Abstract
The tagged quasi-free np→npπ+π− reaction has been studied experimentally with the High Acceptance Di-Electron Spectrometer (HADES) at GSI at a deuteron incident beam energy of 1.25 GeV/nucleon (s∼2.42 GeV/c for the quasi-free collision). For the first time, differential distributions of solid statistics for π+π− production in np collisions have been collected in the region corresponding to the large transverse momenta of the secondary particles. The invariant mass and angular distributions for the np→npπ+π− reaction are compared with different models. This comparison confirms the dominance of the t-channel with ΔΔ contribution. It also validates the changes previously introduced in the Valencia model to describe two-pion production data in other isospin channels, although some deviations are observed, especially for the π+π− invariant mass spectrum. The extracted total cross section is also in much better agreement with this model. Our new measurement puts useful constraints for the existence of the conjectured dibaryon resonance at mass M∼2.38 GeV and with width Γ∼70 MeV.
Highlights
The two-pion production in nucleon–nucleon (NN) collisions is a rich source of information about the baryon excitation spectrum and the baryon–baryon interactions
By giving access to single and double baryon excitation processes, which both play an important role in the NN dynamics in the few GeV energy range and contribute significantly to meson and dilepton production, the two-pion production appears as a key process towards a better understanding of hadronic processes
The different contributions in the OPER and modified Valencia models are shown in Figs. 3 and 5 in comparison to selected experimental distributions exhibited in Figs. 2 and 4
Summary
The two-pion production in nucleon–nucleon (NN) collisions is a rich source of information about the baryon excitation spectrum and the baryon–baryon interactions. By giving access to single and double baryon excitation processes, which both play an important role in the NN dynamics in the few GeV energy range and contribute significantly to meson and dilepton production, the two-pion production appears as a key process towards a better understanding of hadronic processes. The excitation of baryonic resonances coupled to the ρ meson can be studied with the two pions in the isospin 1 channel. This is of utmost interest for a better understanding of the dilepton production in nucleon–nucleon reactions, where these couplings manifest clearly [3,4,5], and in nucleon matter due to the expected modifications of the ρ meson spectral functions [6]. It is important to check the description of the double process in the two-pion production channels
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