Figure 1

(Left panel) Scatter plot of the two proton polar angles in the laboratory system obtained from a simulation of phase space distributed $\mathit{pp}\to \mathit{pp}{\pi }^0$ events. The geometrical coverage of the WASA detector is shown by the overlaid histogram. (Right panel) The relative proton momentum ($p$) distribution obtained from a phase space generated event sample is shown by the unlabeled curve. The labeled curves represent the $p$ distributions for two forward prong events (2FD) and for one forward plus one central prong event (1FD1CD), respectively, in a full simulation of the detector setup. The two trigger conditions used in the experiment are of the types 2FD and 1FD1CD (see Sec. 2).Reuse & Permissions

Figure 2

The invariant mass distribution of the two γ's selected by the 1FD1CD-type trigger. (Left panel) The dashed line represents the events that are excluded due to the constraint that the summed γ energy is within the kinematical limits for ${\pi }^0$ production. The bold line depicts what events are cut out based on the relation between the opening angle and the planarity of the two γ's, representing largely background from elastic scattering (accidentals). (Right panel) The combination of the two cuts is shown by the dashed line and the final resulting invariant mass of the two γ's is drawn by the bold line.Reuse & Permissions

Figure 3

The experimental center of mass ${\pi }^0$ angular distribution, arbitrarily normalized. The black markers represent the data and the unfilled markers correspond to the predicted histogrammed values from a simulation weighted according the microscopic calculation by Hanhart et al. [28, 29]. The line shows the result of a simulated phase space generated isotropic distribution of the $\mathit{pp}\to \mathit{pp}{\pi }^0$ after passing through the detector system.Reuse & Permissions

Figure 4

Acceptance corrected center of mass ${\pi }^0$ angular distribution, normalized to ${\sigma }_{\mathrm{tot}}=92.3\pm 7.2$ μb [22]. The uncertainites are dominated by systematic effects. The solid (black) line corresponds to the microscopic calculation by Hanhart et al. [28, 29]. The dashed (blue) line represents a fit of the dependence on $\cos {}^2{\theta }_{\pi }$, taking only the statistical errors into account.Reuse & Permissions

Figure 5

A compilation of measurements of the slope parameter $b$ below 1 GeV is shown. The definition of $b$ differs by a factor of three in Ref. [22], which is why the values given by that reference have been divided by three for consistency. (The large error bar at 400 MeV is from Ref. [35], and the location of the present data point at the same energy is shifted horizontally by 10 MeV to make it visible).Reuse & Permissions

Figure 6

Acceptance corrected center of mass ${\pi }^0$ angular distribution, for $p<53$ MeV/c, normalized to the cross sections of Ref. [22]. The line represents a fit of the data points to the dependence on $\cos {}^2{\theta }_{\pi }$.Reuse & Permissions