Slow Nucleon Research Articles

Final-state interactions in semi-inclusive deep inelastic scattering off the deuteron

Semi-inclusive deep inelastic scattering off the deuteron with production of a slow nucleon in recoil kinematics is studied in the virtual nucleon approximation, in which the final-state interaction (FSI) is calculated within generalized eikonal approximation. The cross section is derived in a factorized approach, with a factor describing the virtual photon interaction with the off-shell nucleon and a distorted spectral function accounting for the final-state interactions. One of the main goals of the study is to understand how much the general features of the diffractive high-energy soft rescattering accounts for the observed features of FSI in deep inelastic scattering (DIS). Comparison with the Jefferson Lab data shows good agreement in the covered range of kinematics. Most importantly, our calculation correctly reproduces the rise of the FSI in the forward direction of the slow nucleon production angle. By fitting our calculation to the data we extracted the $W$ and ${Q}^{2}$ dependencies of the total cross section and slope factor of the interaction of DIS products, $X$, off the spectator nucleon. This analysis shows the $\mathit{XN}$-scattering cross section rising with $W$ and decreasing with an increase of ${Q}^{2}$. Finally, our analysis points at a largely suppressed off-shell part of the rescattering amplitude.

Spectator scaling in semi-inclusive inelastic lepton scattering off the deuteron and the neutron structure function

The energy and angular distributions of slow nucleons produced in semi-inclusive inelastic lepton scattering off the deuteron are investigated. It is shown that within the spectator mechanism the semi-inclusive cross section exhibits a peculiar scaling property, the spectator scaling, which can be used as a model-independent test of the dominance of the spectator mechanism itself. This property may provide a very interesting tool to obtain model-independent information on the neutron structure function both in the deep-inelastic regime and in the nucleon-resonance production regions.

Semi-inclusive deep inelastic electron scattering off the deuteron and the neutron to proton structure function ratio

The production of slow nucleons in semi-inclusive deep inelastic electron scattering off the deuteron is investigated in the region x ≳ 0.3. It is shown that within the spectator mechanism the semi-inclusive cross section exhibits a scaling property even at moderate values of Q 2 (∼ few (GeV/ c) 2) accessible at present facilities, like CEBAF. Such a scaling property can be used as a model-independent test of the dominance of the spectator mechanism itself and provides an interesting tool to investigate the neutron structure function. The possibility of extracting model-independent information on the neutron to proton structure function ratio from semi-inclusive experiments is illustrated. The application of the spectator scaling to semi-inclusive processes off complex nuclei is outlined.

Final-state interaction in the pd → pnp reaction at 1 GeV

The pd → pnp reaction at 1 GeV in both the direct and charge-exchange channel has been investigated. The experimental data come from a line reversed beam-target experiment with 3.3 GeV/ c deuterons incident on a proton target. In the direct channel data exhibit narrow structures in the np effective-mass spectra: at threshold, at 2.02 and at 2.12 GeV, which have been seen before and we report on a new narrow enhancement at 1.95 GeV. In the charge-exchange channel the data show a somewhat broader peak at 2.18 GeV. The purpose of this work is to explain the data by using a conventional approach, i.e. without sub-nucleonic degrees of freedom, but including the Δ N channel in NN scattering. The calculation is based on a two-channel distorted-wave impulse approximation. The NN → NN spin-dependent scattering amplitudes have been reconstructed by a summation of partial waves with the parameters supplied by phase-shift analyses, whereas the NN → ΔN amplitudes have been calculated from a one-pion-exchange mechanism. The final-state interaction between the slow nucleons is derived from the Paris or Bonn N N potential except for the 1D 2 and 3F 3 states, which are assumed to be coupled with the Δ N channel and have been described here by a two-channel phenomenological potential. The finite width of the delta is included by introducing a continuum of Δ N channels with the delta-mass distribution given by a Breit-Wigner shape. The calculated spectra of the effective mass of the two slow nucleons, imposing various kinematical cuts, are compared with experiment. The effects of the Δ N intermediate state are clearly seen as a broad maximum in the effective-mass distribution in the charge-exchange channel. This effect is less pronounced in the direct channel dominated by the isospin non-flip amplitude. The overall agreement is very good, but in the direct channel narrow peaks are present which are unaccounted for by a conventional approach.

The pion contribution to semi-inclusive deep inelastic scattering with slow baryons in the final state

We analyze the pion-exchange contribution to semi-inclusive leptoproduction on protons, with a slow nucleon or delta in the final state. We calculate the crosssection for slow protons including also the effect of the delta intermediate state, which is the dominant contribution. A comparison with recent experiment shows reasonable agreement for small Bjorkenx, if the cut-off in the pion-nucleon-delta form-factor is chosen to be around 0.7 GeV. This substantiates the recent conjecture that sea-quark distributions can be completely associated with the meson cloud, but suggests that the pion-nucleon-delta form-factor should be softer than the one used in description of nuclear forces by meson exchange. We also compare the pion-exchange and the string-fragmentation models and point out possible future experimental tests.

Six-quark effects in the proton-deuteron break-up reaction

The impulse-approximation model has been used to analyse spectator momentum distributions in dp → pnp reaction with deuterons of 3.3 GeV/ c momentum incident on a hydrogen bubble chamber. The model includes corrections due to (i) final-state interactions between the two slow nucleons, (ii) double scattering (shadowing), and (iii) six-quark effects modifying the deuteron (Paris) wave function. The six-quark correction is obtained by eliminating Pauli-forbidden components from the deuteron wave function. The overall agreement with experiment is very good. We find the six-quark correction indispensable for reproducing the high-momentum tail of the spectator momentum distribution.

Symmetrization Effects in Spectator Momentum Distributions

The traditional identification of the slow nucleon as the in deuteron-breakup collisions is shown to be valid only when there is a significant difference in the speeds of the two nucleons. When both have similar speeds, symmetrization effects cannot be ignored. These effects will be particularly important (a) for low-momentum-transfer events and (b) for high-momentum spectator events. A new comparison technique free of these difficulties is suggested. By studying the transverse momentum spectrum for both final-state nucleons, one avoids ambiguous identification. In addition, the effect of the interference in such a distribution reveals the spatial symmetry of the final two-nucleon state. From this information it is possible to determine the relative contributions of spin-flip and -nonflip processes, and thereby to reconstruct the free-nucleon scattering accurately.

Perturbation approach to the fermi gas model of heavy nuclei

The general perturbation method presented in an earlier paper 1) is applied to a study of some aspects of the Fermi gas model of nuclear matter. The two extreme cases of very low and very high particle density are investigated. It is shown that the theory recently developed by Brueckner 2) can be considered as an improved low density approximation. The validity of some other approximations made in the Brueckner theory, involving the energy denominators in intermediate states are briefly discussed. The study of the case of high density reveals the interesting fact that, contrary to indications presented by Swiatecki 3) and Bethe 4), the convergence of the perturbation expansion gets worse with increasing density, while in that case the Brueckner approximation becomes extremely poor. It is furthermore shown that a slow nucleon, travelling through nuclear matter, can be considered as being in a metastable state and an exact expression for the inverse life-time of such a state is given; this quantity must be identified with the imaginary part of the potential in the optical model.