Q192 · Practice questionComplex unfamiliar6 marks
QUESTION 192 (6 marks)
A proton and an antiproton collide. Consider only conservation of total electric charge, baryon number and lepton number when assessing these candidate final states: A: ; B: ; C: ; D: . Neutrons are neutral baryons; neutrinos are neutral leptons. Antiparticle quantum numbers have opposite signs.
a)[1 mark]
State the three total quantum numbers of the initial state.
b)[4 marks]
For each candidate, state whether it is ruled out by any of these three conservation laws and justify.
c)[1 mark]
Explain why passing these three tests alone does not prove that a channel occurs in an experiment.
WORKED SOLUTION
6 marksPractice marking scheme
ANSWER
(a) , , . (b) A and C are not ruled out by these laws; B violates baryon number; D violates electric charge. (c) These are necessary constraints; energy, momentum and interaction dynamics must also permit the channel.
Worked solution
(a) The proton's charge and baryon number cancel those of the antiproton, and both have zero lepton number.
(b) A has from a neutron and antineutron. B has and violates . C has from electron–positron cancellation. D has : its lepton numbers cancel but charge does not. Thus only A and C pass these three tests.
(c) A candidate can satisfy the three listed quantum-number rules and still fail other requirements such as sufficient collision energy or momentum conservation. Even a kinematically allowed channel has a probability determined by the interaction.
Equivalent physically justified methods and consistent equivalent units accepted.
Displayed decimals are model answers; accept appropriate significant figures and consistent rounding from stated constants.
Part a: State all three initial totals correctly.
Part b: Assess A using its cancelling baryon numbers.
Part b: Reject B using nonzero total baryon number.
Part b: Assess C using cancelling charge and lepton numbers.
Part b: Reject D using nonzero electric charge.
Part c: Distinguish necessary conservation tests from a guarantee of occurrence.
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