Pions Off Research Articles

Diffractive neutrino production of pions on nuclei: Adler relation within the color-dipole description

Effects of coherence in neutrino-production of pions off nuclei are studied employing the color dipole representation and path integral technique. If the nucleus remains intact, the process is controlled by the interplay of two length scales. One is related to the pion mass and is quite long (at low Q^2), while the other, associated with heavy axial-vector states, is much shorter. The Adler relation is found to be broken at all energies, but especially strongly at \nu > 10 GeV, where the cross section is suppressed by a factor ~A^{-1/3}. On the contrary, in a process where the recoil nucleus breaks up into fragments, the Adler relation turns out to be strongly broken at low energies, where the cross section is enhanced by a factor ~A^{1/3}, but has a reasonable accuracy at higher energies, where all the coherence length scales become long.

Production of charged pions off nuclei with [formula omitted] incident protons and pions

We compare calculations for the production of charged pions by pion or proton beams off nuclei calculated within our coupled channel transport model (GiBUU) with recent data of the HARP Collaboration for beam energies from 3 up to 13 GeV. Predictions for the 30 GeV data for pions and kaons from the NA61/SHINE experiment are included.

Why Breakup of Photons and Pions into Forward Dijets Is So Different: Predictions from Nonlinear Nuclear k[sub ⊥] Factorization

Based on an approach to non-Abelian propagation of color dipoles in a nuclear medium we formulate a nonlinear k_t-factorization for the breakup of photons and pions into forward hard dijets in terms of the collective Weizs\"acker-Williams (WW) glue of nuclei. We find quite distinct practical consequences of nonlinear nuclear k_t-factorization for interactions of pointlike photons and non-pointlike pions. In the former case the large transverse momentum p_t of jets comes from the intrinsic momentum of quarks and antiquarks in the photon and nuclear effects manifest themselves as an azimuthal decorrelation with an acoplanarity momentum of the order of the nuclear saturation momentum Q_A. In the breakup of pions off free nucleons to the leading order in pQCD the spectator parton has a small transverse momentum and the hard dijet cross section is suppressed. In the breakup of pions off heavy nuclei the forward hard jets are predicted to be entirely decorrelated. We comment on the sensitivity of the pionic dijet cross section to the pion distribution amplitude. The predicted distinction between the breakup of photons and pions can be tested by the sphericity and thrust analysis of the forward hadronic system in the COMPASS experiment at CERN.

Partonic structure of π and ρ mesons from data on hard exclusive production of two pions off nucleon

We fitted the ππ mass distribution in the range 0.5≤ M ππ ≤1.1 GeV measured in hard exclusive positron–proton reactions at HERA by the form dictated by QCD at leading twist level. Extracted parameters are related to valence quark distribution in the pion, and to the pion and ρ meson distribution amplitudes. We obtain, for the first time, a measurement of the second Gegenbauer coefficient of the ρ meson distribution amplitude: a 2 ( ρ) =−0.10±0.20 for a photon virtuality of 〈 Q 2〉=21.2 GeV 2.

Photoproduction of pions off nucleons and nuclei

Present status of pion photoproduction on nucleons and nuclei is discussed. Transitions on bound nuclear states in charged pion photoproduction and the coherent processes in neutral pion photoproduction are examined from the point of view of self-consistent description of all theory ingredients.

Some approximate symmetries of spin observables in elastic scattering of pions off deuterium

Abstract A number of approximate relations between polarization observables in elastic pion—deuteron scattering below 300 MeV is derived. They shed some light on the degree to which experimental data can be used obtain information on the dynamics of the process.

Measurements of recoil proton polarization in photoproduction of pions off the nucleus12C

Preliminary results on the measurements of recoil photon polarization in photoproduction of pions off protons and neutrons bound in the nucleus carbon-12 at 0.8 and 0.9 GeV maximum energy for incident photons are presented. (JFP)

Exchange Effects in Radiativen−pCapture

The contribution of the one-pion-exchange interaction current to the transition amplitude for radiative $n\ensuremath{-}p$ capture at thermal energies is evaluated using the Chew-Low static model for the amplitude for photoproduction of pions off nucleons. A correct treatment of the pion-rescattering term in that amplitude gives essential agreement with the static-quark-model calculation of Riska and Brown which considers only nucleon excitation to the $\ensuremath{\Delta}(1236)$ state.

Current algebraic sum rules and Veneziano model for photoproduction of pions off nucleons

Sum rules relating the proton and the neutron magnetic moments to amplitudes for γ+N→π+N are analysed in the framework of the dual-resonance model and reasonably good agreement is obtained.

Double-pion electro- and photoproduction viewed from mass-dispersion and current-algebra theory

We discuss the electro- and photoproduction of two pions off nucleons within the frame work of current algebra and the new mass-dispersion relations developed by De Alfaro, Fubini, Furlan, and Rossetti. The results obtained by a one-pion reduction are compared with those obtained in earlier calculations, and the importance of dispersion corrections, not obtained thus far are pointed out. Also noted is that the proposal, suggested by some authors to obtain indirectly by means of the electroproduction process the t-dependence of the axial vector form factor, appears rather dim. Next both pions are reduced out. In the case in which the pions form an iso-singlet important dispersive corrections could be present due to a low-lying σ-meson (if it exists) and the N 11+(1400), Roper resonance, intermediate states. Dispersive corrections which effect the amplitudes in which the pions form an iso-quintet are also discussed. The dipion production amplitudes are given for the kinematical configuration in which the pions are at rest in the Breit frame of the nucleons.