With an effective Lagrangian approach, we analyze several N N → N N ππ channels by including various resonances with mass up to 1.72 GeV. For the channels with the pion pair of isospin zero, we confirm the dominance of N * (1440) → N σ in the near threshold region. At higher energies and for channels with the final pion pair of isospin one, we find large contributions from N * (1440) → ∆π, double-∆, ∆(1600) → N * (1440)π, ∆(1600) → ∆π and ∆(1620) → ∆π. There are also sizeable contributions from ∆ → ∆π, ∆ → N π, N → ∆π and nucleon pole at energies close to the threshold. We well reproduce the total cross sections up to beam energies of 2.2 GeV except for the pp → ppπ 0 π 0 channel at energies around 1.1 GeV and our results agree with the existing data of differential cross sections of pp → ppπ + π − , pp → nnπ + π + and pp → ppπ 0 π 0 which are measured at CELSIUS and COSY.

Hadronic transitions of χ cJ → η c π + π − (J = 0, 1, 2) are searched for using a sample of 1.06 × 10 8 ψ(3686) events collected with the BESIII detector at the BEPCII storage ring. The η c is reconstructed with K 0 S K ± π ∓ and K + K − π 0 final states. No signals are observed in any of the three χ cJ states in either η c decay mode. At the 90% confidence level, the upper limits are determined to be B(χ c0 → η c π + π − ) < 0.07%, B(χ c1 → η c π + π − ) < 0.32%, and B(χ c2 → η c π + π − ) < 0.54%. The upper limit of B(χ c1 → η c π + π − ) is lower than the existing theoretical prediction by almost an order of magnitude. The branching fractions of χ cJ → K 0 S K ± π ∓ π + π − , K + K − π + π − π 0 , ωK + K − and φπ + π − π 0 (J = 0, 1, 2) are measured for the first time.

Using a sample of 1.06 × 10 8 ψ ′ decays collected by the BESIII detector, χ c0 and χ c2 decays into π 0 π 0 and ηη are studied. The branching fraction results are Br(χ c0 → π 0 π 0 ) = (3.23 ± 0.03 ± 0.23 ± 0.14) × 10 −3 , Br(χ c2 → π 0 π 0 ) = (8.8 ± 0.2 ± 0.6 ± 0.4) × 10 −4 , Br(χ c0 → ηη) = (3.44 ± 0.10 ± 0.24 ± 0.2) × 10 −3 , and Br(χ c2 → ηη) = (6.5 ± 0.4 ± 0.5 ± 0.3) × 10 −4 , where the uncertainties are statistical, systematic due to this measurement, and systematic due to the branching fractions of ψ ′ → γχ cJ , respectively. The results provide information on the decay mechanism of χ c states into pseudoscalars.

The pp → pnπ + reaction is a channel with the largest total cross section for pp collision in COSY/CSR energy region. In this work, we investigate individual contributions from various N * and ∆ * resonances with mass up to about 2 GeV for the pp → pnπ + reaction. We extend a resonance model, which can reproduce the observed total cross section quite well, to give theoretical predictions of various differential cross sections for the present reaction at Tp = 2.88 GeV. It could serve as a reference for identifying new physics in the future experiments at HIRFL-CSR.