Observation of flow angle and flow magnitude fluctuations in Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV at the LHC

This Letter reports on the first measurements of transverse momentum dependent flow angle $\Psi_n$ and flow magnitude $v_n$ fluctuations, determined using new four-particle correlators. The measurements are performed for various centralities in Pb-Pb collisions at a centre-of-mass energy of $\sqrt{s_{\rm NN}}$ = 5.02 TeV with ALICE at the LHC. Both flow angle and flow magnitude fluctuations are observed in the presented centrality ranges and are strongest in the most central collisions and for a transverse momentum $p_{\rm T}>2$ GeV/$c$. Comparison with theoretical models, including iEBE-VISHNU, MUSIC, and AMPT, show that the measurements bring novel insights into the fluctuating initial conditions that are not well known. In addition, these new results exhibit unique sensitivities to the specific shear viscosity, $\eta/s$, of the quark--gluon plasma (QGP) and to the initial state of the heavy-ion collisions. As such fluctuations are getting stronger with increasing $p_{\rm T}$, a re-examination of existing models is needed to have a more precise and unbiased extraction of properties of the QGP.

 

Submitted to: PRL
e-Print: arXiv:2206.04574 | PDF | inSPIRE
CERN-EP-2022-121

Figure 1

The flow angle fluctuation \Fpsi{2} in \PbPb collisions at \snn = 5.02 TeV as a function of the associate particle transverse momentum, \pta, in centrality classes 0--5\% (left), 10--20\% (middle), and 30--40\% (right). Comparison with iEBE-VISHNU with \trento initial conditions and $\eta/s(T)$~, iEBE-VISHNU with AMPT initial conditions and $\eta/s=0.08$~ and $\eta/s=0.20$, CoLBT~, and AMPT~ are shown as coloured bands.

Figure 2

The flow magnitude fluctuation \Fvn{2} in \PbPb collisions at \snn = 5.02 TeV as a function of the associate particle transverse momentum, \pta, in centrality classes 0--5\% (left), 10--20\% (middle), and 30--40\% (right). Comparison with iEBE-VISHNU with \trento initial conditions and $\eta/s(T)$~, MUSIC with Glauber initial conditions and $\eta/s=0.08$~, MUSIC with \trento initial conditions and $\eta/s=0.08,0.12,0.16$~, and AMPT~ are shown as coloured bands.

Figure 3

The lower limit of the first-order flow angle fluctuations, upper limit of the first-order flow magnitude fluctuations, and the flow vector fluctuations as a function of centrality for the $3.0/c. The lower and upper limits are denoted by the arrows. The top panel shows the absolute contribution and the bottom panel the contribution relative to the overall flow vector fluctuations. Comparisons to AMPT~ are shown as coloured bands.