Q11.

(a) Complete the table comparing some of the properties of the positive pion, π+, and the proton

Name	π+	Proton
Relative charge	+1	+1
Baryon number	0	+1
Quark composition	u ̅d̅	uud

 

pion is positive and not strange - therefore has 'u' and antidown

proton has a charge of +1 and is a non-strange baryon - must be uud

 

(b)     When a positive pion interacts with a proton, a kaon can be produced, along with another strange particle, as shown in this equation

 

What type of interaction is shown in this equation?

Strong nuclear interaction

Weak nuclear interactions involve leptons - no leptons here

or

the reaction consists of a strange particle decaying without conservation of strangeness - we have no strange quarks on the left hand side.... so it must be strong.

(1mark)

(c)     Deduce the relative charge, baryon number and strangeness of particle X.

conservation of charge:

+1 +1 → +1 + ?

charge of particle X must be +1

conservation of baryon number:

0 + 1 → 0 + ?

baryon number of particle X must be +1

conservation of strangeness:

0 + 0 → 1 + ?

the positive kaon is made of an up and antistrange quark combination, giving it a strangeness of +1

strangeness of particle X must be -1

(3marks)

(d)     Particle X can decay to produce a neutron and positive pion as shown in this equation

 

What type of interaction is shown in this equation?

Weak nuclear interaction

Equations that show no conservation of strangeness occur via the weak interaction

(1mark)

(e)     Explain your answer.

The strangeness of X is -1, the strangeness of the pion and neutron are both zero so strangeness is not conserved. This can only occur in weak interactions.

(2 marks)

(f)     The neutron and positive pion will then decay. The positive pion can decay into a positron and an electron neutrino. Write down the equation for the decay of the neutron.

Neutrons decay into protons (you did this in beta decay at GCSE... but now you know to add in the antineutrino to balance lepton number).

n → p + e^- + ̅ν̅_e

 

(2 marks)

(g)     Explain why no further decays occur.

The proton is a stable particle (in fact it is the only stable baryon!) and the electron and antineutrino are stable fundamental particles. None of them decay.

 (2 marks)

(Total 16 marks)