Using a data-driven technique to estimate the key background in the search for an invisibly decaying dark photon at Belle II*

The dark photon is a well motivated, massive, hypothetical particle that would be the connection between Standard Model (SM) matter and dark matter. It would do this by feebly coupling to the SM photon in addition to coupling to dark matter particles. The dark photon could be produced at particle collider experiments such as Belle II via their coupling to the SM photon. In the invisible decay case it decays to dark matter, which is not detectable, but due to the massive nature of the dark photon and conservation of energy another photon is produced. This photon is detectable, and along with the missing momentum, is the signature of the dark photon. This signature can be mimicked by other particle decays however, in particular two photon production. If one of those photons is not detected, this decay looks exactly like the signature for a sub-GeV mass dark photon. The estimation of this background must be done through a data-driven method as simulations are not reliable enough to estimate the angular distribution and magnitude of this process, as is required. The ABCD method is implemented, and verified through testing on simulations.