Near-threshold Pion Production Research Articles

Near-threshold pion production in diproton reactions with polarised beams and target at ANKE-COSY

An extensive experimental study of near-threshold pion production in dipro- ton reactions is underway at the ANKE-COSY spectrometer (JThe programme is aimed at isolating the four-nucleon-pion contact interaction term appearin g in thePT ex- pansions of these processes. This will establish links between pion production and other low energy phenomena within thePT approach. The first step in the programme was to measure the differential cross-section and the proton analysing power in the~!f ppg s� 0 and~n!f ppg s� − reactions over the full angular range. Heref ppg s denotes a diproton, i.e.,a two-proton system in a 1 S 0 state. These data allow a partial wave analysis to be carried out provided that simplifying assumptions are made when applying the Watson theorem. To make the analysis more robust, and independent of the uncertainties in the relative normalization, the spin-correlation coeffi cients Ax; x and Ay㭹 in the~ n~! f ppg s� − reac- tion were measured in a follow-up experiment. The first results of the data analysis are presented and future developments of the programme are outlined.

Near-threshold pion production in diproton reactions at ANKE

With the advent of chiral perturbation theory (χPT), the low-energy effective field theory of QCD, accurate calculations have become possible for hadronic reactions. The extension of the approach to pion production in nucleon-nucleon collisions requires new high precision experimental information in the near-threshold region.Of especial interest are the processes pp → {pp}sπ0 and pn → {pp}sπ−, with the formation of a 1S0 proton pair (diproton) in the final state. The measurements of dσ/dΩ, Ay and the spin-correlation coefficients Ax,x and Ax,z will permit an amplitude analysis that should provide a non-trivial test of the χPT predictions. A combined study of these processes will lead to the isolation of the strength parameter d of the 4Nπ contact operator in χPT.The ANKE spectrometer is particularly well suited for the study of reactions with a final diproton. The use of the polarised COSY beams and the ANKE polarised internal target allows one to conduct single and double polarisation experiments. Preliminary results on the near-threshold measurements at ANKE of Ay in these processes are presented. The future experimental programme, including the double-polarised experiments, is discussed.

Near-threshold pion production by protons on nuclei

The p + p → d + π+ process is considered in detail along with the p + N → N + N + π process. Such consideration makes it possible to show the difference in the production cross sections for π+ and π− mesons on nuclei. In this case, experimental data can be reproduced both above the pion production threshold at a proton energy of 350 MeV and under the threshold at a proton energy of 180 MeV.

Near-threshold pion production by protons on nuclei

Near-threshold pion production by protons on nuclei

Near-threshold pion production with radioactive beams

Using an isospin- and momentum-dependent transport model we study near-threshold pion production in heavy-ion collisions induced by radioactive beams at the planned rare isotope accelerator (RIA). We revisit the question of probing the high-density behavior of nuclear symmetry energy E{sub sym}({rho}) using the {pi}{sup -}/{pi}{sup +} ratio. It is found that both the total and differential {pi}{sup -}/{pi}{sup +} ratios remain sensitive to the E{sub sym}({rho}) when the momentum-dependence of both the isoscalar and isovector potentials are consistently taken into account. Moreover, the multiplicity and spectrum of {pi}{sup -} mesons are found more sensitive to the E{sub sym}({rho}) than those of {pi}{sup +} mesons. Finally, effects of the Coulomb potential on the pion spectra and {pi}{sup -}/{pi}{sup +} ratio are also discussed.

Near-threshold pion production with storage rings

The first seven years of experience with stored, cooled light-ion beams have clearly demonstrated the benefits of this new technology for nuclear physics. In particular, our knowledge of pion production near threshold has been improved dramatically, leading to important, and sometimes surprising, consequences for a range of nuclear-physics topics.

Core polarization effects in (p

Based on a two-nucleon model, we have studied the core polarization effects in near-threshold pion-production reactions $^{12,14}\mathrm{C}$(p\ensuremath{\rightarrow},${\mathrm{\ensuremath{\pi}}}^{\mathrm{\ensuremath{-}}}$) $^{13,15}\mathrm{O}_{\mathrm{g}.\mathrm{s}.}$. The core polarization reduces the absolute value of the pion-production cross section over all angular directions, while the analyzing power distributions are quite insensitive to it.

Analysis of near-threshold reactions [formula omitted] with two-nucleon model

The reaction cross section and analyzing power for near-threshold pion-production reactions 12,14 C( p , π −) 13,15 O g.s are calculated with the two-nucleon model. We have used the shell-model wave functions to describe the target and the residual nuclear states. It is shown that the dominant two-body amplitudes are those with the final two protons coupled to angular momentum f = 0; p + n →(p + p) I f = 0 + π −. The analyzing power distributions are greatly influenced by the pion distortion effects and exhibit a clear isotope dependence. These are compared with experimental data.