## 20140505

### Online reading assignment: Feynman diagrams

Physics 205B, spring semester 2014
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.
"Each line is a particle and when they meet its an interaction. Lines are matter particles and wavy lines are photons."

"I totally understood what you can and cannot do. You can draw the arrow lines so that they connect as in a 'V' and they connect at the end of a squiggly line. I also know that all lines must be connected and that crazy lines/curves can be simplified."

"I am pretty lost."

"It's interesting to me to entertain the idea of a positron just being an electron moving backwards in time. It intuitively sounds impossible but its fun to think about nonetheless."

"Kinda liking these! I think I understand just about all of the four rules, (two lines, both meet a single squiggly line, diagrams should only contain connected sections with at least one vertex, and ridding our graph of excess curves).

"The direction on the arrows must point in the same direction. The line with arrows are matter particles, and the wiggly line is a force particle."

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.
"Not always 100% on what is correct when using the diagrams."

"I had a pretty hard time following the links provided. Particularly when they went into the background behind where the diagrams came from."

"I dont understand the diagrams and what the paths represent. I don't know how to determine what path will be valid or invalid for the diagram."

"Feynman diagram vertices."

"I'm worried that I didn't understand all of it because normally I don't and this time It seemed simple."

Draw each of these Feynman diagram vertices. Then determine whether these vertices are valid or invalid.
(Only correct responses shown.)
In = wiggly line; out = two straight lines: valid. [58%]
In = two straight lines; out = wiggly line: valid. [61%]
In = straight line; out = two wiggly lines: invalid. [83%]
In = two wiggly lines; out = straight line: invalid. [83%]
In = straight line; out = straight line, wiggly line: valid. [75%]
In = straight line, wiggly line; out = straight line: valid. [64%]
In = wiggly line; out = straight line, wiggly line: invalid. [69%]
In = straight line, wiggly line; out = wiggly line: invalid. [83%]

Describe how the path of an electron and the path of a positron are drawn differently on a Feynman diagram.
"The path of an electron goes from left to right, the positron goes from right to left."

"The electrons are denoted by the solid lines with the arrow pointing in the direction of travel. The antiparticles on the other hand are also denoted by the solid lines but the arrow in their case is reversed. The virtual particles, like photons, are represented either by the wavy or broken line."

"Positrons are drawn as electrons moving backwards in time."

Describe what will happen if an electron meets a positron.
"They will be annihilated?"

"They create a photon."

When reading Feynman diagrams, time runs from:
 bottom to top. [0] top to bottom. [0] left to right. ******************************** [32] right to left. [0] (Unsure/guessing/lost/help!) **** [4]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"It could very well be just me very exhausted this weekend but this was the first assignment that put me to sleep. Definitely am having to go over it a second time."

"I had a hard time following the theory stuff."

"Can we please practice these and go over the information in that second link? Thanks!"

"There has to be something more to this! How can matter and antimatter meet and disappear to nothing? If all matter is made with and then destroyed by antimatter, how does everything exist? I don't get this. Life is so complicated lol." (The equal amounts of matter and antimatter generated from pair production is one of the fundamental questions of cosmology. There are theories that slightly more matter than antimatter had to have been produced in the early universe, such that all matter today is what was leftover after all antimatter was annihilated by almost all of the matter. Indications of this matter-antimatter imbalance may have been observed at the Large Hadron Collider.)