1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Feynman diagram

  1. Nov 11, 2017 #1
    1. The problem statement, all variables and given/known data
    Hi there,
    I am a very beginner in particle physics and now I'm struggling with this question.
    Draw the lowest-order Feynman diagrams for e+e- --> W+W- process

    2. Relevant equations

    3. The attempt at a solution
    There are 3 answers as follows:
    Q1: 1st pic: Why did the gamma and Z take place in s -channel but not t -channel?
    Q2: 2nd pic: Is there a reason why (e-) neutrino is the force carrier? Couldn't it be other entities like gamma or
    Z or sth else?

  2. jcsd
  3. Nov 11, 2017 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Education Advisor

    Q1: You need the electrons going in and the Ws coming out. You can only do that with the s-channel with a photon or Z. To have the t-channel, you'd need to turn the diagram sideways, which would correspond to, for example, an electron and W+ interacting to produce the positron and W-.

    Q2: There are only certain allowed vertices. You can't have an electron, W, and photon at one vertex. That would violate conservation of lepton number, for one thing.

    Do you have a textbook? There's probably a page showing diagrams of the allowed vertices.
  4. Nov 11, 2017 #3
    I am just following my lecturer's note and he didn't say when we are going to use s or t-channel.I apologize but I'm really confused since this is my first course in particle physics, and I have no previous knowledge in QED so that a lot of course notes online are too difficult for me...:sorry:

    I'd like to know, if time is flowing from left to right, in both s and t channels? (1st and 2nd pic)
    To me, the only difference between 1st and 2nd pic is that:
    1) e- doesn't physically touch e+ in 2nd pic
    2) The exchange particle is different , but I don't know why the force carrier is a neutrino ( It could also be a neutrino in 1st pic)
    3) You mentioned the conservation of lepton number, so according to 2nd diagram, we now have ve,e- and W- at the lower vertex. So Le = 1 + 1 = 2
    At the upper vertex,Le = 1 - 1 = 0 ?
    Let say if ve is replaced by a photon, then L is still different at both vertices ( -1 and 1) ?

    Sorry...Please help:frown:
  5. Nov 11, 2017 #4


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Education Advisor


    Like I said in my first reply, there are only certain allowed vertices. Unless you recognize that fact, these diagrams are going to seem random to you.

    Take a look at http://www.hep.phy.cam.ac.uk/~chpotter/particleandnuclearphysics/Lecture_05_FeynmanDiagrams.pdf. It may help you make sense of what's going on.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted