Exclusive Measurements of $pd{\to }^{3}He\ddot{I}€\ddot{I}€$ : the $ABC$ Effect Revisited

Abstract

Exclusive measurements of the reactions $pd\to$ ${}^{3}He\ddot{I}{€}^{+}\ddot{I}{€}^{-}$ and $pd\to$ ${}^{3}He\ddot{I}{€}^0\ddot{I}{€}^0$ have been carried out at $T_p=0.895$ GeV at the CELSIUS storage ring using the WASA detector. The $\ddot{I}{€}^{+}\ddot{I}{€}^{-}$ channel evidences a pronounced enhancement at low invariant $\ddot{I}€\ddot{I}€$ masses - as anticipated from previous inclusive measurements of the ABC effect. This enhancement is seen to be even much larger in the isoscalar $\ddot{I}{€}^0\ddot{I}{€}^0$ channel. The differential distributions prove this enhancement to be of scalar-isoscalar nature. $\hat{I}”\hat{I}”$ calculations give a good description of the data, if a boundstate condition is imposed for the intermediate $\hat{I}”\hat{I}”$ system.

extended version, 8 pages, 7 figures, theoretical model calculations added