共查询到20条相似文献,搜索用时 15 毫秒
1.
The generalized polarizabilities of the nucleon have been calculated in the one-loop approximation of the linear sigma model in the limit of infinite sigma mass. In particular, we have investigated the spin-independent amplitude of virtual Compton scattering off the nucleon. The dependence of the polarizabilities on the momentum transfer Q has been compared with two recent model calculations. It has been shown in the linear sigma model that only two of the three scalar polarizabilities are independent quantities. Particular attention has been paid to the real photon point (Q2 = 0), where the results of the relativistic chiral perturbation theory have been recovered. 相似文献
2.
The paper presents results of perturbative calculations for nucleon Compton scattering. The results are given for the real incoming photon and for the virtual incoming photon. 相似文献
3.
Low energy single nucleon transfer reactions are proposed as a tool to investigate the structure of nuclei far off stability.
Experimental concepts and conditions are discussed, in particular high resolution γ-ray spectroscopy after single nucleon
pickup reactions. Nuclear structure is described by Skyrme Hartree-Fock calculations including pairing. As representative
examples, binding energies, radii and wave functions for Mg and Sn isotopes are calculated. In the neutron deficient Mg isotopes
a proton skin is found. At the neutron driplines the Mg and Sn isotopes develop extended neutron skins. The nuclear structure
results are used in DWBA and EFR-DWBA transfer calculations. Single nucleon transfer reactions of 32,36Mg and exotic Sn beams on targets ranging from 2H to 24Mg in inverse kinematics are explored. The one-nucleon transfer cross sections decrease strongly for high-Z targets. An impact
parameter analysis shows that the transfer process is selective on the tails of the wave functions. The largest cross sections
are obtained for 2H and 9Be targets at incident energies of E
lab
= 2-5 MeV/u. The energy-momentum dependence is closely related to the special properties of wave functions of weakly bound
states. Two-neutron (p,t) stripping reactions are studied for a 6He projectile. A strong competition of sequential and direct processes is found at low energies.
Received: 1 October 1997 / Revised version: 25 November 1997 相似文献
4.
D. Merten U. Löring K. Kretzschmar B. Metsch H.-R. Petry 《The European Physical Journal A - Hadrons and Nuclei》2002,14(4):477-489
We compute electroweak form factors of the nucleon and photon transition form factors of non-strange baryon resonances up
to the third resonance region in a model with instanton-induced interaction. The calculation is based on the Bethe-Salpeter
equation for three light constituent quarks and is fully relativistic (U. L?ring et al., Eur. Phys. J. A 10, 309 (2001)). Static nucleon properties and photon resonance couplings are in good agreement with experiment and the Q2 behaviour of the experimentally known form factors up to large momentum transfer is accounted for.
Received: 4 April 2002 / Accepted: 2 May 2002 相似文献
5.
M. Kopytin 《The European Physical Journal A - Hadrons and Nuclei》2007,31(4):575-577
Measurements of azimuthal cross-section asymmetries from deeply virtual Compton scattering on transversely and longitudinally
polarized hydrogen and longitudinally polarized deuterium targets at HERMES are reported. By comparing the HERMES results
on the transverse target-spin asymmetry with theoretical calculations based on a phenomenological model of generalized parton
distributions, a model-dependent constraint on the total angular momentum carried by quarks in the nucleon is obtained. 相似文献
6.
M.V. Kossov 《The European Physical Journal A - Hadrons and Nuclei》2002,14(3):377-392
Photonuclear interaction cross-sections from the GEANT4 database are approximated for all nuclei and all energies (from the
hadron production threshold to about 40 TeV). The approximation methods in the giant-dipole resonance region, nucleon resonance
region, and high-energy region are improved with respect to existing approximations. As an application of the approximation
for photonuclear cross-sections, an improved method of calculating electronuclear cross-sections is developed. The interaction
cross-section of virtual photons with nuclei at high Q2 are approximated and a simple algorithm for describing the electronuclear reactions, including high-Q2 scattering, is proposed.
Received: 22 February 2002 / Accepted: 6 May 2002 相似文献
7.
B. Pasquini D. Drechsel M. Vanderhaeghen 《The European physical journal. Special topics》2011,198(1):269-285
We review recent developments in the theoretical investigation of the nucleon polarizabilities. We first report on the static
polarizabilities as measured in real Compton scattering, comparing and interpreting the results from various theoretical approaches.
In a second step, we extend the discussion to the generalized polarizabilities which can be accessed in virtual Compton scattering,
showing how the information encoded in these quantities can provide a spatial interpretation of the induced polarization densities
in the nucleon. 相似文献
8.
We computed the twist-4 part of the photon spin-flip amplitude in deeply virtual Compton scattering on a nucleon in the Wandzura–Wilczek
approximation. We found a factorizable contribution, which arises from photon scattering on quarks with non-zero angular momentum
along the collision axis. As the genuine twist-2 amplitude arises at the NLO, for moderate virtualities of the hard photon,
GeV, a kinematical twist-4 correction can give a numerically important contribution to the photon helicity-flip amplitude.
Received: 2 July 2001 / Revised version: 17 July 2001 / Published online: 31 August 2001 相似文献
9.
The generalized polarizabilities of the nucleon have been calculated in the one-loop approximation of the linear sigma model in the limit of infinite sigma mass. In particular, we have investigated the spin-independent amplitude of virtual Compton scattering off the nucleon. The dependence of the polarizabilities on the momentum transferQ has been compared with two recent model calculations. It has been shown in the linear sigma model that only two of the three scalar polarizabilities are independent quantities. Particular attention has been paid to the real photon point (Q 2=0), where the results of the relativisitc chiral perturbation theory have been recovered. 相似文献
10.
In a previous paper virtual Compton scattering off the nucleon has been investigated in the one-loop approximation of the linear sigma model in order to determine the 3 scalar generalized polarizabilities. We have now extended this work and calculated the 7 vector polarizabilities showing up in the spin-dependent amplitude of virtual Compton scattering. The results fulfill 3 model-independent constraints recently derived. Compared to the constituent quark model there exist enormous differences for some of the vector polarizabilities. At vanishing three-momentum of the virtual photon, the analytical results of the sigma model and of chiral perturbation theory can be related. The influence of the π0 exchange in the t channel has been discussed in some detail. Besides, the vector polarizabilities determine 2 linear combinations of the third order spin-polarizabilities appearing in real Compton scattering. 相似文献
11.
E.Ya. Paryev 《The European Physical Journal A - Hadrons and Nuclei》2000,7(1):127-137
The inclusive K+ meson production in photon–induced reactions in the near threshold and subthreshold energy regimes is analyzed with respect
to the one–step (γN→K
+
Y, Y=Λ,Σ) incoherent production processes on the basis of an appropriate new folding model, which takes properly into account
the struck target nucleon removal energy and internal momentum distribution (nucleon spectral function), extracted from recent
quasielastic electron scattering experiments and from many–body calculations with realistic models of the NN interaction.
Simple parametrizations for the total and differential cross sections of the K+ production in photon–nucleon collisions are presented. Comparison of the model calculations of the K+ differential cross sections for the reaction γ+C12 in the threshold region with the existing experimental data is given, that displays the contributions to the K+ production at considered incident energies coming from the use of the single–particle part as well as high momentum and high
removal energy part of the nucleon spectral function. Detailed predictions for the K+ total and differential cross sections from γH2, γC12 and γPb208 reactions at subthreshold and near threshold energies are provided. The influence of the uncertainties in the elementary
K+ production cross sections on the K+ yield is explored.
Received: 12 April 1999 / Revised version: 11 September 1999 相似文献
12.
B. Pasquini M. Gorchtein D. Drechsel A. Metz M. Vanderhaeghen 《The European Physical Journal A - Hadrons and Nuclei》2001,11(2):185-208
We present in detail a dispersion relation formalism for virtual Compton scattering (VCS) off the proton from threshold into
the Δ(1232)-resonance region. Such a formalism can be used as a tool to extract the generalized polarizabilities of the proton
from both unpolarized and polarized VCS observables over a larger energy range. We present calculations for existing and forthcoming
VCS experiments and demonstrate that the VCS observables in the energy region between pion production threshold and the Δ(1232)-resonance
show an enhanced sensitivity to the generalized polarizabilities.
Received: 14 March 2001 / Accepted: 18 June 2001 相似文献
13.
E.R. Berger M. Diehl B. Pire 《The European Physical Journal C - Particles and Fields》2002,23(4):675-689
We investigate the exclusive photoproduction of a heavy timelike photon which decays into a lepton pair, . This can be seen as the analog of deeply virtual Compton scattering, and we argue that the two processes are complementary
for studying generalized parton distributions in the nucleon. In an unpolarized experiment the angular distribution of the
leptons readily provides access to the real part of the Compton amplitude. We estimate the possible size of this effect in
kinematics where the Compton process should be dominated by quark exchange.
Received: 9 October 2001 / Published online: 5 April 2002 相似文献
14.
T. Glasmacher B.V. Pritychenko P.D. Cottle R.W. Ibbotson K.W. Kemper H. Scheit 《The European Physical Journal A - Hadrons and Nuclei》2002,13(1-2):59-62
Projectile fragmentation provides radioactive beams at intermediate velocities (v/c = 0.3-0.5) by physical means of fragment separation. With the development of position-sensitive photon detectors it has become
possible to measure the energies and directions of photons emitted in-flight from such fast-moving exotic beams. This allows
the reconstruction of the photons' energies emitted from an exotic projectile with high accuracy. It can be advantageous to
employ photon detection in experiments with exotic beams since photons can traverse matter easily and their attenuation can
be calculated. Experiments with standard luminosities can be carried out at intermediate beam energies with thick secondary
targets (order of g/cm2) and very low incident beam rates (order of particle/s or less). Experimental success in this field is strongly correlated
with the development of photon detectors such as position-sensitive scintillation detectors or segmented germanium detectors.
In-beam gamma-ray spectroscopy of fast exotic beams has been successfully used at all projectile fragmentation facilities
in intermediate-energy heavy-ion inelastic scattering experiments, knockout reactions and fragmentation reactions. Here, we
focus on experimental results for neutron-rich exotic nuclei in the π(sd )-shell. Measurements and detector developments carried out at the NSCL at Michigan State University during the last four
years are discussed.
Received: 1 May 2001 / Accepted: 4 December 2001 相似文献
15.
The pion cloud renormalises the light-cone wavefunction of the nucleon which is measured in hard, exclusive photon-nucleon reactions. We discuss the leading twist contributions to high-energy exclusive reactions taking into account both the pion cloud and perturbative QCD physics. The nucleon’s electromagnetic form-factor at high Q2 is proportional to the bare nucleon probability Z and the cross-sections for hard (real at large angle or deeply virtual) Compton scattering are proportional to Z2. Our present knowledge of the pion-nucleon system is consistent with Z = 0.7 ± 0.2. If we apply just perturbative QCD to extract a light-cone wavefunction directly from these hard exclusive cross-sections, then the light-cone wavefunction that we extract measures the three valence quarks partially screened by the pion cloud of the nucleon. We discuss how this pion cloud renormalisation effect might be understood at the quark level in terms of the (in-)stability of the perturbative Dirac vacuum in low energy QCD. 相似文献
16.
W. Zuo A. Lejeune U. Lombardo J.F. Mathiot 《The European Physical Journal A - Hadrons and Nuclei》2002,14(4):469-475
Brueckner calculations including a microscopic three-body force have been extended to isospin-asymmetric nuclear matter. The
effects of the three-body force on the equation of state and on the single-particle properties of nuclear matter are discussed
with a view to possible applications in nuclear physics and astrophysics. It is shown that, even in the presence of the three-body
force, the empirical parabolic law of the energy per nucleon vs. isospin asymmetry β = (N - Z)/A is fulfilled in the whole asymmetry range 0≤β≤1 up to high densities. The three-body force provides a strong enhancement
of the symmetry energy which increases with density in good agreement with the predictions of relativistic approaches. The
Lane's assumption that proton and neutron mean fields linearly vary vs. the isospin parameter is violated at high density due to the three-body force, while the momentum dependence of the mean
fields turns out to be only weakly affected. Consequently, a linear isospin split of the neutron and proton effective masses
is found for both cases with and without the three-body force. The isospin effects on multifragmentation events and collective
flows in heavy-ion collisions are briefly discussed along with the conditions for direct URCA processes to occur in the neutron
star cooling.
Received: 18 February 2002 / Accepted: 16 May 2002 相似文献
17.
We give an overview of low-energy Compton scattering γ(∗)
p → γp with a real or virtual incoming photon. These processes allow the investigation of one of the fundamental properties of the
nucleon, i.e. how its internal structure deforms under an applied static electromagnetic field. Our knowledge of nucleon polarisabilities
and their generalization to non-zero four-momentum transfer will be reviewed, including the presently ongoing experiments
and future perspectives. 相似文献
18.
We formulate the low-energy theorem for virtual Compton scattering off a nucleon and examine its consequences for generalized nucleon polarizabilites. As a result of a new, model-independent definition of the low-energy limit for doubly virtual Compton scattering, all generalized sum rules of the nucleon have a continuous limit for real photons and obtain contributions from the t channel that were not included previously. 相似文献
19.
Various aspects of the electromagnetic interactions of hadrons (strongly interacting particles) are reviewed in these lectures. After a discussion of the properties of the electromagnetic current of the hadrons and the idea of vector meson dominance, the general features of photoproduction cross sections are presented and compared with the very similar behavior of purely strong interaction processes. Given this close similarity in behavior, particular photon induced reactions are then considered in some detail, illustrating the application of theoretical ideas used in treating both strong and electromagnetic processes. The particular subjects discussed in some detail are photoproduction of pions at low energy and partial wave analysis, Compton scattering, vector meson photoproduction and tests of the vector dominance model, and photoproduction of charged pions at high energy. In the second half of the lectures inelastic electron-nucleon scattering is the principal topic. After a presentation of the kinematics and structure functions of the nucleon, the principal results of the inelastic electron scattering experiments are given together with the evidence for the remarkable scaling behavior of the structure functions. This leads to a presentation of the parton model of point constituents of the nucleon and the interpretation of the experimental results in terms of the properties of the constituent partons. A different view of inelastic electron scattering in terms of strong interaction ideas is presented in the last two lectures. These concern the high energy behavior of the scattering and the application of duality concepts which tie the behavior of nucleon resonance electroproduction to the behavior of the deep inelastic scattering. 相似文献