1. Physics 4 – April 23, 2019 P3 Challenge – Consider the reaction
Get out #28-38 for
Hmk check
P3 Challenge –
Consider the reaction
e- + p+  n + e
If this a valid reaction, demonstrate how each of the 4 conservation laws are validated. If not, state which conservation law or laws it violates and validate all others.
Objective:
7.3 Structure of Matter
Assignment:
p307 #39-46
Agenda:
Homework Review
Feynman Diagrams
Examples
Feynman Diagrams Activity

2. Topic 7: Atomic, nuclear and particle physics 7
Topic 7: Atomic, nuclear and particle physics 7.3 – The structure of matter
Feynman diagrams
Richard Feynman developed a graphic representation of particle interactions that could be used to predict the probabilities of the outcomes of particle collisions.
A typical Feynman diagram consists of two axes: Space and Time:
© 2006 By Timothy K. Lund
FYI
Some books switch the space and time axis. The IB presentation is as shown above.
SPACE
TIME

3. Topic 7: Atomic, nuclear and particle physics 7
Topic 7: Atomic, nuclear and particle physics 7.3 – The structure of matter e-
Feynman diagrams
Consider two electrons approaching one-another from the top and the bottom of the page…
A purely spatial sketch of this interaction would look like this:
But if we also apply a time axis, the sketch would look like this:
The Time axis allows us to draw the reaction in a spread-out way to make it clearer.
The bubble of ignorance e- e-
SPACE
© 2006 By Timothy K. Lund e- e-
FYI
The “bubble of ignorance” is the actual place in the plot that exchange particles do their thing.
Ingoing and outgoing particles are labeled.
TIME e-

4. Topic 7: Atomic, nuclear and particle physics 7
Topic 7: Atomic, nuclear and particle physics 7.3 – The structure of matter
Feynman diagrams
Particles are represented with straight arrows, as were the two electrons in the previous electron-electron interaction.
Exchange (force) particles are represented with either wavy lines (photons, W+, W- and Z0), or curly lines (gluons).
Particle
Electromagnetic and weak exchange
Strong exchange
© 2006 By Timothy K. Lund
FYI
You may have noticed that the electromagnetic exchange particle and the weak exchange particles all have the same wavy symbol.
Indeed, it has been found that the two forces are manifestations of a single ELECTRO-WEAK force.

5. Topic 7: Atomic, nuclear and particle physics 7
Topic 7: Atomic, nuclear and particle physics 7.3 – The structure of matter
EXAMPLE:
The complete Feynman diagram showing the repulsion of two electrons looks like this:
Here is a diagram for one electron emitting a photon: e- e-
SPACE  e- e-
TIME e- e-
© 2006 By Timothy K. Lund
SPACE 
TIME

6. Topic 7: Atomic, nuclear and particle physics 7
Topic 7: Atomic, nuclear and particle physics 7.3 – The structure of matter
EXAMPLE:
In a Feynman diagram, antimatter points backward in time. This diagram represents two positrons repelling each other:
Here is a diagram for one positron emitting a photon: e+ e+
SPACE  e+ e+
TIME e+
© 2006 By Timothy K. Lund e+
SPACE 
TIME

7. Topic 7: Atomic, nuclear and particle physics 7
Topic 7: Atomic, nuclear and particle physics 7.3 – The structure of matter
EXAMPLE:
Here is a photon producing an electron-positron pair.
Here is an electron-positron pair annihilating to become a photon: e-
SPACE  e+
TIME e+
© 2006 By Timothy K. Lund
SPACE  e-
TIME

8. Topic 7: Atomic, nuclear and particle physics 7
Topic 7: Atomic, nuclear and particle physics 7.3 – The structure of matter
EXAMPLE:
Here is a diagram of a down quark emitting a W particle that decays into an electron and an antineutrino: d u e W-
SPACE e-
TIME
© 2006 By Timothy K. Lund
FYI
One can use Feynman diagrams to map out complete processes – including the bubble of ignorance. Using the conservation rules and the exchange particles, you can predict what kind of processes can occur.

9. Topic 7: Atomic, nuclear and particle physics 7
Topic 7: Atomic, nuclear and particle physics 7.3 – The structure of matter
EXAMPLE: Explain what has happened in this Feynman diagram.
SOLUTION:
It is a diagram of a down quark emitting a W- particle that decays into an electron and an antineutrino:
Recall that a neutron consists of an up-down-down quark combo.
Recall that a proton consists of an up-up-down quark combo.
This is non other than the beta decay (- ) we talked about a long time ago. n p u u d d d u e
SPACE W- e-
TIME
© 2006 By Timothy K. Lund
n  p + e- + e

10. Topic 7: Atomic, nuclear and particle physics 7
Topic 7: Atomic, nuclear and particle physics 7.3 – The structure of matter
EXAMPLE: Write the reaction (including the neutrino) for beta (+) decay.
SOLUTION:
Just know it!
EXAMPLE: Now draw the Feynman diagram for the above + decay: p n u u d d u d
p  n + e+ + e e
SPACE W+ e+
© 2006 By Timothy K. Lund
TIME
FYI
Why is the neutrino not an anti-neutrino as in the - decay?
To conserve lepton number.

11. Feynman Diagrams Trick to reading them: Time flows from left to right.
Symbols.
Segment with arrow in direction of time to right, matter (labeled) often e-
Segment with arrow opposite direction of time, antimatter (labeled) often e+
Wavy line, photon
Helix line, gluon,
Wavy line, W+, W-, or Z0 labeled

12. Practice matching Matter - antimatter annihilation
Absorption of a photon by electron
Particle Pair production
Emission of a photon by electron
Absorption of a photon by antielectron
Now do the Feynman Diagrams Activity

13. Exit slip and homework
Exit Slip – Draw the Feynman Diagram for a up quark that emits a W+ exchange particle becoming a down quark. The W+ then splits into a positron and an electron neutrino.
What’s due? (homework for a homework check next class)
p307 #39-46
Start Studying for U14 Particle Physics Test on Apr 30 (next Tues)
What’s next? (What to read to prepare for the next class)
Read 7.3