arXiv subjects
Publications and source records attributed to J. Speth.
We investigate the role of correlated $πρ$ exchange in the extraction of matrix elements of the strange vector current in the proton. We show that a realistic isoscalar spectral function including this effect leads to sizeably reduced strange vector form factors based on the dispersion--theoretical analysis of the nucleons' electromagnetic form factors.
We study the influence of mass splitting between the charged and neutral pions and kaons in the Juelich meson exchange model for $ππ$ and $πη$ scattering. The calculations are performed in the particle basis, which permits the use of physical masses for the pseudoscalar mesons and a study of the distinct thresholds associated with the neutral and the charged kaons. Within this model we also investigate the isospin violation which arises from the mass splitting and an apparent violation of G-parity in $ππ$ scattering which stems from the coupling to the $K\overline{K}$ channel. Nonvanishing cross sections for $ππ\rightarrow π^{0}η$ indicate a mixing of the $f_{0}(980)$ and $a_{0}(980)$ states.
We explore the missing momentum dependence of the CT effects in quasielastic (e,e'p) scattering. We develop the coupled-channel multiple-scattering theory (CCMST) description of final-state interaction including both the coherent and incoherent rescatterings of the ejectile state. We demonstrate that the contribution of the off-diagonal incoherent rescattering does not vanish at low Q^{2} which is a novel correction to the conventional Glauber theory evaluation of nuclear transparency. We comment on the nontrivial impact of this correction on the onset of CT. The sensitivity of the onset of CT to the 3q-nucleon reggeon amplitudes is discussed for the first time. We present numerical results for nuclear transparency as a function of the missing momentum for exclusive (e,e'p) reaction in the kinematical region of Q^{2} \le 40 GeV^{2} and p_{m} \le 250$ MeV/c. Our evaluations show that at Q^{2}\sim 10 CT effects are substantial only in antiparallel kinematics at p_{m,z}\sim -250 MeV/c. The effect is enhanced on light nuclei and could be observed in a high precision experiment.
We develop the Glauber theory description of initial- and final-state interactions (IFSI) in quasielastic A(p,2p) scattering. We study the IFSI-distortion effects both for the inclusive and exclusive conditions. In inclusive reaction the important new effect is an interaction between the two sets of the trajectories which enter the calculation of IFSI-distorted one-body density matrix for inclusive (p,2p) scattering and are connected with incoherent elastic rescatterings of the initial and final protons on spectator nucleons. We demonstrate that IFSI-distortions of the missing momentum distribution are large over the whole range of missing momentum both for inclusive and exclusive reactions and affect in a crucial way the interpretation of the BNL data on (p,2p) scattering. Our numerical results show that in the region of missing momentum $p_{m}\lsim 100-150$ MeV/c the incoherent IFSI increase nuclear transparency by 5-10\%. The incoherent IFSI become dominant at $p_{m}\gsim 200$ MeV/c.
A brief overview on recent model calculations for near threshold pion production in nucleon-nucleon collisions is given. Results from our own investigations of the reactions $pp \rightarrow ppπ^0$ and $pn \rightarrow dπ^0$ are presented. Direct production, heavy meson exchange and pion rescattering are taken into account. For the latter a T-matrix obtained from a microscopic model of the $πN$ interaction is employed. We obtain a significant contribution from rescattering, but not enough to describe the data for $pp \to ppπ^0$. The missing production rate can be provided by heavy meson exchanges. For the first time the effect of off--shell rescattering is investigated for the reaction $pn \rightarrow dπ^0$. Isoscalar rescattering in combination with isovector rescattering is able to describe the s-wave production data. We confirm that heavy meson exchanges are negligible in this reaction.
We suggest a method of determination of the pion structure function down to x \simeq 10^{-4} based on semi-exclusive deep inelastic scattering off protons. The idea is to exploit the nonperturbative πN$ and πΔFock components of the nucleon, which contribute significantly to deep inelastic scattering and dominate the fragmentation of protons into fast neutrons and deltas. The intrinsic factorization properties of the semi-exclusive cross section give a good test for the validity of this approach.
We discuss the present status of the \bar u-\bar d asymmetry in the nucleon and analize the quantities which are best suited to verify the asymmetry. We find that the Drell-Yan asymmetry is the quantity insensitive to the valence quark distributions and very sensitive to the flavour asymmetry of the sea. We compare the prediction of the meson cloud model with different experimental data including the Fermilab E772 data and recent data of the NA51 Collaboration at CERN and make predictions for the planned Drell-Yan experiments.
We present a complete set of formulas for longitudinal momentum distribution functions (splitting functions) of mesons in the nucleon. It can be applied in the framework of convolution formalism to the deep-inelastic structure functions (quark distributions) of the nucleon viewed as a system composed of virtual 'mesons' and 'baryons'. Pseudoscalar and vector mesons as well as octet and decuplet baryons are included. In contrast to many approaches in the literature the present approach ensures charge and momentum conservation by the construction. We present not only spin averaged splitting functions but also helicity dependent ones, which can be used to study the spin content of the nucleon. The cut-off parameters of the underlying form factors for different vertices are determined from high-energy particle production data. This information allows one to calculate the flavour and spin content of the nucleon. The value of the Gottfried Sum Rule obtained from our model (S_G = 0.224) nicely agrees with that obtained by the NMC. In addition, we calculate the x-dependence of the \bar d - \bar u asymmetry and get an impressive agreement with a recent fit of Martin-Stirling-Roberts. The calculated axial coupling constants for semileptonic decays of the octet baryons agree with the experimental data already with SU(6) wave function for the bare nucleon. Although we get improvements for the Ellis-Jaffe Sum Rules for the proton and neutron in comparison to the naive quark model, the MCM is not sufficient to reproduce the experimental data.
A dynamical model for correlated two-pion and two-kaon exchange in the baryon- baryon interaction is presented, both in the scalar-isoscalar ($σ$) and the vector-isovector ($ρ$) channel. The correlations between the two pseudoscalar mesons are taken into account by means of $ππ- K\bar K$ amplitudes derived from a meson-exchange model, which is in line with the empirical $ππ$ data. It is found that correlated $K\bar K$ exchange plays an important role in the $σ$-channel for baryon-baryon states with non- vanishing strangeness. The strength of correlated $ππ$ plus $K\bar K$ exchange in the $σ$-channel decreases with the strangeness of the baryon- baryon system becoming more negative. The results for correlated $ππ$- exchange in the vector-isovector channel deviate from what is expected in the naive SU(3) picture for genuine $ρ$-exchange. Shortcomings of a simplified description in terms of sharp mass $σ$- and $ρ$-exchange are pointed out.
We evaluate the contribution to the nucleon-nucleon interaction due to correlated $πρ$ exchange in the $π$, $ω$, and $A_1$/$H_1$ channels by means of dispersion-theoretic methods based on a realistic meson exchange model for the interaction between $π$ and $ρ$ mesons. These processes have substantial effects: In the pionic channel it counterbalances the suppression generated by a soft $πNN$ form factor of monopole type with a cutoff mass of about 1 GeV; in the $ω$-channel it provides nearly half of the empirical repulsion, leaving little room for explicit quark-gluon effects.
We perform a momentum-space calculation of the reaction $pp \rightarrow ppπ^0$ near threshold. Direct production and pion rescattering are taken into account. For the latter a T-matrix obtained from a microscopic model of the $πN$ interaction is employed. The effects of some approximations commonly used in studies of $ pp \rightarrow ppπ^0$ \ are discussed. We find that those approximations overestimate the cross section and also modify its energy dependence. We obtain a significant contribution from rescattering, but not nearly enough to describe the data.
Are effects of short range correlations in the ground state of the target nucleus (initial state correlations ISC) observable in experiments on quasielastic $A(e,e'p)$ scattering at large missing momentum $p_{m}$? Will the missing momentum spectrum observed at CEBAF be overwhelmed by final state interactions (FSI) of the struck proton? The recent advances in the theory of FSI and findings of complex interplay and strong quantum-mechanical interference of FSI and ISC contributions to scattering at $p_{m}\gsim 1$\,fm$^{-1}$ are reviewed. We conclude that for $p_m \gsim 1$ \, fm$^{-1}$ quasielastic scattering is dominated by FSI effects and the sensitivity to details of the nuclear ground state is lost.
We use multiple scattering theory to evaluate the $Q^{2}$ dependence of the forward-backward asymmetry, $A_{z}$, of the missing momentum distribution in quasielstic $A(e,e'p)$ scattering, which is expected to be affected by color transparency. The novel features of our analysis are a consistent treatment of the structure of the ejectile state formed after absorption of the virtual photon by the struck proton and a careful evaluation of the background asymmetry induced by the nonvanishing real part of the proton-nucleon scattering amplitude. We find that the absolute magnitude of $A_{z}$ is dominated by this background at the $Q^{2}$'s attainable at the Continuous Electron Beam Accelerator Facility. However, the $Q^{2}$ dependence of the asymmetry is sensitive to the onset of color transparency, whose observation in high statistics experiments appears to be feasible, particularly using light targets such as $^{12}C$ or $^{16}O$.
A dynamical model for S-- and P--wave correlated $2 π$ (and $K \bar K$) exchange between a kaon and a nucleon is presented, starting from corresponding $N \bar N \rightarrow K \bar K$ amplitudes in the pseudophysical region, which have been constructed from nucleon, $Δ$--isobar and hyperon ($Λ$, $Σ$) exchange Born terms and a realistic meson exchange model of the $ππ\rightarrow K \bar K$ and $K \bar K \rightarrow K \bar K$ amplitude. The contribution in the s--channel is then obtained by performing a dispersion relation over the unitarity cut. In the $ρ$--channel, considerable ambiguities exist, depending on how the dispersion integral is performed. Our model, supplemented by short range interaction terms, is able to describe empirical $K^+ N$ data below pion production threshold in a satisfactory way.
The excitation of the proton into undetected multiparticle states (double diffraction dissociation) is an important background to single diffractive deep-inelastic processes $ep \rightarrow e'p'ρ^{0}, ~e'p'J/Ψ, ~e'p'X$ at HERA. We present estimates of the admixture of the double diffraction dissociation events in all diffractive events. We find that in the $J/Ψ$ photoproduction, electroproduction of the $ρ^0$ at large $Q^2$ and diffraction dissociation of real and virtual photons into high mass states $X$ the contamination of the double diffraction dissociation can be as large as $\sim 30\%$, thus affecting substantially the experimental tests of the pomeron exchange in deep inelastic scattering at HERA. We discuss a possibility of tagging the double diffraction dissociation by neutrons observed in the forward neutron calorimeter. We present evaluations of the spectra of neutrons and efficiency of neutron tagging based on the experimental data for diffractive processes in the proton-proton collisions.
We apply the light-cone technique to electroproduction reaction $ep\to eπ^+n$ long considered as a means of measuring the electromagnetic form factor of the pion. We show that the interpretation of the long-standing puzzle of large transverse cross section ($σ_T$) in terms of the $γ^*ρ\to π$ transition on the $ρ$ mesons in the light-cone proton is possible, but requires quite a slow decrease of the $F_{ρπ}(\q2)$ form factor. This interpretation can be tested in the related $ep\to eπ^0p$ reaction. Corrections which are due to the final-state meson-baryon interactions (FSI) are evaluated and are shown to amount to a $25\%$ effect at moderately large $\q2$. Vanishing FSI with increasing $Q^2$ - the color transparency phenomenon - is shown to be very strong.
We develop the Glauber theory description of final-state interaction (FSI) in quasielastic $A(e,e'p)$ scattering. The important new effect is an interaction between the two trajectories which enter the calculation of FSI-distorted one-body density matrix and are connected with incoherent elastic rescatterings of the struck proton on spectator nucleons. We demonstrate that FSI distortions of the missing momentum distribution are large over the whole range of missing momenta. Important finding is that incoherent elastic rescatterings of the ejected proton lead to a strong quantum mechanical distortions of both the longitudinal and transverse missing momentum distributions. It is shown that allowance for finite longitudinal size of the interaction region for proton-nucleon collision neglected in the standard Glauber model drastically affects the theoretical predictions at high longitudinal missing momentum. We also find very large corrections to the missing momentum distribution calculated within the local-density approximation.
Meson-meson correlation effects are investigated in the $\NNb\toρπ$ annihilation process using a realistic meson-exchange model for the $ρπ$ interaction determined previously, together with a conventional baryon-exchange transition model and a consistent $\NNb$ interaction. For $\NNb$ $S$-states, they have a drastic effect and bring the relative (${^1S_0}/{^3S_1}$) branching ratio up to the experimental value, thus resolving the long-standing so-called ``$ρπ$'' puzzle. For $\NNb$ $P$-states, their effect is of minor importance, and discrepancies remain for those ratios involving annihilation from the $\NNb({^3P_J})$ state to $ρπ(l'=2)$.