WASA-at-COSY

The $\eta$ meson production process was studied via measurements of the analyzing power, $A_{y}$ for the $\vec{p}p\to pp\eta$ reaction. The measurement was performed with the WASA-at-COSY detector at excess energies of 15 MeV and 72 MeV. The missing mass and invariant mass techniques were used to identify $\eta$ meson. The angular distribution of the analyzing power of for the $\eta$ meson was determined. The result is more than 5 order of magnitude more precise than achieved in previous experiments [1,2]. The result of the studies shows a disagreement between experiment and the predicted theoretical behavior of $A_{y}$. The data indicate that at the excess energy of 15 MeV there is no contribution from $Sd$ and $Pp$ partial waves. Thus we prove experimentally for the first time that in the $\vec{p}p\to pp\eta$ reaction the $\eta$ meson is produced in a $s$-wave with respect to the protons at least up to 15 MeV. Whereas at an excess energy of 72 MeV contributions from the $Pp$ wave is significant.

[1] R. Czyzykiewicz et al., Phys. Rev. Lett. 98 (2007) 122003.

[2] F. Balestra et al. Phys. Rev. C 69 (2004) 064003.