Spectator Nucleon Research Articles

Color transparency effects in electron deuteron interactions at intermediateQ 2

High momentum transfer electrodisintegration of polarized and unpolarized deuterium targets, $d(e,e'p)n$ is studied. We show that the importance of final state interactions-FSI, occuring when a knocked out nucleon interacts with the other nucleon, depends strongly on the momentum of the spectator nucleon. In particular, these FSI occur when the essential contributions to the scattering amplitude arise from internucleon distances $\sim 1.5~fm$. But the absorption of the high momentum $\gamma^*$ may produce a point like configuration, which evolves with time. In this case, the final state interactions probe the point like configuration at the early stage of its evolution. The result is that significant color transparency effects, which can either enhance or suppress computed cross sections, are predicted to occur for $\sim 4 GeV^2 \ge Q^2\leq~10~(GeV/c)^2$.

Simple classification of final state interaction effects in [formula omitted] scattering

The radius of interaction between the struck proton and spectator nucleons is close to the radius of short-distance two-nucleon correlations in nuclear matter, which makes final state interaction (FSI) an important background to production of protons with large missing momentum. We present a simple classification of the dominant FSI effects in inclusive 4 He(e, e′p) scattering and identify parts of the phase space dominated by FSI. At large missing momentum, final state interaction leads to a striking angular anisotropy of the missing momentum distribution, which has a prominent peak in transverse kinematics and smaller, forward-backward asymmetric, peaks in parallel kinematics.

Polarization phenomena in knock-out reactions from the deuteron

Expressions are obtained in the quasi-free scattering approximation for the asymmetry in (p, pn) and (p, 2p) reactions on polarized deuterons as well as for the polarization of spectator nucleons for the case of an unpolarized target. They guide the way to obtain new information on the D-wave deuteron wave function and thereby to refine tensor terms of the NN-potential. The particular value of noncoplanar investigations is emphasized.

Slow proton production in deep-inelastic neutrino scattering on deuterium.

The cross section for semi-inclusive deep-inelastic charged current neutrino scattering on hydrogen and deuterium in which a slow proton is observed in coincidence with the muon is computed as a function of Bjorken x and light-cone momentum of the detected proton. In the impulse approximation contributions from hadronization and (in case of the deuteron) the emission of a spectator nucleon are considered. In addition the probability for rescattering is computed. The results are compared to a recent analysis of the data from the WA25 (BEBC) experiment at CERN.

Slow proton production in semi-inclusive deep inelastic lepton scattering off nuclei

Nuclear effects in semi-inclusive deep inelastic lepton scattering off complex nuclei are investigated within the spectator mechanism. Results of calculations for forward and backward proton emissions are presented both at x < 1 and x > 1. Contributions from quark fragmentation and final state interaction of the spectator nucleon are estimated and a comparison with existing data on single proton backward production is illustrated.

Rescattering effects in the reaction gamma d to d pi 0 at large pi 0 production angles

The authors determine the reaction amplitude for gamma d to d pi 0 taking into account two mechanisms: the impulse approximation (IA) and the produced pion rescattering (double scattering) on the spectator nucleon (DS). In the calculation they use the non-relativistic diagrams. Employing the factorization hypothesis for the elementary processes gamma N to gamma N pi and pN to N pi amplitudes and the deuteron form factors, they determine the spin structure of both the considered parts of the amplitude. The kinematical conditions for which the amplitudes can be calculated are discussed in detail. They take into account the Fermi motion of nucleons in the deuteron. They show that the method used (the DS effect calculation) is appropriate at small pi 0 production angles, theta <90 degrees and results in better agreement between the measured and calculated cross section. However, at theta >90 degrees the DS contribution to the reaction amplitude significantly exceeds the IA contribution. At large pi 0 production angles, the DS amplitude calculation is rather ambiguous.

Bose-Einstein correlations in the target fragmentation region of 200 A GEV 16O + nucleus collisions

Correlations of positive pions are investigated in the target fragmentation region of 200 A GeV 16O + nucleus collisions. The pions are measured with the Plastic Ball detector in the WA80 experiment at the CERN SPS. The detailed behaviour of the fitted parameters on rapidity, centrality and the number of spectator nucleons is studied. The data indicate a large source in central O + Au collisions for ylab ⩽ 0. The observations are compatible with a substantial amount of rescattering in the target.

Evidence against a strong sub-threshold structure in the�p p amplitude

Spectator nucleon distributions in¯pd →mπN are directly related to the antiproton-nucleon annihilation process below threshold. To demonstrate this quantitatively, we analyse the available¯pd → 3πN and¯pd → 5πN data. For these data, (and others) the shape of the annihilation spectrum in the peak region is well described by the tree diagram, and reflects mainly theS-state part of the deuteron wave function. The data are not consistent with a strong structure in the antinucleon-nucleon sub-thresh-old amplitude as reported in the dispersion analysis of¯pp while a weak structure is consistent with the existing data.

Pion absorption in flight on 4He

We have measured double and triple coincidences following π + and π − absorption on 4He at various pion energies. Differential and integrated cross sections are determined for all possible absorption channels. We study and compare the two-nucleon absorption on nucleon pairs with isospin I = 0 and I = 1. In the dNN final-state two absorption mechanisms are identified, one with only one spectator nucleon and one with phase-space behavior. The existence of a three-nucleon absorption component with a spectator nucleon is established and a four nucleon phase-space-like contribution is also identified and found to be small. The total absorption cross section is directly determined by adding the integrated cross sections of all measured reaction channels.

Electroexcitation of 6(-) states in 32S.

Inelastic electron scattering is used to identify nine stretched isovector [${\mathit{T}}_{&gt;}$ (${\mathit{T}}_{0}$\ensuremath{\rightarrow}${\mathit{T}}_{0}$+1) M6 transitions in $^{32}\mathrm{S}$ (${\mathit{T}}_{0}$=0) which exhaust (77\ifmmode\pm\else\textpm\fi{}17)% of the extreme single-particle-hole-model sum rule. This is the first time that several prominent ${\mathit{T}}_{&gt;}$ electromagnetic stretched transitions have been observed in a self-conjugate nucleus and the first time that such a large fraction of the sum rule has been observed. These effects are primarily due to the presence of spectator nucleons in the 2${\mathit{s}}_{1/2}$ orbit, as substantiated by shell-model calculations.

Cascading in ultrarelativistic nuclear collisions

We sketch the string model VENUS 3 for ultrarelativistic nuclear collisions, which treats the reinteraction of decay products from string fragmentation among themselves or with spectator nucleons. We investigate rapidity and transverse energy distributions in proton-nucleus and nucleus-nucleus collisions. Comparing the spectra with and without interaction we observe no increase in the total pion multiplicity; however, there is a backward shift in rapidity. On the other hand we observe considerably more protons in the target fragmentation region around y=0. Transverse energy distributions are significantly broader once the target fragmentation region is covered.

Probing string fragmentation in nuclear matter: Lepton-nucleus scattering

We introduce a new model for the fragmentation of strings in a “medium” of hadronic objects. These objects may be hadrons, resonances, or string pieces from the fragmentation of other strings. As a first application we treat the fragmentation of a string inside a nuclear medium, taking into account interactions of the fragmenting string with spectator nucleons (spectators of the hard process which created the string in the first place). We compare with neutrino-neon data.

Production of quark matter via a conical shock wave

Exact solutions of the relativistic Taylor-Maccoll equations have been obtained for an idealized model for the flow of nuclear matter where the surface separating participant and spectator nucleons is a cone. Comparison of these solutions for baryonic matter and quark matter suggests that quark matter production will result in an increase in the deflection of nuclear matter for nearly head-on collisions of relativistic heavy ions.

The quasi-elastic charge exchange reaction p 3He→ n Fppp at intermediate energies

The basic characteristics of the quasi-elastic charge exchange reaction p 3He→ n Fppp are investigated using the 80 cm ITEP liquid-hydrogen bubble chamber, which is exposed to beams of 3He nuclei with momenta of 2.5 and 5 GeV/ c (the kinetic energy of primary protons in the rest system of the nucleus are, respectively, T p = 0.318 and 0.978 GeV). The experimental data are compared with the predictions of the Glauber-Sitenko multiple scattering theory and of the pole model, taking into account the interaction between spectator nucleons in the final state. In the mass spectrum of the 3p system at 3.05 GeV a clearly denned structure has been revealed, which is not described within the pole model. A possible resonance origin of the given structure is discussed.

Phase space model of hard-photon production in heavy-ion collisions

We propose a semi-classical model to study the characteristics of the observed hard-photon production in heavy-ion collisions. For this we divide the nucleons in two categories on a geometrical basis: participant nucleons in the overlapping region which can suffer two-body collisions thus producing photons through n-p bremsstrahlung and spectator nucleons. We investigate the relative importance of the gain and loss term of the Uehling-Uhlenbeck collision integral. The elementary pn→pnγ process is included for two different prescriptions: i) classical soft limit, ii) quantum calculation including meson exchange. We also study the influence of the Δ-resonance in hard-photon production. We show that for heavy colliding nuclei and high beam energies a bump should be visible.

Annihilation of antiprotonic deuterium into two pseudoscalar mesons and a nucleon

In conclusion, the annihilation of deuterium contains additional information compared to protonium annihilation. The angular distribution of the relative momentum of the mesons compared to the spectator nucleon momentum is essential. Absolute annihilation threshold helicity amplitudes as well as their relative phases can in principle be determined by experiments.

Corrections to the Fermi matrix element for superallowed beta decay.

Corrections to the Fermi matrix element for superallowed transitions due to isospin nonconservation are reexamined. The sources of theoretical uncertainty and the possibility of a previously neglected correction based on the mismatch between the spectator nucleons are investigated. The consequences to the conserved-vector-current hypothesis and the Kobayashi-Maskawa mixing angles are discussed.

Production of pions and light fragments in 0.8A GeV La

The spectra of pions and light fragments at laboratory angles from 10/sup 0/ to 89/sup 0/ were measured in 0.8A GeV La+La collisions. Event multiplicity was also measured to select central collisions. For inclusive data, the slope of the pion spectrum at theta/sub c.m./ = 90/sup 0/ for La+La is as steep as that for Ar+KCl while the slope of the proton spectrum at theta/sub c.m./ = 90/sup 0/ becomes less steep with increasing mass number of the colliding nuclei. Furthermore, the ..pi../sup -/ yield is about half of the geometrical estimate while total nuclear charge yield agrees well. Since the yield ratio of ..pi../sup -/ to total nuclear charge at theta/sub c.m./ = 90/sup 0/ is significantly larger for high multiplicity events than for inclusive data, it appears that pion absorption by spectator nucleons is important. An azimuthal correlation of charged particle emission was also observed, which is consistent with collective flow of nuclear matter. The coalescence relation between protons and composite fragments holds for inclusive data in La+La collisions.

Longitudinal response of three-body nuclei

The method of correlated basis functions is generalized to describe three-nucleon and deuteron-plus-nucleon continuum states. The correlated basis functions are obtained by incorporating short range correlations between the spectator nucleon and the two nucleons of the interacting pair. Correlated orthonormal wave functions for the continuum states are generated by orthogonalizing the correlated basis functions via a combination of Schmidt and L\owdin transformations, designed so that the correlated orthonormal states have correct energies. The correlated orthonormal continuum states include the minimum interaction effects required to ensure orthogonality and reasonable short range behavior. The longitudinal response of $^{3}\mathrm{He}$ and $^{3}\mathrm{H}$ at k=300 to 600 MeV/c is calculated with the variational ground state wave function and the correlated orthonormal final states. Both the short range correlations and the orthogonality corrections in the final states decrease the response in the quasi-free peak. The calculated response has the correct sum and energy-weighted sum within \ensuremath{\sim}5%, and that of $^{3}\mathrm{He}$ is in reasonable agreement with the Saclay experimental data. We find that the response of $^{3}\mathrm{H}$ is broader than that of $^{3}\mathrm{He}$, and that it is relatively larger at energies greater than that of the quasi-free peak.

Spectator recoil and nucleon emission spectra in intermediate-energy nuclear collisions

The preequilibrium nucleon specra for intermediate-energy heavy-ion collisions are calculated in a hot-zone model based on the concept of equal participation. The recoil spectra of heavy spectators are also calculated. The results are in good agreement with the experimental data.