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Study of the quasi-free np to np π^+π^- reaction with a deuterium beam at 1.25 GeV/nucleon

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arxiv 1503.04013 v2 pith:KQV5OLW2 submitted 2015-03-13 nucl-ex

Study of the quasi-free np to np π^+π^- reaction with a deuterium beam at 1.25 GeV/nucleon

classification nucl-ex
keywords massquasi-freereactionbeambeendeltadistributionsinvariant
verification ladder T0 review T1 audit T2 compute T3 formal T4 reserved
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The tagged quasi-free $np \to np\pi^+\pi^-$ 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 ($\sqrt s \sim$ 2.42 GeV/c for the quasi-free collision). For the first time, differential distributions for $\pi^{+}\pi^{-}$ 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 $np\rightarrow np\pi^{+}\pi^{-}$ reaction are compared with different models. This comparison confirms the dominance of the $t$-channel with $\Delta\Delta$ 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 $\pi^{+}\pi^{-}$ 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$\sim$ 2.38 GeV and with width $\Gamma\sim$ 70 MeV.

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