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| CMS-PAS-LUM-20-002 | ||
| CMS luminosity measurement for nucleus-nucleus collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}}= $ 5.02 TeV in Run 2 | ||
| CMS Collaboration | ||
| 22 July 2024 | ||
| Abstract: The measurements of the luminosity delivered to the CMS experiment during the lead-lead (PbPb) data taking in 2015 and 2018 are presented. The nucleon-nucleon collisions were recorded at a center-of-mass energy of 5.02 TeV, with the 2018 data sample being three times larger than that of 2015. Three subdetectors are used: the fast beam conditions monitor, the forward hadron calorimeter, and the pixel luminosity telescope. The absolute luminosity calibration is determined using the so-called van der Meer (vdM) technique that relies on transverse beam separation scans. The dominant sources of uncertainty are the transverse factorizability of the bunch density profiles, and in 2015, the cross-detector consistency. The total uncertainty on the integrated luminosity, considering also the stability of the vdM-calibrated subdetector response over time, amounts to 3.0% for 2015, and 1.7% for 2018. | ||
| Links: CDS record (PDF) ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Schematic cross section through the CMS detector in the $ r-z $ plane. The main luminometers used during heavy ion runs in Run 2, as described in the text, are highlighted, showing the PLT, BCM1F, and HF. The center of the detector, corresponding to the approximate position of the PbPb collision point, is located at the origin. |
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Figure 2:
Horizontal and vertical beam displacements derived from the LHC corrector magnet currents during the vdM scan program in fill 4689 (upper) in 2015, fill 7442 (middle), and fill 7443 (lower) in 2018. The first x offset scan in the middle plot was performed with smaller then planned maximal separation and was retaken. The last vdM scan in fill 7442 was interrupted and thus not used in the analysis. |
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Figure 2-a:
Horizontal and vertical beam displacements derived from the LHC corrector magnet currents during the vdM scan program in fill 4689 (upper) in 2015, fill 7442 (middle), and fill 7443 (lower) in 2018. The first x offset scan in the middle plot was performed with smaller then planned maximal separation and was retaken. The last vdM scan in fill 7442 was interrupted and thus not used in the analysis. |
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Figure 2-b:
Horizontal and vertical beam displacements derived from the LHC corrector magnet currents during the vdM scan program in fill 4689 (upper) in 2015, fill 7442 (middle), and fill 7443 (lower) in 2018. The first x offset scan in the middle plot was performed with smaller then planned maximal separation and was retaken. The last vdM scan in fill 7442 was interrupted and thus not used in the analysis. |
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Figure 2-c:
Horizontal and vertical beam displacements derived from the LHC corrector magnet currents during the vdM scan program in fill 4689 (upper) in 2015, fill 7442 (middle), and fill 7443 (lower) in 2018. The first x offset scan in the middle plot was performed with smaller then planned maximal separation and was retaken. The last vdM scan in fill 7442 was interrupted and thus not used in the analysis. |
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Figure 3:
Examples of 2015 scan distributions, i.e.,, the normalized rate recorded by PLT in BCID 955 as a function of beam separation ($ \Delta $) in $ x $ (left) and $ y $ (right), respectively. The purple curve corresponds to the single-Gaussian fit. The reduced $ \chi^2 $ is shown on the plots. The bottom panels include the residuals, i.e.,, the difference between the measured and fitted values divided by the statistical uncertainty of the rate. |
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Figure 3-a:
Examples of 2015 scan distributions, i.e.,, the normalized rate recorded by PLT in BCID 955 as a function of beam separation ($ \Delta $) in $ x $ (left) and $ y $ (right), respectively. The purple curve corresponds to the single-Gaussian fit. The reduced $ \chi^2 $ is shown on the plots. The bottom panels include the residuals, i.e.,, the difference between the measured and fitted values divided by the statistical uncertainty of the rate. |
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Figure 3-b:
Examples of 2015 scan distributions, i.e.,, the normalized rate recorded by PLT in BCID 955 as a function of beam separation ($ \Delta $) in $ x $ (left) and $ y $ (right), respectively. The purple curve corresponds to the single-Gaussian fit. The reduced $ \chi^2 $ is shown on the plots. The bottom panels include the residuals, i.e.,, the difference between the measured and fitted values divided by the statistical uncertainty of the rate. |
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Figure 4:
The absolute difference in the displacement of the two beams at IP as measured by the DOROS and arc BPMs during vdM1--5 scans as a function of the nominal beam separation in $ x $ (left) and $ y $ (right), respectively. The values and the statistical uncertainty in the measured beam positions are shown. |
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Figure 4-a:
The absolute difference in the displacement of the two beams at IP as measured by the DOROS and arc BPMs during vdM1--5 scans as a function of the nominal beam separation in $ x $ (left) and $ y $ (right), respectively. The values and the statistical uncertainty in the measured beam positions are shown. |
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Figure 4-b:
The absolute difference in the displacement of the two beams at IP as measured by the DOROS and arc BPMs during vdM1--5 scans as a function of the nominal beam separation in $ x $ (left) and $ y $ (right), respectively. The values and the statistical uncertainty in the measured beam positions are shown. |
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Figure 5:
Difference between the reconstructed beamspot and the nominal positions, as a function of the latter, during the 2015 length scale scans. The results are shown for the $ x $ (left) and $ y $ (right) LSC scans, separately for the forward (purple) and backward (green) directions. The lines denote linear fits with slope and $ \chi^2/\text{dof} $ values shown in the legend. |
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Figure 5-a:
Difference between the reconstructed beamspot and the nominal positions, as a function of the latter, during the 2015 length scale scans. The results are shown for the $ x $ (left) and $ y $ (right) LSC scans, separately for the forward (purple) and backward (green) directions. The lines denote linear fits with slope and $ \chi^2/\text{dof} $ values shown in the legend. |
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Figure 5-b:
Difference between the reconstructed beamspot and the nominal positions, as a function of the latter, during the 2015 length scale scans. The results are shown for the $ x $ (left) and $ y $ (right) LSC scans, separately for the forward (purple) and backward (green) directions. The lines denote linear fits with slope and $ \chi^2/\text{dof} $ values shown in the legend. |
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Figure 6:
Model dependence of the nonfactorization correction on the visible cross section in 2015 for double Gaussian and super-Gaussian models with and without an extra constant (left). The points represent the mean values, with the error bars defined by the RMS over residual OD variations and luminometers of the BCID averaged corrections. The dashed lines indicate the weighted and unweighted averages and the shaded bands show the RMS range around the average for the six models. Time dependence of the nonfactorization correction in 2018 for fill 7443, based on the scan combinations vdM3+offset2, vdM3+diag2, offset2+vdM4, diag2+vdM4, and vdM5+diag3 using the most stable supG model as baseline (right). The error bars signify the combined uncertainty. The dashed blue line shows the trend line over the scan combinations, with the solid grey area denoting the total uncertainty. The hatched areas denote contributions to the uncertainty due to the model choice including supG, DG, SG+supG and supG+supG (orange) and due to the fit (blue) uncertainty of the trend line. The vertical lines mark the times of vdM2--5 scans. The $ x $ axis indicates time relative to the start of fill 7443. |
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Figure 7:
Cross-detector comparison of the fitted $ \Sigma_{x} $ as a function of BCID for the 2015 vdM3 (left) and 2018 vdM5 (right) scans. The horizontal dashed lines indicate the averaged values of the corresponding set of points. |
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Figure 7-a:
Cross-detector comparison of the fitted $ \Sigma_{x} $ as a function of BCID for the 2015 vdM3 (left) and 2018 vdM5 (right) scans. The horizontal dashed lines indicate the averaged values of the corresponding set of points. |
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Figure 7-b:
Cross-detector comparison of the fitted $ \Sigma_{x} $ as a function of BCID for the 2015 vdM3 (left) and 2018 vdM5 (right) scans. The horizontal dashed lines indicate the averaged values of the corresponding set of points. |
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Figure 8:
The measured $ \sigma_{\text{vis}} $ in 2015 (upper) and in 2018 (lower) calculated by averaging over all BCIDs for each vdM scan pair after all corrections are applied. From left to right the results are given for HFOC, PLT, and in 2018 for BCM1F. The error bars represent the standard deviation of the measurements per BCID. To calculate the central value of $ \sigma_{\text{vis}} $ the uncertainty-weighted average of the individual measurements for the various vdM scans are calculated along with its uncertainty. |
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Figure 8-a:
The measured $ \sigma_{\text{vis}} $ in 2015 (upper) and in 2018 (lower) calculated by averaging over all BCIDs for each vdM scan pair after all corrections are applied. From left to right the results are given for HFOC, PLT, and in 2018 for BCM1F. The error bars represent the standard deviation of the measurements per BCID. To calculate the central value of $ \sigma_{\text{vis}} $ the uncertainty-weighted average of the individual measurements for the various vdM scans are calculated along with its uncertainty. |
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Figure 8-b:
The measured $ \sigma_{\text{vis}} $ in 2015 (upper) and in 2018 (lower) calculated by averaging over all BCIDs for each vdM scan pair after all corrections are applied. From left to right the results are given for HFOC, PLT, and in 2018 for BCM1F. The error bars represent the standard deviation of the measurements per BCID. To calculate the central value of $ \sigma_{\text{vis}} $ the uncertainty-weighted average of the individual measurements for the various vdM scans are calculated along with its uncertainty. |
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Figure 8-c:
The measured $ \sigma_{\text{vis}} $ in 2015 (upper) and in 2018 (lower) calculated by averaging over all BCIDs for each vdM scan pair after all corrections are applied. From left to right the results are given for HFOC, PLT, and in 2018 for BCM1F. The error bars represent the standard deviation of the measurements per BCID. To calculate the central value of $ \sigma_{\text{vis}} $ the uncertainty-weighted average of the individual measurements for the various vdM scans are calculated along with its uncertainty. |
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Figure 8-d:
The measured $ \sigma_{\text{vis}} $ in 2015 (upper) and in 2018 (lower) calculated by averaging over all BCIDs for each vdM scan pair after all corrections are applied. From left to right the results are given for HFOC, PLT, and in 2018 for BCM1F. The error bars represent the standard deviation of the measurements per BCID. To calculate the central value of $ \sigma_{\text{vis}} $ the uncertainty-weighted average of the individual measurements for the various vdM scans are calculated along with its uncertainty. |
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Figure 8-e:
The measured $ \sigma_{\text{vis}} $ in 2015 (upper) and in 2018 (lower) calculated by averaging over all BCIDs for each vdM scan pair after all corrections are applied. From left to right the results are given for HFOC, PLT, and in 2018 for BCM1F. The error bars represent the standard deviation of the measurements per BCID. To calculate the central value of $ \sigma_{\text{vis}} $ the uncertainty-weighted average of the individual measurements for the various vdM scans are calculated along with its uncertainty. |
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Figure 9:
Cross-detector comparison of the measured luminosities using the PLT over HFOC luminosity ratio, cumulatively (upper) and as a function of time (middle and lower), for 2015 (upper left and middle), and 2018 (upper right and lower). |
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Figure 9-a:
Cross-detector comparison of the measured luminosities using the PLT over HFOC luminosity ratio, cumulatively (upper) and as a function of time (middle and lower), for 2015 (upper left and middle), and 2018 (upper right and lower). |
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Figure 9-b:
Cross-detector comparison of the measured luminosities using the PLT over HFOC luminosity ratio, cumulatively (upper) and as a function of time (middle and lower), for 2015 (upper left and middle), and 2018 (upper right and lower). |
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Figure 9-c:
Cross-detector comparison of the measured luminosities using the PLT over HFOC luminosity ratio, cumulatively (upper) and as a function of time (middle and lower), for 2015 (upper left and middle), and 2018 (upper right and lower). |
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Figure 9-d:
Cross-detector comparison of the measured luminosities using the PLT over HFOC luminosity ratio, cumulatively (upper) and as a function of time (middle and lower), for 2015 (upper left and middle), and 2018 (upper right and lower). |
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Figure 10:
The ratio of $ \sigma_{\text{vis}} $-normalized PLT to HFOC cross sections from emittance scans recorded during fills 7442--7443 and routine data-taking conditions. The uncertainty is of statistical nature only. |
| Tables | |
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Table 1:
Summary of the Run 2 LHC conditions at IP5 for the scan sessions with PbPb collisions in 2015 and 2018. The column $ \mathcal{L}_{\text{init}} $ corresponds to the initial instantaneous luminosity, with all other variables defined in the text. The columns corresponding to ``Number of scan pairs'' indicate the total number of vdM, length scale calibration, off-axis (offset or diagonal), and emittance scan pairs as well as super separation periods that were performed, counting only the scans that were used in the analysis. |
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Table 2:
Ghost fraction and satellites fraction per beam and total impact on $ \sigma_{\text{vis}} $. |
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Table 3:
The scan combinations used in the nonfactorization measurements. The components of the scan combinations are listed in time order, see also Fig. 2. |
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Table 4:
Summary of the BCID-averaged corrections to $ \sigma_{\text{vis}} $ (in %) obtained with the vdM scan calibrations at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}}= $ 5.02 TeV PbPb collisions in Run 2. When applicable, the average correction applied to $ \sigma_{\text{vis}} $ of the primary Pixel Luminosity Telescope is shown. |
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Table 5:
Summary of contributions to the relative systematic uncertainty in $ \sigma_{\text{vis}} $ (in %) at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}}= $ 5.02 TeV PbPb collisions in Run 2. The systematic uncertainty is divided into groups affecting the description of the vdM profile and the bunch population product measurement (normalization), and the measurement of the rate in physics running conditions (integration). The last column specifies the correlation of the systematic source among years. |
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Table 6:
Summary of the measured recorded luminosity integrated over time periods where the CMS subsystems necessary for physics studies with muons in the final state were in good operational state, as well as contributions from correlated and uncorrelated sources of uncertainties at $ \sigma_{\text{vis}} $ (in %) at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}}= $ 5.02 TeV PbPb collisions in Run 2. The actual amount of recorded data can depend significantly on the data quality requirements imposed on the various subsystems. |
| Summary |
| The measurement of the luminosity delivered to the CMS experiment during the lead-lead (PbPb) data-taking period in Run 2 at a nucleon-nucleon center-of-mass energy of 5.02 TeV is presented. Three subdetectors are used (in the order of their priority to provide the luminosity measurement): the pixel luminosity telescope, the forward hadron calorimeter, and the fast beam conditions monitor. Two groups of uncertainties are formed for effects related to ``normalization'' and ``integration''. The former concerns the visible cross section ($ \sigma_{\text{vis}} $) as determined from the van der Meer scan procedure, and the latter the stability and quality of the measurements by the luminosity subdetectors under the PbPb data-taking conditions. The dominant sources of uncertainty contributing to the normalization and integration, as estimated in 2018, are associated to the transverse factorizability of the colliding-bunch densities and cross-detector stability, respectively. In 2015, transverse factorizability and cross-detector consistency and stability represent the main sources of the uncertainty. The estimated stability in 2018 is found to be consistent with the analysis of short vdM-like scans performed regularly during the PbPb data-taking period. The uncertainty in the normalization is found to be 2.9% and 1.5% in 2015 and 2018, respectively, while it is almost similar and about 0.7--0.8% in the integration in both years. These are treated as uncorrelated and are summed in quadrature. When applying the vdM calibration to the entire periods and requiring that the detectors essential for studying final states including muons took high quality data, the total recorded luminosity is 0.43 $ \pm $ 0.01 nb$^{-1}$ with a relative precision of 3.0% in 2015, and 1.7 $ \pm $ 0.1 nb$^{-1}$ with a relative precision of 1.7% in 2018. Taking into account the correlations among various systematic sources specified in the last column of Table 5, one can find the combined 2015 $ + $ 2018 luminosity measurement. The results are summarized in Table 6. It should be noted that the combined uncorrelated contribution is mainly determined by the 2018 uncorrelated precision since it is the significantly larger data set and the precision in 2015 is similar. In summary, the total uncertainty in the luminosity recorded to the CMS experiment with nucleus-nucleus collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}}= $ 5.02 TeV is estimated to be 3.0% and 1.7% in 2015 and 2018, respectively. The combined data set has a luminosity uncertainty of 1.6%. |
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