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.

Dilepton production in nucleon-nucleon collisions around 1 GeV/nucleon: A theoretical update

We present a fully relativistic and gauge invariant framework for calculating the cross sections of dilepton production in nucleon-nucleon (NN) collisions which is based on the meson-exchange approximation for the NN scattering amplitudes. Predictions of our model are compared with those of other covariant models that have been used to describe this reaction. We discuss the comparison of our calculations with the old DLS and the recent HADES data.

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 $NN\to NN\pi$.

OPEN AND HIDDEN STRANGENESS PRODUCTION IN NUCLEON-NUCLEON COLLISIONS

Strange meson production reactions are expected to provide information on the manifestation of quantum chromodynamics in the non-perturbative regime of energies larger than that of the low energy pion physics. The K meson contains a strange quark (s) or antiquark [Formula: see text] while the η meson has hidden strangeness as it contains some component of the [Formula: see text] pair. In this lecture we present an overview of describing the production of K and η mesons in nucleon-nucleon collisions within an effective Lagrangian model (ELM) where meson production proceeds via excitation, propagation and subsequent decay of intermediate baryonic resonant states. Specific examples are discussed where proper understanding of the data is still lacking.

Dilepton production in nucleon-nucleon collisions reexamined

We present a fully relativistic and gauge-invariant framework for calculating the cross sections of dilepton production in nucleon-nucleon ($\mathit{NN}$) collisions that is based on the meson-exchange approximation for the $\mathit{NN}$-scattering amplitudes. Predictions of our model are compared with those of other covariant models that have been used earlier to describe this reaction. Our results are also compared with those of the semiclassical models that are employed to get the input elementary cross sections in the transport model calculations of the dilepton production in nucleus-nucleus collisions. It is found that cross sections obtained within the semiclassical and quantum mechanical models differ noticeably from each other.

η′ meson production in nucleon-nucleon collisions near the threshold

The production of η′ mesons in the reactions pp → ppη′ and pn → pnη′ at threshold-near energies is analyzed within a covariant effective meson-nucleon theory. The description of cross-section and angular distributions of the available data in this kinematical region in the pp channel is accomplished by including meson currents and nucleon currents with the resonances S 11(1650), P 11(1710) and P 13(1720). Predictions for the pn channel are given. The di-electron production from subsequent η′ Dalitz decay η′ → γγ * → γe + e − is also calculated and numerical results are presented for intermediate energy and kinematics of possible experiments with HADES, CLAS and KEK-PS.

Meson production in nucleon-nucleon collisions

I review some recent works on meson production in nucleon-nucleon collisions. In particular, I report on an investigation of the reaction NN → NNπ near threshold where the production amplitude has been evaluated within chiral perturbation theory up to next-to-leading order, using a counting scheme that takes into account the large scale introduced by the initial momentum. With regard to other meson-production reactions I focus on a phenomenological analysis of the Λ/Σ0 production ratio in pp collisions.

Excitation of the Roper resonance in single- and double-pion production in nucleon-nucleon collisions⋆

In most investigations the Roper resonance is sensed only very indirectly via complex partial-wave analyses. We find indications for its excitation in the invariant nπ+ mass spectrum of the pp → npπ+ reaction at M ≈ 1360 MeV with a width of ≈ 150 MeV . The values fit very favorably to the most recent phase shift results as well as to the observations at BES. In the near-threshold two-pion production pp → ppπ0π0 , where the Roper excitation and its subsequent decays via the routes N * → Δπ → Nππ and N * → Nσ are the only dominant processes, we find its direct decay into the Nσ channel to be the by far dominating decay process --in favor of a monopole excitation of the Roper resonance.

η-meson production in nucleon-nucleon collisions within an effective Lagrangian model

We investigate the pp → ppη and pn → pnη reactions within an effective Lagrangian model for laboratory kinetic energies ranging from very close to the eta meson production threshold to about 10 GeV. Production amplitudes include contributions from the mechanism of excitation, propagation and decay of N ∗ (1535), N ∗ (1650), and N ∗ (1710) baryonic resonances. The initial interaction between two incoming nucleons is modeled by the exchange of π, ρ, ω and σ mesons where the vertex parameters are taken to be the same as those used in the previous applications of this model. Parameters of the resonance vertices also have been taken from our earlier studies wherever applicable. Calculations have been done for total as well as differential η production cross sections. To describe the data for energies closer to the production threshold final state interactions among the out-going particles have been included by means of a generalized WatsonMigdal method. Terms corresponding to the excitation of N ∗ (1535) resonance and pion exchange process dominate the cross sections. With a single set of vertex parameters our model is able to describe the available data well on total cross sections for beam energies ranging from close to threshold to upto 10 GeV.

Mechanism of Near-Threshold Production of theηMeson

Measurements of the analyzing power for the pp-->pp eta reaction have been performed at excess energies of Q=10 and 36 MeV. The determined analyzing power is essentially consistent with zero, implying dominance of the s wave at both excess energies. The angular dependence of the analyzing power, combined with the isospin dependence of the total cross section for the eta meson production in nucleon-nucleon collisions, reveal that the excitation of the nucleon to the S11(1535) resonance is predominantly due to the exchange of the pi meson between the colliding nucleons.

Hyperon production in near-threshold nucleon-nucleon collisions

We study the mechanism of the associated \ensuremath{\Lambda}-kaon and \ensuremath{\Sigma}-kaon production in nucleon-nucleon collisions over an extended range of near-threshold beam energies within an effective Lagrangian model, to understand the new data on $\mathit{pp}\ensuremath{\rightarrow}p\ensuremath{\Lambda}{K}^{+}$ and $\mathit{pp}\ensuremath{\rightarrow}p{\ensuremath{\Sigma}}^{0}{K}^{+}$ reactions published recently by the COSY-11 Collaboration. In this theory, the hyperon production proceeds via the excitation of ${N}^{*}(1650),{N}^{*}(1710)$, and ${N}^{*}(1720)$ baryonic resonances. Interplay of the relative contributions of various resonances to the cross sections is discussed as a function of the beam energy over a larger near threshold energy domain. Predictions of our model are given for the total cross sections of $\mathit{pp}\ensuremath{\rightarrow}p{\ensuremath{\Sigma}}^{+}{K}^{0},\mathit{pp}\ensuremath{\rightarrow}n{\ensuremath{\Sigma}}^{+}{K}^{+}$, and $\mathit{pn}\ensuremath{\rightarrow}n\ensuremath{\Lambda}{K}^{+}$ reactions.

Spin and isospin effects in the NN → NKΛ reaction near threshold

The spin and isospin structure of the amplitudes and observables for K+Λ production in nucleon-nucleon collisions in the near-threshold region is analysed. Five experiments are required in order to isolate the amplitudes, up to an overall phase and two discrete ambiguities, in a model-independent way. It is shown that, with reasonable values for the relative strengths of the π and ρ terms in a meson-exchange model, one expects production on the neutron to be significantly stronger than that on the proton. Negative values of the spin-transfer coefficient DNN are also predicted due to π-ρ interference.

Field-theory approach to the dibaryon model of nuclear forces

A covariant field-theory formulation of the recently developed dibaryon model of nuclear forces is given. The model involves an intermediate dibaryon dressed with π, σ, ρ, and other fields; the corresponding mesons can be in various orbitally excited states with respect to a six-quark bag; and scalar, pseudoscalar, vector, and axial-vector fields are taken into account for the dressed dibaryon. Particular attention is given to nucleon-nucleon interaction in even partial waves. A relativistically covariant nucleon-nucleon potential is derived. It is shown that a simple generalization of the single-channel model makes it possible to take into account coupling to other baryon channels of the NΔ, ΔΔ, NN*(1535), etc., types, this in turn leading to a description of the production of various mesons within this approach. The model also enables one to describe, in a natural way, 2π production in nucleon-nucleon collisions in the isoscalar and in the isovector channel and provides a new formulation of the theory of meson-exchange currents in the physics of electromagnetic interactions. The possible applications of the developed approach in the physics of hadron interactions and in nuclear physics on the whole are discussed.

Scalar-meson production in nucleon-nucleon collisions near threshold

We establish the model-independent spin structure of the matrix elements for the near-threshold scalar-meson production in pp and np collisions, when the final particles are emitted in S-state. Polarization phenomena are derived in a general form. The properties of the t-channel dynamics, which is based on different meson exchanges, are studied in terms of the s-channel parametrization of the matrix element. The predictions of a model, based on $\pi + \sigma$ exchanges are also presented.

Experiments on the search for the NN-decoupled $NN\pi$ -resonance d’

The status on the search for N N-decoupled N Nπ-resonances is reviewed with regard to recent experimental searches in the pionic double-charge exchange in nuclei, the two-pion production in nucleon-nucleon collisions, the photo pion-production on the deuteron and the electro pion-production in nuclei.