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If isospin were a perfect symmetry, then the neutral pion would have equal $u\bar u$ and $d\bar d$ content, but since up quarks are slightly lighter than down quarks, the neutral pion, being the lightest neutral meson, should have a slightly higher proportion of $u\bar u$ than of $d\bar d$. What does experiment have to say about this? Is pion-neutron scattering (perhaps using deuterium targets) measurably different from pion-proton scattering?

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    $\begingroup$ Neutral pions have a lifetime around $0.1\,\mathrm{fs}$. You can't make a beam of them, so measurements would have to be productions measurements. I find lots of references to production cross-section measurements of various sorts, so there is probably sufficient data to extract an estimate, but I don't know of such a paper off the top of my head and a quick google didn't turn one up. $\endgroup$ – dmckee Dec 12 '17 at 22:37
  • $\begingroup$ Could it connect to this? $\endgroup$ – Cosmas Zachos Dec 12 '17 at 23:52
  • $\begingroup$ Or this? Apologies for the indiscriminate scattershot ... $\endgroup$ – Cosmas Zachos Dec 13 '17 at 0:01
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    $\begingroup$ @CosmasZachos Not just "no", but "not even close". Those two papers involve measurement of the quark content of nucleons and have nothing to do with mesons. Drell-Yan measurements are useful because you can build the detector to exclude meson junk. In E866 (the subject of the arXiv paper, and a project I was peripherally involved with but not at a level that would lead to authorship of their papers) that was accomplished by stacking up tons of copper, carbon and borated-plastic in the hollow of the motor-coach sized main magnet. $\endgroup$ – dmckee Dec 13 '17 at 5:25
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    $\begingroup$ (We built the absorber wall by passing bars, blocks and sheets of copper, graphite and borated-poly from hand-to-hand while sitting in a row inside the magnet. Not enormously hard work (except for the thesis student who was stacking the wall), but sweaty because the meson east experimental hall is only moderately well air conditioned.) $\endgroup$ – dmckee Dec 13 '17 at 5:29

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