Q190 · Practice questionComplex unfamiliar6 marks
QUESTION 190 (6 marks)
The interaction diagram shows a down quark in a neutron changing to an up quark. The emitted boson produces an electron and an unidentified particle X. A neutron is and a proton is . Leptons have lepton number and antileptons have .
a)[2 marks]
Identify X and justify using lepton-number conservation.
b)[2 marks]
Identify the boson and the fundamental interaction it mediates.
c)[2 marks]
Write the complete neutron decay and state the initial and final baryon numbers.
WORKED SOLUTION
6 marksPractice marking scheme
ANSWER
(a) X is an electron antineutrino, . (b) ; the weak interaction. (c) ; total on both sides.
Worked solution
(a) The neutron initially has . The outgoing electron has , so X must have . In neutron beta decay, the neutral partner is the electron antineutrino. This gives total final lepton number zero.
(b) The down quark changes charge from to , so the emitted boson has charge to conserve charge: . W bosons mediate the weak interaction.
(c) Changing one to turns into , producing a proton. Thus . The neutron and proton each have baryon number one, while the emitted leptons have zero baryon number.
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: Identify the electron antineutrino.
Part a: Justify with cancelling electron and antineutrino lepton numbers.
Part b: Identify the negatively charged W boson.
Part b: Identify the weak interaction.
Part c: Write the complete decay with correct particle identities.
Part c: Account for baryon number one before and after.
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