Baryon number
A quantum number conserved in all interactions, defined as +1 for baryons (protons, neutrons), -1 for antibaryons, and 0 for non-baryonic particles (leptons, photons, W/Z bosons). Baryon number conservation is an exact law in the Standard Model, preventing processes like proton decay.
Formula
B = +1 (baryons), B = −1 (antibaryons), B = 0 (leptons, photons)
Real World
Grand Unified Theories predict proton decay, but experiments like Super-Kamiokande in Japan have searched for it for decades without success, strongly supporting baryon number conservation in the Standard Model.
Exam Focus
Assign baryon number to every particle in a reaction and sum both sides — a mismatch means the process is forbidden, which is a common 'explain why' question.
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