Azimuthal anisotropy of charged hadrons at very high $p_{T}$ in PbPb collisions at 2.76 TeV with CMS

Measurements of the azimuthal anisotropy of charged hadrons are presented for PbPb collisions at 2.76 TeV over an extended transverse momentum range up to approximately 60 GeV/c. The data were collected with the CMS detector at the LHC. Utilizing a novel and unique high-$p_{T}$ single-track high-level trigger, the analysis explores the full 2011 PbPb data set corresponding to an integrated luminosity of 150$\mub^{-1}$. Anisotropy parameters ($v_{2}$, $v_{3}$ and $v_{4}$) are extracted by correlating charged tracks with the event plane reconstructed using the energy deposited in the forward calorimeters. By utilizing the broad coverage of the CMS forward calorimetry, contamination from back-to-back dijets is suppressed. The results presented in this talk significantly improve on the statistical precision of previous $v_{2}$ measurements for $p_{T} > 12$ GeV/c, and explore for the first time the harmonic components of the azimuthal dependence in the very high $p_{T}$ region beyond 20 GeV/c. Beyond $p_{T} > $10 GeV/c, the observed $v_{2}$ values show a moderate decrease with $p_{T}$, being consistent with zero only above $p_{T}\approx40$ GeV/c and for mid-central (30-60\%) collisions. A common trend in the centrality dependence of $v_{2}$ is observed for particles over a wide range of $p_{T}$ up to approximately 48 GeV/c that is independent of pseudorapidity, suggesting a potential connection to the initial geometry. These new data can impose quantitative constraints on the details of in-medium parton energy loss models, particularly the influence of the path length and the shape of the interaction region.