Classification of particles: Baryon number as a quantum number.

The idea

Baryon number is a conserved quantum number that tracks the net number of baryons in an interaction. Each baryon has baryon number +1, each antibaryon has baryon number −1, and mesons and leptons have baryon number 0. In any allowed interaction the total baryon number before and after must be equal, so a proton (B = +1) cannot simply disappear; it can only change into another baryon, such as a neutron in beta-plus decay, where the positron and neutrino both have B = 0. Applying the rule means writing the baryon number of every particle on each side of an equation and checking that the totals match. This conservation law explains why the proton is stable in the Standard Model and why baryon–antibaryon pairs are produced together.

What earns the marks

• Define baryon number as a quantum number with value +1 for each baryon, −1 for each antibaryon and 0 for mesons and leptons.
• State that baryon number is conserved in all interactions.
• Apply conservation of baryon number to check whether a proposed particle interaction or decay is allowed.
• Use baryon number together with charge and other quantum numbers to deduce an unknown particle in a reaction.

Common mistakes and how to put them right

• Giving mesons a baryon number of +1.

  Put it right by stating that mesons contain a quark–antiquark pair and have baryon number 0.

• Forgetting the negative sign for antibaryons.

  Put it right by assigning baryon number −1 to each antibaryon.

• Balancing charge but ignoring baryon number when testing a decay.

  Put it right by checking every conserved quantum number, including baryon number.

Exam tips

• Write the baryon number beneath each particle in an equation before adding the totals.
• If a decay appears to create a baryon from nothing, check whether an antibaryon on the other side balances it.
• Show the conservation check explicitly so the examiner can see that the totals match.