Adapted from A. Mocsy, Eur. Phys. J. C 61, 705 (2009)

Figure 1: Thermometry with Upsilon mesons. The Upsilon ($Υ$) meson (a bound state of a bottom quark and an antibottom quark) and its excited states emerge from the heavy-ion collisions that produce the quark-gluon plasma. However, the free quarks and gluons in the plasma weaken the binding force of the mesons: The more highly excited Upsilon states ($Υ(2S)$ and $Υ(3S)$) are progressively less tightly bound and have a larger effective radius ($$) than the $Υ$ ground state ($Υ(1S)$) and are therefore more highly suppressed by the hot plasma. The relative suppressions of the different states can, in principle, be used to measure the plasma’s temperature.