A plausible particle arrow is rejected when its ledgers do not close. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Charge does not disappear
The input scaled charge is -3 but two photons total 0.
−3=0
Lepton number also fails
The input electron-family ledger is 1 while photons total 0.