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.
120110530
Deep inelastic scattering as a probe of nucleon and nuclear structure
Abstract We briefly review recent thinking about the meaning of various phenomenologlcal models of hadronic structure in the light of modern deep-inelastic scattering data. As a particularly topical example, we argue that a widely accepted explanation of the EMC effect, in terns of a change of confinement scale, is unlikely to be correct.
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.
16608552
Kaon-nucleus Drell-Yan processes and kaon structure functions.
Abstract We investigate the information which could be obtained from Drell-Yan processes with sufficiently intense beams of charged kaons on isoscalar targets. It is found that combinations of K + and K − Drell-Yan measurements on isoscalar nuclear targets would allow one to extract the kaon sea quark distributions. These cross sections are also sensitive to the strange valence quark distribution in the kaon, although one would need a significant increase over the presently available kaon fluxes in order to extract this quantity with sufficient accuracy.
124728025
A microscopic understanding of the structure functions of finite nuclei
Abstract A number of relatively successful quark model descriptions of nuclear matter have recently been developed, based on a mean-field description of non-overlapping nucleon bags bound by the self-consistent exchange of scalar (σ) and vector (ω) mesons. The meson fields modify the internal quark motion in the nucleon bag in medium and this induces a saturation mechanism for nuclear matter. The model gives a nuclear compressibility around 220 MeV and provides a rather satisfactory interpretation of nuclear matter properties. Furthermore, it has recently been shown that the twist-2 valence quark distributions of the free nucleon calculated in the MIT bag model are in reasonable agreement with world data. By using a local density approximation we combine these developments to microscopically investigate the structure functions of finite nuclei. The effect of nuclear shadowing and antishadowing is taken into account phenomenologically. The model can quantitatively reproduce the existing experimental data - the so-called European Muon Collaboration (EMC) effect. We show that the quark degrees of freedom in nuclei and the final state interaction between the debris of the struck nucleon and the residual nucleus both play important roles in the nuclear structure functions. Thus the conventional treatment based on nucleon binding within the impulse approximation is quantitatively unreliable. We also study the effects of the self-coupling terms of the σ field and nucleon-nucleon correlations on some nuclear matter properties and the nuclear structure functions.
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.
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.
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.
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.
15366431
Upgrade of the Glasgow photon tagging spectrometer for Mainz MAMI-C
The Glasgow photon tagging spectrometer at Mainz has been upgraded so that it can be used with the 1500MeV electron beam now available from the Mainz microtron MAMI-C. The changes made and the resulting properties of the spectrometer are discussed.
121408728
Behaviour of polarized ammonia in an intense electron beam
Abstract Polarized target experiments with intense particle beams suffer from the depolarization effects due to radiation damage and beam heating. The local depolarization of the target material 14 NH 3 at the electron beam spot has been investigated by hitting it with 1.2 GeV electrons. This beam has been delivered from the electron stretcher accelerator ELSA of the Bonn University. The polarized target consists of a 4 T superconducting magnet with a Helmholtz configuration and a 4 He evaporation cryostat which operates at a temperature around 1 K. The heating of the target chips by the electron beam has been measured up to an intensity of 70 nA using a specific set-up of the nuclear magnetic resonance apparatus for the polarization measurements.