Two-Pion Production in Proton-Proton Collisions -- Experimental Total Cross Sections and their Isospin Decomposition

Understand

The two-pion production in pp-collisions has been investigated at CELSIUS in exclusive measurements from threshold up to $T_p$ = 1.36 GeV.

• Total and differential cross sections have been obtained for the channels $pn\pi^+\pi^0$, $pp\pi^+\pi^-$, $pp\pi^0\pi^0$ and also $nn\pi^+\pi^+$.
• For intermediate incident energies $T_p >$ 1 GeV, i.e.
• in the region which is beyond the Roper excitation but at the onset of $\Delta\Delta$ excitation, the total $pp\pi^0\pi^0$ cross section falls behind theoretical predictions by as much as an order of magnitude near 1.2 GeV, whereas the $nn\pi^+\pi^+$ cross section is a factor of five larger than predicted.