  Exclusive measurements of the quasi-free $pp \to pp\pi^+\pi^-$ reaction have
been performed by means of $pd$ collisions at $T_p$ = 1.2 GeV using the WASA
detector setup at COSY. Total and differential cross sections have been
obtained covering the energy region $T_p = 1.08 - 1.36$ GeV ($\sqrt s$ = 2.35 -
2.46 GeV), which includes the regions of $N^*(1440)$ and
$\Delta(1232)\Delta(1232)$ resonance excitations. Calculations describing these
excitations by $t$-channel meson exchange are at variance with experimental
differential cross sections and underpredict substantially the measured total
cross section. An isotensor $\Delta N$ dibaryon resonance with $I(J^P) =
2(1^+)$ produced associatedly with a pion is able to overcome these
deficiencies. Such a dibaryon was predicted by Dyson and Xuong and more
recently calculated by Gal and Garcilazo.
