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 �
122049860
Finite-range regularisation and chiral extrapolation
We study the expansion of the nucleon mass in chiral effective field theory. We describe finite-range regularisation and demonstrate its application to the chiral extrapolation problem for lattice QCD.
119454330
HiX2000: Theory Summary
We summarize the theoretical consensus of the HiX workshop concerning those measurements which seem most appropriate for inclusion in the ``white paper'' being prepared to justify the proposed 12 GeV upgrade at Jefferson Lab. The criteria for inclusion are that such measurements should be decisive, not possible elsewhere and should answer crucial physics questions with broad implications.
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.
54190998
Scale setting, sigma terms and the Feynman-Hellman theorem
The authors recently presented new values for the octet baryon sigma terms. These were extracted using the Feynman-Hellman theorem from a chiral perturbation theory fit to octet baryon mass data from the PACS-CS collaboration. Of particular interest is the precise determination of the strangeness sigma term $\sigma_s = 21 \pm 6$ MeV. In this work, we elaborate on the critical effect which the choice of scale setting has on this value. We discuss the prospect that the comparison of direct and 'spectrum' determinations of the sigma terms from the lattice can provide insight not only into scale setting on the lattice, but into QCD itself.
120073991
Role of the U(1) ghost beyond leading order in a large-Nc expansion
The 1/Nc expansion is one of the very few methods we have for generating a systematic expansion of QCD at the energy scale relevant to hadron structure. The present formulation of this theory relies on the double-line notation for calculating the leading order of a diagram in the 1/Nc expansion, where the local SU(Nc) gauge symmetry is substituted by a U(Nc) symmetry and the associated U(1) ghost field is ignored. In the current work we demonstrate the insufficiency of this formulation for describing certain non-planar diagrams of interest in current attempts to model QCD. We derive a more complete set of Feynman rules that include the U(1) ghost field and provide a useful tool for calculating both color factors and 1/Nc orders of all color-singlet diagrams.
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.
120805738
The Bonn polarized target NMR-system
Abstract The Bonn frozen spin target is used for meson photoproduction experiments in combination with the facility PHOENICS at ELSA. The maximum polarization field of 7 T is provided by a superconducting solenoid. For electron scattering experiments a target apparatus which uses a superconducting 4 T magnet in a Helmholtz configuration is employed. For dynamic nuclear polarization of protons or deuterons various microwaves sources with frequencies of 70 GHz, 98 GHz or 140 GHz are available. Ammonia, butanol and lithium hydrids are used as target materials. The main part of the polarization detection system is the so-called Liverpool nuclear magnetic resonance module, which ensures a high flexibility for the polarization measurements of different nuclear species, e.g. 15 N at 2.5 T ( f L =10.8 MHz) and protons at 50 T ( f L =213 MHz).
122999945
Pfeiffer et al. Reply
43984541
Performance of PWO-II Prototype Arrays for the EMC of PANDA
The response function of a 3 times 3 matrix of 200-mm-long PWO-II crystals has been measured with energy-marked photons up to 675 MeV energy. For the first time the crystals were readout individually with one large area avalanche photo diode of the second generation operating at a temperature of 0degC. The reconstructed electromagnetic shower distributions show excellent energy resolutions down to 40 MeV. Reducing the temperature and increasing the light collection by an additional large-area avalanche photo diode can achieve further improvement.
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.