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Bottomonium production in Pb + Pb collisions with ATLAS
Available online at www.sciencedirect.com
Nuclear Physics A 1005 (2021) 121860 www.elsevier.com/locate/nuclphysa
XXVIIIth International Conference on Ultrarelativistic Nucleus-Nucleus Collisions (Quark Matter 2019)
Bottomonium production in Pb + Pb collisions with ATLAS Songkyo Lee, on behalf of the ATLAS collaboration Iowa State University, Ames, IA, 50010, USA
Abstract Bottomonium, a bound state of a bottom quark and its antiquark, is an excellent probe of the hot and dense medium created in heavy-ion collisions. The ATLAS collaboration at the LHC collected two large datasets of pp and Pb + Pb collisions in 2017 and 2018 corresponding to integrated luminosities of 0.26 fb−1 and 1.38 nb−1 respectively, at a centerof-mass energy per nucleon pair of 5.02 TeV. Bottomonium states are reconstructed via the dimuon decay channel in the absolute rapidity range of |y| < 1.5, and their production in Pb + Pb collisions is compared to that in pp collisions to extract the nuclear modification factor, RAA , as a function of transverse momentum, absolute rapidity, and event centrality. In addition, the relative suppression of the excited states Υ(nS) to the ground state Υ(1S) is studied. Keywords: Bottomonia, Quarkonia, QGP, nuclear modification factor, heavy-ion
1. Introduction In ultra-relativistic heavy-ion collisions, hadronic matter can experience a phase transition and turn into a state of deconfined quarks and gluons, Quark-Gluon Plasma (QGP). In such collisions, heavy-flavor quarks, especially charm and bottom, are produced at a very early stage and hence can be used to characterize the properties of the QGP. The production of quarkonia, a bound state of a heavy quark and its antiquark, in nucleus-nucleus collisions is predicted to be suppressed relative to that in proton-proton (pp) collisions due to the modification to the heavy-quark potential [1]. In particular, a sequential suppression of quarkonium states depending on their binding energies has been proposed as a thermometer of the QGP. It was found, for example, by the lattice calculation that the Υ(1S) survives well above the critical temperature T c needed to form the QGP, while the Υ(2S) melts at about 1.1T c and Υ(3S) cannot exist above T c [2]. The modifications of bottomonium yields in Pb + Pb collisions with respect to those in pp collisions can be quantified by the nuclear modification factor, RAA , which can be defined as RAA =
https://doi.org/10.1016/j.nuclphysa.2020.121860 0375-9474/© 2020 Elsevier B.V. All rights reserved.
NAA , T AA × σ pp
(1)
S. Lee
Nuclear Physics A 1005 (2021) 121860
where NAA is the per-event yield of bottomonium states, T AA is a mean value of the nuclear overlap function defined by the ratio of the number of binary nucleon-nucleon collisions to the inelastic nucleonnucleon cross-section, and σ pp is the bottomonium production cross section in pp collisions. The ALICE [3] and CMS [4] at the LHC and PHENIX [5] at the RHIC have studied bottomonium production in heavy-ion collisions, and this proceeding reports the measurement performed with the ATLAS detector [6] which will provide complementary information as each experiment covers a different kinematic range. The analysis is based on data collected during Pb + Pb collisions at a center-of-mass energy of 5.02 TeV per nucleon pairs in 2018 with an integrated luminosity of 1.38 nb−1 , and pp collisions in 2017 with an integrated luminosity of 0.26 fb−1 [7]. Three bottomonium states, Υ(nS), are reconstructed in their dimuon decay channel, within the absolute rapidity range of |y| < 1.5, transverse momentum range of pT < 30 GeV, and event centrality range of 0 − 80%. In addition to RAA , the double ratio of the excited-to-ground state nS)/Υ(1S) = [σΥ(nS) /σΥ(1S) ]/[σΥ(nS) /σΥ(1S) ], is production cross sections in Pb + Pb to pp collisions, ρΥ( pp pp AA Pb+Pb Pb+Pb also investigated to quantify binding-energy dependent suppression. 2. Analysis prodecure In Pb + Pb collisions, the candidate events are collected with a trigger which requires one muon to pass the Level-1 Trigger (L1) and then confirmed at the High-Level Trigger (HLT) with pμT > 4 GeV, and at least another muon candidate with pμT > 4 GeV to be found in the full Muon Spectrometer (MS) system. In pp collisions, a more strict trigger is used, which requires at least two L1 muon candidates with pμT > 4 GeV, subsequently confirmed at the HLT. Muon candidates are reconstructed based on a combination of chargedparticle tracks in the Inner Detector (ID) and the MS. A standard quality cut and vertex-association cut are applied to reduce fake muons and muons coming from other sources such as b-hadron decay. The Pb + Pb events are characterized by centrality using the total transverse energy deposit in the forward hadronic calorimeter. In this analysis, the interval 80–100% is excluded to suppress dimuon production from electromagnetic processes [8]. Each candidate is then weighted by wtotal (Υ(nS)) = 1/[A(Υ(nS)) · εreco (μ1 μ2 ) · εtrig (μ1 μ2 ) · εpvAsso (μ1 μ2 )], where A(Υ(nS)) is the acceptance for Υ(nS) → μ+ μ− decay, εreco is the muon reconstruction efficiency which also includes muon identification, εtrig is the trigger efficiency, and εpvAsso is the efficiency related to the primary vertex association. Monte Carlo (MC) simulations [9] of pp collision events with and without overlaying minimum-bias Pb + Pb data are used to study acceptance and efficiencies as well as signal yields extraction. Events are generated using PYTHIA 8 [10] with the CTEQ6L1 [11] parton distribution functions, and the response of the ATLAS detector is simulated using GEANT 4 [12]. The Υ(nS) yields are extracted performing an unbinned maximum-likelihood fit to the weighted dimuon μμ invariant mass distribution in each pμμ T , |y | or centrality interval. The probability distribution function for the fit is defined as a normalized sum of three Υ(nS) signals and the background components. Each signal shape is described by a combination of Crystal Ball [13] and Gaussian functions with a common mean but μμ μμ different widths. The background parameterizations vary with dimuon pμμ T . At low pT bins (pT < 6 GeV), μμ μμ or integrated over the whole pT range (pT < 30 GeV), an error function multiplied by an exponential function is used to describe the mμμ turn-on effects due to the single-muon pμT requirement, pμT > 4 GeV. At μμ higher pμμ T (pT > 6 GeV), such turn-on effects are not present, and a second-order polynomial is used to model the background contribution. 3. Results Figure 1 shows the RAA of Υ(nS) as functions of the number of participating nucleons, Npart (left), dimuon pT (middle), and the dimuon |y| (right). The centrality integrated results are also shown on the right panel of the left plot. In addition to the results for Υ(1S) and Υ(2S), the combined results of excited states, Υ(2S+3S), are presented as Υ(3S) peaks are barely seen in Pb + Pb data. The Υ(nS) states are observed to be suppressed over the whole kinematic range investigated, and the RAA values of Υ(2S) and Υ(2S+3S) are
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Fig. 1. The nuclear modification factor RAA of Υ(1S), Υ(2S), and Υ(2S+3S) as functions of centrality (left), dimuon pT (middle), and the dimuon |y| (right) at 5.02 TeV. The error bars indicate the statistical uncertainties and the boxes represent the systematic uncertainties. The grey boxes around RAA = 1 correspond to the correlated systematic uncertainty. For the left panel, data points for Υ(2S+3S) are slightly shifted to the right in order to avoid overlap with those for Υ(2S) [7].
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always lower than those of Υ(1S), which is consistent with the sequential suppression expectation. The RAA decreases with centrality for all three states. No strong pT or |y| dependence is observed. Figure 2 shows the double ratio of Υ(2S) and Υ(2S+3S) as functions of Npart (left), dimuon pT (middle), and the dimuon |y| (right). The centrality integrated results are also shown on the right panel of the left plot. The advantage of measuring the double ratios is that the acceptance and efficiency corrections are partially canceled, and the systematic uncertainty on the corrections is reduced. The Υ(2S) and Υ(2S+3S) results are always lower than unity, indicating the excited states are more suppressed than the ground state. The centrality-dependent result shows a slightly decreasing trend, but is also consistent with a flat behavior within uncertainties. No strong pT or |y| dependence is observed.
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nS)/Υ(1S) of Υ(2S), and Υ(2S+3S) as functions of centrality (left), dimuon p (middle), and the dimuon |y| Fig. 2. The double ratio ρΥ( T AA (right) at 5.02 TeV. The error bars indicate the statistical uncertainties and the boxes represent the systematic uncertainties. For the left panel, data points for Υ(2S+3S) are slightly shifted to the right in order to avoid overlap with those for Υ(2S) [7].
The left panel of Figure 3 shows the comparison of Υ(1S) RAA as a function of Npart results to the CMS measurements at the same collisions energy [14]. The centrality integrated results are also shown on the right panel of the left plot. The CMS results are systematically higher, but both measurements are consistent within uncertainties. It is worth noting that the kinematic ranges and the centrality intervals are different between the two experiments. The CMS results are obtained in pT < 30 GeV and |y| < 2.4. On the right panel of Figure 3, the Υ(1S) RAA as a function of dimuon pT is compared to those of prompt and nonprompt J/ψ mesons from the previous ATLAS measurement [15]. The resulting Υ(1S) RAA is found to be comparable with prompt J/ψ RAA for 9 < pT < 30 GeV, although Υ(1S) meson is more tightly bound and expected to be less suppressed than J/ψ meson according to the sequential suppression scenario. Unlike
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Fig. 3. The nuclear modification factor RAA of Υ(1S) as a function of centrality compared to the CMS results at 5 TeV (left). The RAA of Υ(1S) as a function of dimuon pT compared to RAA of prompt and nonprompt J/ψ from the previous ATLAS measurement at 5 TeV (right). The error bars indicate the statistical uncertainties and the boxes represent the systematic uncertainties. The boxes around RAA = 1 correspond to the correlated systematic uncertainties [7].
the clear binding-energy dependence observed in the suppression of three Υ(nS) states, there might be other effects that affect the production yields of charmonia and bottomonia differently, such as a different amount of regeneration, feed-down fractions, etc. Nonprompt J/ψ RAA , on the other hand, reflects the suppression of b-hadron driven by a different mechanism from the suppression of quarkonia, e.g., parton energy loss in the medium. 4. Summary A measurement of bottomonium production in pp and Pb + Pb collisions at 5.02 TeV with ATLAS at the LHC is presented. The measurement uses pp data collected in 2017 and Pb + Pb data collected in 2018 with a total integrated luminosity of 0.26 fb−1 and 1.38 nb−1 , respectively. The pp and Pb + Pb measurements are combined to obtain the nuclear modification factor and double ratio as functions of dimuon transverse momentum, dimuon rapidity, and centrality. Both Υ(1S) and Υ(2S) yields are suppressed with increasing centrality in Pb + Pb compared to those in pp collisions, and the excited state shows stronger suppression of yields than the ground state. The measurements are found to be consistent with the previous CMS results. A constraint on Υ(3S) yields in Pb + Pb collision is also provided via a combined measurement of Υ(2S+3S) yields. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
T. Matsui, H. Satz, Phys. Lett. B 178 (1986) 416. S. Digal, P. Petreczky, H. Satz, Phys. Lett. B 514 (2001) 57. arXiv:hep-ph/0105234. ALICE Collaboration, Phys. Lett. B 790 (2019) 89. arXiv:1805.04387. CMS Collaboration, Phys. Rev. Lett. 109 (2012) 222301, [Erratum: Phys. Rev. Lett.120,no.19,199903(2018)]. arXiv:1208.2826. PHENIX Collaboration, Phys. Rev. C 91 (2) (2015) 024913. arXiv:1404.2246. ATLAS Collaboration, JINST 3 (2008) S08003. ATLAS Collaboration, ATLAS-CONF-2019-054. CMS Collaboration, Eur. Phys. J. C 79 (2019) 277. arXiv:1809.11080, doi:10.1140/epjc/s10052-019-6774-8. ATLAS Collaboration, Eur. Phys. J. C 70 (2010) 823. arXiv:1005.4568. T. Sj¨ostrand, et al. 191 (2015) 159. arXiv:1410.3012. J. Pumplin, et al., JHEP 07 (2002) 012. arXiv:hep-ph/0201195. S. Agostinelli, et al., Nucl. Instrum. Meth. A 506 (2003) 250. M. Oreglia, SLAC-R-0236. CMS Collaboration, Phys. Lett. B 790 (2019) 270. arXiv:1805.09215. ATLAS Collaboration, Eur. Phys. J. C 78 (2018) 762. arXiv:1805.04077.
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Nuclear Physics A 1005 (2021) 121860 www.elsevier.com/locate/nuclphysa
XXVIIIth International Conference on Ultrarelativistic Nucleus-Nucleus Collisions (Quark Matter 2019)
Bottomonium production in Pb + Pb collisions with ATLAS Songkyo Lee, on behalf of the ATLAS collaboration Iowa State University, Ames, IA, 50010, USA
Abstract Bottomonium, a bound state of a bottom quark and its antiquark, is an excellent probe of the hot and dense medium created in heavy-ion collisions. The ATLAS collaboration at the LHC collected two large datasets of pp and Pb + Pb collisions in 2017 and 2018 corresponding to integrated luminosities of 0.26 fb−1 and 1.38 nb−1 respectively, at a centerof-mass energy per nucleon pair of 5.02 TeV. Bottomonium states are reconstructed via the dimuon decay channel in the absolute rapidity range of |y| < 1.5, and their production in Pb + Pb collisions is compared to that in pp collisions to extract the nuclear modification factor, RAA , as a function of transverse momentum, absolute rapidity, and event centrality. In addition, the relative suppression of the excited states Υ(nS) to the ground state Υ(1S) is studied. Keywords: Bottomonia, Quarkonia, QGP, nuclear modification factor, heavy-ion
1. Introduction In ultra-relativistic heavy-ion collisions, hadronic matter can experience a phase transition and turn into a state of deconfined quarks and gluons, Quark-Gluon Plasma (QGP). In such collisions, heavy-flavor quarks, especially charm and bottom, are produced at a very early stage and hence can be used to characterize the properties of the QGP. The production of quarkonia, a bound state of a heavy quark and its antiquark, in nucleus-nucleus collisions is predicted to be suppressed relative to that in proton-proton (pp) collisions due to the modification to the heavy-quark potential [1]. In particular, a sequential suppression of quarkonium states depending on their binding energies has been proposed as a thermometer of the QGP. It was found, for example, by the lattice calculation that the Υ(1S) survives well above the critical temperature T c needed to form the QGP, while the Υ(2S) melts at about 1.1T c and Υ(3S) cannot exist above T c [2]. The modifications of bottomonium yields in Pb + Pb collisions with respect to those in pp collisions can be quantified by the nuclear modification factor, RAA , which can be defined as RAA =
https://doi.org/10.1016/j.nuclphysa.2020.121860 0375-9474/© 2020 Elsevier B.V. All rights reserved.
NAA , T AA × σ pp
(1)
S. Lee
Nuclear Physics A 1005 (2021) 121860
where NAA is the per-event yield of bottomonium states, T AA is a mean value of the nuclear overlap function defined by the ratio of the number of binary nucleon-nucleon collisions to the inelastic nucleonnucleon cross-section, and σ pp is the bottomonium production cross section in pp collisions. The ALICE [3] and CMS [4] at the LHC and PHENIX [5] at the RHIC have studied bottomonium production in heavy-ion collisions, and this proceeding reports the measurement performed with the ATLAS detector [6] which will provide complementary information as each experiment covers a different kinematic range. The analysis is based on data collected during Pb + Pb collisions at a center-of-mass energy of 5.02 TeV per nucleon pairs in 2018 with an integrated luminosity of 1.38 nb−1 , and pp collisions in 2017 with an integrated luminosity of 0.26 fb−1 [7]. Three bottomonium states, Υ(nS), are reconstructed in their dimuon decay channel, within the absolute rapidity range of |y| < 1.5, transverse momentum range of pT < 30 GeV, and event centrality range of 0 − 80%. In addition to RAA , the double ratio of the excited-to-ground state nS)/Υ(1S) = [σΥ(nS) /σΥ(1S) ]/[σΥ(nS) /σΥ(1S) ], is production cross sections in Pb + Pb to pp collisions, ρΥ( pp pp AA Pb+Pb Pb+Pb also investigated to quantify binding-energy dependent suppression. 2. Analysis prodecure In Pb + Pb collisions, the candidate events are collected with a trigger which requires one muon to pass the Level-1 Trigger (L1) and then confirmed at the High-Level Trigger (HLT) with pμT > 4 GeV, and at least another muon candidate with pμT > 4 GeV to be found in the full Muon Spectrometer (MS) system. In pp collisions, a more strict trigger is used, which requires at least two L1 muon candidates with pμT > 4 GeV, subsequently confirmed at the HLT. Muon candidates are reconstructed based on a combination of chargedparticle tracks in the Inner Detector (ID) and the MS. A standard quality cut and vertex-association cut are applied to reduce fake muons and muons coming from other sources such as b-hadron decay. The Pb + Pb events are characterized by centrality using the total transverse energy deposit in the forward hadronic calorimeter. In this analysis, the interval 80–100% is excluded to suppress dimuon production from electromagnetic processes [8]. Each candidate is then weighted by wtotal (Υ(nS)) = 1/[A(Υ(nS)) · εreco (μ1 μ2 ) · εtrig (μ1 μ2 ) · εpvAsso (μ1 μ2 )], where A(Υ(nS)) is the acceptance for Υ(nS) → μ+ μ− decay, εreco is the muon reconstruction efficiency which also includes muon identification, εtrig is the trigger efficiency, and εpvAsso is the efficiency related to the primary vertex association. Monte Carlo (MC) simulations [9] of pp collision events with and without overlaying minimum-bias Pb + Pb data are used to study acceptance and efficiencies as well as signal yields extraction. Events are generated using PYTHIA 8 [10] with the CTEQ6L1 [11] parton distribution functions, and the response of the ATLAS detector is simulated using GEANT 4 [12]. The Υ(nS) yields are extracted performing an unbinned maximum-likelihood fit to the weighted dimuon μμ invariant mass distribution in each pμμ T , |y | or centrality interval. The probability distribution function for the fit is defined as a normalized sum of three Υ(nS) signals and the background components. Each signal shape is described by a combination of Crystal Ball [13] and Gaussian functions with a common mean but μμ μμ different widths. The background parameterizations vary with dimuon pμμ T . At low pT bins (pT < 6 GeV), μμ μμ or integrated over the whole pT range (pT < 30 GeV), an error function multiplied by an exponential function is used to describe the mμμ turn-on effects due to the single-muon pμT requirement, pμT > 4 GeV. At μμ higher pμμ T (pT > 6 GeV), such turn-on effects are not present, and a second-order polynomial is used to model the background contribution. 3. Results Figure 1 shows the RAA of Υ(nS) as functions of the number of participating nucleons, Npart (left), dimuon pT (middle), and the dimuon |y| (right). The centrality integrated results are also shown on the right panel of the left plot. In addition to the results for Υ(1S) and Υ(2S), the combined results of excited states, Υ(2S+3S), are presented as Υ(3S) peaks are barely seen in Pb + Pb data. The Υ(nS) states are observed to be suppressed over the whole kinematic range investigated, and the RAA values of Υ(2S) and Υ(2S+3S) are
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Fig. 1. The nuclear modification factor RAA of Υ(1S), Υ(2S), and Υ(2S+3S) as functions of centrality (left), dimuon pT (middle), and the dimuon |y| (right) at 5.02 TeV. The error bars indicate the statistical uncertainties and the boxes represent the systematic uncertainties. The grey boxes around RAA = 1 correspond to the correlated systematic uncertainty. For the left panel, data points for Υ(2S+3S) are slightly shifted to the right in order to avoid overlap with those for Υ(2S) [7].
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always lower than those of Υ(1S), which is consistent with the sequential suppression expectation. The RAA decreases with centrality for all three states. No strong pT or |y| dependence is observed. Figure 2 shows the double ratio of Υ(2S) and Υ(2S+3S) as functions of Npart (left), dimuon pT (middle), and the dimuon |y| (right). The centrality integrated results are also shown on the right panel of the left plot. The advantage of measuring the double ratios is that the acceptance and efficiency corrections are partially canceled, and the systematic uncertainty on the corrections is reduced. The Υ(2S) and Υ(2S+3S) results are always lower than unity, indicating the excited states are more suppressed than the ground state. The centrality-dependent result shows a slightly decreasing trend, but is also consistent with a flat behavior within uncertainties. No strong pT or |y| dependence is observed.
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nS)/Υ(1S) of Υ(2S), and Υ(2S+3S) as functions of centrality (left), dimuon p (middle), and the dimuon |y| Fig. 2. The double ratio ρΥ( T AA (right) at 5.02 TeV. The error bars indicate the statistical uncertainties and the boxes represent the systematic uncertainties. For the left panel, data points for Υ(2S+3S) are slightly shifted to the right in order to avoid overlap with those for Υ(2S) [7].
The left panel of Figure 3 shows the comparison of Υ(1S) RAA as a function of Npart results to the CMS measurements at the same collisions energy [14]. The centrality integrated results are also shown on the right panel of the left plot. The CMS results are systematically higher, but both measurements are consistent within uncertainties. It is worth noting that the kinematic ranges and the centrality intervals are different between the two experiments. The CMS results are obtained in pT < 30 GeV and |y| < 2.4. On the right panel of Figure 3, the Υ(1S) RAA as a function of dimuon pT is compared to those of prompt and nonprompt J/ψ mesons from the previous ATLAS measurement [15]. The resulting Υ(1S) RAA is found to be comparable with prompt J/ψ RAA for 9 < pT < 30 GeV, although Υ(1S) meson is more tightly bound and expected to be less suppressed than J/ψ meson according to the sequential suppression scenario. Unlike
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Fig. 3. The nuclear modification factor RAA of Υ(1S) as a function of centrality compared to the CMS results at 5 TeV (left). The RAA of Υ(1S) as a function of dimuon pT compared to RAA of prompt and nonprompt J/ψ from the previous ATLAS measurement at 5 TeV (right). The error bars indicate the statistical uncertainties and the boxes represent the systematic uncertainties. The boxes around RAA = 1 correspond to the correlated systematic uncertainties [7].
the clear binding-energy dependence observed in the suppression of three Υ(nS) states, there might be other effects that affect the production yields of charmonia and bottomonia differently, such as a different amount of regeneration, feed-down fractions, etc. Nonprompt J/ψ RAA , on the other hand, reflects the suppression of b-hadron driven by a different mechanism from the suppression of quarkonia, e.g., parton energy loss in the medium. 4. Summary A measurement of bottomonium production in pp and Pb + Pb collisions at 5.02 TeV with ATLAS at the LHC is presented. The measurement uses pp data collected in 2017 and Pb + Pb data collected in 2018 with a total integrated luminosity of 0.26 fb−1 and 1.38 nb−1 , respectively. The pp and Pb + Pb measurements are combined to obtain the nuclear modification factor and double ratio as functions of dimuon transverse momentum, dimuon rapidity, and centrality. Both Υ(1S) and Υ(2S) yields are suppressed with increasing centrality in Pb + Pb compared to those in pp collisions, and the excited state shows stronger suppression of yields than the ground state. The measurements are found to be consistent with the previous CMS results. A constraint on Υ(3S) yields in Pb + Pb collision is also provided via a combined measurement of Υ(2S+3S) yields. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
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