Multiple soft pion production in nonlinear chiral sigma model

Multiple soft pion production in the baryon scattering reactions is considered in the framework of chiral nonlinear sigma model neglecting the pion mass. Treating baryons in the eikonal approximation as classical sources, a set of analytical solutions for the pion field is found. A tree S-matrix is constructed on the basis of these solutions describing the emission (or absorption) of any number of soft pions. Then the contribution of soft virtual pions is taken into account in a closed form. It is shown that the loop corrections strongly suppress the pion radiation, and for the two limiting cases of nonrelativistic and ultra-relativistic baryon scatterings there is no pion emission from the “ends”. Thus, the mechanism similar to the soft photon bremsstrahlung in the quantum electrodynamics seems to be unable to create a state with a large number of the soft pions.     

High-pT pion production in ππ interactions

Charged pion production in high energy π^-π^- interactions is studied in the p_T region of 1–2 GeV/c. The characteristics of pion production in π^-π^- interactions are compared with those in π^-p and pp interactions. The p_T dependence of pion production in these reactions follows a systematic trend which indicates that high-p_T pion production for ππ interactions as well as for πp and pp interactions proceeds via quark-quark scattering as predicted by QCD.     

An analysis of pion-nucleus scattering lengths within an α-cluster model

Pion-nucleus s-wave scattering lengths are calculated from a multiple scattering series in terms of the π-α scattering length. Agreement on the level of 10–20% with the values obtained from mesic-atom data is achieved for all nuclei with A ≦ 20. This lends weight to the speculation that pion absorption on light nuclei takes place primarily on α-particle substructures.     

Quenching in the basic πN amplitude for pion-nucleus scattering

An upward shift of the 3,3 resonance energy in pion-nucleus scattering is discussed, which arises from the absorption of the transferred pion in the Chew-Low πN amplitude, when the πN system is embedded in the nuclear medium.     

Pions interact with nuclei

This is a pedagogically oriented report about the interaction of pions with nuclei. The review is restricted to low and medium energy phenomena, i.e., to pion energies below 300 MeV. Pionic atoms, pion absorption and production, and the scattering around the (3,3) resonance are the main topics. The experimental data are interpreted within the frame of multiple scattering theory. The basic ideas are critically analysed and open problems are stressed.     

π-π scattering in a hot pion gas at URHIC-conditions

Pion-pion scattering in a hot pion environment is considered. In medium effects considerably diminish the pion — pion scattering rates in the σ — and ρ — meson channels at temperatures and chemical potential of the pion gas close tom _π     

Theory of pion-nucleus scattering lengths

We calculate the scattering lengths for pions on light nuclei, in a multiple scattering theory given earlier. We include all effects through second order in the pion-nucleon scattering amplitudes, using a simple nuclear model. We show how to calculate the binding corrections (BC) consistently, and find large cancellations between differentBC terms. The resulting scattering lengths are smaller than those obtained from measurements of pi-atomic level shifts. The size of the difference is consistent with the contribution of pion absorption, which we estimate.     

A relativistic description of πd scattering: Effects of small πN waves,NN rescattering, ρ-exchange and π absorption and emission

Using a relativistic three-body theory we calculated elastic differential cross sections, polarisations and total cross sections for pion-deuteron scattering in the (3, 3) resonance region. Effects of pion absorption and emission have been calculated rigorously, including pion rescattering effects and ρ-meson exchange to all orders. Inclusion of pion absorption and emission produces appreciable changes in the large angle differential cross section and in all polarisation parameters. The inclusion of non-resonant πN partial waves leads to significantly improved agreement with experiment for T_π = 142 MeV and at forward angles for 256 MeV. None of these effects, nor the inclusion of selected NN partial waves other than ³S₁-³D₁, produces the deep minimum observed at 256 MeV.     

The absorptive pion-nucleus optical potential

We calculate s-wave and p-wave absorptive pion-nucleus optical potentials assuming that a pion is absorbed by a pair of nucleons. Employing a model which takes into account both a single nucleon absorption with nucleon-nucleon correlations and rescattering, we obtain simple analytic expressions for Im B₀ sid Im C₀ of the pion-nucleus optical potential. The off-shell effect on the s-wave pion absorption is examined and shown to be strongly modified by short range correlations. The result for the p-wave absorptive part Im C₀ clearly shows the importance of the tensor correlations. The enhanced nn emission after π^? absorption is shown to be related with a large p-wave πN scattering length a₃₃ via the tensor correlations.     

Nucleons after pion absorption

We calculate energy spectra and angular distributions of single and correlated spectra after the absorption of stopped pions in nuclei. We assume: The pion is abosrbed by a pair of nucleons; these primary nucleons may leave the nucleus directly or only after one or more collisions with other nucleons. We propose a multiple scattering expansion for the intranuclear cascade after pion absorption. Various experimental data are successfully described in shape and absolute magnitude. The ratio R_np = R(π^?np → nn)/R (π^? pp → np) is determined from the experimental proton spectra: R_np = 13 ± 6.     

Pionic deuterium

The strong-interaction shift ε _1s ^πD and broadening Γ _1s ^πD in pionic deuterium have been determined in a high statistics study of the πD(3p-1s) X-ray transition using a high-resolution crystal spectrometer. The pionic deuterium shift will provide constraints for the pion-nucleon isospin scattering lengths extracted from measurements of shift and broadening in pionic hydrogen. The hadronic broadening is related to pion absorption and production at threshold. The results are ε _1s ^πD = (?2356 ± 31) meV (repulsive) and Γ _1s ^πD meV yielding for the complex πD scattering length a _πD = [?(24.99±0.33)+i(6.22 _?0.26 ^+0.12 )] × 10^?3 m _π ^?1 . From the imaginary part, the threshold parameter for pion production is obtained to be α = (251 _?11 ⁺⁵ ) μb. This allows, in addition, and by using results from pion absorption in ³He at threshold, the determination of the effective couplings g ₀ and g ₁ for s-wave pion absorption on isoscalar and isovector NN pairs.     

Nuclear effects in massive lepton pair production

Precise measurements of nuclear effects in the production of massive lepton pairs from heavy targets by pion and proton beams would extend knowledge of the nuclear dependence of antiquark and quark structure functions of nucleons into important intervals of the fractional longitudinal momentum variable x_Bj not easily accessible in deep inelastic scattering. Isolating nuclear dependence is also essential before definite conclusions can be drawn on the behavior of the pion structure function and on the magnitude and kinematic variation of the perturbative QCD K-factor obtained from heavy target data.     

Calculation of differential Cross Section for pion absorption on diproton

The asymmetry of the differential cross section for the process of pion absorption onT=1 nucleon pairs in³He and³H is calculated using the experimental partial amplitudes for pion scattering prior to the absorption. The asymmetry is found to be a consequence of the even and odd partial-wave interference.     

Isobar-hole doorway states and π-16O scattering

We describe and apply the isobar-hole approach to intermediate energy pion-nucleus reactions. Pion propagation, nucleon and isobar binding, Pauli restrictions and Δ propagation are calculated explicitly within a shell model framework. Intermediate coupling to multihole channels, for example through pion absorption, is treated phenomenologically through an isobar spreading potential. We find strongly collective Δ-hole states, leading to a reformulation of the approach in an extended schematic model. This entails systematic construction of a Δ-hole doorway state basis within which the Δ-hole propagator is evaluated. We find that this doorway space can be truncated at very low dimensionality while preserving accuracy, thereby simplifying the calculations appreciably. We make a detailed comparison between the theoretical results and recent data for π⁺ — ¹⁶O scattering in the pion energy range 50–340 MeV. Nonresonant πN interactions and the π-nucleus Coulomb interaction are included in the calculations. The data is reproduced quite well both below and in the resonance region, and we discuss in detail the role of various dynamical mechanisms. Above the resonance, the calculations are far less successful. We discuss possible shortcomings, stressing the role of inclusive pion-nucleus reactions for revealing the important dynamics. As a test of the Δ spreading potential used for describing elastic scattering, we calculate the total cross section for pion absorption. The result agrees reasonably well with the available data.     

Possible exotic resonance effects in two-photon—two-nucleon processes

The discussion is given of a structure in the cross section of double-photon production pp→2γX below the pion threshold, observed recently by the Dib2γ Collaboration (JINR) and interpreted as a positive indication of the excitation of an exotic NN-decoupled resonance in this reaction, as well as of the related questions on possible exotic resonance effects in elastic Compton scattering on the deuteron and radiative pion capture in the pionic deuterium.     

Single-pion production and the structure of the weak neutral current

We investigate the restrictions on the structure of the weak neutral current imposed by single pion production cross sections on single nucleons. A general vector (V), axial-vector (A) neutral current with |ΔI|?1 is assumed, where the isovector V,A neutral currents are the neutral members of the isotriplets containing the charged weak currents. From neutrino cross sections alone we derive bounds for the neutral current coupling constants. These bounds supplement the known constraints from inclusive scattering in a very useful way. More specific assumptions about the isoscalar neutral current are also considered. We discuss the resulting bounds using the existing neutrino data. Finally, it is shown that with the advent of antineutrino data for single pion production the neutral current coupling constants will be determined uniquely.