Double Pion Production Research Articles

Neutrino-nucleon cross-section model tuning in GENIE v3

We summarise the results of a study performed within the GENIE global analysis framework, revisiting the GENIE bare-nucleon cross-section tuning and, in particular, the tuning of a) the inclusive cross-section, b) the cross-section of low-multiplicity inelastic channels (single-pion and double-pion production), and c) the relative contributions of resonance and non-resonance processes to these final states. The same analysis was performed with several different comprehensive cross-section model sets available in GENIE Generator v3. In this work we performed a careful investigation of the observed tensions between exclusive and inclusive data, and installed analysis improvements to handle systematics in historic data. All tuned model configurations discussed in this paper are available through public releases of the GENIE Generator. With this paper we aim to support the consumers of these physics tunes by providing comprehensive summaries of our alternate model constructions, of the relevant datasets and their systematics, and of our tuning procedure and results.

Isospin symmetry breaking in double-pion production in the region of d*(2380) and the scalar σ meson

The first attempt is made to provide a quantitative theoretical interpretation of the WASA-at-COSY experimental data on the basic double-pion production reactions $pn\ensuremath{\rightarrow}d{\ensuremath{\pi}}^{0}{\ensuremath{\pi}}^{0}$ and $pn\ensuremath{\rightarrow}d{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}$ in the energy region ${T}_{p}=1--1.3\text{ }\text{ }\mathrm{GeV}$ [Adlarson et al., Phys. Lett. B 721, 229 (2013)]. The data are analyzed within a model based on the production and decay of an intermediate $I({J}^{P})=0({3}^{+})$ dibaryon resonance ${\mathcal{D}}_{03}$ [also denoted as ${d}^{*}(2380)$]. The observed decrease of the near-threshold enhancement (the so-called ABC effect) in the reaction $pn\ensuremath{\rightarrow}d{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}$ in comparison to that in the reaction $pn\ensuremath{\rightarrow}d{\ensuremath{\pi}}^{0}{\ensuremath{\pi}}^{0}$ is explained (at least partially) to be due to isospin symmetry violation in the two-pion decay of an intermediate near-threshold scalar $\ensuremath{\sigma}$ meson emitted from the ${\mathcal{D}}_{03}$ dibaryon resonance under conditions of partial chiral symmetry restoration.

Nature of $$\varvec{S}$$-wave $$\varvec{NN}$$ interaction and dibaryon production at nucleonic resonance thresholds

Phase shifts and inelasticity parameters for $NN$ scattering in the partial-wave channels ${}^3S_1$--${}^3D_1$ and ${}^1S_0$ at energies $T_{\rm lab}$ from zero to about 1 GeV are described within a unified $NN$ potential model assuming the formation of isoscalar and isovector dibaryon resonances near the $NN^*(1440)$ threshold. Evidence for these near-threshold resonances is actually found in the recent WASA experiments on single- and double-pion production in $NN$ collisions. There, the excitation of the Roper resonance $N^*(1440)$ exhibits a structure in the energy dependence of the total cross section, which corresponds to the formation of dibaryon states with $I(J^\pi)=0(1^+)$ and $1(0^+)$ at the $NN^*(1440)$ threshold. These two $S$-wave dibaryon resonances may provide a new insight into the nature of the strong $NN$ interaction at low and intermediate energies.

Dibaryon resonances in hadron and nuclear physics: New results

The old and new experimental studies of dibaryon resonances are reviewed, and their results are interpreted theoretically. Main emphasis is made on the research in this and related areas of relativistic nuclear physics carried out at the Joint Institute for Nuclear Research (JINR) in Dubna, starting from its foundation in mid-1950s. The dibaryon studies are shown to be pivotal for JINR together with the research in such related areas as the cumulative processes, subthreshold production of mesons, single and double pion production in NN collisions, and short-range NN correlations in nuclei.

NN and Nd Scattering with Intermediate Dibaryons

Excitation of intermediate dibaryon resonances in hadronic processes accompanied by high momentum transfers is considered. It is shown that some basic features of NN inelastic scattering with single- or double-pion production can be effectively described by taking into account the short-range s-channel dibaryon excitation along with peripheral t-channel meson-exchange processes. In particular, isovector dibaryons with \(I(J^P)=1(2^+)\), \(1(3^-)\) and \(1(2^-)\) are shown to play an important role in reproducing the cross sections and especially polarization observables in the basic single-pion production reaction \(pp \rightarrow d\pi ^+\), while an isoscalar \(I(J^P)=0(3^+)\) dibaryon is crucial for describing the basic double-pion production process \(pn \rightarrow d (\pi \pi )_0\). Possible generalization of resonance (dibaryon) mechanisms considered in NN scattering to Nd elastic and inelastic scattering is also discussed.

Dynamical coupled-channels model for neutrino-induced meson productions in resonance region

A dynamical coupled-channels (DCC) model for neutrino-nucleon reactions in the resonance region is developed. Starting from the DCC model that we have previously developed through an analysis of $\pi N, \gamma N\to \pi N, \eta N, K\Lambda, K\Sigma$ reaction data for $W\le 2.1$ GeV, we extend the model of the vector current to $Q^2\le$ 3.0 (GeV/$c$)$^2$ by analyzing electron-induced reaction data for both proton and neutron targets. We derive axial-current matrix elements that are related to the $\pi N$ interactions of the DCC model through the Partially Conserved Axial Current (PCAC) relation. Consequently, the interference pattern between resonant and non-resonant amplitudes is uniquely determined. We calculate cross sections for neutrino-induced meson productions, and compare them with available data. Our result for the single-pion production reasonably agrees with the data. We also make a comparison with the double-pion production data. Our model is the first DCC model that can give the double-pion production cross sections in the resonance region. We also make comparison of our result with other existing models to reveal an importance of testing the models in the light of PCAC and electron reaction data. The DCC model developed here will be a useful input for constructing a neutrino-nucleus reaction model and a neutrino event generator for analyses of neutrino experiments.

Study of the reaction npπ → npπ+π− at 1.0 and 1.5 GeV

The np → npπ + π − reaction has been studied at the incident neutron energies of 1.0 GeV and 1.5 GeV in 4π-geometry using the neutron beam produced by the breakup of the relativistic deuterons at the JINR Synchrophasotron. The np → npπ + π − data obtained at 1.5 GeV demonstrate the dominance of the ΔΔ excitation process with a significant contribution of the higher-lying baryonic resonances, while the data at 1.0 GeV show a non-resonant behavior. The angular and mass distributions are compared with the predictions of different models suggested for the double pion production in the NN collisions. The calculations performed within the model based on the reggeized π-exchange taking one baryon exchange into account are in good agreement with the experimental data obtained at both energies.

MEASUREMENT OF POLARIZATION OBSERVABLES IN NEUTRAL DOUBLE PION PHOTOPRODUCTION OFF THE PROTON WITH THE CBELSA/TAPS EXPERIMENT

The Crystal Barrel/TAPS experiment is ideally suited to measure neutral mesons decaying into photons due to its high detection efficiency for photons, good energy resolution, and the nearly complete solid angle coverage. In combination with a longitudinally or transversely polarized target and an energy tagged, linearly or circularly polarized photon beam the experiment allows the measurement of a large set of polarization observables. The presented preliminary results for polarization observables of neutral double pion production obtained with a transversely polarized target show significant deviations from current predictions of partial wave analyses. This clearly indicates that 2π0-photoproduction is not yet understood.

Photoproduction of mesons off nuclei

Photoproduction of mesons off nuclei, in particular the deuteron, helium-, and lithium-isotopes, but also heavier ones up to lead, has been studied at the tagged photon beams of the electron accelerators ELSA in Bonn and MAMI in Mainz with the Crystal Barrel/TAPS and Crystal Ball/TAPS experiments. The aim of these measurements was twofold: the study of meson production reactions off quasi-free neutrons and the study of meson–nucleus interactions. The measurements of quasi-free reactions off the neutron include the observation of a narrow structure, of as yet unknown nature, in the excitation function of η production off the neutron. These experiments continue now with the measurement of single and double polarization observables. The main topics for the meson–nucleus interactions were the search for η-mesic nuclei and the investigation of nuclear double-pion production reactions in view of possible in-medium modifications of the σ-meson.

Exclusive measurement of two-pion production in the [formula omitted] reaction

The results from the first kinematically complete measurement of the d d → He 4 π π reaction are reported. The aim was to investigate a long standing puzzle regarding the origin of the peculiar ππ-invariant mass distributions appearing in double pion production in light ion collisions, the so-called ABC effect. The measurements were performed at the incident deuteron energies of 712 MeV and 1029 MeV, with the WASA detector assembly at CELSIUS in Uppsala, Sweden. We report the observation of a characteristic enhancement at low ππ-invariant mass at 712 MeV, the lowest energy yet. At the higher energy, in addition to confirming previous experimental observations, our results reveal a strong angular dependence of the pions in the overall centre of mass system. The results are qualitatively reproduced by a theoretical model, according to which the ABC effect is described as resulting from a kinematical enhancement in the production of the pion pairs from two parallel and independent N N → d π sub-processes.

Hadron Spectroscopy at CLAS and the Evolution of Strong Degrees of Freedom

Understanding the strong force constitutes one of the biggest challenges in fundamental science that we can and have to tackle now as the needed experimental and theoretical tools become available. Perturbative Quantum ChromoDynamics (pQCD) at small distances, which is governed by quark and gluon fields, and Chiral Perturbation Theory (ChPT) at larger distances, which is governed by pion fields, are both already experimentally validated. However, strong fields at intermediate distances, where they generate about 98% of the total mass of nucleons and therefore of all normal matter, are not understood on similarly firm grounds. Electron scattering in particular serves as an ideal tool to investigate this intermediate region by measuring the resonance transition form factors of three-quark systems with varying momentum transfer and spatial resolution of the probe. The status of the research program at Jefferson Lab to study baryon transition form factors and the evolution of the underlying effective degrees of freedom, or the origin of mass, will be exemplified by recent results in single and double-pion production obtained with CLAS by the Hall B collaborations. These results demonstrate that the separation of resonance and background contributions and therefore the extraction of the electro-coupling amplitudes of resonances become easier and cleaner at higher four-momentum transfers (Q2). Furthermore, the double-pion in comparison to the single-pion channel shows a higher sensitivity to energetically higher lying resonances and a distinctly different dependence on the contributing background amplitudes. The combined analysis of the single- and double-pion data thus reduces model dependent uncertainties significantly, which allows us to extract the resonant electro-coupling amplitudes in an unprecedented quality.

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.

Roper resonance—Its static and dynamic properties from single- and double-pion production

In most investigations the Roper resonance is sensed only very indirectly via complex partial wave analyses. We find indications for its excitation in the p p → n p π + reaction, where some resonance-like structure is observed in the invariant n π + mass spectrum 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 near-threshold two-pion production p p → p p π 0 π 0 the Roper excitation and its subsequent decay into N π π is the only dominant process. There we find its decay into the N σ channel as the dominating decay process — in favor of a monopole nature of the Roper excitation.

Single- and double-pion production in nucleon collisions on the nucleon and on nuclei—the ABC effect and its origin in a dibaryonic resonance

The ABC effect–an intriguing low-mass enhancement in the π π invariant mass spectrum–is known from inclusive measurements of two-pion production in nuclear fusion reactions. First exclusive measurements carried out at CELSIUS-WASA for the fusion reactions leading to d or 3He reveal this effect to be a σ -channel phenomenon associated with the formation of a Δ Δ system in the intermediate state and combined with a resonance-like behavior in the total cross-section. Together with the observation that the differential distributions do not change in shape over the resonance region the features fulfill the criteria of an isoscalar s-channel resonance in p n and N N π π systems, if the two emitted nucleons are bound. It obviously is robust enough to survive in nuclei as a dibaryonic resonance configuration. In this context also the phenomenon of N Δ resonances is reexamined.

Measurement of the cross sections for π + π − production in pp collisions at energies below 1 GeV

The cross sections for the reaction pp → ppπ + π − are measured at several values of the incident-proton energy. The resulting values are compared with other results, including values obtained previously for a deuteriumtarget, as well as with the predictions of a theoreticalmodel. This comparison reveals that, at energies below 850 MeV, the use of the impulse approximation in extracting the cross sections for double pion production on a free nucleon from data obtained with deuteron targets is illegitimate.

Nucleon Transition Form Factors and New Perspectives

The status of the electro-excitation program to study baryon resonances at Jefferson Lab will be exemplified by the most recent results on resonance parameters and transition form factors in single and double-pion production. These results demonstrate that the separation of resonance and background contributions and therefore the extraction of the electro-coupling amplitudes of resonances become easier and cleaner at higher four-momentum transfers (Q2). Furthermore, the double-pion in comparison to the single-pion channel shows a higher sensitivity to higher excited resonances and a distinctly different Q2 dependence of the background amplitudes. The combined analysis of the single- and double-pion data reduces model dependent uncertainties significantly, which allows us to extract the resonant electrocoupling amplitudes with an unprecedented quality.

Two Pion Production from Nuclei

Differences in the photoproduction of mesons on the free proton and on nuclei are expected to reveal changes in the properties of hadrons. Double pion photoproduction from nuclei has been used to investigate the in medium modification of meson-meson interactions with the TAPS photon spectrometer at MAMI-B. Recent developments are reported. Hadron-hadron interactions and meson production and decays involving light nuclei are also studied in hadron induced experiments. In particular, the physics program of the new WASA-at-COSY facility includes double pion production from light nuclei. Here, the efforts focus on the ABC effect, the puzzling low-mass enhancement in the pi-pi invariant mass spectrum, first observed in inclusive measurements of two-pion production in nuclear collisions.

The Gerasimov-Drell-Hearn sum rule at MAMI

An extended experimental program to investigate the Gerasimov-Drell-Hearn (GDH) sum rule and related partial reaction cross sections on proton and neutron has been carried out by the GDH collaboration at the electron accelerators MAMI (Mainz) and ELSA (Bonn). The GDH sum rule connects the helicity-dependent photoabsorption cross section with the anomalous magnetic moment of the nucleon. The GDH collaboration has measured the total cross section of circularly polarised photons with longitudinally polarised protons at MAMI and ELSA to check this sum rule experimentally. In addition partial reaction channels like pion, double pion and eta production were determined. This provides new information on the helicity-dependent excitation spectrum of the nucleon. With the help of partial wave analyses it is possible to extract new, complementary information on the broad, overlapping resonances in this energy region. The double polarisation observable E measured in this experiment enhances the smaller multipoles via interference terms. The analysis of our data provides a new possibility to study the photon couplings to the nucleon resonances, especially above the Δ resonance where many properties of the observed states (e.g., coupling constants, branching ratios, helicity amplitudes) are only poorly known. In this paper we present several new results from our measurements on polarised proton and deuteron targets.

Double-pion photoproduction from nuclei

Differences in the photoproduction of mesons on the free proton and on nuclei are expected to reveal changes in the properties of hadrons. Inclusive studies of nuclear photoabsorption have provided evidence of medium modifications. However, the results have not been explained in a model independent way. A deeper understanding of the situation is anticipated from a detailed experimental study of meson photoproduction from nuclei in exclusive reactions. In the energy regime above the Delta(1232) resonance, the dominant double pion production channels are of particular interest. Double pion photoproduction from nuclei is also used to investigate the in-medium modification of meson-meson interactions.

Two-pion production in photon-induced reactions

Differences in the photoproduction of mesons on the free proton and on nuclei are expected to reveal changes in the properties of hadrons. Inclusive studies of nuclear photoabsorption have provided evidence of medium modifications. However, the results have not been explained in a model independent way. A deeper understanding of the situation is anticipated from a detailed experimental study of meson photoproduction from nuclei in exclusive reactions. In the energy regime above the Delta(1232) resonance, the dominant double pion production channels are of particular interest. Double pion photoproduction from nuclei is also used to investigate the in-medium modification of meson-meson interactions.