Measurement of nuclear modification factors of $\Upsilon$(1S), $\Upsilon$(2S), and $\Upsilon$(3S) mesons in PbPb collisions at $\sqrt{s_{_\mathrm{NN}}} =$ 5.02 TeV

The CMS collaboration
Sirunyan, Albert M , Tumasyan, Armen , Adam, Wolfgang , Ambrogi, Federico , Asilar, Ece , Bergauer, Thomas , Brandstetter, Johannes , Brondolin, Erica , Dragicevic, Marko , Erö, Janos
Phys.Lett.B 790 (2019) 270-293, 2019.

Abstract (data abstract)
CERN-LHC. The cross sections for $\Upsilon(\mathrm{1S})$, $\Upsilon(\mathrm{2S})$, and $\Upsilon(\mathrm{3S})$ production in lead-lead (PbPb) and proton-proton (pp) collisions at $ {\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV have been measured using the CMS detector at the LHC. The nuclear modification factors, $ {R_{\mathrm{AA}}} $, derived from the PbPb-to-pp ratio of yields for each state, are studied as functions of meson rapidity and transverse momentum, as well as PbPb collision centrality. The yields of all three states are found to be significantly suppressed, and compatible with a sequential ordering of the suppression, $ {R_{\mathrm{AA}}}( \Upsilon(\mathrm{1S}) ) > {R_{\mathrm{AA}}}( \Upsilon(\mathrm{2S})) > {R_{\mathrm{AA}}} (\Upsilon(\mathrm{3S}) )$ . The suppression of $ \Upsilon(\mathrm{1S})$ is larger than that seen at $ {\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 2.76 TeV, although the two are compatible within uncertainties. The upper limit on the ${R_{\mathrm{AA}}}$ of $ \Upsilon(\mathrm{3S})$ integrated over $ {p_{\mathrm{T}}} $ and rapidity and centrality is 0.096 at 95% confidence level, which is the strongest suppression observed for any hadron species in heavy ion collisions to date.

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