Off-shell effects in laser-assisted scattering

We present calculations of laser-assisted differential cross-sections in the second Born approximation. We explore effects of off-the-energy-shell propagation, and we conclude, contrary to other works, that this effect, although significant when compared to the second Born on-shell approximation, is too small for the second Born approximation to account for experimental results.Received: 20 January 2004, Published online: 9 March 2004PACS: 34.80.Qb Laser-modified scattering - 34.50.Rk Laser-modified scattering and reactionsS. Hokland: Present address: The MR Reseach Centre, Inst. of Exp. Clinical Research, University of Aarhus, Aarhus University Hospital, 8200 Århus N, Denmark.     

Off-shell effects in the three-nucleon system

The triton binding energy and the neutron-deuteron doublet scattering length have been studied with phase-equivalent rank-2 separable potentials in S-waves. The potentials have been constructed from the phase shifts using the solution of the inverse scattering problem. Large off-shell effects are connected with the appearance of additional nodes in the deuteron or the low-energy singlet wave functions.     

Off-shell effects in nucleon-deuteron polarization observables

A separable representation of theS-wave Paris potential and a phase-shift equivalent Yamaguchi-type potential significantly differing in their off-shell behaviours are used to calculate second-order polarization observables of elastic nucleon-deuteron scattering atE _D=10 and 20MeV. Off-shell effects are found that stem from differences in the nuclear interaction in the inner regionr?1.2 fm. Adding approximate Coulomb-distortion contributions to the purely nuclear Faddeev amplitudes proton-deuteron predictions are obtained. Coulomb effects are not found to be negligible. Comparison with experimental data, in particular, the spin-correlation parameterC _y,y of the reaction \({}^1\vec H(\vec d,d)^1 H\) forE _D=10MeV vector-polarized deuterons and the spin-transfer coefficientsK _y ^y′ ,K _x ^x′ andK _z ^x′ for \({}^2H(\vec p,\vec p)^2 H\) atE _p=10MeV, prefer the interaction model that contains an intermediate-range repulsion.     

Off-shell effects in the three-nucleon problem

Using a previously reported solution of the inverse scattering problem for rank two separable potentials, off-shell effects on the binding energy of the model triton are investigated. Much larger effects are found than previously reported in the literature.     

Off-shell effects in the three-nucleon bound state

Phase-shift-equivalent separable potentials are used to study the sensitivity of the triton binding energy E_T to the off-shell behavior of the two-nucleon T matrix. It is found that the deuteron wave function significantly constrains the theoretical uncertainty in E_T and that the properties of the off-shell T matrix at intermediate energies are only weakly reflected in E_T.     

Off-shell effects in scattering from the deuteron

A simple model for scattering from a deuteron is studied. Within this model exact coupled channel solutions, including all multiple scattering series terms, are obtained for a fixed separation of the two scattering potentials. These solutions are compared with approximations which involve a spherically averaged or non-overlapping potential. Our results indicate that the non-negligible contributions arising from the overlapping potential region can be adequately reproduced by a combination of simple approximations thereby facilitating calculations of scattering from a more physical deuteron model.     

Off-shell effects in quasielastic electron nucleus scattering

Using the Ward-Takahashi identity, on-shell condition, bound Dirac equation and off-shell expansion, a reduced version of half off-shell virtual photon nucleon vertex has been suggested. The vertex are decomposed into several different order terms: the on-shell terms, first and second off-shell terms. The off-shell behaviour of the form factors is discussed in the one meson loop model. Using the reduced vertex and parametrized off-shell form factors the quasielastic response functions are calculated for several nuclei at ¦q¦–k_f and for⁵⁶Fe at large ¦q¦ up to 1.14 GeV/c and at ?q²=0.9 (GeV/c)². The Coulomb sums are evaluated and a comparison of the theoretical prediction with data is given. The off-shell electron nucleon cross section is calculated and compared with the “cc1” off-shell extrapolation.     

Off-shell effects in the (p, 2p) reaction

It is shown that, within the limitations of the distorted-wave off-shell impulse approximation, future accurate (p, 2p) experiments near 150 MeV incident energy can be helpful in distinguishing between some of six phenomenological nuclear potentials that are examined. On-shell differences between the Reid soft-core and five different one-boson-exchange potentials are amplified in the (p, 2p) context in such a way that an accurately measurable feature, the peak height ratio for 1p knockout, is sensitive to them.     

Off-shell phenomena in Coulomb scattering

In the framework of off-shell quantum electrodynamics — the quantum field theory of a covariant symplectic mechanics, in which events evolve according to a Poincaré-invariant parameter τ — we study the low energy scattering of identical scalar particles. It is shown that exchange of mass is permitted in the formalism, and we calculate scattering cross sections for this case. In these cross sections, the usual forward pole of standard scalar QED splits into two poles and a zero, slightly offset from the forward direction. As mass exchange vanishes, a pole-zero pair cancel, the remaining pole moves to θ = 0, and the standard cross section is recovered.     

Off-shell effective actions in string theory

A new, “off-shell”, approach to conformal invariance, β-functions and field equations in first quantized string theory is presented. The method, valid to all orders in the weak-field expansion and non-perturbatively in α′, is illustrated for a tachyon background at genus zero and one level. For a particular choice, we derive a Zamolodchikov-type equation for the effective action.     

Off-shell states in dual resonance theory

We discuss ooff-shell states guided by an analogue model approach. This leads us to a more complete understanding of a model proposed recently by Schwarz with critical dimension 16. We are led, by algebraic considerations, to off-shell states in the Neveu- Schwarz-Ramond model, which obey the gauge conditions in the same critical dimension as the on-shell theory, the amplitudes factorizing on the usual positive definite states in 10 dimensions. Brief calculations reveal that some of the divergences present in the orbital model disappear in the fermion theory.     

Off-shell effects in the (p, 2p) reaction using optimum geometrical conditions

Calculations of off-shell effects in the (p, 2p) reaction are presented using a fixed condition geometry and the Reid soft core potential for the proton-proton interaction. It is shown that ratios of cross sections in the chosen geometry clearly exhibit the off-shell behaviour of the p-p interaction, and that this method therefore offers a means of discriminating between phase equivalent nucleon-nucleon potentials.     

Off-shell fields and pauli-villars regularization

We analyze the correspondence between a five-dimensional U(1)gauge invariant theory and four-dimensional scalar QED, where the fifth dimension ()is an invariant parameter of evolution of the manifestly covariant one-particle sector as well as for the full Fock space. The correspondence is represented by the limit in which the width of the photon mass distribution s tends to zero and large correlations occur. In the limiting procedure, calculation of a twopoint diagram shows that the PauliVillars regularization is intrinsically related to these longrange correlations.