Particle Questions that include Feynman Diagrams
Q8. A positron is emitted from a nucleus when a proton changes to a neutron in the nucleus.
The Feynman diagram for the quark interaction is shown in the diagram.
![](80a.png)
(a) Identify the particles labelled A, B, C and D in the diagram.
A = d (down quark) ![](../../../graphics/symbols/nuclides/ticksmall.png)
![](../../../graphics/equations/dataSheet/quarks.png)
B = w+ ![](../../../graphics/symbols/nuclides/ticksmall.png)
C and D (either way round) = e+ (positron) and νe ![](../../../graphics/symbols/nuclides/ticksmall.png)
(3 marks)
(b)
(i) State the interaction responsible for this process.
weak ![](../../../graphics/symbols/nuclides/ticksmall.png)
(1 mark)
(ii) State which letter in the diagram above represents an exchange particle.
B ![](../../../graphics/symbols/nuclides/ticksmall.png)
(1 mark)
(iii) State one difference between this exchange particle and a photon.
The w-boson has charge whereas a photon does not have charge. ![](../../../graphics/symbols/nuclides/ticksmall.png)
The w-boson has (rest) mass but the photon has zero (rest) mass. ![](../../../graphics/symbols/nuclides/ticksmall.png)
The photon has an infinite range but the range of the w+ boson is much shorter. ![](../../../graphics/symbols/nuclides/ticksmall.png)
(1 mark)
(c) Energy and momentum have to be conserved in this process.
State two other quantities that need to be conserved and show that they are conserved in the process.
|
u |
→ |
d |
+ |
e+ |
+ |
ve |
|
|
charge ![](../../../graphics/symbols/nuclides/ticksmall.png) |
+2/3 |
→ |
-1/3 |
+ |
1 |
+ |
0 |
balances |
|
lepton number ![](../../../graphics/symbols/nuclides/ticksmall.png) |
0 |
→ |
0 |
+ |
-1 |
+ |
+1 |
balances |
|
baryon number ![](../../../graphics/symbols/nuclides/ticksmall.png) |
+1/3 |
→ |
+1/3 |
+ |
0 |
+ |
0 |
balances |
|
(4 marks)
(Total 10 marks)