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Particle Physics Questions that involve Feynman Diagrams

Q3. decays, forming a new nucleus, by releasing a particle and one other particle which is difficult to detect.

In beta-plus (positron) decay:

  • a proton changes into a neutron - baryon number therefore balances
  • proton and positron are both positive therefore there is already a balance of charge.
  • lepton number doesn't balance - we have an anti-lepton (positron) so we also need a lepton, that must be neutral, so as not to disturb the balance of the charge - must be a neutrino (or the electron variety!)

(a) Name the particle which is difficult to detect. (1 mark)

neutrino

(b) Write down the proton number and the nucleon number of the new nucleus. (clearly indicate which is which!) (2 marks)

proton number = 10 (one less as a proton changed into a neutron)

nucleon number = 22 (the same - as proton loss is balanced out by neutron gain)

(c) Name the baryon and each of the leptons (clearly indicate which is a baryon and which is a lepton) formed as a result of this decay. (3 marks)

Note that it says 'name' - they want the word not the symbol - no marks for the symbols alone!

baryon = neutron

lepton = positron

lepton = (electron) neutrino

(d) Give the quark structure for

(i) a neutron and udd

(ii) a proton. uud

(1 mark)

(e) Complete the following Feynman diagram so that it represents decay.

(3 marks)
(Total 10 Marks)

To learn how to work out how to draw these diagrams go to my page on Feynman diagrams - then use my powerpoint it takes you through the reasoning step by step!