CMS observes Y suppression in lead–lead collisions

The heavy-ion collision data collected in November 2010 at the LHC continue to provide exciting new physics results. Recently, at the time of the Quark Matter 2011 conference, the CMS collaboration released the first results on the observation of a suppression of the excited Υ resonances in the lead–lead collisions at 2.76 TeV per nucleon pair. The suppression of heavy quarkonia is considered to be one of the “candle” signatures for the possible formation of a quark gluon plasma (QGP).

The Υ, a quarkonium system consisting of a bottom and an antibottom quark, exists in three states known as 1S, 2S and 3S, in decreasing order of how tightly the quarks are bound. The 1S is the ground state of the Υ, while the others are excited states. Because they are more loosely bound, the 2S and 3S states are less likely to survive in QGP matter. This means that the number of Υ(2S) and Υ(3S) particles observed relative to Υ(1S) in heavy-ion collisions is expected to be less than the corresponding numbers from proton collisions.

CMS studied pairs of muons that are part of the post-collision debris in the detector, in which pairs of muons produced from the decays of particles such as the Υ will outnumber the pairs that are created by random processes. Thanks to the excellent momentum resolution of the CMS detector, a spectrum can be produced from the masses of each pair, with clear peaks corresponding to the masses of the particles from which they decayed.

The results show a dramatic difference in the number of Υ(2S) and Υ(3S) produced in the heavy-ion and proton–proton collisions. From the data collected from both runs at 2.76 TeV, CMS has observed that the relative production of the excited states of the Υ particle in heavy-ion collisions is only about 30% that of the comparable rates from proton collisions, with an uncertainty of about 20% (see figure). The probability of obtaining the measured value, or a lower one, if the true double ratio of the heavy ion and proton results is unity, has been calculated to be less than 1%.

The CMS collaboration is looking forward to the next lead–lead run later this year when more data will allow study of the suppression of the excited Υ states with even higher statistics.