We show that the incoherent elastic scattering of partons, as present in a multi-phase transport model (AMPT), with a modest parton-parton cross-section of $\sigma$=1.5-3 mb, naturally explains the long-range two-particle azimuthal correlations as observed in p+p and p+Pb collisions for all measured $N_{track}$ and $p_T$ bins at the Large Hadron Collider. We calculate the elliptic, $v_2$, and triangular, $v_3$, Fourier coefficients of the two-particle azimuthal correlation function in p+Pb and peripheral Pb+Pb collisions. Our results for $v_3$ are in a good agreement with the CMS data. The $v_2$ coefficient is very well described in p+Pb collisions and is underestimated for higher $p_T$ in Pb+Pb collisions. The characteristic mass ordering of $v_2$ in p+Pb is reproduced whereas for $v_3$ such ordering is not observed. We further predict the pseudorapidity dependence of the two-particle azimuthal correlation function. Predictions for the higher order Fourier coefficients, $v_4$ and $v_5$, in p+Pb are also presented. Our numerical results indicate an emergence of collectivity in p+p, p+Pb and peripheral Pb+Pb collisions from parton scatterings.

References:
[1] Guo-Liang Ma and Adam Bzdak, arXiv:1404.4129.
[2] Adam Bzdak and Guo-Liang Ma, arXiv:1406.2804.