16742385
The spin structure function of the neutron
Abstract The neutron spin structure function, g 1 n , has been of considerable interest recently in connection with the Bjorken sum rule and the proton spin crisis. Work on this problem has concentrated on measurements at low- x . We recall the important, non-perturbative physics to be learnt by going instead to larger values of x and especially from a determination of the place where the expected sign change occurs. Of course, in order to obtain neutron data one must use nuclear targets and apply appropriate corrections. In this regard, we review recent progress concerning the various nuclear corrections that must be applied to measurements on polarised 3 He.
119930609
The EMC spin effect
Attempts to understand the breakdown of the Ellis-Jaffe sum-rule observed by the European Muon Collaboration have led a new insight into the QCD parton model. The authors provide an overview of this work with emphasis on the role of the U(1) axial anomaly. Particular attention is given to the role that future experimental work might play in resolving the key theoretical questions.
55909393
Quarks in finite nuclei
We describe the development of a theoretical description of the structure of finite nuclei based on a relativistic quark model of the structure of the bound nucleons which interact through the (self-consistent) exchange of scalar and vector mesons.
122821193
The NJL‐jet model for quark fragmentation functions
A description of fragmentation functions which satisfy the momentum and isospin sum rules is presented. We concentrate on the pion fragmentation function, taking into account cascade‐like processes in a generalized jet‐model approach. Numerical results obtained in this NJL‐jet model are presented.
54008667
Anatomy of relativistic pion loop corrections to the electromagnetic nucleon coupling
We present a relativistic formulation of pion loop corrections to the coupling of photons with nucleons on the light front. Vertex and wave function renormalization constants are computed to lowest order in the pion field, including their nonanalytic behavior in the chiral limit, and studied numerically as a function of the ultraviolet cutoff. Particular care is taken to explicitly verify gauge invariance and Ward-Takahashi identity constraints to all orders in the mexpansion. The results are used to compute the chiral corrections to matrix elements of local operators, related to moments of deep-inelastic structure functions. Finally, comparison of results for pseudovector and pseudoscalar coupling allows the resolution of a longstanding puzzle in the computation of pion cloud corrections to structure function moments. I. INTRODUCTION The importance of chiral symmetry in hadron physics has been understood for more than 50 years. As an explicit Lagrangian representation of the approximate chiral sym- metry that had been recognized in low-energy pion- nucleon interactions, Gell-Mann and Levy (1) constructed the extremely successful linear sigma model. In that model the pion couples to the nucleon through pseudoscalar coupling, while an additional scalar (� ) field is also coupled linearly. At a more formal level, Gell-Mann (2) proposed SUð2 Þ� SUð2Þ as an exact algebra for the charges associated with the Hamiltonian governing the strong interaction, even though chiral symmetry was not an exact symmetry of that Hamiltonian. On the basis of current algebra one can show very gen- erally that the amplitude for pion scattering or production must vanish as the four-momentum of the pion vanishes (3,4). Within the linear sigma model, this important result for low-energy pion-nucleon scattering, for example, is only possible through a subtle cancellation of two large contri- butions, the first involving pion emission and absorption through the pseudoscalar (PS) coupling, and the second involvingexchange in the t channel. Keeping track of the necessary cancellations between such large terms in the linear sigma model is tedious and for that reason modern formulations of chiral effective field theory tend to prefer a Lagrangian formulation based on a nonlinear realization of chiral symmetry (5-8). In such a formulation the natural �NN vertex involves pseudovector (PV) coupling and the vanishing of pion-nucleon scattering amplitudes as the pion four-momentum vanishes emerges trivially. In this work, motivated by the phenomenological simplicity of enforcing soft-pion theorems, we focus on the case of PV coupling. However, since the linear realization of chiral symmetry is still used in the literature, for completeness we also compare our results with those for PS coupling. More recently, chiral symmetry and the pion cloud of the nucleon have been shown to play a central role in under- standing various flavor and spin asymmetries in quark distribution functions measured at high energies. Most prominent of these has been the SU(2) flavor asymmetry in the proton sea, with the large excess of � d quarks over �
122078954
Proton pole contribution in proton decay
We show that nucleon pole terms contribute significantly to proton decay, increasing the branching ratio for e+π0 and νRπ+ modes. The total lifetime is decreased to the point where keeping the SU(5) value of the lifetime below the experimental upper bound forces us to the choice of QCD scale parameter and proton radius parameter which are near the edge of the allowed region.
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.
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.
94396828
Butanol in the Frozen Spin Mode
Measurements of the polarization build-up in butanol at 3.5 T are presented. Measurements of the proton relaxation times in the frozen spin mode were performed at magnetic fields of 0.5 and 0.35 Tesla, leading to the conclusion that butanol is a suitable material for the Bonn Frozen Spin Target.
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.
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.