Measurement of the suppression and elliptic anisotropy of heavy flavor muons in Pb+Pb collisions at $\sqrt{s_{NN}} =2.76$ TeV with the ATLAS detector

ATLAS has measured the production of heavy flavor muons over $|\eta| {<} 1$ in $\sqrt{s_{NN}} = 2.76$ TeV Pb+Pb collisions and $\sqrt{s} = 2.76$ TeV $pp$ collisions at the LHC. The measurements are performed over the transverse momentum range $4 {<} p_T {<} 14$ GeV and for five Pb+Pb centrality intervals. Backgrounds arising from in-flight pion and kaon decays, hadronic showers, and mis-reconstructed muons are statistically removed using a template fitting procedure. The heavy flavor muon differential cross-sections and per-event yields are measured in $pp$ and Pb+Pb collisions, respectively. The nuclear modification factor, $R_{AA}$, is observed to be in- dependent of $p_T$ within uncertainties and to be less than unity, which indicates suppressed production of heavy flavor muons in Pb+Pb collisions. For the most central interval measured (0–10%), $R_{AA} {\sim} 0.4$. The heavy flavor muon yield is measured as a function of the azimuthal angle difference, $\phi − \Psi_2$, where the experimental event plane angle, $\Psi_2$, is measured in forward calorimeters that cover $3.2 < |\eta| < 4.9$. Fourier coefficients associated with the $\cos{(2(\phi − \Psi_2))}$ modulation, $v_2$, are measured as a function of $p_T$ and centrality. They vary slowly with $p_T$ and show a systematic variation with centrality that is characteristic of other elliptic anisotropy measurements.