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| CMS-HIN-16-008 ; CERN-EP-2017-080 | ||
| Suppression of excited ${\Upsilon} $ states relative to the ground state in PbPb collisions at ${\sqrt{{s_{_{\mathrm{NN}}}}}} = $ 5.02 TeV | ||
| CMS Collaboration | ||
| 19 June 2017 | ||
| Phys. Rev. Lett. 120 (2018) 142301 | ||
| Abstract: The relative yields of $ {\Upsilon} $ mesons produced in pp and PbPb collisions at ${\sqrt{{s_{_{\mathrm{NN}}}}}} = $ 5.02 TeV and reconstructed via the dimuon decay channel are measured using data collected by the CMS experiment. Double ratios are formed by comparing the yields of the excited states, $ {\Upsilon(\mathrm{2S})} $ and $ {\Upsilon(\mathrm{3S})} $, to the ground state, $ {\Upsilon(\mathrm{1S})} $, in both PbPb and pp collisions at the same center-of-mass energy. The double ratios, $ [{\Upsilon(\mathrm{nS})} /{\Upsilon(\mathrm{1S})} ]_{{\mathrm{PbPb}} }/ [{\Upsilon(\mathrm{nS})} /{\Upsilon(\mathrm{1S})} ]_{pp}$, are measured to be 0.308 $\pm$ 0.055 (stat) $\pm$ 0.019 (syst) for the $ {\Upsilon(\mathrm{2S})} $ and less than 0.26 at 95% confidence level for the $ {\Upsilon(\mathrm{3S})} $. No significant $ {\Upsilon(\mathrm{3S})} $ signal is found in the PbPb data. The double ratios are studied as a function of collision centrality, as well as dimuon transverse momentum and rapidity. No significant dependencies are observed. | ||
| Links: e-print arXiv:1706.05984 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Measured dimuon invariant mass distributions in PbPb data. The total fit (solid blue line) and the background component (dot-dashed blue line) are also shown, as are the individual $ {\Upsilon (\mathrm {1S})} $, $ {\Upsilon (\mathrm {2S})} $, and $ {\Upsilon (\mathrm {3S})} $ signal shapes (dotted gray lines). The dashed red line represents the pp signal shape added to the PbPb background and normalized to the $ {\Upsilon (\mathrm {1S})} $ mass peak in PbPb. |
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Figure 2:
Double ratio of the $ {\Upsilon (\mathrm {2S})} $ as a function of centrality. The centrality-integrated value is shown in the right panel. The error bars represent the statistical uncertainty in the PbPb data while the boxes represent the systematic uncertainty due to signal and background variations. The box drawn around the line at unity depicts the systematic and statistical uncertainties from pp data, as well as the systematic uncertainties due to the combined detection efficiency; it is 3.1% and applies to all measurements (including the centrality-integrated one). Calculations by Krouppa and Strickland (orange curves [21]) and by Emerick, Zhao, and Rapp (green hatched band [35]) are also shown. |
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Figure 3:
Double ratio of the $ {\Upsilon (\mathrm {2S})} $ as functions of $ {p_{\mathrm {T}}} ^{\mu \mu }$ (left) and $|y^{\mu \mu }|$ (right). The error bars depict the statistical uncertainty while the boxes represent the systematic uncertainties in the signal and background models as well as the combined detection efficiency. Calculations by Krouppa and Strickland (orange curves [21]) and by Emerick, Zhao, and Rapp (green hatched band [35]) are also shown. |
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Figure 3-a:
Double ratio of the $ {\Upsilon (\mathrm {2S})} $ as functions of $ {p_{\mathrm {T}}} ^{\mu \mu }$. The error bars depict the statistical uncertainty while the boxes represent the systematic uncertainties in the signal and background models as well as the combined detection efficiency. Calculations by Krouppa and Strickland (orange curves [21]) and by Emerick, Zhao, and Rapp (green hatched band [35]) are also shown. |
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Figure 3-b:
Double ratio of the $ {\Upsilon (\mathrm {2S})} $ as functions of $|y^{\mu \mu }|$. The error bars depict the statistical uncertainty while the boxes represent the systematic uncertainties in the signal and background models as well as the combined detection efficiency. Calculations by Krouppa and Strickland (orange curves [21]) and by Emerick, Zhao, and Rapp (green hatched band [35]) are also shown. |
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Figure 4:
Confidence intervals at 95% CL (blue arrows) and 68% CL (red boxes) of the $ {\Upsilon (\mathrm {3S})} $ double ratio as a function of centrality. The centrality-integrated limit is shown in the right panel. |
| Summary |
| In summary, the $ \Upsilon(\mathrm{2S}) $ and $ \Upsilon(\mathrm{3S}) $ double ratios have been measured at 5.02 TeV, using pp and PbPb data samples significantly larger than those used in the corresponding 2.76 TeV measurements. The centrality-integrated double ratios are 0.308 $\pm$ 0.055 (stat) $\pm$ 0.019 (syst) for the $ \Upsilon(\mathrm{2S}) $ and $<$0.26 at 95% CL for the $ \Upsilon(\mathrm{3S}) $. The large relative suppression of the $ \Upsilon(\mathrm{2S}) $ does not show significant variations with $p_{\mathrm{T}}^{\mu\mu}$ or $|{y^{\mu\mu}} |$ within the explored phase space window of $p_{\mathrm{T}}^{\mu\mu} < $ 30 GeV/$c^2$ and $|{y^{\mu\mu}}| < $ 2.4. The $ \Upsilon(\mathrm{2S}) $ double ratio is compatible with unity in the most peripheral collisions (70--100%) and with zero in the most central ones (0--5%), but a flat centrality dependence is not excluded, given the current uncertainties. The 95% CL intervals for the $ \Upsilon(\mathrm{3S}) $ double ratio exclude unity in the four centrality bins of this analysis, including the most peripheral collisions (50--100%). |
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