| MadSci Network: Physics |
Hi Simon,
No, although this point confused me a lot when I was taking particle physics. The neutral pion (pi0) is made of a mixture of up-antiup pairs and down-antidown pairs. Its mass is about 135 MeV. A mixture of up-antiup, down-antidown, and strange-antistrange results in an eta meson. The eta mass is larger, about 547 MeV.
Why can't you mix strange quarks into a pion? And why can't you have an isolated, distinguishable up-antiup meson, down-antidown meson, or strange-antistrange meson? The answer has to do with symmetry. You can combine these three quark flavors in a large number of different pairs (up-antidown, up-antiup, strange-antidown, etc.). If two pairs have the same charge and the same overall flavor, nature allows you to "mix" them together quantum mechanically. Many of the mesons are in fact mixtures of different allowable quark combinations. There are two ways of mixing: symmetric and antisymmetric. It turns out that you can't have a u-ubar, d-dbar, and s-sbar all together in an antisymmetric state, so the antisymmetric mesons (pi, b, rho, a) mix together just uu and dd. (Why not just uu and ss, or just dd and ss? The answer to this is not so clear---it seems to "just work out" that way, due to the quark's specific masses and couplings.) You can have ss in the symmetric mesons (eta, eta', h, h', omega, phi, f, and f') but I believe they are always mixed, to some extent, with uu+dd.
To learn more about this---in addition to searching the Madsci.org archives---you may have to go straight to the horse's mouth, the Particle Data Group. This is a group of physicists who compile and carefully average together experimental results from all over the world. They also write (for a technical audience) concise summaries of current theory. Here's a link to their quark model summary in PDF format.
Hope this helps,
-Ben Monreal
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