Measurements of azimuthal anisotropies at forward and backward rapidity with muons in high-multiplicity p–Pb collisions at s_NN=8.16 TeV

The study of the azimuthal anisotropy of inclusive muons produced in p–Pb collisions at s_NN=8.16 TeV, using the ALICE detector at the LHC is reported. The measurement of the second-order Fourier coefficient of the particle azimuthal distribution, v₂, is performed as a function of transverse momentum p_T in the 0–20% high-multiplicity interval at both forward (2.03<y_CMS<3.53) and backward (−4.46<y_CMS<−2.96) rapidities over a wide p_T range, 0.5<p_T<10 GeV/c, in which a dominant contribution of muons from heavy-flavour hadron decays is expected at p_T>2 GeV/c. The v₂ coefficient of inclusive muons is extracted using two different techniques, namely two-particle cumulants, used for the first time for heavy-flavour measurements, and forward–central two-particle correlations. Both techniques give compatible results. A positive v₂ is measured at both forward and backward rapidities with a significance larger than 4.7σ and 7.6σ, respectively, in the interval 2<p_T<6 GeV/c. Comparisons with previous measurements in p–Pb collisions at s_NN=5.02 TeV, and with AMPT and CGC-based theoretical calculations are discussed. The findings impose new constraints on the theoretical interpretations of the origin of the collective behaviour in small collision systems.