I wonder eta and pion could exist containing up quark (anti up quark)? Isn't there pair annihilation in this meson?

  • $\begingroup$ Hi HERO, and welcome to Physics Stack Exchange! I've removed a number of comments that were attempting to answer the question and/or responses to them. Commenters, please keep in mind that comments should be used for suggesting improvements and requesting clarification on the question, not for answering. $\endgroup$
    – David Z
    Jul 2 '20 at 16:54

Indeed, they don't exist for long, as I'm sure you have checked from their lifetimes and decay modes.

The neutral pion has its valence quarks (which include your $\bar u u$) annihilate each other to produce two photons, in less than $10^{-16}$ s.

The η has the same thing happen about 40% of the time, but since it is so heavy, it can also have the quarks rearrange and combine with more pairs popping out of the gluonic vacuum of QCD to produce pions, which then decay. It is 3 orders of magnitude shorter-lived than the neutral pion!

  • $\begingroup$ but for pion+ known as $\pi^+$, do we have $u \overline{d}$ which already contains the antimatter $\overline{d}$? also +1 I voted up $\endgroup$ Jul 6 '20 at 15:04
  • $\begingroup$ you may also shed a little light physics.stackexchange.com/questions/563825 $\endgroup$ Jul 6 '20 at 15:04
  • $\begingroup$ The $\pi^+$ is 8 orders of magnitude longer lived than the $\pi^0$, since it decays weakly instead of electromagnetically, via a virtual $W^+$. That is, the $\bar d$ annihilates the u, maintaining B=0, quite nicely. All mesons are unstable, as they should be. $\endgroup$ Jul 6 '20 at 15:12
  • $\begingroup$ @annie the doublet-splitting problem is a long story, best suited for reviews. WP starts it, but it gets technical too quickly. $\endgroup$ Jul 6 '20 at 15:15
  • $\begingroup$ I am not sure I understand this "That is, the 𝑑¯ annihilates the u, maintaining B=0, quite nicely" @ Cosmas Zachos $\endgroup$ Jul 6 '20 at 17:55

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