Andre Yoon (Massachusetts Inst. of Technology (MIT))
The nuclear modification factor R_AA is one of the key signatures for the energy loss of fast partons traversing a QCD medium. Charged particle transverse momentum (pT) spectra have been measured by CMS for pp collisions at sqrt(s)=0.9 and 7 TeV, corresponding to integrated luminosities of 231 ub-1 and 2.96 pb-1, respectively. Calorimeter-based high-transverse-energy jet triggers are employed to enhance the statistical reach of the high-pT measurements. The results are compared to various generator tunes and also to an empirical scaling of different collision energies with xT=2pT/sqrt(s) over the pT range up to 200 GeV/c. Using a combination of xT scaling and direct interpolation at fixed pT, a reference pT spectrum at sqrt(s)=2.76 TeV is constructed from pT = 1-100 GeV/c with less than 13% systematic uncertainty. We have obtained R_AA in bins of collision centrality for a PbPb data sample with an integrated luminosity of 7 ub-1, dividing by the interpolated reference spectrum. As seen at lower energies, the charged particle spectrum in central PbPb collisions is suppressed by at least a factor of five (R_AA < 0.2) compared to binary scaling around pT = 5-10 GeV/c. Above pT = 10 GeV/c, however, there is a significant rise in the nuclear modification factor, which will be presented out to pT = 80 GeV/c.
Collaboration CMS (UCLouvain)