Which of the following decays are possible? For those that...

Key Concepts

Conservation Laws

In any particle decay, fundamental quantities such as energy, momentum, electric charge, and baryon and lepton numbers must be conserved. In addition, quantum numbers like strangeness (or more generally flavor) and isospin are conserved in strong and electromagnetic interactions while they can change (typically by one unit) in weak interactions. A careful check of these conservation laws helps determine whether a proposed decay is allowed or forbidden.

Weak Interaction and Strangeness Change

Weak interactions can change the flavor quantum numbers of particles (for example, they can change strangeness by ±1), which is why many decays that violate strangeness conservation in strong or electromagnetic processes are permitted via the weak interaction. However, the weak force very rarely (if ever) allows processes that involve a change of strangeness by more than one unit, so decays requiring a ?S = 2 typically violate the selection rules of the Standard Model and are forbidden.

Electromagnetic Transitions and Selection Rules

Electromagnetic decays proceed via the emission of photons and must obey conservation laws for energy, momentum, angular momentum, and parity. The photons themselves have specific properties (such as having spin 1 and negative intrinsic parity) and when more than one photon is involved, additional constraints such as Bose symmetry and the Landau–Yang theorem (which restricts how particles of certain spins can decay into two photons) can come into play. These constraints can forbid or highly suppress multiphoton decays if the overall quantum numbers of the final state cannot match those of the initial state.

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