Adapted from A. Mocsy, Eur. Phys. J. C 61, 705 (2009)
Thermometry with Upsilon mesons. The Upsilon () 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 ( and ) are progressively less tightly bound and have a larger effective radius () than the ground state () 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.