Observation of Y(nS) suppression

Abstract

We report the observation of the suppression of the individual Υ(nS) states in PbPb collisions. The dimuon invariant mass spectra from the 150 μb⁻¹ PbPb and 230 nb⁻¹ pp datasets, both produced at a center-of-mass energy per nucleon pair of 2.76 TeV and collected in 2011 by the CMS experiment at the LHC, are explored, for muons of transverse momentum above 4 GeV/c and pseudorapidity between -2.4 and 2.4. The yield double ratios of excited states Υ(2S) and Υ(3S) to Υ(1S) state, for PbPb relative to pp collisions, [Υ(nS)/Υ(1S)]_PbPb/[Υ(nS)/Υ(1S)]_pp, are measured to be 0.21 ± 0.07 (stat.) ± 0.02 (syst.) for n = 2 and less than 0.1 at 95% confidence level for n = 3. Measurements of the Υ(nS) individual states are performed: their suppression is ordered as Υ(3S) > Υ(2S) > Υ(1S), and increases with the centrality of the PbPb collision.

Additional Numerical Results

Double ratio of the combined excited states, Y(2S)+Y(3S), relative to the ground state Y(1S), for the centrality-integrated data:

[Υ(2S+3S)/Υ(1S)]_PbPb/[Υ(2S+3S)/Υ(1S)]_pp = 0.15 ± 0.05 (stat.) ± 0.02 (syst.)

Raw signal yields:

PbPb Y(1S): 1317 ± 73 (stat.)

PbPb Y(2S): 156 ± 38 (stat.)

pp Y(1S): 88 ± 11 (stat.)

pp Y(2S): 49 ± 10 (stat.)

Final Results Plots

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Figure	Abbreviated Caption
	Dimuon invariant-mass distributions from the PbPb data at sqrt(sNN) = 2.76 TeV. The same reconstruction algorithm and analysis criteria are applied to PbPb and pp datasets, including a transverse momentum requirement on single muons of pT > 4 GeV/c. The solid (signal + background) and dashed (background-only) lines show the results of the simultaneous fit performed to the pp and PbPb datasets.
	Dimuon invariant-mass distributions from the pp data at sqrt(sNN) = 2.76 TeV. The same reconstruction algorithm and analysis criteria are applied to PbPb and pp datasets, including a transverse momentum requirement on single muons of pT > 4 GeV/c. The solid (signal + background) and dashed (background-only) lines show the results of the simultaneous fit performed to the pp and PbPb datasets.
	Centrality dependence of the double ratio [Υ(2S)/Υ(1S)]PbPb/[Υ(2S)/Υ(1S)]pp. The centrality bins are 0-5%, 5-10%, 10-20%, 20-30%, 30-40%, 40-50%, and 50-100%. For each bin, the centrality percentile is specified; the point abscissae used correspond to the mean of the distribution of the expected number of participants Npart in the respective bin. The global uncertainty represented by a box at unity corresponds to the quadrature sum of statistical and systematic uncertainties from the fit to the pp data, and do not affect the point-to-point trend.
	Centrality dependence of the nuclear modification factors, RAA, for the individual Υ(1S) and Υ(2S) states. The centrality bins are 0-5%, 5-10%, 10-20%, 20-30%, 30-40%, 40-50%, and 50-100%. The uncertainties of 14% for Υ(1S) and 21% for Υ(2S), from Npart-independent quantities (luminosity, pp yields and efficiency), are represented by the boxes at unity, and are not included in the data points as these do not affect the point-to-point trend.
	Variant of the previous plot, but also including the centrality-integrated values (minimum bias).

Comparison Plots

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Figure	Abbreviated Caption
	Comparison of RAA results for Y(nS) and 2011-run J/Ψ (preliminary results). The results are shown as a function of centrality and for the centrality-integrated case, displaying the quarkonium sequential suppression observed.
	Comparison of the Υ(1S) and Υ(2S) nuclear modification factor RAA centrality dependence results to the 2011-run J/Ψ RAA measurement (preliminary).
	Comparison of the Υ(1S) nuclear modification factor RAA centrality dependence result to the 2010-run Y(1S) RAA previous measurement (Published in JHEP 1205 (2012) 063, preprint available at arXiv:1201.5069).
	Comparison of the Υ(1S) nuclear modification factor RAA centrality dependence result to theory prediction M. Strickland (arXiv:1207.5327v2).
	Comparison of the Υ(1S) nuclear modification factor RAA centrality dependence result to the Y(1S) RAA measurent by STAR (arXiv:1312.3675). The STAR Results are binned in classes of centrality 0-10%, 10-30%, 30-60%.
	Strong-binding scenario (SBS) prediction, produced using the calculations performed in the paper Eur. Phys. J. A48 (2012) 72, based upon the approach developed in Phys.Rev. C 73 (2006) 064906. The significance of cold-nuclear-matter effects has been simulated by employing two nuclear absorption cross sections to estimate an upper and lower bound. For LHC 0.0 mb and 2.0 mb are used to produce the bands seen in the plots. The regeneration component is calculated during plotting as "Total RAA" - "Primordial RAA".
	Comparison of the Υ(1S) and Υ(2S) nuclear modification factor RAA centrality dependence results to the Y(1S) RAA measured in the forward rapidity range (2.5<y<4) by ALICE (arXiv:1405.4493). The grey box at unity displays correlated systematic errors relative to the pp reference. On the right hand side a smaller panel is included, suggesting a comparison between centrality-integrated results.

Technical Plots

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Figure	Abbreviated Caption
	Dimuon invariant-mass distribution from the PbPb data at sqrt(sNN) = 2.76 TeV. The red line shows the fit to the PbPb data. The blue dashed line shows the shape obtained from the fit to the pp data. For a better comparison, the background shape, background yield, mass peak width, mass peak tail shape and the Y(1S) yields in the blue line are fixed to the PbPb fit, while the Y(2S)/Y(1S) and Y(3S)/Y(1S) ratios are fixed to the pp fit values.
	Variant of the previous plot. Here, the dashed curve illustrates the corresponding signals in pp data, scaled by the RAA values. The Y(1S) state is apparently suppressed in PbPb with respect to pp, while the Y(2S) and Y(3S) states are suppressed significantly more.
	Dimuon invariant-mass distribution from the PbPb data at sqrt(sNN) = 2.76 TeV, in the centrality bin 0-5%. Resolution and final state radiation fit parameters are fixed to the centrality-integrated (aka minimum bias) sample.
	Dimuon invariant-mass distribution from the PbPb data at sqrt(sNN) = 2.76 TeV, in the centrality bin 5-10%. Resolution and final state radiation fit parameters are fixed to the centrality-integrated (aka minimum bias) sample.
	Dimuon invariant-mass distribution from the PbPb data at sqrt(sNN) = 2.76 TeV, in the centrality bin 10-20%. Resolution and final state radiation fit parameters are fixed to the centrality-integrated (aka minimum bias) sample.
	Dimuon invariant-mass distribution from the PbPb data at sqrt(sNN) = 2.76 TeV, in the centrality bin 20-30%. Resolution and final state radiation fit parameters are fixed to the centrality-integrated (aka minimum bias) sample.
	Dimuon invariant-mass distribution from the PbPb data at sqrt(sNN) = 2.76 TeV, in the centrality bin 30-40%. Resolution and final state radiation fit parameters are fixed to the centrality-integrated (aka minimum bias) sample.
	Dimuon invariant-mass distribution from the PbPb data at sqrt(sNN) = 2.76 TeV, in the centrality bin 40-50%. Resolution and final state radiation fit parameters are fixed to the centrality-integrated (aka minimum bias) sample.
	Dimuon invariant-mass distribution from the PbPb data at sqrt(sNN) = 2.76 TeV, in the centrality bin 50-100%. Resolution and final state radiation fit parameters are fixed to the centrality-integrated (aka minimum bias) sample.
	The total signal efficiency, evaluated from PbPb and pp Monte Carlo simulation; shown as a function of the centrality of the PbPb collision (expressed by the number of participant nucleons). The error bars reflect the statistics of the MC sample, and the systematic uncertainties related to the kinematic distributions and to the MC validation through comparison with data.
	The Y(1S)/Y(2S) efficiency ratio, evaluated from PbPb and pp Monte Carlo simulation; shown as a function of the centrality of the PbPb collision (expressed by the number of participant nucleons). The error bars reflect the statistics of the MC sample, and the systematic uncertainties related to the kinematic distributions.
	Estimations of the background shape used in the evaluation of systematic fit uncertainties, based on: (top) like-sign, (middle) track-rotated, and (bottom) opposite-sign dimuon spectra.