10596461
The sigma commutator from lattice QCD
As a direct source of information on chiral symmetry breaking within QCD, the sigma commutator is of considerable importance. Since hadron structure is a non-perturbative problem, numerical calculations on a space-time lattice are currently the only rigorous approach. With recent advances in the calculation of hadron masses within full QCD, it is of interest to see whether the sigma commutator can be calculated directly from the dependence of the nucleon mass on the input quark mass. We show that, provided the correct chiral behaviour of QCD is respected in the extrapolation to realistic quark masses, one can indeed obtain a fairly reliable determination of the sigma commutator using present lattice data. For two-flavour dynamical fermion QCD the sigma commutator lies between 45 and 55 MeV based on recent data from CP-PACS and UKQCD.
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 �
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.
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.
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.
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.
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.
122280609
The calorimeter project for the Mu2e experiment
The Mu2e experiment at Fermilab aims to measure the charged lepton flavor violating neutrinoless conversion of a negative muon into an electron. The conversion results in a monochromatic electron with an energy slightly below the rest mass of the muon (104.97 MeV). We expect to set a limit of ∼ 6×10^(−17) at 90% CL in three years of running, using an intense and clean pulsed μ^− beam providing ∼10^(18) stopped muons on target in three years of running. The experiment performs a strong suppression of potential background by gating off the prompts and performing precise momentum determination in conjunction with an highly efficient cosmic veto. The calorimeter should confirm that the candidates reconstructed by the tracker system are indeed conversion electrons and provide an independent trigger (or event reduction filter) for the experiment. It should also provide standalone muon to electron rejection. Moreover, it must be able to keep functionality in a high radiation dose environment inside a 10^(−4)_(torr) vacuum enclosure and in a presence of 1 T axial magnetic field. In order to accomplish all these tasks, a LYSO crystals calorimeter has been chosen. We show the proposed design and the experimental results obtained by exposing a small size calorimeter prototype to a tagged photon beam from 40 to 300 MeV at the A2 photon facility of the Mainz Microton (MAMI), Germany.
117125906
FOL: A Proof Checker for First-order Logic
Abstract : This manual describes the use of the interactive proof checker FOL. FOL implements a version of the system of natural deduction described by Prawitz, augmented in the following ways: (1) it is a many-sorted first-order logic and a partial order over sorts may be declared: this reduces the size of formulas; (2) purely propositional deductions can be made in a single step; (3) the truth values of assertions involving numerical and LISP constants can be derived by computation; (4) there is a limited ability to make metamathematical arguments; and (5) there are many operational conveniences. The goal of FOL is to use formal proof techniques as practical tools for checking proofs in pure mathematics and proofs of the correctness of programs. It is also intended to be used as a research tool in modelling common-sense reasoning in the representation theory of artificial intelligence.
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.
59806645
After Leibniz...: discussions on philosophy and artificial intelligence.
This is an edited transcript of informal conversations which we have had over recent months, in which we looked at some of the issues which seem to arise when artificial intelligence and philosophy meet. Our aim was to see what might be some of the fundamental principles of attempts to build intelligent machines. The major topics covered are the relationship of AI and philosophy and what help they might be to each other: the mechanisms of natural inference and deduction; the question of what kind of theory of meaning would be involved in a successful natural language understanding program, and the nature of models in AI research.