Phys. Rev. Lett. 109, 222301 (2012)


For part of each data-taking year, CERN's Large Hadron Collider is given over to the acceleration of lead ions, rather than protons. These heavy-ion collisions potentially create conditions of such extreme temperature and density that the ion components 'melt' into a quark–gluon plasma (QGP). Heavy quarks, such as bottom quarks, that would usually bind into quark–antiquark pairs ('bottomonium', denoted Υ) in the interaction region are less likely to do so, their confining potential screened by the soup of quarks and gluons.

Hence the suppression of bottomonium states is a useful probe of QGP. Through a comparison of Υ production in proton–proton and lead–lead collisions collected in 2011, the CMS collaboration (S. Chatrchyan et al.) have updated their results proving the suppression of Υ states in heavy-ion collisions, relative to proton interactions. Moreover, they are now able to resolve three states — Υ(1S), Υ(2S) and Υ(3S) — and show that the suppression is sequential: the most excited, least tightly bound 3S state being most greatly suppressed, and the 1S state the least, as expected.