Pseudorapidity dependence of the anisotropic flow of charged particles in Pb-Pb collisions at $\sqrt{s_{\rm NN}}=2.76$ TeV

TL;DR

This paper measures the pseudorapidity dependence of elliptic, triangular, and quadrangular flow in Pb-Pb collisions at 2.76 TeV, revealing that higher harmonics decrease more steeply with pseudorapidity and challenging current models.

Contribution

It provides detailed measurements of flow harmonics over a wide pseudorapidity range and compares them to models, highlighting discrepancies and the behavior of flow coefficients.

Findings

Flow harmonics are largely independent of centrality.

Higher harmonics decrease more steeply with increasing pseudorapidity.

Models struggle to accurately describe the observed flow patterns.

Abstract

We present measurements of the elliptic ($\mathrm{v}_2$), triangular ($\mathrm{v}_3$) and quadrangular ($\mathrm{v}_4$) anisotropic azimuthal flow over a wide range of pseudorapidities ($-3.5< \eta < 5$). The measurements are performed with Pb-Pb collisions at $\sqrt{s_{\text{NN}}} = 2.76$ TeV using the ALICE detector at the Large Hadron Collider (LHC). The flow harmonics are obtained using two- and four-particle correlations from nine different centrality intervals covering central to peripheral collisions. We find that the shape of $\mathrm{v}_n(\eta)$ is largely independent of centrality for the flow harmonics $n=2-4$, however the higher harmonics fall off more steeply with increasing $|\eta|$. We assess the validity of extended longitudinal scaling of $\mathrm{v}_2$ by comparing to lower energy measurements, and find that the higher harmonic flow coefficients are proportional to the charged particle densities at larger pseudorapidities. Finally, we compare our measurements to both hydrodynamical and transport models, and find they both have challenges when it comes to describing our data.