Abstract
ANKE@COSY data on the cross section of the reaction pp → {pp}sπ0, where {pp}s is the proton pair in the 1S 0 state at small excitation energy Epp = 0 – 3 MeV, obtained at beam energies 0.5 - 2.0 GeV are analyzed within the one-pion exchange model. The model involves the subprocess π0 p → π0 p and accounts for the final state pp-interaction. A broad maximum observed in the cross section of the reaction pp → {pp}sπ0 at 0.5 - 1.4 GeV in the forward direction is explained by this model as a dominant contribution of the isospin $\cfrac{3}{2}$ in the π0 p-scattering. The second bump in data at 2 GeV is underpredicted within this model by one order of magnitude. An explicit excitation of the Δ(1232)-isobar using the box-diagram is also considered in the region of the first maximum.
Highlights
Study of the reaction pp → {pp}sπ0, where {pp}s is the proton pair in the 1S 0 state at small excitation energy Epp = 0 − 3 MeV, is motivated by several reasons
While for the reaction pp → dπ+ there are a lot of data including spin observables [1], which are used to test theoretical models in the GeV region [2, 3], data on the reaction pp → {pp}sπ0 above 0.4 GeV were absent until recent measurements at COSY [4,5,6]
If θ is the angle between the cms momenta of the diproton and the proton p1, which emits the virtual pion in the direct OPE diagram in Fig. 1, and φ is the cms scattering angle of the π0-meson in the process π0(kπ) + p2 → p4 + π0(qπ), one can find the following relation: p20q0 + |p2||qπ| cos φ = m2p + p2i m2π + p2f − pi p f cos θ, (10)
Summary
The contribution of the ∆-mechanism to the reaction pp → {pp}sπ0 is expected to be suppressed as compared to the reaction pp → dπ+ This argument was applied in [7] to explain a very small ratio (less of few percents) of the spin-singlet to spin-triplet pn-pairs observed in the LAMPF data [8] in the final state interaction region of the reaction pp → pnπ+ at proton beam energy 0.8 GeV. In view of qualitative arguments given above, this disagreement would mean that the observed maximum of the cross section of the reaction pp → {pp}sπ0 at 0.5 - 1.4 GeV is of non-∆-isobar origin We analyze these data employing a simpler model, which includes the subprocess π0 p → π0 p and the final state pp(1S 0)-interaction (Fig. 1). 3 shows [15] that the agreement between this mechanism and the data both in a shape and absolute value of the cross section is only qualitative
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