19 research outputs found
Nucleon form factors with dynamical twisted mass fermions
The electromagnetic and axial form factors of the nucleon are evaluated in
twisted mass QCD with two degenerate flavors of light, dynamical quarks. The
axial charge g_A, magnetic moment and the Dirac and Pauli radii are determined
for pion masses in the range 300 MeV to 500 MeV
Baryon masses from lattice QCD: Beyond the perturbative chiral regime
Consideration of the analytic properties of pion-induced baryon self-energies leads to new functional forms for the extrapolation of light baryon masses. These functional forms reproduce the leading non-analytic behavior of chiral perturbation theory, the correct non-analytic behavior at the threshold and the appropriate heavy-quark limit. They involve only three unknown parameters, which may be obtained by fitting to lattice data. Recent dynamical fermion results from CP-PACS and UKQCD are extrapolated using these new functional forms. We also use these functions to probe the limit of applicability of chiral perturbation theory to the extrapolation of lattice QCD results.Derek B. Leinweber, Anthony W. Thomas, Kazuo Tsushima, and Stewart V. Wrigh
Virtual Compton Scattering and Neutral Pion Electroproduction in the Resonance Region up to the Deep Inelastic Region at Backward Angles
We have made the first measurements of the virtual Compton scattering (VCS)
process via the H exclusive reaction in the nucleon resonance
region, at backward angles. Results are presented for the -dependence at
fixed GeV, and for the -dependence at fixed near 1.5 GeV.
The VCS data show resonant structures in the first and second resonance
regions. The observed -dependence is smooth. The measured ratio of
H to H cross sections emphasizes the different
sensitivity of these two reactions to the various nucleon resonances. Finally,
when compared to Real Compton Scattering (RCS) at high energy and large angles,
our VCS data at the highest (1.8-1.9 GeV) show a striking -
independence, which may suggest a transition to a perturbative scattering
mechanism at the quark level.Comment: 20 pages, 8 figures. To appear in Phys.Rev.
Strangeness nuclear physics: a critical review on selected topics
Selected topics in strangeness nuclear physics are critically reviewed. This
includes production, structure and weak decay of --Hypernuclei, the
nuclear interaction and the possible existence of bound
states in nuclei. Perspectives for future studies on these issues are also
outlined.Comment: 63 pages, 51 figures, accepted for publication on European Physical
Journal
Moments of isovector quark distributions from lattice QCD
We present a complete analysis of the chiral extrapolation of lattice moments of all twist-2 isovector quark distributions, including corrections from Nπ and Δπ loops. Even though the Δ resonance formally gives rise to higher order non-analytic structure, the coefficients of the higher order terms for the helicity and transversity moments are large and cancel much of the curvature generated by the wave function renormalization. The net effect is that, whereas the unpolarized moments exhibit considerable curvature, the polarized moments show little deviation from linearity as the chiral limit is approached
Strong evidences of hadron acceleration in Tycho's Supernova Remnant
Very recent gamma-ray observations of G120.1+1.4 (Tycho's) supernova remnant
(SNR) by Fermi-LAT and VERITAS provided new fundamental pieces of information
for understanding particle acceleration and non-thermal emission in SNRs. We
want to outline a coherent description of Tycho's properties in terms of SNR
evolution, shock hydrodynamics and multi-wavelength emission by accounting for
particle acceleration at the forward shock via first order Fermi mechanism. We
adopt here a quick and reliable semi-analytical approach to non-linear
diffusive shock acceleration which includes magnetic field amplification due to
resonant streaming instability and the dynamical backreaction on the shock of
both cosmic rays (CRs) and self-generated magnetic turbulence. We find that
Tycho's forward shock is accelerating protons up to at least 500 TeV,
channelling into CRs about the 10 per cent of its kinetic energy. Moreover, the
CR-induced streaming instability is consistent with all the observational
evidences indicating a very efficient magnetic field amplification (up to ~300
micro Gauss). In such a strong magnetic field the velocity of the Alfv\'en
waves scattering CRs in the upstream is expected to be enhanced and to make
accelerated particles feel an effective compression factor lower than 4, in
turn leading to an energy spectrum steeper than the standard prediction
{\propto} E^-2. This latter effect is crucial to explain the GeV-to-TeV
gamma-ray spectrum as due to the decay of neutral pions produced in nuclear
collisions between accelerated nuclei and the background gas. The
self-consistency of such an hadronic scenario, along with the fact that the
concurrent leptonic mechanism cannot reproduce both the shape and the
normalization of the detected the gamma-ray emission, represents the first
clear and direct radiative evidence that hadron acceleration occurs efficiently
in young Galactic SNRs.Comment: Minor changes. Accepted for publication in Astronomy & Astrophysic
Equation of state of hot dense hyperonic matter in the Quark–Meson-Coupling (QMC-A) model
International audienceWe report a new equation of state (EoS) of cold and hot hyperonic matter constructed in the framework of the quark–meson-coupling (QMC-A) model. The QMC-A EoS yields results compatible with available nuclear physics constraints and astrophysical observations. It covers the range of temperatures from T = 0 to 100 MeV, entropies per particle S/A between 0 and 6, lepton fractions from Y_L = 0.0 to 0.6, and baryon number densities n_B = 0.05–1.2 fm^−3. Applications of the QMC-A EoS are made to cold neutron stars (NSs) and to hot proto-neutron stars (PNSs) in two scenarios: (i) lepton-rich matter with trapped neutrinos (PNS-I) and (ii) deleptonized chemically equilibrated matter (PNS-II). We find that the QMC-A model predicts hyperons in amounts growing with increasing temperature and density, thus suggesting not only their presence in PNS but also, most likely, in NS merger remnants. The nucleon–hyperon phase transition is studied through the adiabatic index and the speed of sound c_s. We observe that the lowering of (c_s/c)^2 to and below the conformal limit of 1/3 is strongly correlated with the onset of hyperons. Rigid rotation of cold and hot stars, their moments of inertia and Kepler frequencies are also explored. The QMC-A model results are compared with two relativistic models, the chiral mean field model (CMF), and the generalized relativistic density functional (GRDF) with DD2 (nucleon-only) and DD2Y-T (full baryon octet) interactions. Similarities and differences are discussed
Constraints and excited states of the bag
SIGLECNRS RP 120 (143) / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc
Nucleon form factors and moments of parton distributions in twisted mass lattice QCD
The proceedings of the conference will be published in the on-line journal "Proceedings of Science".International audienc
A non-static bag model for the Roper resonances
SIGLECNRS RP 120 (144) / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc