## 20150504

### Online reading assignment: Feynman diagrams

Physics 205B, spring semester 2015
Cuesta College, San Luis Obispo, CA

Students have a weekly online reading assignment (hosted by SurveyMonkey.com), where they answer questions based on reading their textbook, material covered in previous lectures, opinion questions, and/or asking (anonymous) questions or making (anonymous) comments. Full credit is given for completing the online reading assignment before next week's lecture, regardless if whether their answers are correct/incorrect. Selected results/questions/comments are addressed by the instructor at the start of the following lecture.

The following questions were asked on reading presentations Feynman diagrams (Phillip "Flip" Tanedo, Cornell University/USLHC Collaboration) and quantum electrodynamics (QED) (Christopher "Bot" Skilbeck, cronodon.com).

Selected/edited responses are given below.

Describe what you understand from the assigned textbook reading or presentation preview. Your description (2-3 sentences) should specifically demonstrate your level of understanding.
"Squiggles are a photons, lines with arrows are positrons and electrons."

"Feynman diagrams are graphical ways to represent exchange forces. Each point at which lines come together is called a vertex, and at each vertex one may examine the conservation laws which govern particle interactions. Each vertex must conserve charge, baryon number and lepton number."

Describe what you found confusing from the assigned textbook reading or presentation preview. Your description (2-3 sentences) should specifically identify the concept(s) that you do not understand.
"Well, this is pretty weird!"

"Having trouble with understanding what the different parts of a Feynman diagram are and could use some help going over those as well."

"I am confused as to what exactly we are modeling. As mentioned, I suppose I need to see some examples worked out by hand."

"I didn't find anything confusing about the assigned reading. I understood all of the concepts."

"These diagrams seem confusing."

"All of the Feynman diagrams and how they differ. I think I'm stuck on a basic level of understanding."

When reading Feynman diagrams, time runs from:
 bottom to top. [0] top to bottom. **** [4] left to right. ********************************* [33] right to left. *** [3] (Unsure/guessing/lost/help!) ***** [5]

Describe how the path of an electron and the path of a positron are drawn differently on a Feynman diagram. (Note that both paths have the same "e" labels.)
"Electrons have an arrow pointing to the right, and positrons have an arrow pointing to the left."

"One goes in and the other goes out?"

Describe what will happen if an electron meets a positron.
"They annihilate each other."

"A photon is emitted."

The above (valid) Feynman diagram depicts an electron:
 absorbing a photon. *** [3] emitting a photon. ******************************* [31] annihilated by a positron. **** [4] (Unsure/guessing/lost/help!) ******* [7]

The above (valid) Feynman diagram depicts an electron:
 absorbing a photon. ***** [5] emitting a photon. *********** [11] annihilated by a positron. ********************* [21] (Unsure/guessing/lost/help!) ******** [8]

The above (valid) Feynman diagram depicts an electron:
 absorbing a photon. *********************** [23] emitting a photon. *** [3] annihilated by a positron. ******* [7] (Unsure/guessing/lost/help!) ************ [12]

Explain why the above Feynman diagram is invalid.
"Electrons cannot travel towards each other to form a photon?"

"Two electrons can't absorb each other."

"Both are drawn going in."

"They would be annihilated."

"No idea."

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"Nerd hieroglyphs."

"This is going to be an interesting topic I bet."

"Please draw out these diagrams and explain what the components mean."

"I'm just going to wait until I see something that confuses me."

"An electron is passing by a positron and says, 'You can have this photon, I'm tired of its backtalk.'"