First measurement of polarizations in the decay $D^0\to \ddot{I}‰\ddot{I}•$

Abstract

Using a data sample corresponding to an integrated luminosity of 2.93 fb${}^{-1}$ collected at a center-of-mass energy $\sqrt{s}=3.773$ GeV by the BESIII detector, the decay $D^0\to \ddot{I}‰\ddot{I}•$ is observed for the first time. The branching fraction is measured to be $(6.48\pm 0.96\pm 0.40)\times 10^{-4}$ with a significance of $6.3\ddot{I}ƒ$, where the first and second uncertainties are statistical and systematic, respectively. An angular analysis reveals that the $\ddot{I}•$ and $\ddot{I}‰$ mesons from the $D^0\to \ddot{I}‰\ddot{I}•$ decay are transversely polarized. The $95\%$ confidence level upper limit on longitudinal polarization fraction is set to be less than $0.24$, which is inconsistent with current theoretical expectations and challenges our understanding of the underlying dynamics in charm meson decays.

8 pages, 3 figures, to be accepted by PRL