This talks might be interesting to you: https://indico.cern.ch/event/827066/contributions/3590884/attachments/1938785/3213894/gninenko-na64-pbc-05-11-2019.pdf h

This talks might be interesting to you: https://indico.cern.ch/event/827066/contributions/3590884/attachments/1938785/3213894/gninenko-na64-pbc-05-11-2019.pdf https://indico.cern.ch/event/827066/contributions/3590884/at... (see page 10) There is hopefully also a DarkLight run at JLAB or something similar at MAMI to test the remaining phase space.