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| CMS-HIN-15-001 ; CERN-EP-2016-248 | ||
| Suppression of $\Upsilon$(1S), $\Upsilon$(2S), and $\Upsilon$(3S) quarkonium states in PbPb collisions at ${\sqrt{{s_{_{\mathrm{NN}}}}}} = $ 2.76 TeV | ||
| CMS Collaboration | ||
| 4 November 2016 | ||
| Phys. Lett. B 770 (2017) 357 | ||
| Abstract: The production yields of $\Upsilon$(1S), $\Upsilon$(2S), and $\Upsilon$(3S) quarkonium states are measured through their decays into muon pairs in the CMS detector, in PbPb and pp collisions at the centre-of-mass energy per nucleon pair of 2.76 TeV. The data correspond to integrated luminosities of 166 $\mu$b$^{-1}$ and 5.4 pb$^{-1}$ for PbPb and pp collisions, respectively. Differential production cross sections are reported as functions of $\Upsilon$ rapidity $y$ up to 2.4, and transverse momentum $p_{\mathrm{T}}$ up to 20 GeV/$c$. A strong centrality-dependent suppression is observed in PbPb relative to pp collisions, by factors of up to ${\approx} 2$ and 8, for the $\Upsilon$(1S) and $\Upsilon$(2S) states, respectively. No significant dependence of this suppression is observed as a function of $y$ or $p_{\mathrm{T}}$. The $\Upsilon$(3S) state is not observed in PbPb collisions, which corresponds to a suppression for the centrality-integrated data by at least a factor of ${\approx} 7$ at a 95% confidence level. The observed suppression is in agreement with theoretical scenarios modeling the sequential melting of quarkonium states in a quark gluon plasma. | ||
| Links: e-print arXiv:1611.01510 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Dimuon invariant mass distributions in pp (left) and centrality-integrated PbPb (right) data at $ {\sqrt {{s_{_{\mathrm {NN}}}}}} = $ 2.76 TeV, for muon pairs having one ${p_{\mathrm {T}}}$ greater than 4 GeV/$c$ and the other greater than 3.5 GeV/$c$. The solid (signal + background) and dashed (background only) lines show the result of fits described in the text. |
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Figure 1-a:
Dimuon invariant mass distributions in pp data at $ {\sqrt {{s_{_{\mathrm {NN}}}}}} = $ 2.76 TeV, for muon pairs having one ${p_{\mathrm {T}}}$ greater than 4 GeV/$c$ and the other greater than 3.5 GeV/$c$. The solid (signal + background) and dashed (background only) lines show the result of fits described in the text. |
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Figure 1-b:
Dimuon invariant mass distributions incentrality-integrated PbPb data at $ {\sqrt {{s_{_{\mathrm {NN}}}}}} = $ 2.76 TeV, for muon pairs having one ${p_{\mathrm {T}}}$ greater than 4 GeV/$c$ and the other greater than 3.5 GeV/$c$. The solid (signal + background) and dashed (background only) lines show the result of fits described in the text. |
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Figure 2:
Differential cross section for $\Upsilon$ states as a function of their transverse momentum and per unit of rapidity in pp (left) and PbPb (right) collisions. Statistical (systematic) uncertainties are displayed as error bars (boxes). Global relative uncertainties of 3.7% (pp) and 6.5% (PbPb) are not displayed. |
png pdf |
Figure 2-a:
Differential cross section for $\Upsilon$ states as a function of their transverse momentum and per unit of rapidity in pp collisions. Statistical (systematic) uncertainties are displayed as error bars (boxes). Global relative uncertainties of 3.7% (pp) and 6.5% (PbPb) are not displayed. |
png pdf |
Figure 2-b:
Differential cross section for $\Upsilon$ states as a function of their transverse momentum and per unit of rapidity in PbPb collisions. Statistical (systematic) uncertainties are displayed as error bars (boxes). Global relative uncertainties of 3.7% (pp) and 6.5% (PbPb) are not displayed. |
png pdf |
Figure 3:
Differential cross section for $\Upsilon$ states as a function of their rapidity and integrated over transverse momentum in pp (left) and PbPb (right) collisions. Statistical (systematic) uncertainties are displayed as error bars (boxes). Global relative uncertainties of 3.7% (pp) and 6.5% (PbPb) are not displayed. |
png pdf |
Figure 3-a:
Differential cross section for $\Upsilon$ states as a function of their rapidity and integrated over transverse momentum in pp collisions. Statistical (systematic) uncertainties are displayed as error bars (boxes). Global relative uncertainties of 3.7% (pp) and 6.5% (PbPb) are not displayed. |
png pdf |
Figure 3-b:
Differential cross section for $\Upsilon$ states as a function of their rapidity and integrated over transverse momentum in PbPb collisions. Statistical (systematic) uncertainties are displayed as error bars (boxes). Global relative uncertainties of 3.7% (pp) and 6.5% (PbPb) are not displayed. |
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Figure 4:
Nuclear modification factor for $\Upsilon$(1S) and $\Upsilon$(2S) states in PbPb collisions as a function of ${p_{\mathrm {T}}}$ (left) and $ {| y | }$ (right). Statistical (systematic) uncertainties are displayed as error bars (boxes), while the global (fully correlated) uncertainty (7.5%) is displayed as a grey box at unity. |
png pdf |
Figure 4-a:
Nuclear modification factor for $\Upsilon$(1S) and $\Upsilon$(2S) states in PbPb collisions as a function of ${p_{\mathrm {T}}}$. Statistical (systematic) uncertainties are displayed as error bars (boxes), while the global (fully correlated) uncertainty (7.5%) is displayed as a grey box at unity. |
png pdf |
Figure 4-b:
Nuclear modification factor for $\Upsilon$(1S) and $\Upsilon$(2S) states in PbPb collisions as a function of $ {| y | }$. Statistical (systematic) uncertainties are displayed as error bars (boxes), while the global (fully correlated) uncertainty (7.5%) is displayed as a grey box at unity. |
png pdf |
Figure 5:
Nuclear modification factors for $\Upsilon$(1S) and $\Upsilon$(2S) meson production in PbPb collisions, as a function of centrality, displayed as the average number of participating nucleons. The upper limit derived on the nuclear modification factor for $\Upsilon$(3S) is represented with an arrow in the centrality integrated panel. Statistical (systematic) uncertainties are displayed as error bars (boxes), while the global (fully correlated) uncertainties from the PbPb data (3.2%) or from the pp reference (6.3 and 6.9% for $\Upsilon$(1S) and $\Upsilon$(2S) states, respectively) are displayed at unity as empty, filled red, and filled black boxes, respectively. |
| Tables | |
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Table 1:
Average values of the number of participating nucleons ($ {N_{\text {part}}} $, with the root-mean-square of its distribution in each bin), and nuclear overlap function ($ {T_\mathrm {AA}} $, with its systematic uncertainty) for the $\Upsilon$(1S) (upper), $\Upsilon$(2S) (middle) and centrality-integrated binning (last row). |
| Summary |
| The $\Upsilon$(1S), $\Upsilon$(2S), and $\Upsilon$(3S) yields have been measured in PbPb and pp collisions at ${\sqrt{{s_{_{\mathrm{NN}}}}}} = $ 2.76 TeV with the CMS detector, using integrated luminosities of 166 $\mu$b$^{-1}$ and 5.4 pb$^{-1}$, respectively. For the first time, differential production cross sections are derived for individual $\Upsilon$ states as functions of their rapidity and transverse momentum in heavy ion collisions. The $\Upsilon$(1S) and $\Upsilon$(2S) states are suppressed in PbPb relative to pp collisions scaled by the number of nucleon-nucleon collisions, by factors of ${\approx}2$ and 8, respectively, while the absence of a significant $\Upsilon$(3S) signal corresponds to a suppression by a factor larger than ${\approx}7$ at a 95% confidence level. While a strong centrality dependence of the suppression is found for the $\Upsilon$(1S) and $\Upsilon$(2S) states, no clear dependence is observed as a function of either transverse momentum or rapidity. The level of suppression measured in this analysis is compatible with theoretical models of a sequential melting of quarkonium states in a hot medium. |
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