Measurement of mixing-induced CP violation in the decay $B^0\to \ddot{I}{€}^0\ddot{I}{€}^0$

Abstract

Measurements of charge-parity (CP) violation in quark transitions provide stringent tests of the current theory and sensitive probes of particles or interactions beyond it. We report the first measurement of mixing-induced CP violation in $B^0\to \ddot{I}{€}^0\ddot{I}{€}^0$ decays. The measurement uses 190 million $\hat{I}\yen (4S)\to B^0\overline{B}{}^0$ events collected with the Belle II experiment at the SuperKEKB asymmetric-energy electron-positron collider. A new approach that exploits the quantum entanglement of the $B^0\overline{B}{}^0$ pair enables a decay-time-dependent analysis without reconstructing the $B^0\to \ddot{I}{€}^0\ddot{I}{€}^0$ decay position. We measure the mixing-induced CP-violating coefficient to be $S_{\ddot{I}{€}^0\ddot{I}{€}^0}=0.61_{-0.79}^{+0.75}(\mathrm{stat})\pm 0.11(\mathrm{syst})$, attaining a precision that would require a data set twenty times as large with a conventional analysis. We also measure the direct CP-violating coefficient to be $C_{\ddot{I}{€}^0\ddot{I}{€}^0}=0.05\pm 0.28(\mathrm{stat})\pm 0.07(\mathrm{syst})$. These results substantially improve the constraints from $B\to \ddot{I}€\ddot{I}€$ decays on the CP-violating phase $\ddot{I}{•}_2$ of the quark-mixing matrix using far less data than had previously been assumed necessary.

20 pages, 11 figues, 2 tables