Great summary. To add to that: 1. They see a bump (aka "resonance") which can generically be modeled as a new particle/state (whether it's a dark force particl

Great summary. To add to that: 1. They see a bump (aka "resonance") which can generically be modeled as a new particle/state (whether it's a dark force particle, or a nuclear state or atomic state or whatever). 2. (Small correction to the above post: the mass of the resonance is 16.6 MeV not GeV) Atomic physics operates at the eV scale, and the mass of a Be nucleus is roughly 10 GeV (10^9 eV). So, given that this bump fits the 16.6 MeV (10^7 eV) scale which between the other two scales, this might either be some detailed nuclear physics (eg: some Berellium resonance) or an honest-to-god new particle. 3. Since the angular separation is roughly 135 degrees, it seems reasonable that the intermediate state is moving fast ("boosted") [1]. That means that it cannot be as heavy as a whole nucleus -- therefore, if this was a Berellium resonance then the angle between the electron-positron pair would leave back-to-back (180 degree separation). That is what pushes the interpretation towards a new particle/force. [1]: https://en.wikipedia.org/wiki/Particle_decay#From_two_different_frames https://en.wikipedia.org/wiki/Particle_decay#From_two_differ...