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Description
The reaction $pp → dπ^+$, which is a classical method of studying $NN$ interactions, has two peaks in the energy dependence of the differential cross section at zero angle $dσ(0)/dΩ$. The first one is located in the region of the sum of the $NΔ(1232)$ masses, $√s ≈ 2.15$ GeV. When analyzing the ANKE-COSY data, a similar peak was found in the reaction $pp →\{pp\}_Sπ^0$, the spin-isospin partner of $pp → dπ^+$. Here $\{pp\}_S$ denotes a diproton, i.e. an unrelated interacting proton pair in the $^1S_0$ state. The previously published ANKE-COSY data also indicate the possibility of the existence of a similar second peak.
Partial wave analysis shows that the first peak for the $pp → dπ^+$ reaction is caused by three dominant transitions exhibiting resonant behavior in the intermediate two-baryon system: $^1D_2p$, $^3F_3d$ and $^3P_2d$; and for the reaction $pp →\{pp\}_Sπ^0$ by two transitions $^3P_2d$ and $^3P_0s$. The transitions responsible for the second peak remain much less clear.
In order to study the second peak on the ANKE-COSY facility, the reaction $pp →\{pp\}_Sπ^0$ was additionally investigated at small angles in the beam energy range $T_{beam} = 1.0 - 2.8$ GeV. For the reaction $pp →\{pp\}_Sπ^0$ we obtained the angular dependences of the differential cross section $dσ/dΩ$ at forward angles for several energies, the energy dependences of the cross section at zero angle $dσ(0)/dΩ$ and of the forward cross section angular slope $k$.
The obtained results confirm the existence of a second peak in the $dσ(0)/dΩ$ energy dependence for the reaction $pp →\{pp\}_Sπ^0$ and allow us to estimate its maximum, mass and width. The change in the sign of the slope $k$ in comparison with the region of the first peak indicates a change in the reaction dynamics at the energies $√s ≈ 2.3−2.6$ GeV.
The obtained results can contribute to the development of a theoretical understanding of the hadron interaction mechanisms in this energy range.
