Q283 · Practice questionComplex familiar4 marks
QUESTION 283 (4 marks)
In a neutron, a down quark changes into an up quark by emitting a boson. The boson produces an electron and an electron antineutrino. Up and down charges are and .
a)[2 marks]
Show quantitatively that electric charge and baryon number are conserved at the quark vertex.
b)[2 marks]
Explain why the antineutrino is required for lepton-number conservation, and why conservation does not imply a visually mirror-symmetric diagram.
WORKED SOLUTION
4 marksPractice marking scheme
ANSWER
(a) Charge: . Baryon number: . (b) has ; has , balanced by with . Conservation concerns physical quantities, not drawing geometry.
Worked solution
(a) The initial d quark has charge and . The final u and have total charge and baryon number .
(b) The two final leptons have total L = 0. In this syllabus context, symmetry is connected to conservation principles; it does not require matching shapes on either side of a page. Vertex and overall totals, rather than visual appearance, are the relevant checks.
Equivalent physically justified methods, alternative valid sketches and consistent units accepted.
Accept sensible significant figures and consistent rounding from stated constants.
Part a: Show equal initial and final electric charge.
Part a: Show equal initial and final baryon number.
Part b: Explain the +1/-1 lepton-number balance.
Part b: Distinguish conserved physical quantities from visual mirror symmetry.
Practice question aligned to the current QCAA syllabus; review the worked solution and marking criteria.
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