Q191 · Practice questionComplex familiar6 marks
QUESTION 191 (6 marks)
The particle interaction diagram shows two electrons scattering by exchange of a wavy line. Time increases upward. Incoming and outgoing external particles are all electrons.
a)[2 marks]
Identify the exchanged gauge boson and the interaction.
b)[2 marks]
State and compare total electric charge, lepton number and baryon number before and after the interaction.
c)[2 marks]
Explain why substituting a gluon for the exchanged line would not describe a direct interaction between these external electrons.
WORKED SOLUTION
6 marksPractice marking scheme
ANSWER
(a) A photon; the electromagnetic interaction. (b) Both before and after: , , . (c) Electrons lack colour charge; gluons mediate the strong interaction between colour-charged particles.
Worked solution
(a) Electrons are electrically charged leptons and their electromagnetic scattering is mediated by photon exchange. The wavy internal line is the exchanged photon.
(b) There are two electrons on each side. Each has charge , lepton number and baryon number zero. Therefore all three totals are unchanged: .
(c) Electrons are leptons and do not carry the colour charge involved in the strong interaction. A gluon therefore cannot directly couple to the electron lines as shown. A quark, unlike an electron, carries colour charge.
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 photon.
Part a: Identify the electromagnetic interaction.
Part b: Calculate the incoming totals correctly.
Part b: Calculate the matching outgoing totals and identify conservation.
Part c: Identify gluons with the strong interaction.
Part c: Explain why electron external lines cannot directly couple through a gluon.
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