First Measurement of the Absolute Branching Fraction of ${\eta }_c\to \gamma \gamma$

BESIII Collaboration, M. Ablikim, M. N. Achasov +691

We apply a tag-and-probe method to precisely measure the absolute branching fraction of the decay ${\eta }_c\to \gamma \gamma$ with the BESIII experiment at BEPCII. Starting with a large initial s…