  In order to study further the long-range correlations ("ridge") observed
recently in p+Pb collisions at sqrt(s_NN) =5.02 TeV, the second-order azimuthal
anisotropy parameter of charged particles, v_2, has been measured with the
cumulant method using the ATLAS detector at the LHC. In a data sample
corresponding to an integrated luminosity of approximately 1 microb^(-1), the
parameter v_2 has been obtained using two- and four-particle cumulants over the
pseudorapidity range |eta|<2.5. The results are presented as a function of
transverse momentum and the event activity, defined in terms of the transverse
energy summed over 3.1<eta<4.9 in the direction of the Pb beam. They show
features characteristic of collective anisotropic flow, similar to that
observed in Pb+Pb collisions. A comparison is made to results obtained using
two-particle correlation methods, and to predictions from hydrodynamic models
of p+Pb collisions. Despite the small transverse spatial extent of the p+Pb
collision system, the large magnitude of v_2 and its similarity to hydrodynamic
predictions provide additional evidence for the importance of final-state
effects in p+Pb reactions.
