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Why don't quark-antiquark pairs annihilate?

  1. Sep 27, 2011 #1
    i just learned that quarks are always paired with antiquarks, and i wonder what effect keeps them from turning into energy? is it some kind of quantum pressure? thanks
     
  2. jcsd
  3. Sep 27, 2011 #2
    Actually, this is not true. In baryons (e.g. proton) there are 3 'normal' quarks, so they are not going to annihilate themselves. Mesons, however, ARE made of quark-antiquark pairs, but they are not all made of the same flavour quark and anti-quark. Consider the pions: charged pions are made of up-antidown and down-antiup pairs. However, the neutral pion is made of a superposition of up-antiup and down-antidown. This means the charged pion quarks can't annihilate themselves but the neutral ones can.

    Now, I lied a little bit. The charged pions CAN annihilate themselves, but they first have to change say an antidown quark into an antiup quark, and they can only do that via the weak force. This makes the charged pions way more stable.

    Check out their relative lifetimes:

    http://en.wikipedia.org/wiki/Pion#Neutral_pion_decays

    The neutral pion decays a billion times faster than the charged pion! This is super cool I think :). Also I lied again a little bit, they don't really change an antidown into an antiup, the up and antidown can just annihilate straight away through a weak interaction, but this is still weak so happens way more slowly than if they could do it by strong or electromagnetic interactions.
     
    Last edited: Sep 27, 2011
  4. Sep 27, 2011 #3
    thanks for the information, that really cleared up a lot
     
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