Q244 · Practice questionComplex unfamiliar5 marks
QUESTION 244 (5 marks)
A proton and an antiproton collide with enough total energy for any listed final state. Consider the three candidate channels below. Use electric charge, baryon number B and total lepton number L only; passing these tests does not guarantee that a process occurs.
| Channel | Final particles |
|---|---|
| I | |
| II | |
| III |
a)[3 marks]
Determine the total charge (in units of e), B and L for each candidate final state.
b)[2 marks]
Identify which channels are excluded by these conservation laws, and state whether the remaining channel is thereby guaranteed.
WORKED SOLUTION
5 marksPractice marking scheme
ANSWER
(a) I: . II: . III: . (b) I is excluded by B, II by charge and L. III passes these tests but is not guaranteed.
Worked solution
(a) A neutron has , , ; an antineutron has . An electron has and , while is neutral with . Adding each pair gives the stated triples.
(b) The initial proton-antiproton pair has , and . Channel I violates baryon conservation; channel II violates charge and total lepton conservation. Channel III matches all three totals. These are necessary conditions, not a prediction of which dynamically possible channel actually occurs.
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: Calculate all three totals correctly for channel I.
Part a: Calculate all three totals correctly for channel II.
Part a: Calculate all three totals correctly for channel III.
Part b: Identify I and II as excluded with the violated laws.
Part b: Recognise that III passes the tests but that conservation alone does not guarantee occurrence.
Practice question aligned to the current QCAA syllabus; review the worked solution and marking criteria.
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