Measurement of correlated azimuthal anisotropy Fourier harmonics with subevent cumulants in pPb collisions at 8.16 TeV

Event-by-event correlations of azimuthal anisotropy Fourier coefficients ($v_n$) in 8.16 TeV pPb data are extracted using a subevent four-particle cumulant technique with the CMS experiment at the LHC. Each combination of four particles is required to fall into two, three or four distinct subevents within the full pseudorapidity range from -2.4 to 2.4 using tracks with transverse momentum between 0.3 and 3.0 GeV/c. Using the subevent technique, $v_n$ correlations are measured as a function of particle multiplicity and compared to the standard method without using subevents. At high multiplicity, $v_2$ and $v_3$ anticorrelation exhibits a consistent behavior among the various methods. The $v_2$ and $v_4$ correlation strength is found to depend on the number of subevents used in the calculation. Toward the low-multiplicity region, data from different methods diverge because of different contributions of few-particle correlations. The results presented represent a key step in understanding the onset of collective behavior from low to high multiplicities in small systems.