Narrow Results

We study the threshold production of two pions induced by neutrinos in nucleon targets. The contribution of nucleon pole, pion and contact terms is calculated using a chiral Lagrangian. The contribution of the Roper resonance, neglected in earlier studies, has also been taken into account.

We study neutrino-induced one-pion production off the nucleon in and around the Delta resonance region. Apart from the Delta-pole mechanism we include background terms required by chiral symmetry. These background terms give sizeable contributions in all channels. To better reproduce the ANL q²-differential cross section data, we make a new fit of the axial nucleon to Delta form factor. The new result is some 30% smaller than the commonly accepted value. This correction is compatible with most quark model estimates and a recent lattice calculation.¹

The results of an extensive campaign of measurements of the π→ππ process in the nucleon and nuclei at intermediate energies are presented. Several differential cross-sections were measured following the detection of multiparticle (π⁺,π⁺π^±) and (π⁺,π⁺π^-p) events, which allowed to understand the general properties of the pion production reaction inside the nucleus. Particular attention was devoted to the study of the effects of nuclear matter on the π-π scalar-isoscalar system. The most relevant results are reviewed and compared to theories as well as to other recent experimental results.

The experimental program on meson production in light-ion collisions at the CELSIUS cooler/storage ring of the The Svedberg Laboratory in Uppsala, is reviewed. Production of pions and eta-mesons have been studied in pp and pd collisions using the PROMICE/WASA detector. This program is now continued at CELSIUS with the WASA4π detector together with studies of meson decays.

Invoking the formation of a fireball we analyze the experimental data on pion multiplicity from La + La collision at and its kinetic energy spectrum. We discuss the freeze-out scenario, the possible role of the delta and strange particles and whether the hadronic repulsion (leading to an excluded volume effect) plays any role at the thermochemical and the hydrodynamical freeze-out stage. The importance of collective flow is realized in explaining the pion spectrum at this collision energy. The analysis also provides evidence of the presence of strange particles and the delta resonance in partial chemical equilibrium, at an initial chemical freeze-out stage when the hydrodynamic flow has not developed to have any significant effect. At the final "hydrodynamical freeze-out" we do not find any evidence of the deltas and the strange particles when the thermal temperature has dropped considerably. The importance of the collective flow vis a vis the resonance decay contribution is also discussed in explaining the pion kinetic energy spectra. Further, all the model parameters are determined uniquely, unlike previous analysis, thus avoiding any arbitrariness.

The possibility to select reliably the diproton final state with small excitation energy with the ANKE spectrometer at COSY-Juelich provides a new tool to study hadron interactions at intermediate energies. In this case the final proton pairs are found presumably in the ¹S₀ state which simplifies significantly the theoretical analysis. This approach has been successfully applied to the reactions pd → {pp}_sn at high and low momentum transfer kinematics, as well as to the pp → {pp}_sπ⁰ and pp → {pp}_sγ processes.

Spallation is one of possible scenarios of proton - nucleus reaction considered nowadays. Pions are the most abundantly produced mesons during the reactions. Boltzmann-Uehling-Uhlenbeck (BUU) model was used to describe pion spectra, results will be presented.

Charge symmetry breaking effects can be used as an experimental tool to study quark mass effects. Using recent high precision data from TRIUMF and IUCF an international theory collaboration is working on a consistent description within the framework of chiral pertubation theory. The WASA-at-COSY collaboration has started an experimental program extending the studies on the reaction dd → ⁴Heπ⁰ towards higher excess energies in order to provide necessary information on the contribution of p-waves. First preliminary results on the total cross section at Q = 60 MeV are presented.

Preliminary results on charged pion production in np collisions at an incident beam energy of 1.25 GeV obtained with HADES are presented. The np reactions were studied in dp collisions at 1.25 GeV/u using a Forward Wall hodoscope, aimed at registering spectator protons. The separation of np → ppπ^-, np → npπ⁺π^- and np → dπ⁺π^- channels is demonstrated.

It is important to consider the charged current weak interaction in the (semi-)inclusive deeply inelastic scattering to determine (transverse momentum dependent) parton distribution functions with flavor identifications. To this end, we calculate the charged current semi-inclusive deeply inelastic neutrino and anti-neutrino nucleus scattering and propose a measurable quantity in this paper to determine the transverse momentum dependent parton distribution functions. Semi-inclusive means that an unpolarized or a spin-0 hadron is detected in the current fragmentation region in addition to the scattered charged lepton. The target nucleus is assumed to be spin-1 for the complete calculation. We find that only the chiral-even terms survive and the chiral-odd terms vanish in this charged current deeply inelastic (anti-)neutrino nucleus scattering process. The measurable quantity is the yield difference or asymmetry of the positive pion and the negative pion. We notice that it is only determined by (anti-)strange quark distribution functions under isospin symmetry and would be reduced to a function of kinematic variable y if (anti-)strange quark distribution functions are neglected. The kaon meson production can be discussed in a similar way. Our calculations are limited at the leading twist level.

The ANKE spectrometer at COSY-Juelich provides the possibility to select proton pairs with small excitation energy in the final state. This opens a new way to study hadron interactions at intermediate energies. The final proton pairs with small excitation energy are found presumably in the ¹S₀ state which simplifies significantly the theoretical analysis. This approach has been successfully applied to the reactions pd → {pp}_sn at high and low momentum transfer kinematics, as well as to the pp → {pp}_sπ⁰ and pp → {pp}_sγ processes.

The collision centrality dependencies of the average kinematical characteristics of the negative and positive pions, produced in ${}^{12}\mathrm{\text{C}}+{}^{181}\mathrm{\text{Ta}}$ collisions at $4.2A\mathrm{\text{GeV}}/c$, were investigated. The ratio $〈n({\pi }^{-})〉/〈n({\pi }^{+})〉$ proved to be $1.20\pm 0.05$, $1.08\pm 0.04$ and $1.04\pm 0.03$ in the peripheral, semicentral, and central ${}^{12}\mathrm{\text{C}}+{}^{181}\mathrm{\text{Ta}}$ collision events, respectively, decreasing noticeably with increasing collision centrality. The suppression (decrease) of the ratio $〈n({\pi }^{-})〉/〈n({\pi }^{+})〉$ was observed in the semicentral and central ${}^{12}\mathrm{\text{C}}+{}^{181}\mathrm{\text{Ta}}$ collisions as compared to the ratio $(\approx 1.22)$, estimated using the simple model for ${}^{12}\mathrm{\text{C}}+{}^{181}\mathrm{\text{Ta}}$ collision system. The ratio $〈n({\pi }^{-})〉/〈n({\pi }^{+})〉$ estimated using the simple model agreed well with the corresponding ratio $(\approx 1.24)$, estimated for ${}^{12}\mathrm{\text{C}}+{}^{181}\mathrm{\text{Ta}}$ collisions at $4.2A\mathrm{\text{GeV}}/c$ based on the Wounded Nucleon Model (WNM). Comparison of the emission angle as well as momentum distributions of the charged pions in the peripheral and central ${}^{12}\mathrm{\text{C}}+{}^{181}\mathrm{\text{Ta}}$ collisions revealed the significant decrease of the fraction of the relatively fast charged pions (with smaller emission angles) and increase of the fraction of the relatively slow charged pions (with larger emission angles) with an increase in collision centrality. The results of the present analysis can be useful for analysis of the centrality dependence of the charged pion production in heavy ion collisions at high energies.

The new experimental data on various characteristics of the secondary charged pions produced in $n^{12}$C collisions at 4.2GeV/c are presented. A comparative analysis of the average multiplicities and various kinematic characteristics of the charged pions produced in $n^{12}$C and $p^{12}$C collisions at 4.2GeV/c is made. The experimental data are compared systematically with the predictions of the modified FRITIOF model.

The recent measurements of the reactions $pp\to pp{\pi }^0$ and $pn\to pp{\pi }^{-}$ by the ANKE collaboration at COSY are analyzed with the focus on the p-wave pion production amplitudes. These amplitudes are known to provide an important connection between $NN\to NN\pi$ and other low-energy few-nucleon reactions. The results of the recent partial wave analysis of the ANKE data are discussed and compared with the theoretical predictions.

We analyze pion production from nucleon-deuteron collisions, with the outgoing three-nucleon system in bound state. Potentially, these reactions could be used to dissection three-nucleon force diagrams, which lead to three-nucleon potential operators. The experimental data, from threshold up to the Δ resonance, are compared with calculations using accurate nuclear wavefunctions. Pion production amplitudes are obtained through matrix elements involving pion-nucleon rescattering mechanisms in S- and P-waves. We assume the hypothesis that S-wave rescattering includes an isoscalar contribution which is generally suppressed for low-energy pion-nucleon scattering, but is enhanced for pion production because the kinematical regime is different, with involvment of high-momenta contributions. P-wave rescattering includes also explicitly the Δ degrees of freedom. Initial-state interactions (ISI) between the proton and the deuteron have sizable effects on the spin-averaged observables. These ISI effects become important for spin observables such as the deuteron tensor analyzing powers T₂₀. For spin observables involving interference terms amongst the various helicity amplitudes, such as for the nucleon vector analyzing power A_y, ISI effects are very important.

Recent progress towards an understanding of the πNN system within chiral perturbation theory is reported. The focus lies on an effective field theory calculation and its comparison to phenomenological calculations for the reaction NN → dπ. In addition, the resulting absorptive and dispersive corrections to the πd scattering length are discussed briefly.

We have searched for an enhancement in the excitation function of the yield of pions in the pion production reaction by protons and deutrons at bombarding energies near 350 MeV/nucleon. The measurements was carried out on the JINR Nuclotron internal beam and target for Ag, Cu, Al and C targets.

Energy dependence of slow pion yield in proton-nucleus interactions is analyzed. Experimental data of the HARP collaboration is fitted using two-fireball model with χ²/n_d ~ 1 from 3 to 8 GeV/c. It is shown that low momentum negative pion yield rises almost linearly with proton kinetic energy. Normalized low momentum positive pion yield is larger at lower proton energy.