# Number of $\pi ^+$ and $\pi^-$ in collision experiments

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?

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. 