Figure 1: Thermometry with Upsilon mesons. The Upsilon ($\mathrm{{\rm Y}}$) 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 ($\mathrm{{\rm Y}}(2S)$ and $\mathrm{{\rm Y}}(3S)$) are progressively less tightly bound and have a larger effective radius ($<r>$) than the $\mathrm{{\rm Y}}$ ground state ($\mathrm{{\rm Y}}(1S)$) 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.