The $\Upsilon$(1S) $v_2$ coefficient integrated over the transverse momentum range 2 $< \pt <$ 15 GeV/$c$ in three centrality intervals

Scope: PWG
PWG-DQ (Dileptons and Quarkonia)
Energy
5.02 TeV
System
Pb-Pb
Figure Image
Figure image (.eps)
Figure image (.pdf)
Figure Caption

The $\Upsilon$(1S) $v_2$ coefficient integrated over the transverse momentum range 2 $< \pt <$ 15 GeV/$c$ in three centrality intervals compared to that of inclusive ${\rm J}/\psi$. Error bars (open boxes) represent the statistical (systematic) uncertainties.

Detail description

Given that any $v_2$ originating either from recombination or from path-length dependent dissociation vanishes at zero \pt{}, the observed difference between $\Upsilon$(1S) and J/$\psi$ $v_2$ is quantified by performing the \pt-integrated measurement excluding the low \pt{} range. Figure~\ref{fig:v2_int_cen} presents the $\Upsilon$(1S) $v_2$ coefficient integrated over the transverse momentum range $2 < \pt < 15$~GeV/$c$ for three centrality intervals compared with that of the inclusive ${\rm J}/\psi$. The $\Upsilon$(1S) $v_2$ is found to be $-0.003 \pm 0.030(\rm{stat}) \pm 0.006 (\rm{syst})$ in the $2 < \pt < 15$~GeV/$c$ and 5--60\% centrality interval. This value is lower than the corresponding ${\rm J}/\psi$ $v_2$ by 2.6$\sigma$. This observation, coupled to the different measured centrality and $p_{\rm T}$ dependence of the $\Upsilon$(1S) and J/$\psi$ suppression in Pb--Pb collisions at the LHC~\cite{Acharya:2018mni,Adam:2016rdg}, can be interpreted within the models used for comparison as a sign that unlike $\Upsilon$(1S), J/$\psi$ production has a significant regeneration component. Nevertheless, no firm conclusions can be drawn, given that currently the transport models can not explain the significant J/$\psi$ $v_2$ for $p_{\rm T}$~$>$~4-5 GeV/$c$ observed in the data~\cite{Acharya:2017tgv}.