A Level Physics Practice Exam

When checking for conservation in a decay process, which quantity is NOT typically checked?

1. Charge (Q)

2. Baryon number (B)

3. Neutron number (N)

4. Lepton number (L)

The correct answer is: Neutron number (N)

In the context of particle decay processes, conservation laws govern how certain quantities must remain constant before and after a decay event. Charge, baryon number, and lepton number are key quantities that typically must be conserved. Charge conservation ensures that the total electric charge before decay equals the total charge after the decay. Baryon number conservation indicates that the number of baryons (like protons and neutrons) remains unchanged in interactions that do not involve significant transformations of these particles into other types. Lepton number conservation means that the total number of leptons (such as electrons and neutrinos) remains constant in the decay process. However, neutron number is not a conserved quantity in the same sense. While individual decays may involve neutrons and could initially suggest conservation in a specific instance, the overall neutron number is not a fundamental conservation law across all types of decays. Neutrons may convert to other particles or types of nucleons during decay, making neutron number a less reliable measure for checking conservation during decay processes. Thus, it is the neutron number that is not typically checked for conservation in these scenarios.