Nucleon-nucleon Final-state Interactions Research Articles

Near-threshold π− photoproduction on the deuteron

The first experimental investigation of the near-threshold cross section for incoherent $\pi^-$ photoproduction on the deuteron $\gamma$ d -> $\pi^-$ pp is presented. The experimental technique involved detection of the ~131 MeV gamma ray resulting from the radiative capture of photoproduced $\pi^-$ in the target. The total cross section has been measured using an unpolarized tagged-photon beam, a liquid-deuterium target, and three very large NaI(Tl) spectrometers. The data are compared to theoretical models that give insight into the elementary reaction $\gamma$ n -> $\pi^-$ p and pion-nucleon and nucleon-nucleon final-state interactions.

On the migdal-watson approach to FSI effects in meson production in NN collisions

The influence of the nucleon-nucleon final-state interaction (FSI) on properties of the meson-production amplitude near threshold is discussed. For nucleon-nucleon interaction, a simple Yamaguchi potential and realistic potential models are considered. It is shown that FSI effects cannot be factorized from the production amplitude. The absolute magnitude of FSI effects depends on the momentum transfer (or on the mass of the produced meson) and hence is not universal. Only in the case of the production of rather heavy mesons like η′ or φ FSI do effects become universal. The Jost function approach to FSI effects is critically examined.

Spin polarization effects in the 3He(d, 3He p)n and the 3He(d, t p)p reactions at Ed = 17 MeV

Triple-differential cross sections and vector analyzing powers were measured for the 3He(d, 3He p)n and the 3He(d, t p)p reactions at E d = 17 MeV. Data are presented for four pairs of coplanar detection angles for the 3He/triton and proton where quasi-free scattering (QFS) or quasi-free reaction (QFR) processes are expected to be dominant, and for five sets of angles where the kinematics favor the nucleon-nucleon final-state interaction (NN FSI). Tensor-analyzing-power measurements, T 20 and T 22, are also presented for two sets of angles. For the angle pairs where the QFS/QFR mechanism is important, the data are compared with the predictions of PWIA and DWIA calculations. While both calculations are only partly successful in describing the shapes of the quasi-free peaks, the DWIA better predicts their magnitudes. The vector analyzing powers (VAP) in the quasi-free region are at best only qualitatively reproduced by both calculations for the QFS, while for the QFR there is quantitative agreement between both calculations and the data only in the energy region near the center of the QFR maximum. Data obtained for the angle pairs where the NN FSI is present are compared to the predictions of a simple two-step model. The reaction is assumed to proceed by pickup of a nucleon from the 3He by the deuteron, followed by breakup of the recoil dinucleon system. These calculations describe the shapes and magnitudes of the NN FSI peaks reasonably well. The VAP predictions are for the most part also consistent with the two-step assumption. In all of the data, strong contributions are present from nucleon-trion FSI corresponding to the 0 − and 2 − resonances in 4He at E x = 21.1 and 22.1 MeV, respectively.

New pion-absorption modes observed from triple coincidences in 4He.

The first triple-coincidence data resulting from pion absorption in $^{4}\mathrm{He}$ are reported. The existence of a three-nucleon absorption component in the four-nucleon channel is established. A four-nucleon phase-space-like contribution and nucleon-nucleon final-state interactions are also identified.

A three-body calculation of the πd → πnp reaction in the Δ-resonance region

The reaction d( π ±, p π ±)n at incident pion momentum 340 MeV/ c is analyzed based on a relativistic three-body formalism. The contributions of the various reaction mechanisms such as impulse processes, pion multiple scattering and nucleon-nucleon final-state interactions are investigated for several cases of typical kinematics. The impulse term is dominant when the recoil neutron momentum is small. On the other hand, the NN final-state interaction is found to be relatively important when the recoil neutron momentum is large. The effects of dibaryon resonances which have been suggested in πd elastic scattering are estimated using a phenomenological model. Comparisons with other work are also made.

Bound-pion absorption followed by emission of two nucleons

The bound-pion absorption reaction, viz. 12C( π −, NN), is studied using Hartree-Fock (HF) wave functions obtained with the unitary-model-operator approach starting with the hard-core nucleon-nucleon (NN) interaction. The inequality of the energies of the two outgoing nucléons is treated exactly and calculations are done using the “1N model” for π-absorption. Other effects taken into account are: NN scattering in the final state, contributions of all excited states of 10B and 10Be with E < 5 MeV, and effects of the strong π-nucleus interaction and the finite nuclear size on the bound-π wave function. Branching ratios and angular distributions of absorption rates are in better agreement with experimental data. The correct order of magnitude of the total absorption rate is reproduced. Whatever the effects of short-range correlations present in the HF wave functions, they are not masked by the NN final-state interaction. The contribution of excited states in 10B and 10Be is found to be quite large. Absorption rates obtained with the HF and oscillator wave functions differ significantly both in size and shape.

Effects of exchange and final-state interactions in the reactions 2H( 3He, 3H p)p and 2H( 3He, 3He p)n

Absolute coincidence cross sections for the 2H( 3He, 3He p)n and 2H( 3He, 3H p)p reactions were measured at E He = 35.9 MeV. Spectra dominated by the nucleon-nucleon final-state interaction (FSI) are fitted by a fully antisymmetrized PWBA theory which includes the effects of FSI in all its matrix elements. Previously reported 26.8 MeV data showing both FSI and quasi-elastic scattering (both with and without charge exchange) are also fitted by the theory, which qualitatively describes the shapes of all these spectra and the ratios of the cross sections for the various processes. Predictions of Watson-Migdal theory are fitted to the FSI spectra and differences between the two theories are analyzed.

Measurement of theH2(p,2p)nReaction over a Large Volume of Phase Space

A kinematically complete measurement of the deuteron breakup cross section has been made using a 39.5-MeV proton beam. The cross section was simultaneously measured at approximately 2000 angle pairs chosen to uniformly sample the entire phase-space volume available to the two protons. A portion of the data in the form of a partial Dalitz plot is compared with a simple model calculation. In the model, the breakup cross section is expressed as an incoherent sum of terms to separately represent the dependence of the cross section on statistical processes, $pp$ quasifree scattering, and nucleon-nucleon final-state interactions. The model calculation does not reproduce the minimum in the partial Dalitz plot, consistent with the previously published hypothesis that the minimum is due to a destructive interference effect. The results of the model calculation are also compared with spectra at a representative set of individual angle pairs.

Analysis of the reaction np → d π+π− below 3.5 GeV/ c

The reaction np → d π + π − was studied from threshold up to 3.5 GeV/ c neutron beam momentum. Cross-section values for this reaction are evaluated as a function of the beam momentum. The deuteron production features and the enhancement observed in the dπ mass spectrum at 2.17 GeV are adequately described in terms of an OPE model and nucleon-nucleon final-state interactions.

A unitary model for three-body collisions

A connected 3 → 3 formalism for three-body collision processes is reduced to a hierarchy of three on-energy-shell integral equations and one off-energy-shell integral equation. Only the on-energy-shell equations, which involve only on-energy-shell three-body and two-body amplitudes, need be solved exactly in order to obtain elastic and break-up amplitudes satisfying the unitarity constraints exactly. Applied to n-d break-up, the on-energy-shell equations ensure that the n-d initial-state interaction, the nucleon-nucleon final-state interactions, and more complicated 3 → 3 processes are correctly described. After angular momentum analysis the on-energy-shell equations are one-dimensional integral equations, even in the case of local two-body potentials. This unitary model provides a practical scheme for calculating approximate three-body elastic and break-up amplitudes when two-body local potentials are used to describe the two-body subsystems.

Three-body break-up in the 3He+d reaction

Abstract The three-body break-up of the five-nucleon system 3 He+d has been investigated through the reactions 3 He + d → 3 He + n + p and 3 He + d → 3 H + 2 p . In both cases the mass-3 particle and a proton were detected in a kinematically complete experiment for a deuteron bombarding energy of 10.92 MeV. A 3 He gas target in a small gas cell was used. Particle identification of the mass-3 particle was done with a three-detector d E /d x — E telescope. The data show the presence of nucleon-nucleon final-state interactions as well as those leading to excited states of the mass-4 system. The extraction of nucleon-nucleon scattering lengths and the effect of the excited mass-4 states are discussed.

Nucleon-nucleon final-state interactions in the reactions 3He(p, d)2p, 4He(p, t)2p and 4He(p, 3He)pn

The energy spectra of deuterons, tritons and 3He particles from the reactions 3He(p, d)2p, 4He(p, t)2p and 4He(p, 3He)pn have been measured at angles between 6° and 60° lab. The 3He(p, d)2p reaction was studied at both 30.5 and 49.5 MeV incident proton energies, while the other two reactions were studied at 49.5 MeV only. The energy spectra are compared with calculations based on the Watson-Migdal model of final-state interactions.