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| CMS-PAS-MUO-21-001 | ||
| Performance of CMS muon reconstruction in heavy ion collisions | ||
| CMS Collaboration | ||
| 21 August 2023 | ||
| Abstract: The performance of muon tracking, identification, triggering, momentum scale, and momentum resolution has been studied with the CMS detector at the LHC using data collected in proton-proton (pp) and lead-lead (PbPb) collisions at $ \sqrt {\smash [b]{s_{_{\mathrm {NN}}}}} = $ 5.02 TeV in 2017 and 2018 respectively, and in proton-lead (pPb) collisions at $ \sqrt {\smash [b]{s_{_{\mathrm {NN}}}}} = $ 8.16 TeV in 2016. Muon efficiencies, momentum scales, and momentum resolutions are presented and compared across the aforementioned collision systems, focusing on how the muon reconstruction performance varies from relatively small occupancy pp collisions, to pPb collisions, to the highest track multiplicity PbPb collisions. The muon-tracking, identification, and trigger efficiencies are above 90% throughout most of the studied multiplicity range. The momentum scale and resolution are unaffected by the occupancy. The excellent muon reconstruction of the CMS detector enables precision studies across all available collision systems. | ||
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Links:
CDS record (PDF) ;
CADI line (restricted) ;
These preliminary results are superseded in this paper, Submitted to JINST. The superseded preliminary plots can be found here. |
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| Figures | |
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Figure 1:
Longitudinal layout of one quadrant of the CMS detector. The drawing shows the four DT stations in the muon barrel (MB1-MB4, yellow), the four CSC stations in the muon endcap (ME1-ME4, green) and the RPC stations (blue). |
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Figure 2:
Left: Distribution of $ N_{\mathrm{tracks}} $ in pp at $ \sqrt{s}= $ 5.02 TeV (black), pPb at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}}= $ 5.02 TeV (green), pPb at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}}= $ 8.16 TeV (blue), and PbPb at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}}= $ 5.02 TeV (red). The distribution of $ N_{\mathrm{tracks}} $ in the most central PbPb collisions (0-20% centrality) is shown in magenta. The values are uncorrected. Right: Translation of the centrality in PbPb collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}}= $ 5.02 TeV into an equivalent pileup in pp collisions at $ \sqrt{s}= $ 14 TeV. |
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Figure 2-a:
Left: Distribution of $ N_{\mathrm{tracks}} $ in pp at $ \sqrt{s}= $ 5.02 TeV (black), pPb at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}}= $ 5.02 TeV (green), pPb at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}}= $ 8.16 TeV (blue), and PbPb at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}}= $ 5.02 TeV (red). The distribution of $ N_{\mathrm{tracks}} $ in the most central PbPb collisions (0-20% centrality) is shown in magenta. The values are uncorrected. Right: Translation of the centrality in PbPb collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}}= $ 5.02 TeV into an equivalent pileup in pp collisions at $ \sqrt{s}= $ 14 TeV. |
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Figure 2-b:
Left: Distribution of $ N_{\mathrm{tracks}} $ in pp at $ \sqrt{s}= $ 5.02 TeV (black), pPb at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}}= $ 5.02 TeV (green), pPb at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}}= $ 8.16 TeV (blue), and PbPb at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}}= $ 5.02 TeV (red). The distribution of $ N_{\mathrm{tracks}} $ in the most central PbPb collisions (0-20% centrality) is shown in magenta. The values are uncorrected. Right: Translation of the centrality in PbPb collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}}= $ 5.02 TeV into an equivalent pileup in pp collisions at $ \sqrt{s}= $ 14 TeV. |
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Figure 3:
Dimuon event from a PbPb\ collision in the CMS detector. |
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Figure 4:
Example fit to the data in PbPb collisions. The three panels show invariant mass distributions of the tag-and-probe pair in central 0-10% collisions fitted with signal and background components. Left panel: total spectrum. Middle panel: spectrum for pairs where probe passed the muon identification selection. Right panel: spectrum for muon pairs where probe failed the cut. The vertical scale of the failing probes is zoomed-in by a factor of 5 to enlarge the peak. The efficiency is obtained by dividing the number of pairs in the passing peak by number of pairs in the total peak. The efficiency is retrieved directly from the simultaneous fit and in this particular case is $ \epsilon = $ 0.968 $ \pm $ 0.002. |
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Figure 5:
Muon reconstruction efficiency (defined as the probability that a given muon with a tracker track will be reconstructed as both a global and a PF muon) plotted as a function of the number of tracks in pp, pPb, and PbPb collisions. Open symbols are the MC results corresponding to each data set. Only statistical uncertainties are shown. |
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Figure 6:
Regions of the CMS detector commonly used in the heavy ion muon analyses for pPb (left) and PbPb (right). For each panel, the simulated single-muon identification efficiency is plotted as a function of generated muon $ |\eta| $ and $ p_{\mathrm{T}} $. The lower-threshold curves are for muon identification, and are used only by those analyses that do not use a dedicated muon trigger. The higher-threshold curves are used by most analyses (those using the muon trigger information). |
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Figure 6-a:
Regions of the CMS detector commonly used in the heavy ion muon analyses for pPb (left) and PbPb (right). For each panel, the simulated single-muon identification efficiency is plotted as a function of generated muon $ |\eta| $ and $ p_{\mathrm{T}} $. The lower-threshold curves are for muon identification, and are used only by those analyses that do not use a dedicated muon trigger. The higher-threshold curves are used by most analyses (those using the muon trigger information). |
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Figure 6-b:
Regions of the CMS detector commonly used in the heavy ion muon analyses for pPb (left) and PbPb (right). For each panel, the simulated single-muon identification efficiency is plotted as a function of generated muon $ |\eta| $ and $ p_{\mathrm{T}} $. The lower-threshold curves are for muon identification, and are used only by those analyses that do not use a dedicated muon trigger. The higher-threshold curves are used by most analyses (those using the muon trigger information). |
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Figure 7:
Hybrid-soft ID efficiency for global muons as a function of $ \eta $ (left) and $ p_{\mathrm{T}} $ (right) in pp and PbPb collisions. The muons are restricted in acceptance as shown by the red line in the right panel of Fig. 6. Only statistical uncertainties are shown. |
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Figure 7-a:
Hybrid-soft ID efficiency for global muons as a function of $ \eta $ (left) and $ p_{\mathrm{T}} $ (right) in pp and PbPb collisions. The muons are restricted in acceptance as shown by the red line in the right panel of Fig. 6. Only statistical uncertainties are shown. |
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Figure 7-b:
Hybrid-soft ID efficiency for global muons as a function of $ \eta $ (left) and $ p_{\mathrm{T}} $ (right) in pp and PbPb collisions. The muons are restricted in acceptance as shown by the red line in the right panel of Fig. 6. Only statistical uncertainties are shown. |
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Figure 8:
Tight ID efficiency for global and PF muons as a function of the number of tracks in pp, pPb, and PbPb collisions. Open symbols are the MC results corresponding to each data set. Only statistical uncertainties are shown. |
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Figure 9:
Tight ID efficiency as a function of $ \eta $ (left) and $ p_{\mathrm{T}} $ (right) in pp, pPb, and PbPb collisions. The probe is both a global muon and a PF muon. Open symbols are the MC results corresponding to each data set. Only statistical uncertainties are shown. |
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Figure 9-a:
Tight ID efficiency as a function of $ \eta $ (left) and $ p_{\mathrm{T}} $ (right) in pp, pPb, and PbPb collisions. The probe is both a global muon and a PF muon. Open symbols are the MC results corresponding to each data set. Only statistical uncertainties are shown. |
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Figure 9-b:
Tight ID efficiency as a function of $ \eta $ (left) and $ p_{\mathrm{T}} $ (right) in pp, pPb, and PbPb collisions. The probe is both a global muon and a PF muon. Open symbols are the MC results corresponding to each data set. Only statistical uncertainties are shown. |
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Figure 10:
Trigger efficiency of tight muons as a function of the number of tracks. The trigger requires a single muon with $ p_{\mathrm{T}} $ above 12 GeV. The efficiency is calculated for muons with $ p_{\mathrm{T}} $ above 15 GeV in order to avoid threshold effects. Open symbols are the MC results corresponding to each data set. Only statistical uncertainties are shown. |
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Figure 11:
Trigger efficiency of tight muons as a function of $ \eta $ (left) and $ p_{\mathrm{T}} $ (right) in pp, pPb, and PbPb collisions. Bottom panel shows the ratio between data and MC simulation (MC points are omitted in the top panels for clarity). Only statistical uncertainties are shown. |
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Figure 11-a:
Trigger efficiency of tight muons as a function of $ \eta $ (left) and $ p_{\mathrm{T}} $ (right) in pp, pPb, and PbPb collisions. Bottom panel shows the ratio between data and MC simulation (MC points are omitted in the top panels for clarity). Only statistical uncertainties are shown. |
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Figure 11-b:
Trigger efficiency of tight muons as a function of $ \eta $ (left) and $ p_{\mathrm{T}} $ (right) in pp, pPb, and PbPb collisions. Bottom panel shows the ratio between data and MC simulation (MC points are omitted in the top panels for clarity). Only statistical uncertainties are shown. |
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Figure 12:
Purity for the L1, L2, and L3 trigger steps, compared among pp, pPb, and PbPb collisions. The online muon must have $ p_{\mathrm{T}} > $ 15 GeV, while the offline muon matched to it must pass the tight ID selection and have $ |\eta| < $ 2.4. Details of our purity definition are given in the text. Statistical uncertainties are smaller than the symbols. |
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Figure 13:
Mass resolution at the $ \mathrm{J}/\psi $ (left) and Z (right) peaks as a function of the number of tracks from MC simulations (open points) and real data (closed points) in pp, pPb, and PbPb collisions. Only statistical uncertainties are shown. |
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Figure 13-a:
Mass resolution at the $ \mathrm{J}/\psi $ (left) and Z (right) peaks as a function of the number of tracks from MC simulations (open points) and real data (closed points) in pp, pPb, and PbPb collisions. Only statistical uncertainties are shown. |
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Figure 13-b:
Mass resolution at the $ \mathrm{J}/\psi $ (left) and Z (right) peaks as a function of the number of tracks from MC simulations (open points) and real data (closed points) in pp, pPb, and PbPb collisions. Only statistical uncertainties are shown. |
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Figure 14:
Mass scale at the $ \mathrm{J}/\psi $ (left) and Z (right) peaks as a function of the number of tracks from MC simulations (open points) and real data (closed points) in pp, pPb, and PbPb collisions, calculated as the measured mass $ m_{\textrm{Fit}} $ divided by the PDG mass $ m_{\textrm{PDG}} $. Only statistical uncertainties are shown. |
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Figure 14-a:
Mass scale at the $ \mathrm{J}/\psi $ (left) and Z (right) peaks as a function of the number of tracks from MC simulations (open points) and real data (closed points) in pp, pPb, and PbPb collisions, calculated as the measured mass $ m_{\textrm{Fit}} $ divided by the PDG mass $ m_{\textrm{PDG}} $. Only statistical uncertainties are shown. |
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Figure 14-b:
Mass scale at the $ \mathrm{J}/\psi $ (left) and Z (right) peaks as a function of the number of tracks from MC simulations (open points) and real data (closed points) in pp, pPb, and PbPb collisions, calculated as the measured mass $ m_{\textrm{Fit}} $ divided by the PDG mass $ m_{\textrm{PDG}} $. Only statistical uncertainties are shown. |
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Figure 15:
Mass resolution at the $ \mathrm{J}/\psi $ (left) and Z (right) peaks as a function of $ |y^{\mu\mu}| $ from MC simulations (open points) and real data (closed points) in pp, pPb, and PbPb collisions. Only statistical uncertainties are shown. |
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Figure 15-a:
Mass resolution at the $ \mathrm{J}/\psi $ (left) and Z (right) peaks as a function of $ |y^{\mu\mu}| $ from MC simulations (open points) and real data (closed points) in pp, pPb, and PbPb collisions. Only statistical uncertainties are shown. |
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Figure 15-b:
Mass resolution at the $ \mathrm{J}/\psi $ (left) and Z (right) peaks as a function of $ |y^{\mu\mu}| $ from MC simulations (open points) and real data (closed points) in pp, pPb, and PbPb collisions. Only statistical uncertainties are shown. |
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Figure 16:
Mass scale at the $ \mathrm{J}/\psi $ (left) and Z (right) peaks as a function of $ |y^{\mu\mu}| $ from MC simulations (open points) and real data (closed points) in pp, pPb, and PbPb collisions, calculated as the measured mass $ m_{\textrm{Fit}} $ divided by the PDG mass $ m_{\textrm{PDG}} $. Only statistical uncertainties are shown. |
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Figure 16-a:
Mass scale at the $ \mathrm{J}/\psi $ (left) and Z (right) peaks as a function of $ |y^{\mu\mu}| $ from MC simulations (open points) and real data (closed points) in pp, pPb, and PbPb collisions, calculated as the measured mass $ m_{\textrm{Fit}} $ divided by the PDG mass $ m_{\textrm{PDG}} $. Only statistical uncertainties are shown. |
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Figure 16-b:
Mass scale at the $ \mathrm{J}/\psi $ (left) and Z (right) peaks as a function of $ |y^{\mu\mu}| $ from MC simulations (open points) and real data (closed points) in pp, pPb, and PbPb collisions, calculated as the measured mass $ m_{\textrm{Fit}} $ divided by the PDG mass $ m_{\textrm{PDG}} $. Only statistical uncertainties are shown. |
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Figure 17:
Mass resolution (left) and mass scale (right) versus the probe muon pseudorapidity $ |\eta^\mu| $ at the $ \mathrm{J}/\psi $ peak in pp collisions. The solid colored points are real data and the open points are MC simulations. Our results (black diamonds) are compared to measurements from a previous pp analysis done at $ \sqrt{s}= $ 7 TeV (purple crosses) [30]. Only statistical uncertainties are shown. |
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Figure 17-a:
Mass resolution (left) and mass scale (right) versus the probe muon pseudorapidity $ |\eta^\mu| $ at the $ \mathrm{J}/\psi $ peak in pp collisions. The solid colored points are real data and the open points are MC simulations. Our results (black diamonds) are compared to measurements from a previous pp analysis done at $ \sqrt{s}= $ 7 TeV (purple crosses) [30]. Only statistical uncertainties are shown. |
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Figure 17-b:
Mass resolution (left) and mass scale (right) versus the probe muon pseudorapidity $ |\eta^\mu| $ at the $ \mathrm{J}/\psi $ peak in pp collisions. The solid colored points are real data and the open points are MC simulations. Our results (black diamonds) are compared to measurements from a previous pp analysis done at $ \sqrt{s}= $ 7 TeV (purple crosses) [30]. Only statistical uncertainties are shown. |
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Figure 18:
Distributions of $ N_{\mathrm{tracks}} $ in the PbPb collisions for various ranges of centrality in a minimum bias data set (left) and a triggered data set (right). |
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Figure 18-a:
Distributions of $ N_{\mathrm{tracks}} $ in the PbPb collisions for various ranges of centrality in a minimum bias data set (left) and a triggered data set (right). |
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Figure 18-b:
Distributions of $ N_{\mathrm{tracks}} $ in the PbPb collisions for various ranges of centrality in a minimum bias data set (left) and a triggered data set (right). |
| Tables | |
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Table 1:
Average number of reconstructed tracks, $ \langle N_{\mathrm{tracks}} \rangle $, in our data sets. The values are not corrected for pileup. Two columns are shown for PbPb. The left column shows the values that are obtained from a minimum-bias data set, and correspond to distributions shown in the left panel of Fig. 2. The right column shows the values in muon-triggered data, and correspond to the location of points shown in the performance plots of this note. Details in Appendix 11. |
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Table 2:
Characteristics of selected collisions collected during LHC Run 2. |
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Table 3:
Overview of muon identification and triggers used in $ \mathrm{J}/\psi $ and $ \Upsilon $ analyses. Descriptions of these are given in the text. |
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Table 4:
Overview of muon identification and triggers used in Z, W, and t analyses. The settings are comparable among the collision systems. A description of muon identification and triggers is given in the text. |
| Summary |
| We have presented efficiencies of muon reconstruction, identification, and triggering, as well as measurements of the dimuon mass scale and resolution of the CMS detector. The efficiencies were estimated using the data-driven tag-and-probe technique discussed in Section 5. The fits to the invariant mass spectra were also used to derive the mass scale and resolution. We have extended previous studies of the muon performance in pp collisions to the heavy ion environment using PbPb data at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}}= $ 5.02 TeV and pPb data at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}}= $ 8.16 TeV. The efficiencies were also measured in pp data at $ \sqrt{s}= $ 5.02 TeV for comparison across all collision systems as a function of charged particle multiplicity. In all cases, the efficiencies are high (typically above 90%), even with extremely high occupancies. We observe a slight drop (1-2%) in the muon-identification efficiency at high $ p_{\mathrm{T}} $ in the most central PbPb events at multiplicities that are unattainable in pPb or pp events. In the low-$ p_{\mathrm{T}} $ region, the muon-identification efficiency is comparable between pp and PbPb collisions except in the region of highest occupancy at very low $ p_{\mathrm{T}} $ ($ < $ 5 GeV) and forward rapidity ($ |\eta| > $ 2). This drop in efficiency is expected because the high number of tracks in the inner parts of the detector complicates the matching of tracks between the muon chambers and the tracker. A slight decrease ($ {\approx}$3%) in reconstruction efficiency at high occupancies is also observed. Additionally, the trigger efficiency decreases in the most central PbPb events. A relative reduction of $ {\approx}8% $ in trigger efficiency occurs between the lowest and highest $ N_{\mathrm{tracks}} $ bins in PbPb. This reduction is more pronounced than the corresponding decrease in muon identification and reconstruction efficiencies, suggesting that the CMS single muon trigger is more sensitive to detector occupancy. In most cases, Monte Carlo calculations of the corresponding efficiencies capture the trends seen in data, indicating that the main features are contained in the detector simulation. In a few instances, the MC efficiencies overestimate those obtained from real data (by up to 4 percentage points), highlighting the need for independent data-driven techniques of efficiency estimation. The excellent muon performance of the CMS detector has allowed us to have a robust muon and dimuon program in the heavy ion environment, leading to many muon-based measurements, including heavy flavor [31,5,32,33,34,35,36,37,38,39,40], electroweak bosons [41,25,42,43,44,45], and jets [46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63]. |
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Compact Muon Solenoid LHC, CERN |
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