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

TL;DR

This study experimentally investigates the quasi-free np to np pi+ pi- reaction at 1.25 GeV/nucleon, providing new differential distributions, validating and refining theoretical models, and offering constraints on possible dibaryon resonances.

Contribution

First measurement of differential distributions for np to np pi+ pi- at this energy, testing and refining reaction models, and constraining dibaryon resonance hypotheses.

Findings

Dominance of t-channel with DeltaDelta contribution confirmed.

Valencia model modifications are validated but some deviations observed.

Total cross section aligns better with the refined model.

Abstract

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.