Abstract
The flow ansatz states that the single-particle distribution of a given event can be described in terms of the complex flow coefficients Vn. Multi-particle distributions can therefore be expressed as products of these single-particle coefficients; a property commonly referred to as factorization. The amplitudes and phases of the coefficients fluctuate from event to event, possibly breaking the factorization assumption for event-sample averaged multi-particle distributions. Furthermore, non-flow effects such as di-jets may also break the factorization assumption. The factorization breaking with respect to pseudorapidity η provides insights into the fluctuations of the initial conditions of heavy ion collisions and can simultaneously be used to identify regions of the phase space which exhibit non-flow effects. These proceedings present a method to perform a factorization of the two-particle Fourier coefficients VnΔ(ηa, ηb) which is largely independent of detector effects. AMPT model calculations of Pb-Pb collisions at TeV are used to identify the smallest |Δη|-gap necessary for the factorization assumption to hold. Furthermore, a possible Δη-dependent decorrelation effect in the simulated data is quantified using the empirical parameter . The decorrelation effect observed in the AMPT calculations is compared to results by the CMS collaboration for Pb-Pb collisions at TeV.
Highlights
IntroductionWhere vn are the flow coefficients and ψn are the symmetry planes
The Fourier coefficients of an event-sample averaged two-particle distribution are commonly described asVn∆(ηa, ηb) = Vn(ηa)Vn∗(ηb), (1)= vn(ηa)vn(ηb)ein(ψn(ηa)−ψn(ηb)), (2)where vn are the flow coefficients and ψn are the symmetry planes
Factorization The functional form of Vn∆(ηa, ηb) in the-plane is assessed with two different models. Both models focused on the flow ansatz i.e., individual events can be described in terms of single particle distributions
Summary
Where vn are the flow coefficients and ψn are the symmetry planes. Either of these two quantities may fluctuate from event to even due to varying initial conditions, thereby breaking the factorization of the sample average even for simulations of ideal hydrodynamics [1]. Flow related analyses commonly assume that non-flow contributions decrease with an increasing η-separation of the particles. In order to minimize the impact of non-flow effects on the measurement a minimal longitudinal separation between particles, referred to as |∆η|-gap, is often applied.
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