an energy of ~1.5GeV and a polarized Frozen Spin Target. We use a 4 π detection system with the 'Crystal Ball' as central part. Our Frozen Spin Target provides both longitudinally and transversely polarized protons and deuterons. In this paper the apparatus will be described. Double polarized experiments for the investigation of the nucleons excitation spectrum via meson production and our Compton program to measure the proton polarizabilities will be briefly discussed.
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.
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.
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.
121648376
Structure functions of the nucleon in a covariant scalar spectator model
Nucleon structure functions, as measured in deep-inelastic lepton scattering, are studied within a covariant scalar diquark spectator model. Regarding the nucleon as an approximate two-body bound state of a quark and diquark, the Bethe–Salpeter equation (BSE) for the bound state vertex function is solved in the ladder approximation. The valence quark distribution is discussed in terms of the solutions of the BSE.
14453178
Lambda polarization in polarized proton-proton collisions at BNL RHIC
We discuss Lambda polarization in semi-inclusive proton-proton collisions, with one of the protons longitudinally polarized. The hyperfine interaction responsible for the �-N and �-� mass splittings gives rise to flavor asymmetric fragmentation functions and to sizable polarized non-strange fragmentation functions. We predict large positive Lambda polarization in polarized proton-proton collisions at large rapidities of the produced Lambda, while other models, based on SU(3) flavor symmetric fragmentation functions, predict zero or negative Lambda polarization. The effect of � 0 and � ∗ decays is also discussed. Forthcoming experiments at RHIC will be able to differentiate between these predictions.
6056940
The spin of the proton
doi: http://dx.doi.org/10.1016/j.ppnp.2007.12.039 The twenty years since the announcement of the proton spin crisis by the European Muon Collaboration has seen tremendous progress in our knowledge of the distribution of spin within the proton. The problem is reviewed, beginning with the original data and the suggestion that polarized gluons may play a crucial role in resolving the problem through the U(1) axial anomaly. The discussion continues to the present day where not only have strong limits have been placed on the amount of polarized glue in the proton but the experimental determination of the spin content has become much more precise. It is now clear that the origin of the discrepancy between experiment and the naive expectation of the fraction of spin carried by the quarks and anti-quarks in the proton lies in the non-perturabtive structure of the proton. We explain how the features expected in a modern, relativistic and chirally symmetric description of nucleon str
55392142
Study of lattice QCD form factors using the extended Gari-Krümpelmann model
We explore the suitability of a modern vector meson dominance (VMD) model as a method for chiral extrapolation of nucleon electromagnetic form factor simulations in lattice QCD. It is found that the VMD fits to experimental data can be readily generalized to describe the lattice simulations. However, the converse is not true. That is, the VMD form is unsuitable as a method of extrapolation of lattice simulations at large quark mass to the physical regime.
16756851
Revealing dressed quarks via the proton's charge distribution.
The proton is arguably the most fundamental of nature's readily detectable building blocks. It is at the heart of every nucleus and has never been observed to decay. It is nevertheless a composite object, defined by its valence-quark content: u+u+d--i.e., two up (u) quarks and one down (d) quark; and the manner by which they influence, inter alia, the distribution of charge and magnetization within this bound state. Much of novelty has recently been learned about these distributions; and it now appears possible that the proton's momentum-space charge distribution possesses a zero. Experiments in the coming decade should answer critical questions posed by this and related advances; we explain how such new information may assist in charting the origin and impact of key emergent phenomena within the strong interaction. Specifically, we show that the possible existence and location of a zero in the proton's electric form factor are a measure of nonperturbative features of the quark-quark interaction in the standard model, with particular sensitivity to the running of the dressed-quark mass.