CMS-PAS-HIN-19-002

CMS-PAS-HIN-19-002
Measurement of the elliptic flow of $\Upsilon\textrm{(1S)}$ and $\Upsilon\textrm{(2S)}$ mesons in PbPb collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}}=$ 5.02 TeV
CMS Collaboration
November 2019

Abstract: The second-order Fourier coefficients ( $v_{2}$ ) characterizing the azimuthal dependence of $\Upsilon\textrm{(1S)}$ and $\Upsilon\textrm{(2S)}$ mesons arising from PbPb collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}}=$ 5.02 TeV are determined. The $\Upsilon$ mesons are reconstructed using the di-muon decay channel as measured using the CMS detector. The dataset was obtained in 2018 and corresponds to an integrated luminosity of 1.7 nb $^{-1}$ . The scalar product method is used for the $v_{2}$ analysis. Results are reported for the rapidity range $\|y\| <$ 2.4, with 0 $< p_{\mathrm{T}} <$ 50 GeV, and in four centrality classes of 0-10%, 10-30%, 30-50% and 50-90%, with 0-10% corresponding to the most central collisions. The average $v_{2}$ value for $\Upsilon\textrm{(1S)}$ mesons is 0.007 $\pm$ 0.011 (stat) $\pm$ 0.005 (syst) in the 10-90% centrality interval. The observed $v_{2}$ values are consistent with zero for all $p_{\mathrm{T}}$ and centrality ranges, with a maximum offset of 2.5 standard deviations. The $v_{2}$ value for $\Upsilon\textrm{(2S)}$ mesons is measured for the first time and found to be -0.063 $\pm$ 0.085 (stat) $\pm$ 0.037 (syst) in the 10-90% centrality interval.
Links: CDS record (PDF) ; CADI line (restricted) ; These preliminary results are superseded in this paper, PLB 819 (2021) 136385. The superseded preliminary plots can be found here.

Figures	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Simultaneous fit of the dimuon invariant mass spectrum and the $v_{2}^{S+B}$ distribution, as defined in Eq. 3, for ${p_{\mathrm {T}}} <$ 50 GeV and with centrality 10-30%. The solid (signal + background) and dashed (background only) blue lines show the result of the mass fit, and the solid and dashed red lines show the corresponding results for the fit to the $v_{2}$ distribution.
png pdf	Figure 2: (Left) ${p_{\mathrm {T}}}$ integrated $v_{2}$ values for ${\Upsilon \textrm {(1S)}}$ mesons measured in four centrality bins and for the ${\Upsilon \textrm {(2S)}}$ meson in the 10-90% centrality range. (Right) $v_{2}$ as a function of ${p_{\mathrm {T}}}$ in the 10-90% centrality range. All results are for the rapidity range of $\|y\| <$ 2.4. The vertical bars denote statistical uncertainties, and the rectangular bands show the total systematic uncertainties.
png pdf	Figure 2-a: ${p_{\mathrm {T}}}$ integrated $v_{2}$ values for ${\Upsilon \textrm {(1S)}}$ mesons measured in four centrality bins and for the ${\Upsilon \textrm {(2S)}}$ meson in the 10-90% centrality range.
png pdf	Figure 2-b: $v_{2}$ as a function of ${p_{\mathrm {T}}}$ in the 10-90% centrality range. All results are for the rapidity range of $\|y\| <$ 2.4. The vertical bars denote statistical uncertainties, and the rectangular bands show the total systematic uncertainties.
png pdf	Figure 3: The green shaded area (Du, Rapp [29]) shows the results of a model calculation using the kinetic-rate equation to simulate the time evolution of bottom quarks. In this model, the medium effect is applied with a lattice-QCD based equation of state. The red band (Yao [30]) is the result of a real-time simulation of heavy quarks using coupled Boltzmann transport equations. The dashed violet line (Hong, Lee [31]) is the calculation with the potential Non-Relativistic QCD (pNRQCD) theory. In this approach, a diffusion constant $D(2\pi T)=$ 6 was used to have the derived $\mathrm {R_{AA}}$ value being consistent with the CMS result in Ref. [8]. The dashed brown line (Bhaduri, Borghini, Jaiswal, Strickland [32]) shows the results of using a 3+1d aHydro model. The calculations provide the QGP temperature evolution with the initial conditions and shear viscosity to entropy density ratio tuned to LHC 5.02 TeV identified hadron spectra and flow harmonics.
png pdf	Figure 4: $v_{2}$ coefficients for ${\Upsilon \textrm {(1S)}}$ mesons as a function of ${p_{\mathrm {T}}}$ in three centrality bins: 0-10% (Left), 10-50% (Middle) and 50-90% (Right). The rapidity range is $\|y\| <$ 2.4. The vertical lines indicate the statistical uncertainties and the rectangular bands show the total systematic uncertainties.

Summary

In summary, the elliptic flow coefficient

$v_{2}$ for

$\Upsilon\textrm{(1S)}$ and

$\Upsilon\textrm{(2S)}$ mesons are measured for

$|y|<$ 2.4 in PbPb collision at

${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} =$ 5.02 TeV. Results are reported for the rapidity range

$|y| <$ 2.4, with 0

$< p_{\mathrm{T}} <$ 50 GeV, and in four centrality classes of 0-10%, 10-30%, 30-50% and 50-90%, with 0-10% corresponding to the most central collisions. The

$v_{2}$ values found for

$\Upsilon\textrm{(1S)}$ mesons are consistent with zero over the kinematic range studied, within a maximum of 2.5 standard deviations. This observation contrasts with the measured

$\mathrm{J}/\psi$

$v_{2}$ results in PbPb collisions [13,14], suggesting different medium effects for charmonia and bottomonia. The data are compared to several theoretical models, all consistent with the results. In addition the first measurement of the elliptic flow coefficient for

$\Upsilon\textrm{(2S)}$ mesons in a heavy ion experiment is also reported and the result is consistent with zero. Because the contribution of regeneration to

$\Upsilon\textrm{(2S)}$ meson production in PbPb collisions is expected to be different from that of

$\Upsilon\textrm{(1S)}$ meson and to occur at a later stage of the collision, this study provides new inputs to the production mechanisms of bottomonia in heavy ion collisions.

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