The color screening of quarkonia states like $J/\psi$ (bound states of charm and anti-charm) and $\Upsilon$ (bound states of beauty and anti-beauty) is considered one signal of QGP (quark-gluon plasma) formation. This signal is actively measured by experiments at LHC and RHIC.
Why don't we measure the screening on the bound states of light quarks like pions?
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2 Answers
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From the introduction to this paper, it's because bound states of heavy quarks have smaller radii than mesons and nucleons. This allows colour screening to effectively shield the whole state, whereas it would not manage to do so for larger structures.
answered Sep 14, 2019 at 19:53
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$\begingroup$ pion have same radius as of J/psi and Upsilon states $\endgroup$ Commented Sep 14, 2019 at 21:06
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Charmonium($J/\psi$) and bottomonium($\Upsilon$) states are not produced as abundantly as pions, kaons and other lighter particles. On an average 30 $c\bar{c}$ pairs are produced in a heavy-ion collision. The number of $b\bar{b}$ pair will be even lesser. The lighter quarks do not hadronize until freezeout. They remain unbound and constitutes the hydrodynamic flow of QGP. Whereas quarkonia can hadronize within QGP. Hence if charm quarks are screened in QGP, we would observe lesser charmonium than otherwise. This will tell us something about the QGP. Lighter particles do suffer screening in QGP, but they are not helpful in providing any information about QGP as they won't hadronize in it.
