CERN Accelerating science

The angular distribution of particles produced in relativistic heavy-ion collisions is commonly described in terms of their complex flow coefficients $V_n(\eta, p_{\text{T}})$. This description implicitly assumes that two-particle distributions of a single collision can be described by the product of the complex flow coefficients; a property commonly referred to as factorization. The amplitude and phase of the coefficients fluctuate event-by-event and thereby break the factorization assumption for distributions which are averaged over many events. Additionally, factorization may also be broken by non-flow processes such as di-jets. This analysis studies the factorization of sample-average two-particle distributions in the ($\eta_a, \eta_b$)-plane in Pb--Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV. The analysis is performed over the large pseudorapidity range of $-3 < \eta < 5$ by combining the Forward Multiplicity Detector (FMD) and the Inner Tracking System (ITS) of the ALICE detector in a novel analysis method. The original factorization assumption is found to hold for particle pairs with a minimal longitudinal separation of $\Delta\eta_{\text{min}} = 2.6 \pm 0.2$. A modified factorization assumption which accounts for a $|\Delta \eta|$-dependent attenuation of the two-particle Fourier coefficients due to fluctuations is also investigated. The attenuation effect is quantified by the empirical parameter $F_2^\eta$ which is found to be in agreement with previous CMS observation at $\sqrt{s_{\rm NN}} = 2.76$ TeV as well as with AMPT model calculations.