Measurement of the Azimuthal Anisotropy of Neutral Pions in Pb-Pb Collisions at $\sqrt{s_{NN}}\mathbf{=}2.76\mathrm{TeV}$

First measurements of the azimuthal anisotropy of neutral pions produced in Pb-Pb collisions at a center-of-mass energy of $\sqrt{s_{NN}}=2.76\mathrm{TeV}$ are presented. The amplitudes of the second Fourier component ($v_2$) of the ${\pi }^0$ azimuthal distributions are extracted using an event-plane technique. The values of $v_2$ are studied as a function of the neutral pion transverse momentum ($p_T$) for different classes of collision centrality in the kinematic range $1.6<p_T<8.0\mathrm{GeV}/c$, within the pseudorapidity interval $|\eta |<0.8$. The CMS measurements of $v_2(p_T)$ are similar to previously reported ${\pi }^0$ azimuthal anisotropy results from $\sqrt{s_{NN}}=200\mathrm{GeV}$ Au-Au collisions at RHIC, despite a factor of $\sim 14$ increase in the center-of-mass energy. In the momentum range $2.5<p_T<5.0\mathrm{GeV}/c$, the neutral pion anisotropies are found to be smaller than those observed by CMS for inclusive charged particles.

Figure 1

Top panel: ${\pi }^0$ meson invariant mass distribution with combinatorial background for $2.5<p_T<3.0\mathrm{GeV}/c$ for (40–50)% centrality interval in Pb-Pb collisions at $\sqrt{s_{NN}}=2.76\mathrm{TeV}$. The solid line represents the combinatorial background estimated from the event-mixing technique. Middle panel: Combinatorial background-subtracted ${\pi }^0$ invariant mass distribution (solid circles) in the same $p_T$ bin and centrality interval. Open squares show HYDJET simulation results. Bottom panel: HYDJET simulation results obtained after rejecting converted photons. The HYDJET simulation results have been scaled up by a factor of 556 to match the data.

Figure 2

CMS ${\pi }^0$ meson $v_2$ (solid circles) compared to PHENIX ${\pi }^0$ meson $v_2$ [9] (open circles) for midrapidity ($|\eta |<0.8$ and $|\eta |<0.35$, respectively) and CMS charged particle $v_2$ (open squares, $|\eta |<0.8$). Results are presented as a function of $p_T$ for six centrality intervals [(20–30)% to (70–80)%]. Green (gray) shaded bands represent systematic uncertainties associated with CMS ${\pi }^0$ meson (charged particle) $v_2$ measurements. Only statistical uncertainties are shown for the PHENIX results. The systematic uncertainties for the (50–60)% centrality on the PHENIX data points are 10.4%.