Scattering of pions on pions in threshold region

It is shown that the integral equations for isoscalar-scattering, obtained from the Mandelstam representation by the method of so-called differential approximation, admit the threshold behaviour of partial amplitudes which is in agreement with the quantum theory of scattering and, moreover, they also admit solutions with other possible threshold behaviours. This result does not depend on whether the conformal mapping technique is used or not.     

Topological pions

We study the collider signatures of new pions, composite particles which emerge from a TeV-scale, confining gauge theory with vector-like matter. Similar to the neutral pion in QCD, these new pions mainly decay into a pair of standard model (SM) gauge bosons via triangular anomaly diagrams. One of the new pions, which decays to a gluon plus a photon, has excellent discovery potential at the LHC.     

Inclusive production of soft pions

Amplitudes for inclusive production of pions at zero four-momentum ine ⁺?e ^?, hadron-hadron, and lepton-hadron collisions are computed using traditional soft pion techniques. However, the extrapolation to physical momenta appears to be problematical. Tests involving difference cross sections at relatively low energies are suggested.     

Transverse-momentum dependence of cumulative pions

The transverse-momentum dependence of the cumulative-pion-production rates is studied on the basis of the recently proposed QCD-based parton model for cumulative phenomena. The mean transversemo-mentum value is found to grow with x in the cumulative region. The results are in agreement with available experimental data.     

Dipole polarizabilities of charged pions

We discuss main experimental works, where dipole polarizabilities of charged pions have been determined. Possible reasons for the differences between the experimental data are discussed. In particular, it is shown that the account of the -meson gives a significant correction to the value of the polarizability obtained in the latest experiment of the COMPASS collaboration.     

Hard exclusive electroproduction of pions

We investigate the exclusive electroproduction of mesons from nucleons. To leading-twist, leading-order accuracy, the corresponding production amplitude can be decomposed into pseudovector and pseudoscalar parts. Both can be expressed in terms of quark double distribution functions of the nucleon. While the pseudovector contribution is connected to ordinary polarized quark distributions, the pseudoscalar part can be related to one-pion t-channel exchange. We observe that the pseudovector part of the production amplitude is important at . On the other hand, for , contributions from one-pion exchange dominate. Received: 4 January 1999 / Published online: 15 July 1999     

Tau decays into four pions

We compute branching ratios and invariant mass distributions of the tau decays into four pions. The hadronic matrix elements are obtained by starting from the structure of the hadronic current in the chiral limit and then implementing low-lying resonances in the different channels. Reasonable agreement with experiment is obtained both for the τ → v_τ+(4π) decay rates and the e⁺ e^? → (4π) cross sections. Furthermore we supply an interface to use our matrix elements within the Tauola Monte-Carlo program.     

Light quark fragmentations into pions

We discuss a process of hadronization of light quarks into charged pions in e ⁺ e ⁻ annihilations and in deep inelastic scatering of charged leptons and neutrino off nucleons. The corresponding semi-inclusive cross-sections of pions production we write in terms of quark fragmentation functions and fracture functions. We suggest a new method of measurements of fragmentation and fracture functions based on analysis of semiinclusive data.     

On the radiative decays of pions

A critical analysis of the theory of the radiative decays π → ?νγ (?: electron or muon) is given. Earlier results are corrected through a consistent application of gauge invariance and low-energy theorems. Predictions of specific models for the axial-vector form factor are discussed. The paper concludes with remarks on the relate$\text{π →}\text{e}\text{ν}\text{e}\text{e}$.     

Scattering of negative pions on helium

Differential cross sections for negative pion scattering on ⁴He have been measured at five pion kinetic energies between 110 and 260 MeV in the angular range from 5° to 180°. Total cross sections have also been measured at eleven energies between 67 and 285 MeV. The differential cross sections have been fitted with a phenomenological expression for the nuclear scattering amplitude. Conventional phase shifts have been reconstructed starting from the parameters of the fits.     

Charge form factors of quark-model pions

We demonstrate that Poincaré invariant quark models of the pion can give charge form factors in agreement with all available data. The quark-antiquark model wave functions are spin-zero eigenfunctions of the four-momentum. The relativistic features of the model are essential for the result. The confinement scale is much smaller than the charge radius.     

The interaction of pions with deuterons

Summary There has been remarkable experimental and theoretical progress in the field ofd interactions in the last few years. Within the uncertainties of the theory the experimental data in thed breakup and thed elastic scattering reaction can be fairly well reproduced. Theoretical problems remain in the absorption channel, but there is a hope that with a few additional measurements of tensor polarization and spin transfer observables in this reaction a partial wave amplitude analysis may localize the theoretical difficulties. In addition it is very important to provide data on the deuteron tensor polarizations and the spin transfer parameters for thed elastic scattering reaction in order to obtain a reliable set of phase shifts, which may reveal resonance structures in this reaction channel.Invited talk to the symposium Mesons and Light Nuclei, Bechyn, Czechoslovakia, May 27–June 1, 1985.     

Photoproduction of pions at low energy

Presented in this paper is a field theory model for photoproduction of pions at low energy. In particular, it is shown that certain interaction terms, which were previously considered to be essential in order to explain the experimental situation corresponding to the spin-3/2 resonances, are really unnecessary and that the presence of these terms are not justifiable from the theoretical point of view. Further, the free parameters in the theory representing the off-mass-shell effects of the spin-3/2 particles are fixed on the basis of theoretical considerations.     

Photoproduction of pions in chiral bag models

Threshold pion photoproduction has been calculated for the nucleon, within the framework of chiral bag models. The results of low-energy theorems (LET) are recovered only by taking account of Lorentz ivariance and center-of-mass (CM) correlations of the composite system. In particular the static bag model describes only the Kroll-Ruderman term of the amplitude. The suppression of spurious CM excitations by the concept of effective charges and the use of properly boosted wave functions are necessary in order to obtain also the higher-order contributions as predicted by LET.     

Production of positive pions by neutron-proton collisions

Spectra of positive pions from the processn+p→π ⁺+n+n have been measured for incident neutron energies from 470 MeV to 590 MeV and for laboratory angles up to 20°. The rather broad pion energy spectra and the pronounced anisotropy of the differential cross sections, both indicate an appreciable non-resonant, isoscalar (T=0) contribution to the pion production.     

Elastic photoproduction of neutral pions from helium

Zeitschrift für Physik A Hadrons and nuclei - Differential cross sections are calculated for the elastic photoproduction of neutral pions from He4 under the impulse approximation. We use a...