# Correlations of azimuthal anisotropy Fourier harmonics with subevent cumulants in pPb collisions at $\sqrt{s_\mathrm{NN}} =$ 8.16 TeV

## TL;DR

This study measures event-by-event azimuthal anisotropy correlations in pPb collisions at 8.16 TeV using a subevent cumulant technique, revealing collective behavior at low multiplicities and differences from standard methods.

## Contribution

It introduces a subevent four-particle cumulant method to analyze azimuthal correlations, extending the understanding of collective effects in small collision systems at lower multiplicities.

## Key findings

- Anticorrelation of $v_2$ and $v_3$ at high multiplicity.
- Correlation strength depends on the number of subevents used.
- Evidence for collective multiparticle correlations at low multiplicity.

## Abstract

Event-by-event correlations of azimuthal anisotropy Fourier coefficients ($v_n$) in 8.16 TeV pPb collision data, collected by the CMS experiment at the LHC, are extracted using a subevent four-particle cumulant technique. Each combination of four charged particles are selected from either two, three, or four distinct subevent regions of a pseudorapidity range from $-$2.4 to 2.4 of the CMS tracker, and with transverse momentum between 0.3 and 3.0 GeV. Using the subevent cumulant technique, correlations between $v_n$ of different orders are measured as functions of particle multiplicity and compared to the standard cumulant method without subevents over a wide event multiplicity range. At high multiplicity, the $v_2$ and $v_3$ coefficients exhibit an anticorrelation; this behavior is observed consistently using various methods. The $v_2$ and $v_4$ correlation strength is found to depend on the number of subevents used in the calculation. As the event multiplicity decreases, the results from different subevent methods diverge because of different contributions of non-collective or few-particle correlations. Correlations extracted with the four-subevent method exhibit a tendency to diminish monotonically toward the lowest multiplicity region (about 20 charged tracks) investigated. These findings extend previous studies to a significantly lower event multiplicity range and establish the evidence for the onset of long-range collective multiparticle correlations in small system collisions.