Resonance-like coherent production of a pion pair in the reaction $pd \rightarrow pd\pi\pi$ in the GeV region

TL;DR

This study investigates the resonance-like production of pion pairs in the $pd ightarrow pd\pi\pi$ reaction at GeV energies, revealing a narrow bump near the ABC effect threshold and a resonance at approximately 2.36 GeV/$c^2$.

Contribution

It presents the first observation of a resonance-like peak in the $d\pi\pi$ system associated with the ABC effect in proton-deuteron collisions at GeV energies.

Findings

Identification of a narrow bump in the missing mass distribution near the ABC threshold.

Observation of a resonance peak at $M_{d\pi\pi} \\approx 2.36$ GeV/$c^2$ with a width of about 0.10 GeV/$c^2$.

Evidence supporting a resonance interpretation of the ABC effect in this reaction.

Abstract

The reaction $p + d \rightarrow p + d + X$ was studied at 0.8-2.0 GeV proton beam energies with the ANKE magnetic spectrometer at the COSY synchrotron storage ring. The proton-deuteron pairs emerging with high momenta, 0.6-1.8 GeV/$c$, were detected at small angles with respect to the proton beam. Distribution over the reaction missing mass $M_x$ reveals a local enhancement near the threshold of the pion pair production specific for the so-called ABC effect. The enhancement has a structure of a narrow bump placed above a smooth continuum. The invariant mass of the $d\pi\pi$ system in this enhancement region exhibits a resonance-like peak at $M_{d\pi\pi} \approx 2.36$ GeV/$c^2$ with the width $\Gamma \approx 0.10$ GeV/$c^2$. A possible interpretation of these features is discussed.