When a $\pi^-$ decays, the products may include:
- A positron
- A muon
- A proton
- All of the above
This is College Physics Answers with Shaun Dychko. When a negative pion decays, the products have to include something that has a negative charge in order for charge conservation to apply and charge conservation is one of these strong conservation or exact conservation rules; exact meaning there are no exceptions to it. So if this negative thing decays, it has to produce a negative thing and so that means a muon, which is negatively charged could be one of the products.