## 20170501

### Online reading assignment: Feynman diagram vertices

Physics 205B, spring semester 2017
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).

The Big Bang Theory (Pilot)
Warner Bros. Television (2007)
bigbangtheory.wikia.com/wiki/User_blog:Sheldon_Cooper_PhD/Pilot

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.
"Feynman diagrams give us an easier way to understand the interactions of subatomic particles. Time is represented as going upwards and space is represented as going from left to right. These diagrams tell us what is happening between the interaction of two electrons."

"I was able to follow the drawing of Feynman diagrams, and I understand that if the arrow points from left-to-right, then it is an electron, but if the arrow is from right-to-left, then it is a positron. The wiggle line is a photon. Other than that it was kind of a lot of information to take in."

"Arrowed lines are electrons. Positrons while the wiggly lines are photons."

"You can only draw lines that touch if their arrows are not pointing in the same direction. One has to be going in while the other is moving out."

"The basic concepts and rules of Feynman diagrams, and basic examples."

"In theoretical physics, the Feynman diagrams is used as a representations of the mathematical expressions describing the behavior of subatomic particles using images."

"What’s really important are the endpoints of each line, so we can get rid of excess curves. You should treat each line as a shoelace and pull each line taut to make them nice and neat. They should be as straight as possible. (But the wiggly line stays wiggly!)"

"Not a single thing."

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.
"I'm still confused as to how orientation of the diagram relates to what is going on."

"Still a little unsure on how to read Feynman diagrams."

"I could use some more examples of the double-action diagrams because it's pretty hard just figuring out a one-vertex diagram."

"The diagrams and what the paths represent. I am having a difficult time to determine what path will be valid or invalid for the diagram."

"I need more clarification about the second part of the reading that talks about momentum conservation, loop diagrams, and attractive and repulsive forces. I want to be able to understand how it all fits with the diagrams that we learned to draw in the first section of the reading."

"I didn't find to much confusing. Although reading these diagrams can be challenging."

"Almost everything haha..."

When reading Feynman diagrams, time runs from:
 bottom to top. * [1] top to bottom. ** [2] left to right. ******************* [19] right to left. [0] (Unsure/guessing/lost/help!) *** [3]

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 move left to right, positrons move right to left."

"e are electrons and their arrows point from left to right and e+ are positrons (antimatter) and their arrows point in the other direction."

"The antiparticles are denoted by solid lines, but the arrow is reversed. The virtual particles, such as protons, can be seen as wavy or broken lines. Electrons are denoted by a solid lines with an arrow pointing in the direction of the travel"

"Drawn in opposite ways."

Describe what will happen if an electron meets a positron.
"An annihilation event occurs and gamma rays are produced."

"They will cancel each other out."

"From what I understand a photon will be produced."

"Boom!"

"Will turn into an Autobot or Decepticon?"

"They cancel each other out to make an 'anti-tron?'"

The above (valid) Feynman diagram depicts an electron:
 absorbing a photon. * [1] emitting a photon. ****************** [18] annihilated by a positron. ** [2] (Unsure/guessing/lost/help!) **** [4]

The above (valid) Feynman diagram depicts an electron:
 absorbing a photon. ***** [5] emitting a photon. **** [4] annihilated by a positron. ************* [13] (Unsure/guessing/lost/help!) *** [3]

The above (valid) Feynman diagram depicts an electron:
 absorbing a photon. *************** [15] emitting a photon. ** [2] annihilated by a positron. **** [4] (Unsure/guessing/lost/help!) **** [4]

Explain why the above Feynman diagram is invalid.
"One of the straight lines needs to point to the left to denote that it is a positron."

"The arrows are pointing towards the vertex."

"The lines are pointing in the same direction. One should point inwards and the other outwards."

"There must be one arrow going into the vertex and one arrow coming out."

"Electrons can not crash into each other?"

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"Help reading the diagrams and giving correct interpretations...like in this homework assignment."

"So can we create positrons?" (Well, you just need to find unstable isotopes that undergo beta-plus decay.)

"How do they detect antimatter when electrons collide with positrons?" (An electron-positron annihilating each other will emit gamma rays, which have energies exactly equal to the mass that "disappears" (E = m·c2). This is how a PET scan (positron emission tomography) works, you are injected with a sugar solution that has unstable fluorine isotopes, which undergoes beta-plus decay (emitting positrons), and when this happens in certain places in your body that metabolizes this sugar, those positrons combine with nearby electrons to annihilate, and emit gamma rays from that location that are triangulated by a surrounding detector.)

"Is a squiggly line always a gamma ray?" (Yes, or more generally, any type of photon (light).)

"Can we hard core review this in class?"