PhysicsLAB: Particle Interactions and Feynman Diagrams

Background Information

On this worksheet, time is on the vertical axis, space is on the horizontal axis. Anti-particles "move" backwards in time so their arrows would point down, not up.

When drawing Feynman diagrams the following conventions are followed:

fermions are straight lines with arrows

photons are represented by wavy lines that can be horizontal since they occur at the speed of light (electromagnetic force)

bosons [W^-, W⁺, Z^o] are represented by dotted lines with a slight vertical tilt (weak nuclear force)

gluons are represented by spiraled lines (strong nuclear force)

At each vertex the following properties are always conserved:

charge [quarks carry a fractional charge of +/- 1/3]

baryon number [quarks are +1/3, anti-quarks are -1/3]

lepton number [electrons L_e, muons L_µ and tau L_t are +1 as well as their associated neutrinos while their anti-matter counterparts have lepton numbers of -1]

Protons always change into neutrons and vice versa.

Strangeness does not have to be conserved in weak interactions but must be conserved in strong nuclear interactions. Strangeness can only increase or decrease by 1 in a consecutive series of reactions.

1. At the left vertex, why should this weak interaction involve a W- boson?

2a. At the left vertex, why does this interaction require a W⁺ boson?

2b. Why are both arrows on the right side vertiex of the interaction pointed "downward?"

2c. Write the equation for this interaction.

2d. Time stamp lines can be used to verify the charges of particles participating in an interaction. At all "times" the total charge must be the same. What is the required total charge for all time stamp lines in this proton anti-neutrino interaction?

3a. Write this decay as an equation.

3b. In terms of quarks, a neutron has two down quarks and an up quark while a proton has two up quarks and a down quark. What is taking place in this interaction?

3c. A down quark has a charge of (-1/3)e while an up quark has a charge of (+2/3)e. Does the W^- exchange particle still make sense?

3d. What is another name for a beta-minus particle?

3e. Pair production occurs when matter is simultaneously created with anti-matter. Does pair production occur during a beta-minus decay?

3f. Historically, why was this pair production significant?

4a. Why is the arrow reversed on the beta-plus particle?

4b. Is baryon number conserved at the left vertex?

4c. Is lepton number conserved at the right vertex?

5a. What is meant by a virtual photon?

5b. Would the 'shape' of this Feynman diagram significantly 'change' if the protons were to be replaced with electrons allowing it to then become a representation of the repulsion between two electrons?

6a. Why is this interaction not called proton decay?

6b. Does this interaction obey conservation of baryon number?

6c. What is the required total charge for all time stamp lines in this electron capture interaction?

7a. Write the equation for muon decay.

7b. What is the required total charge for all time stamp lines for this muon decay?

7c. Confirm that Lepton number is conserved in this interaction.

8a. What is the required total charge for all time stamp lines for this annihilation?

8b. True of false. This particle interaction could have been drawn with a photon as the exchange particle.

9a. Shown above is a positive pion decaying. What quarks compose a positive pion?

9b. What is the charge of an anti-down quark?

9c. Is muon lepton number conserved in this decay?

10a. True or False. The diagram shown above could involve electrons?

10b. True or false. The top left particle has less energy compared to the bottom left particle.