Pion Production In Nucleon-nucleon Collisions Research Articles

Pion production in nucleon-nucleon collisions and the issue of dibaryons

Pion production in nucleon-nucleon collisions is tightly connected with the issue of dibaryons. After having established the existence of the narrow dibaryon resonance d* (2380) with I(JP) = 0(3+) by two-pion production and elastic proton-neutron scattering experiments, the discussion focuses now on its structure, whether it is a dilute molecular-like or a compact hexaquark object. These scenarios are confronted with experimental branching ratios and other observables. New WASA data for the pp → ppπ+π− reaction give evidence for the existence of an isotensor ΔN threshold state with JP = 1+ as calculated recently by Gal and Garcilazo and predicted earlier by Dyson and Xuong. New ANKE results suggest the dibaryon spectrum to be richer than expected.

Neutron properties from light nuclei

We review the achievements of the project B.6 . Topics addressed include pion photoproduction of three-nucleon systems, electromagnetic and axial currents in chiral nuclear EFT, aspects of EFTs with spin-3/2 fields, nuclear structure and dynamics from the pionless EFT, the development and applications of high-precision two-nucleon forces and pion production in nucleon-nucleon collisions.

Pion production in nucleon-nucleon collisions in chiral effective field theory: Next-to-next-to-leading order contributions

A complete calculation of the pion-nucleon loops that contribute to the transition operator for $NN\to NN\pi$ up-to-and-including next-to-next-to-leading order (N$^2$LO) in chiral effective field theory near threshold is presented. The evaluation is based on the so-called momentum counting scheme, which takes into account the relatively large momentum of the initial nucleons inherent in pion-production reactions. We show that the significant cancellations between the loops found at next-to-leading order (NLO) in the earlier studies are also operative at N$^2$LO. In particular, the $1/m_N$ corrections (with $m_N$ being the nucleon mass) to loop diagrams cancel at N$^2$LO, as do the contributions of the pion loops involving the low-energy constants $c_i$, i=1...4. In contrast to the NLO calculation however, the cancellation of loops at N$^2$LO is incomplete, yielding a non-vanishing contribution to the transition amplitude. Together with the one-pion exchange tree-level operators, the loop contributions provide the long-range part of the production operator. Finally, we discuss the phenomenological implications of these findings. In particular, we find that the amplitudes generated by the N$^2$LO pion loops yield contributions comparable in size with the most important phenomenological heavy-meson exchange amplitudes.

The Beauty of Spin

I review recent developments in theoretical spin physics. Topics include pion production in nucleon-nucleon collisions, the implications of heavy quark spin symmetry for heavy hadron molecules, the nucleon electric dipole form factors and ab initio calculations of the width of hadron resonances. A few spin physics high-lights from experiments at the COSY accelerator are also discussed.

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.

The role of spin in NN → NNπ

Chiral effective field theory (EFT) of Quantum Chromodynamics (QCD) provides a model independent understanding of the nature at low energies. The short range physics in such an EFT is encoded in the so called low energy constants (LECs). These LECs, once determined from one process, can be applied to make predictions for many other processes. In this talk we focus on pion production in nucleon-nucleon collisions which provides access to the LEC that plays an important role connecting different few-nucleon reactions. We demonstrate that this LEC can be extracted from NN → NNπ by studying double polarization observables in the reaction The corresponding measurement is planned in the near future at the COSY accelerator.

P-wave pion production from nucleon-nucleon collisions

We investigate p-wave pion production in nucleon-nucleon collisions up to next-to-next-to-leading order in chiral effective field theory. In particular, we show that it is possible to describe simultaneously the p-wave amplitudes in the $pn\to pp\pi^-$, $pp\to pn\pi^+$, $pp\to d\pi^+$ channels by adjusting a single low-energy constant accompanying the short-range operator which is available at this order. This study provides a non-trivial test of the applicability of chiral effective field theory to reactions of the type $NN\to NN\pi$.

Complete next-to-leading-order calculation for pion production in nucleon-nucleon collisions at threshold

Based on a counting scheme that explicitly takes into account the large momentum sqrt(M m_pi) characteristic for pion production in nucleon-nucleon collisions we calculate all diagrams for the reaction NN --> NN pi at threshold up to next-to-leading order. At this order there are no free parameters and the size of the next-to-leading order contributions is in line with the expectation from power counting. The sum of loop corrections at that order vanishes for the process pp --> pp pi^0 at threshold. The total contribution at next-to-leading order from loop diagrams that include the delta degree of freedom vanishes at threshold in both reaction channels pp --> pp pi^0, pn pi^+.

Toy model for pion production in nucleon-nucleon collisions. II. The role of three-particle singularities

The influence of three-particle breakup singularities on s-wave meson production in nucleon-nucleon collisions is studied within the distorted wave Born approximation. This study is based on a simple scalar model for the two-nucleon interaction and the production mechanism. An algorithm for the exact numerical treatment of the inherent three-body cuts, together with its straightforward implementation is presented. It is also shown that two often-used approximations to avoid the calculation of the three-body breakup are not justified. The possible impact on pion production observables is discussed.

Toy model for pion production in nucleon-nucleon collisions

We develop a toy model of pion production in nucleon-nucleon collisions that reproduces some of the features of the chiral Lagrangian calculations. We calculate the production amplitude and examine some common approximations.

Threshold pion production in nucleon-nucleon collisions

Threshold pion production in nucleon-nucleon collisions

Single pion production in nucleon-nucleon collisions.

In this work, we explore the NN\ensuremath{\rightarrow}\ensuremath{\pi}NN reaction within a relativistic model for the NN and \ensuremath{\pi}NN coupled systems consistent with two- and three-body unitarity. After describing the theoretical input, we concentrate on some exclusive pp\ensuremath{\rightarrow}pn${\mathrm{\ensuremath{\pi}}}^{+}$ observables. These are compared with the available data and with predictions from other models. In particular, we examine systematically the dependence of some spin observables on various components of the dynamical input. We identify and isolate some problems related to the present approach and we point out possible directions for future research.

Pion production in nucleon-nucleon collisions and dibaryon resonances

Recent data on π production in NN collisions are compared to predictions from the Deck model. It is shown that the model fails spectacularly for Δσ L ( pp «r NNπ) and for the A LL parameter. This is typical for conventional π-exchange models. However, the Deck model with an admixture of a dibaryon resonance ( I = 1, J P = 3 −) leads to a satisfactory description of the data.

A study of single pion production in nucleon-nucleon collisions

A pion exchange model for NN pion production from threshold up to about 6 GeV/ c is constructed. In contrast to other models of this type the full information from πN scattering is fed in whereby the πN partial waves on- and off-shell are identified with each other. Detailed comparison with the data for both the channels isospin one and zero, reveals that the shapes of angle and momentum distributions are, in general, well reproduced but the overall magnitude comes out too small for p L ≲ 1.5 GeV/ c. It is argued that this defect may be due to dibaryon resonances. For reaction cross sections their effects are quantitatively estimated.

Off-shell effects in s-wave pion absorption

The effect of s-wave pion absorption on the pion-nucleus optical potential is calculated. We assume absorption by two uncorrelated nucleons with off-shell pion rescattering. For the pion-nucleon interaction we develop a field theoretic model which can be used on- and off-mass shell. A fully relativistic calculation of the pion polarization operator then gives Uopt = −4πB0ρ2(2μ)−1, with B0 = (0.094 + i 0.036)μ−4, as contribution from s-wave absorption to the optical potential. The imaginary part agrees well with the experimentally determined value, while pion dispersion (real part of B0) does not explain the observed repulsion in the s-wave pion nucleus interaction. We demonstrate the relevance of off-shell dynamics in pion-nucleon scattering. The ratio Rs of π−-absorption rates by neutron-proton and proton-proton pairs is much smaller than predicted by on-shell models. For equal numbers of neutrons and protons we get Rs3.0. We also apply the formalism to s-wave pion production in nucleon-nucleon collisions and obtain qualitative agreement with the data. In particular the longstanding puzzle of the small imaginary pion deuteron scattering length compared to Im B0 is explained. Our calculation gives lm απd = 3.7 × 10−3μ−1.

Theory of pion production

A new time-dependent theory for pion production in nucleon-nucleon collisions is presented, in which each production process is interpreted as a random « measurement » suffered by the nucleon-nucleon system. The approach has the advantage of being defined in a perfectly consistent way from a quantum-mechanical point of view. The obtained results have a remarkable resemblance with those obtained from other models of high-energy production.

The absorption of pion in 3He

The two-nucleon absorption model is submitted to a quantitative test by considering pion absorption in 3He. The wave functions of 3He are explicitly constructed in terms of wave functions for the S-wave two-nucleon systems. Several prescriptions for the range dependence of the pion absorption interaction are considered, the effective coupling constant of which are determined from the data on pion production in nucleon-nucleon collisions. We show that the usual assumption of no short-range repulsion and of a zero-range absorption interaction is too naive to give reliable results. When the effects of the nuclear short-range repulsion and the finite range of the pion absorption interaction are included, the model gives a ratio for the nnp and nd production rates which agrees with experiment. The theoretical absorption rates for S-wave pions are W S ( nd)=(0.9±0.4)·10 16 sec −1, W S ( nnp)=(3.2±0.8)·10 16 sec −1 . These results agree with experiment if a radiative absorption rate of 4.43·10 15 sec −1 is assumed and if the ratio for absorption from a 2p and from a ls state is 0.65·10 −4. This agreement indicates that the basic idea of two-nucleon absorption could be correct.

A model of diffractive production in hadron-hadron collisions

A model for diffractive production off hadronic and nuclear targets is proposed. This model is a development of the Good and Walker description of such processes and is also analogous to the model of Cheng and Wu for high-energy quantum-electrodynamical processes. The diffractive production process consists here of two steps: (i) the incident particle fluctuates and (ii) it interacts elastically with the target either before or after the fluctuation. The vector-dominance model of the rho photoproduction and the Drell-Hiida-type models of diffractive production are special cases of the model proposed here. Results of numerical calculations for the diffractive pion production in nucleon-nucleon collisions are discussed for the cases of some simple fluctuations. A fair agreement with the existing data for this process is found.

An approach to the theory of single pion production in nucleon-nucleon collisions

An approach to the theory of single pion production in nucleon-nucleon collisions

An approach to the theory of single pion production in nucleon-nucleon collisions

The matrix element for the process of single pion production in nucleon-nucleon collisions is constructed under the hypothesis of a dominant contribution from the one-pion exchange term. The effects of the virtuality of the π-meson are fully taken into account. It is found that the finite size of the nucleon cannot be neglected. The method used allows the determination of the «pionic form factor» of the nucleon. It is shown that, by the choice of a form factor similar to the e.m. ones, a satisfactory agreement with the experiment is obtained for incident energies of (1÷3) GeV. For higher energies, the connection with a possible «Regge pole behaviour» for such reactions is briefly discussed.