WASA-at-COSY

In 1986 Haider and Liu postulated a new kind of exotic nuclear matter - $\eta$-mesic nuclei in which the $\eta$ meson is bound in a nucleus by means of the strong interaction [1]. However, till now there is no clear experimental evidence confirmed empirically its existence. In November 2010, the search for the $^4\mathrm{He}$-$\eta$ bound state was performed with high statistics and high acceptance with the WASA-at-COSY detector [2]. The experimental method is based on the measurement of the excitation functions for the two reaction channels: $dd\to$ $^3\mathrm{He} n \pi^0$ and $dd\to$ $^3\mathrm{He} p \pi^-$ near the $\eta$ production threshold. The measurement was carried out using a ramped beam technique. The beam momentum was varying continuously from 2.127 GeV/c to 2.422 GeV/c corresponding to the excess energy range $Q\in(-70,30)$ MeV. The presentation will include description of the experimental method and next steps leading to determination of preliminary excitation functions from the 2010 data.

[1] Q. Haider, L.C. Liu, Phys. Lett. B 172 (1986) 257. [2] M. Skurzok, P. Moskal, W. Krzemien, Prog. Part. Nucl. Phys. (2012), arXiv:1112.2521.