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

The measurement of four-particle cumulant elliptic flow coefficients $c_2\{4\}$ and $v_2\{4\}$ are presented using 0.17 $pb^{-1}$ p+p data at $\sqrt{s_{NN}}$ = 5.02 TeV, 0.9 $pb^{-1}$ p+p data at $\sqrt{s_{NN}}$ = 13 TeV and 28 $nb^{-1}$ p+Pb data $\sqrt{s_{NN}}$ = 5.02 TeV. The $c_2\{4\}$ are calculated as a function of average number of charged particles, $\langle N_{\text{ch}}\rangle$, using standard cumulant method, as well as the recently proposed two-subevent and three-subevent cumulant methods. The three-subevent method is found to be much less sensitive to the short range correlations originating mostly from jets and di-jets compared to the other two methods, which can lead to the observation of negative $c_2\{4\}$ and a real $v_2\{4\} = -\left( c_2\{4\}\right)^{1/4}$ in all collision systems. The magnitude of $c_2\{4\}$ is nearly independent of $\langle N_{\text{ch}}\rangle$. The $v_2\{4\}$ is found to be smaller than the $v_2\{2\}$ measured from two-particle correlation method, as expected for long-range collective ridge. Following a recent model framework, the values of $v_2\{4\}$ and $v_2\{2\}$ are used to probe the number of sources in the initial state collision geometry.