A reaction can look visually simple and still fail a ledger audit: one electron has scaled charge -3, while two photons total 0. The exact mismatch rejects this toy arrow, but in an experiment the candidate particles and missing energy would first be reconstructed from measured detector data.
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.