Episode 535: Particle reactions

This episode considers both hadrons and leptons in particle reactions. Students must take account of both conservation of lepton number and conservation of baryon number.


  • Student activity: Applying conservation rules (20 minutes)
  • Discussion: Identifying conservation rules (10 minutes)
  • Student questions: Questions on conservation rules (30 minutes)

Student activity: Applying conservation rules
Students should first check on the conservation of (electric) charge, baryon number, lepton number and strangeness in real reactions. They should also note that the mass/energy of products should be less/equal to the mass/energy of reactants.

Use the first sheet of hadron cards from the previous episode and the four leptons from the ‘lepton cards’ document to decide which particle is needed to complete several reactions.

Episode 535-1: Applying conservation of baryon and lepton number (Word, 27 KB)

You should expect some ambiguity as to which neutrino or antineutrino is involved: after all, this ambiguity was not resolved until recently. Ask students what they would expect, from symmetry, in each case.

Now add the second sheet of cards from the ‘hadron cards’ document, containing all the anti-baryons and some more mesons.

Episode 535-2: Particle card student activities (Word, 28 KB)

A quick sort of all cards should reveal that all baryons and leptons have their anti-particles, all with obvious names except electron/positron. Tell students that all mesons – those with baryon number and lepton number of zero – have antiparticles, but that some are their own anti-particles; they can then sort out which is which.

Students are now able to check whether reactions can proceed according to the conservation rules met so far.

Discussion: Identifying conservation rules
Invite students to sum up what they now know about particles and particle reactions. Look for the following points:

  • Baryon number is always conserved
  • (Electric) charge is always conserved
  • Lepton number is always conserved
  • Mass on the left hand side of the equation must be bigger than the mass on the right hand side
  • Strangeness may be conserved, but not always. (In weak interactions it can change by 1)

Student questions: Questions on conservation rules
Here are some suggestions for questions that could be given as student exercises at this point.

Episode 535-3: Things that don’t change (Word, 33 KB)

Questions about creation and annihilation (but note that this question uses E = mc2; however, you may find that more mathematical students will accept mass values quoted as e.g. 939.6 MeV/c2 once they have done a calculation of this sort.

Episode 535-4: Creation and annihilation (Word, 28 KB)

Questions about creation from annihilation. (To reduce the demand, you may wish to delete all the text from ‘Exotic forms of matter can occur fleetingly …’ and dropping questions 7 to 11.)

Episode 535-5: Creation from annihilation (Word, 32 KB)

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Episode 535: Particle reactions (Word, 90 KB)