Measurement of multi-particle azimuthal correlations with the subevent cumulant method in pp and p+Pb collisions with the ATLAS detector

The measurement of four-particle cumulant and anisotropic elliptic flow coefficient for the second harmonic, c2{4} and v2{4}, are presented using pp data at s=5.02 and s=13 TeV and p+Pb data at sNN=5.02 TeV. The values of c2{4} are calculated using the standard cumulant method and recently proposed two and three subevent methods. The three subevent method gives a negative c2{4}, and thus a well-defined v2{4} (which equals (−c2{4})1/4), in these collision systems. v2{4} is found to be smaller than v2 measured using the two-particle correlation method (denoted as v2{2}), as expected for a collective behavior. Following a recent model framework, the measured values of v2{4} and v2{2} are used to probe the number of sources in the initial state collision geometry.