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W- or W+ boson

Discussion in 'Science' started by Orion, Dec 18, 2011.

  1. Orion

    Orion New commenter

    Here is a question I have searched low and high for and I cannot seem to find an answer. When looking at Feynman diagrams for AQA AS I have an issue that when you look at electron capture why use the W+ boson instead of W- when looking at a proton-electron collision.
    When you look at cons of charge etc.. the electron capture is wrong with W+
    The answer I got from the textbook is simply that p-e collision is high energy so W- is used so not convincing!
    Any ideas, this is a self taught topic that I never did at Uni so I am a bit blind.
    Also I need an AS answer as to why this is, as all the others can be reasoned out with cons of charge/mass etc..
  2. AshgarMary

    AshgarMary New commenter

    Does this help you at all?

    http://www.cyberphysics.co.uk/topics/particle/feynman.htm#electron capture

    MANY years (28!) since I looked at this, so I really need to brush it up! Thanks for bringing to my attention, but anyway, here goes:

    BEFORE : proton + electron (overall charge neutral)

    Step 1: proton changes to neutron and releases W+ boson -> neutron + (W+) + electron (overall charge neutral)

    Step 2: boson and electron combine and form neutrino -> neutron + neutrino (overall charge neutral)

    PS If I have misremembered please be gentle, as I say, it's been a LONG time!
  3. Word of warning -if you are using the AQA AS physics book by Jim Breithaupt there are errors in the Feynman diagrams & descriptions. There ia also some unhelpful 'vagueness' when the book deals with muons. Use another source to help you teach.
    You could try to write a polite letter to JB pointing this out. We pointed out an error in the Waves section of the A2 book & were told (to paraphrase) that it did not matter since the exam papers accommodated the misleading information. In other words, he knew about the error & fixed the exam questions (he is the Chief Examiner after all) so that the pupils who had learned the incorrect physics (from his book) & reproduced it in the exam were awarded full marks. So, now we are left wondering how pupils who are taught the correct physics fare in exams written by this Chief Examiner.
    As far as I know we have not pointed out the Feynman errors to the author of the book. Given the conflict of interest between making money & teaching correct physics, I fear the former takes precedence in the Kafkaesque world of Exam boards, Chief examiners & text book authors.

  4. Orion

    Orion New commenter


    Thanks for that, do you mean about the arrows for bosons and also what I said before?

    I did note that the muon section is too brief and I had to add some more.

    What did you guys spot?


  5. Orion

    Orion New commenter

    Also what was the waves error you found, can you remember page and para etc..

    I noticed a massive formulae error for fields which confused all of us.

  6. Some of the errors I have noticed:
    AS book page 15:diagram for beta-plus decay. Neutron should be labelled proton & proton should be labelled as a neutron.
    AS book page 25 : quark combination of an antiproton is given incorrectly about half way down the page.
    AS book page 25: Text & both diagrams underneath 'Quarks & beta decay' heading are incorrect: the beta-minus decay should be accompanied by an antineutrino while beta-plus decay is accompanied by a neutrino.
    The whole Feynman section: when I studied this at university, the antiparticles were drawn travelling backwards in time and bosons were also given direction with an arrow. Neither of these conventions seem to be included here. Am I just being fussy or have things moved on?
    AS book page 193: (spotted by a colleague - we teach different parts of the syllabus) Figure 1. the top 2 diagrams are incorrect, you always see some internal reflection. The middle diagram is doubly incorrect, you never see light refracted at 90 degrees. This line is a mathematically useful tool not a real physics phenomenon. The 'Examiner's tip' at the bottom of the page is correct but not supported by the diagram above it.
    Would be pleased to have details of any errors you have spotted. Thanks.

  7. Mis-remembered the 'waves' section. Thought it was included in A2, now realise it is AS - have lost track of it since it was not in the secion I was teaching!

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