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My first idea is that number of $\pi ^+$ and $\pi ^-$ mesons in collision of heavy ions with some target made of heavy nuclei should be close to 50/50. But after that I start to think that mass of $d$ and $u$ quarks are different so maybe it will be easier for some mesons to be produced. Heavy ions have more neutrons then protons. Meaning that in nuclei there is. more d quarks than $u$ quarks and to create $\pi ^-$ we only need one anti $u$ quark. So in my thinking it is easier to produce u anti-$u$ pair and form $\pi ^-$. I know that putting it this way is very oversimplified, but I just started learning High energy physics and this question raised in my head. Maybe there are some articles about this topic that you can advise me to read?

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In the complicated environment of ion ion collisions charge conservation, which is the basis of your argument, is taken into account in complicated mathematical models, in order to fit the data. Yes, $π^+$ and $π^-$ distributions differ as you expect.

pions

A quantitative comparison of the results of a modern transport calculation (URQMD(15)) to pion spectra measured in Au+Au collisions at 1.5 AGeV (14) is presented in figure5. It can be seen that theory and experiment differ both in yield and spectral slope. The model calculation underestimates the yield of soft pions and overpredicts the number of hard pions.

By the way, I found the review paper by googling "heavy ion pion multiplicity LHC" . The quoted paper is from 2004, and there have been a lot of experiments at CERN since then. If you are interested in ion collisions you could search the cern document server.

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