1
See however KLOET 1996 who fit ${{\mathit \pi}^{+}}{{\mathit \pi}^{-}}$ only and find waves only up to $\mathit J = 3$ to be important but not significantly resonant.
2
$\mathit I(\mathit J{}^{P}) = 0(2{}^{+})$ from simultaneous analysis of ${{\mathit p}}$ ${{\overline{\mathit p}}}$ $\rightarrow$ ${{\mathit \pi}^{-}}{{\mathit \pi}^{+}}$ and ${{\mathit \pi}^{0}}{{\mathit \pi}^{0}}$.
3
$\mathit I{}^{G}(\mathit J{}^{P}) = 0{}^{+}(2{}^{+})$ from partial-wave amplitude analysis.
References:
OAKDEN
1994
NP A574 731
Amplitude Analysis of Data on ${{\overline{\mathit p}}}$ ${{\mathit p}}$ $\rightarrow$ ${{\mathit \pi}}{{\mathit \pi}}$ at Low Energy
MARTIN
1980C
NP B169 216
Exploration of the ${{\overline{\mathit p}}}$ ${{\mathit p}}$ $\rightarrow$ ${{\mathit \pi}}{{\mathit \pi}}$ Channel in the Resonance Region
MARTIN
1980B
NP B176 355
Structures in ${{\overline{\mathit p}}}$ ${{\mathit p}}$ $\rightarrow$ ${{\mathit \pi}}{{\mathit \pi}}$
DULUDE
1978B
PL 79B 335
Observation of Structure in ${{\overline{\mathit p}}}$ ${{\mathit p}}$ $\rightarrow$ ${{\mathit \pi}^{0}}{{\mathit \pi}^{0}}$