Measurements of azimuthal anisotropies at forward and backward rapidity with muons in high-multiplicity p-Pb collisions at $\sqrt{s_{\mathrm{N}\mathrm{N}}}=8.16$ TeV

The study of the azimuthal anisotropy of inclusive muons produced in p-Pb collisions at $\sqrt{s_{\mathrm{N}\mathrm{N}}}=8.16$ TeV, using the ALICE detector at the LHC is reported. The measurement of the second-order Fourier coefficient of the particle azimuthal distribution, $v_2$, is performed as a function of transverse momentum $p_{\mathrm{T}}$ in the 0-20% high-multiplicity interval at both forward ($2.03<y_{\mathrm{C}\mathrm{M}\mathrm{S}}<3.53$) and backward ($-4.46<y_{\mathrm{C}\mathrm{M}\mathrm{S}}<-2.96$) rapidities over a wide $p_{\mathrm{T}}$ range, $0.5<p_{\mathrm{T}}<10$ GeV/$c$, in which a dominant contribution of muons from heavy-flavour hadron decays is expected at $p_{\mathrm{T}}>2$ GeV/$c$. The $v_2$ 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 $v_2$ is measured at both forward and backward rapidities with a significance larger than $4.7\sigma$ and $7.6\sigma$, respectively, in the interval $2<p_{\mathrm{T}}<6$ GeV/$c$. Comparisons with previous measurements in p-Pb collisions at $\sqrt{s_{\mathrm{N}\mathrm{N}}}=5.02$ 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.