Meson exchange and isobar current effects in nuclear photon scattering

Explicit expressions of meson exchange and isobar current contributions to the nuclear two-photon amplitude are derived in the non-relativistic limit. In connection with this amplitude the requirement of gauge invariance on the photon scattering amplitude and on the nuclear electromagnetic interaction operator is discussed in detail.

Formal theory for two-particle channels

The general formalism has been developed over many years by various authors. One starting point is the work of de Swart (DSw 59) who has considered electric multipoles in the long-wave-length limit using the Siegert theorem and as magnetic contribution only the dipole spin-flip transition. The T-matrix is then expanded in terms of reduced multipole amplitudes. This approach has been generalized by Donnachie (Don 62a) and Partovi (Par 64) by including higher electric and magnetic multipoles. Furthermore, the electric multipoles are not restricted to the long-wave-length limit and the additional terms besides the Siegert operators (see section 4.1) are included. Using techniques from angular …

Electro- and photodisintegration of deuterium

Deuteron break-up by inelastic electron scattering or photoabsorption is discussed with emphasis on effects from meson exchange currents and isobar configurationgurations.

Isobar configurations in4He

Deuteron electrodisintegration near threshold at high momentum transfer

Electrodisintegration of the deuteron near the break-up threshold is a particularly striking example of the existence of exchange currents (EC). While the conventional one-body current contribution grossly underestimates the experimental cross-section up to a momentum transfer of aboutq μ 2 =15 fm−2, the inclusion of EC—dominantly π-MEC—leads to a drastic increase in satisfactory agreement with experimental data up toq 2=9 fm−2.

The neutron electric form factor and quasi-elastic electron-deuteron scattering

Quasi-elastic electron-deuteron scattering and the electric form factor of the neutron

Methods of calculation for the T-matrix

In the preceding section we have shown how the observables can be expressed in terms of the T-matrix elements or in terms of the multipole amplitudes OLλ(μjls) which contain all the relevant information on the dynamical properties of the system. For the calculation of these amplitudes a variety of different methods have been developed utilizing various kinds of approximations.

Measurement of the magnetic form factor of the neutron

The [sup 2]H([ital e],[ital e][prime][ital n])[sup 1]H quasielastic cross section was measured at [ital Q][sup 2] values of 0.109, 0.176, and 0.255 (GeV/[ital c])[sup 2]. The neutron detection efficiency was determined by the associated particle technique with the [sup 2]H([gamma],[ital pn]) reaction for each of the three neutron kinetic energies. These [sup 2]H([ital e],[ital e][prime][ital n]) measurements of the coincidence cross sections are the first at low [ital Q][sup 2]. The cross sections are sensitive primarily to the neutron magnetic form factor [ital G][sub [ital M]][sup [ital n]] at these kinematics. The extracted [ital G][sub [ital M]][sup [ital n]] values have smaller uncerta…

Polarization Effects in Electromagnetic Deuteron Break-Up

Polarization observables of deuteron photo- and electrodisintegration with polarized photons and electrons, respectively, and oriented deuterons are investigated with respect to their sensitivity to subnuclear degrees of freedom and relativistic effects, to the different electric and magnetic multipole contributions, to various realistic potential models and — in electrodisintegration — to e.m. form factors.

Comparison of theory and experiment

In this section we are finally going to confront experimental data on total cross section, angular distributions and polarization observables with theoretical results in order to establish the present status of our knowledge of this important process. We hope this will enable us to set directions and guidelines for future experimental and theoretical studies, which kind of experiments are needed and what accuracy should be aimed for, where theoretical studies should be extended, on what they should focus and whether some basic assumptions need to be modified. For the comparison we shall use mostly the data of experiments selected in sections 6.3 and 6.4, the relative quantities and the best…

Polarization observables for elastic electron scattering off a moving nucleon

General expressions for all parity-conserving polarization observables of elastic electron-nucleon scattering in the one-photon exchange approximation are derived for a general frame of reference, i.e.\ not assumming scattering off a nucleon at rest and not specializing to a specific system of coordinates. Essentially, the given expressions are also valid for the inverse process, i.e.\ nucleon scattering off electrons.

Elementary theory and brief history

In the history of the theory of deuteron photodisintegration one may distinguish roughly three periods: (i) the primitive period of the elementary theory using very simple wave functions and forces and considering lowest multipoles (E1, M1) only, (ii) the classical period still in the framework f conventional nuclear physics, but using realstic forces with correspondingly elaborate wave functions and considering also higher multipole transitions, (ii) the post-classic period with explicit treatment of subnuclear degrees of freedom like meson and isobar degrees of freedom and very recently quark-gluon degrees of freedom.

Introduction to the Discussion Session on Photo- and Electrodisintegration of the Deuteron

This session is exclusively devoted to the electromagnetic break-up of the deuteron. It reflects the fact that notwithstanding its long history deuteron photo- and electrodisintegration is still an active and interesting field of research (for a recent review on photodisintegration see [1]). The purpose of this introduction is — on the one hand — to put the various contributions to this session into a general perspective and — on the other hand — to supply complimentary material of interesting recent experimental results which is of relevance to the general theme.

Meson and isobar degrees of freedom in light nuclei

The role of mesonic and isobar degrees of freedom in various electromagnetic processes is reviewed for light nuclei. Special emphasis is laid on the deuteron, which allows the cleanest answers within the non-relativistic framework. The origin of the photonuclear enhancement with respect to both exchange forces and exchange currents is discussed in detail.

Polarization Effects in Photo- and Electrodisintegration of the Deuteron

Polarization observables in deuteron photodisintegration like photon and target asymmetries and one- and two nucleon polarization of the final state are studied with respect to their sensitivity to subnuclear degrees of freedom. Furthermore it is shown that the electric form factor of the neutron has a sizeable influence on the polarization asymmetry in d(\(\overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\rightharpoonup}$}}{e}\),e’n)p with polarized electrons. Uncertainties arising from potential model dependence are discussed.

Experimental results for two-particle break-up

In this section we shall review the present status of experiments on two-body photo-disintegration of deuteron up to and above the Δ region. The results of different experiments will be discussed and a guideline will be given for a selection of the experiments to be used in the comparison of experimental data with theoretical results. Moreover, since a large variety of observables exists, which have been studied experimentally, we shall discuss which kind of measured quantities, having a smaller systematic error, can give more accurate information.

Three-Body Analysis of Incoherent Photoproduction of η Mesons on the Deuteron near Threshold

The importance of three-body dynamics in the ηnp system in elastic and inelastic η-deuteron scattering as well as coherent and incoherent η photo-production on the deuteron in the energy region from threshold up to 30 MeV above has been investigated. It is shown that a restriction to first order rescattering with respect to the NN- and ηN-final state interactions, i.e., restriction to rescattering in the two-body subsystems, does not give a sufficiently accurate approximation to the s-wave reaction amplitude and that higher order terms, as described by the three-body dynamics give very substantial contributions.

On screening effects in pair production at intermediate and high energies

The effect of screening in atomic field pair production is calculated in Born approximation without approximation as to energy or angle and compared with the results of previous calculations based on the high energy approximation. The difference between the two calculations is comparable to the errors in recent experiments. The effect of nuclear excitation on the total pair production cross section is also investigated, but is found to be extremely small.

Pion Exchange Current Effects in ν μ + d → μ − +p + p

The effect of pion exchange currents in the process νμ + d → μ− +p + p is estimated using realistic wave functions for the deuteron and closure over the final dinucleon states. Its implications on the determination of axial vector form factor FA(q2) are discussed.

The γd → π0 d and γd → pn Reactions in the Δ-Resonance Region

Coherent pion photoproduction on the deuteron and deuteron photodisintegration in the Δ-resonance region is studied treating the final state interaction within a NN−NΔ coupled channel approach.

Unpolarized and polarized beam sources

The cross sections of photon-induced nuclear processes were usually studied with the aid of a bremsstrahlung beam produced by fast electrons hitting a suitable radiator. Such bremsstrahlung radiation contains photons of all energies from zero up to the kinetic energy of the incoming electrons and so the desired cross section had to be deduced from the integral yield by taking the bremsstrahlung spectrum of the photon beam from the theory. In this section we shall show that this method can produce significant systematic errors in the absolute value of the cross sections due to the lack of a precise knowledge of the bremsstrahlung spectrum.