Azimuthal Anisotropy of Charged Particles at High Transverse Momenta in Pb-Pb Collisions at $\sqrt{s_{NN}}\mathbf{=}2.76\mathrm{TeV}$

The azimuthal anisotropy of charged particles in Pb-Pb collisions at $\sqrt{s_{NN}}=2.76\mathrm{TeV}$ is measured with the CMS detector at the LHC over an extended transverse momentum ($p_{\mathrm{T}}$) range up to approximately $60\mathrm{GeV}/c$. The data cover both the low-$p_{\mathrm{T}}$ region associated with hydrodynamic flow phenomena and the high-$p_{\mathrm{T}}$ region where the anisotropies may reflect the path-length dependence of parton energy loss in the created medium. The anisotropy parameter ($v_2$) of the particles is extracted by correlating charged tracks with respect to the event-plane reconstructed by using the energy deposited in forward-angle calorimeters. For the six bins of collision centrality studied, spanning the range of 0–60% most-central events, the observed $v_2$ values are found to first increase with $p_{\mathrm{T}}$, reaching a maximum around $p_{\mathrm{T}}=3\mathrm{GeV}/c$, and then to gradually decrease to almost zero, with the decline persisting up to at least $p_{\mathrm{T}}=40\mathrm{GeV}/c$ over the full centrality range measured.

Figure 1

The single-particle azimuthal anisotropy $v_2$ as a function of the charged particle transverse momentum from 1 to $60\mathrm{GeV}/c$ with $|\eta |<1$ for six centrality ranges in Pb-Pb collisions at $\sqrt{s_{NN}}=2.76\mathrm{TeV}$, measured by the CMS experiment (solid markers). Error bars denote the statistical uncertainties, while the gray bands correspond to the small systematic uncertainties. Comparison to results from the ATLAS (open squares) and CMS (open circles) experiments using data collected in 2010 is also shown.

Figure 2

The single-particle azimuthal anisotropy $v_2$ as a function of the number of participating nucleons with $|\eta |<1$ (solid circles) and $1<|\eta |<2$ (open squares) for six selected $p_{\mathrm{T}}$ ranges in Pb-Pb collisions at $\sqrt{s_{NN}}=2.76\mathrm{TeV}$, measured by the CMS experiment. Error bars denote the statistical uncertainties, while the gray bands correspond to the systematic uncertainties.