Beam normal spin asymmetries: Theory

The beam normal spin asymmetry in elastic electron-nucleon scattering is discussed. This beam normal spin asymmetry depends on the imaginary part of two-photon exchange processes between electron and nucleon, and measures the non-forward structure functions of the nucleon. After briefly reviewing the theoretical formalism, we discuss calculations in the threshold region, in the resonance region, as well as in the diffractive region, corresponding with high energy and forward angles.     

Isospin breaking in the vector current of the nucleon

Inclusive electron scattering cross-sections in the quasielastic and resonance regions for few GeV electrons are well represented in terms of scaling functions and scaling variables, the so-called superscaling analysis (SuSA). The concepts of scaling of the first and second kinds and superscaling are discussed, as are several mechanisms which are known to yield scaling violations. Given the high quality of scaling for cross-sections at appropriate kinematics, it is shown how the ideas can be turned around to provide predictions for both charge-changing and neutral current neutrino reactions with nuclei at comparable kinematics.     

The role of the pion pair term in the theory of the weak axial meson exchange currents

The structure of the weak axial pion exchange current is discussed in various models. It is shown how the interplay of the chiral invariance and the double-counting problem restricts uniquely the form of the pion potential term, to the case when the nuclear dynamics is described by the Schrödinger equation with static nucleon-nucleon potential.     

Properties of low-lying 9He state

The spectrum of ⁹He was studied by means of the ⁸He(d,p)⁹He reaction at a lab energy of 25 MeV/n and small center of mass (c.m.) angles. Energy and angular correlations were obtained for the ⁹He decay products by complete kinematical reconstruction. The data analysis was done in the assumption of a direct one-neutron transfer reaction mechanism. The ⁹He structure was analysed in the framework of a simple single-particle model. The lowest resonant state of ⁹He is found at about 2 MeV with a width of ∼2MeV and is identified as 1/2^-. The observed angular correlation pattern is uniquely explained by the interference of the 1/2^- resonance with a virtual state 1/2⁺ (a limit on the scattering length is obtained as a > -20fm), and with a 5/2⁺ resonance at energy ≥4.2MeV.     

Nuclear effects on lepton polarization in charged-current quasielastic neutrino scattering

We use a correlated local Fermi gas (LFG) model, which accounts also for long distance corrections of the RPA type and final-state interactions, to compute the polarization of the final lepton in charged-current quasielastic neutrino scattering. The present model has been successfully used in recent studies of inclusive neutrino nucleus processes and muon capture. We investigate the relevance of nuclear effects in the particular case of τ polarization in tau-neutrino induced reactions for several kinematics of relevance for neutrino oscillation experiments.     

Role of multi-step processes in 16O(11B,12C)15N at 41.25 MeV

The ¹⁶O(¹¹B,¹²C)¹⁵N reaction at 41.25 MeV has been investigated using the kinematical coincidence method. Polarization tensors t₂₀ and t₄₀ of ¹²C[2⁺ ₁] for the quantization axis taken along the direction of propagation have been measured at center-of-mass angles (Θ_c.m.) between 48^° and 62^° by analyzing the energy spectrum of ¹²C[2⁺ ₁] modulated by the effect of γ-ray emission. The cross-sections of the transfer reactions leading to the ¹²C[g.s.]+¹⁵N[g.s.], ¹²C[2⁺ ₁]+¹⁵N[g.s.] and ¹²C[g.s.]+¹⁵N[3/2^- ₁] final states have also been measured in the range 48^°≤Θ_c.m.≤ 120^°. The polarization tensor terms of ¹²C[2⁺ ₁] largely deviating from zero have been observed, contrary to the prediction by the distorted-wave Born approximation (DWBA). The one-step DWBA calculation also fails in describing the transfer reaction cross-sections. It is shown that the coupled channel model calculation including excitations and reorientations in ¹¹B and ¹²C satisfactorily reproduces both the tensor terms and the cross-sections of the transfer reactions. The multi-step processes passing through the excited states of ¹¹B are found to significantly contribute to the reaction. Received: 3 July 2000 / Accepted: 15 September 2000     

Polarization observables in (γ,NN) reactions

The formalism of ( γ, NN) reactions is given where the incident photon is polarized and the outgoing-nucleon polarization is detected. Sixteen structure functions and fifteen polarization observables are found in the general case, while only eight structure functions and seven polarization observables survive in coplanar kinematics. Numerical examples are presented for the ¹⁶O(γ, np) and ¹⁶O(γ, pp) reactions. The transitions to the ground state of ¹⁴C and ¹⁴N are calculated in a model where realistic short-range and tensor correlations are taken into account for the np pair, while short-range and long-range correlations are included in a consistent way for pp pairs. The effects of the one-body and two-body components of the nuclear current and the role of correlations in cross-sections and polarizations are studied and discussed. Received: 7 June 2001 / Accepted: 26 September 2001     

Tensor polarization of 12C[2+ 1] in the 16O(13C,12C)17O reaction at 50 MeV

The ¹⁶O(¹³C,¹²C)¹⁷O reaction at 50 MeV has been investigated using the kinematical coincidence method. Polarization tensors t ₂₀ and t ₄₀ of ¹²C[2⁺ ₁] for the quantization axis taken along the direction of propagation have been measured by analyzing the energy spectrum of ¹²C[2⁺ ₁], modulated by the effect of γ ray emission. The deduced t ₄₀ values significantly deviate from zero, contrary to the prediction of the distorted-wave Born approximation theory based on one-step p shell neutron stripping without spin-dependent interactions. The phenomenological spin–orbit interaction necessary to reproduce the magnitude of measured t ₄₀ is found to be much larger than the folding model prediction. It is shown that the experimental polarization tensors as well as the cross sections can be reproduced by introducing multi-step processes involving excitations in ¹²C and ¹³C without introducing spin-dependent interactions. Received: 2 August 1999 / Revised version: 3 February 2000     

Dynamics of polarization buildup by spin filtering

There has been much recent research into polarizing an antiproton beam, instigated by the recent proposal from the PAX (Polarized Antiproton eXperiment) project at GSI Darmstadt. It plans to polarize an antiproton beam by repeated interaction with a polarized internal target in a storage ring. The method of polarization by spin filtering requires many of the beam particles to remain within the ring after scattering off the polarized internal target via electromagnetic and hadronic interactions. We present and solve sets of differential equations which describe the buildup of polarization by spin filtering in many different scenarios of interest to projects planning to produce high-intensity polarized beams. These scenarios are: 1) spin filtering of a fully stored beam; 2) spin filtering while the beam is being accumulated, i.e. unpolarized particles are continuously being fed into the beam; 3) the particle input rate is equal to the rate at which particles are being lost due to scattering beyond the ring acceptance angle, the beam intensity remaining constant; 4) increasing the initial polarization of a stored beam by spin filtering; 5) the input of particles into the beam is stopped after a certain amount of time, but spin filtering continues. The rate of depolarization of a stored polarized beam on passing through an electron cooler is also shown to be negligible.     

Analyzing powers Ayy , Axx , Axz and Ay in the dd → 3Hen reaction at 270 MeV

The data on the tensor A_yy, A_xx, A_xz and vector A_y analyzing powers in the dd → ^3Hen obtained at T _d = 270 MeV in the angular range 0^° - 110^° in the c.m. are presented. The observed negative sign of the tensor analyzing powers A_yy, A_xx and A_xz at small angles clearly demonstrate the sensitivity to the ratio of the D - and S -wave component of the ³He wave function. However, the one-nucleon exchange calculations by using the standard ³He wave functions have failed to reproduce the strong variation of the tensor analyzing powers as a function of the angle in the c.m.     

Contribution of the Δ(1232) to the (e, e'p) reaction along with Coulomb distortion effects

The Δ-resonance contribution has been included in the (e, e'p) reaction along with Coulomb distortion effects. We treat the resonance via a non-relativistic Δ current operator and use a Dirac Hartree single-particle model for the ground-state single-particle wave function and a relativistic optical model for the knocked-out proton wave function. It is assumed that the π-meson created by the virtual photon is absorbed in the target nucleus following the production of a Δ-resonance. Our DWBA calculation shows that the Δ-resonance contribution to the (e, e'p) reaction cross-section is 10-15% for an energy of 250 MeV transfered to the proton knocked out of the s-shell of ⁴⁰Ca, in the parallel and perpendicular kinematics. Received: 21 July 2001 / Accepted: 21 March 2002     

General survey of polarization observables in deuteron electrodisintegration

Polarization observables in inclusive and exclusive electrodisintegration of the deuteron using a polarized beam and an oriented target are systematically surveyed using the standard nonrelativistic framework of nuclear theory but with leading-order relativistic contributions included. The structure functions and the asymmetries corresponding to the various nucleon polarization components are studied in a variety of kinematic regions with respect to their sensitivity to realistic NN-potential models, to subnuclear degrees of freedom in terms of meson exchange currents, isobar configurations and to relativistic effects in different kinematical regions, serving as a benchmark for a test of present standard nuclear theory with effective degrees of freedom.     

The transparency of nuclei to nucleons and pions in a relativistic Glauber approximation

We present a selection of results obtained within the context of a relativistic eikonal model. First, results of relativistic Glauber calculations for the nuclear transparency extracted from photon-induced pion production are presented. Second, computed differential cross-sections for the ¹² C(p, 2p) are compared to data.     

Nucleon elastic form factors

The nucleon form factors are still the subject of active investigation even after an experimental effort spanning 50 years. This is because they are of critical importance to our understanding of the electromagnetic properties of nuclei and provide a unique testing ground for QCD motivated models of nucleon structure. Progress in polarized beams, polarized targets and recoil polarimetry have allowed an important and precise set of data to be collected over the last decade. I will review the experimental status of elastic electron scattering from the nucleon along with an outlook for future progress.     

Nuclear moments of neutron-rich aluminum isotopes

Ground-state magnetic-dipole moments (μ) of ^30-32Al and electric quadrupole moments (Q) of ^31,32Al have been measured with the β-NMR method using spin-polarized radioactive-isotope beams produced in projectile-fragmentation reactions. Beams of ^30-32Al were obtained by using RIKEN projectile-fragment separator RIPS after the fragmentation of ⁴⁰Ar projectiles at an energy of E = 95A MeV on a ⁹³Nb target. The obtained μ_exp[^30-32Al] and values agree well with shell-model calculations within the sd shell using the USD interaction. Also, Q_exp[³¹Al] was found to be small. Thus, we can conclude that these aluminum isotopes are located outside the island of inversion.