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| CMS-BPH-22-009 ; CERN-EP-2024-155 | ||
| Measurement of the polarizations of prompt and non-prompt $ \mathrm{J}/\psi $ and $\psi \text{(2S)} $ mesons produced in pp collisions at $ \sqrt{s} = $ 13 TeV | ||
| CMS Collaboration | ||
| 20 June 2024 | ||
| Submitted to Phys. Lett. B | ||
| Abstract: The polarizations of prompt and non-prompt $ \mathrm{J}/\psi $ and $\psi \text{(2S)} $ mesons are measured in proton-proton collisions at $ \sqrt{s} = $ 13 TeV, using data samples collected by the CMS experiment in 2017 and 2018, corresponding to a total integrated luminosity of 103.3 fb$ ^{-1} $. Based on the analysis of the dimuon decay angular distributions in the helicity frame, the polar anisotropy, $ \lambda_\vartheta $, is measured as a function of the transverse momentum, $ p_{\mathrm{T}} $, of the charmonium states, in the 25-120 and 20-100 GeV ranges for the $ \mathrm{J}/\psi $ and $\psi \text{(2S)} $, respectively. The non-prompt polarizations agree with predictions based on the hypothesis that, for $ p_{\mathrm{T}} \gtrsim $ 25 GeV, the non-prompt $ \mathrm{J}/\psi $ and $\psi \text{(2S)} $ are predominantly produced in two-body B meson decays. The prompt results clearly exclude strong transverse polarizations, even for $ p_{\mathrm{T}} $ exceeding 30 times the $ \mathrm{J}/\psi $ mass, where $ \lambda_\vartheta $ tends to an asymptotic value around 0.3. Taken together with previous measurements, by CMS and LHCb at $ \sqrt{s} = $ 7 TeV, the prompt polarizations show a significant variation with $ p_{\mathrm{T}} $, at low $ p_{\mathrm{T}} $. | ||
| Links: e-print arXiv:2406.14409 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Measured dimuon decay length vs. mass distributions for the $ \mathrm{J}/\psi $ (left) and $\psi \text{(2S)} $ (right) samples, showing the rectangular regions used in the analysis. The prompt and non-prompt signal regions are labeled PRS and NPS, respectively, while the remaining regions are non-signal (sideband) regions. |
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Figure 1-a:
Measured dimuon decay length vs. mass distributions for the $ \mathrm{J}/\psi $ (left) and $\psi \text{(2S)} $ (right) samples, showing the rectangular regions used in the analysis. The prompt and non-prompt signal regions are labeled PRS and NPS, respectively, while the remaining regions are non-signal (sideband) regions. |
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Figure 1-b:
Measured dimuon decay length vs. mass distributions for the $ \mathrm{J}/\psi $ (left) and $\psi \text{(2S)} $ (right) samples, showing the rectangular regions used in the analysis. The prompt and non-prompt signal regions are labeled PRS and NPS, respectively, while the remaining regions are non-signal (sideband) regions. |
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Figure 2:
Dimuon mass distributions measured for the non-prompt $ \mathrm{J}/\psi $ (left) and $\psi \text{(2S)} $ (right) event samples, in the mentioned $ p_{\mathrm{T}} $ bins. The total fit function (blue), the sum of the two CB functions and, only in the $ \mathrm{J}/\psi $ case, the Gaussian function (red), and the background continuum (black) are also shown. |
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Figure 2-a:
Dimuon mass distributions measured for the non-prompt $ \mathrm{J}/\psi $ (left) and $\psi \text{(2S)} $ (right) event samples, in the mentioned $ p_{\mathrm{T}} $ bins. The total fit function (blue), the sum of the two CB functions and, only in the $ \mathrm{J}/\psi $ case, the Gaussian function (red), and the background continuum (black) are also shown. |
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Figure 2-b:
Dimuon mass distributions measured for the non-prompt $ \mathrm{J}/\psi $ (left) and $\psi \text{(2S)} $ (right) event samples, in the mentioned $ p_{\mathrm{T}} $ bins. The total fit function (blue), the sum of the two CB functions and, only in the $ \mathrm{J}/\psi $ case, the Gaussian function (red), and the background continuum (black) are also shown. |
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Figure 3:
Fraction of events in the NPS region due to continuum muon pairs, versus $ p_{\mathrm{T}} $, for the non-prompt $ \mathrm{J}/\psi $ and $\psi \text{(2S)} $ events. |
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Figure 4:
Dimuon mass distributions measured for the prompt $ \mathrm{J}/\psi $ (left) and $\psi \text{(2S)} $ (right) event samples, in the mentioned $ p_{\mathrm{T}} $ bins. The total fit function (blue), the sum of the two CB functions and (only in the $ \mathrm{J}/\psi $ case) the Gaussian function (red), and the background continuum (black) are also shown. |
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Figure 4-a:
Dimuon mass distributions measured for the prompt $ \mathrm{J}/\psi $ (left) and $\psi \text{(2S)} $ (right) event samples, in the mentioned $ p_{\mathrm{T}} $ bins. The total fit function (blue), the sum of the two CB functions and (only in the $ \mathrm{J}/\psi $ case) the Gaussian function (red), and the background continuum (black) are also shown. |
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Figure 4-b:
Dimuon mass distributions measured for the prompt $ \mathrm{J}/\psi $ (left) and $\psi \text{(2S)} $ (right) event samples, in the mentioned $ p_{\mathrm{T}} $ bins. The total fit function (blue), the sum of the two CB functions and (only in the $ \mathrm{J}/\psi $ case) the Gaussian function (red), and the background continuum (black) are also shown. |
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Figure 5:
Fraction of events in the PRS region due to continuum muon pairs, versus $ p_{\mathrm{T}} $, for the prompt $ \mathrm{J}/\psi $ and $\psi \text{(2S)} $ events. |
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Figure 6:
Dimuon decay length distributions, integrated in $ p_{\mathrm{T}} $, measured for the sideband mass ranges mentioned in the legends, in the $ \mathrm{J}/\psi $ (left) and $\psi \text{(2S)} $ (right) cases. |
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Figure 6-a:
Dimuon decay length distributions, integrated in $ p_{\mathrm{T}} $, measured for the sideband mass ranges mentioned in the legends, in the $ \mathrm{J}/\psi $ (left) and $\psi \text{(2S)} $ (right) cases. |
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Figure 6-b:
Dimuon decay length distributions, integrated in $ p_{\mathrm{T}} $, measured for the sideband mass ranges mentioned in the legends, in the $ \mathrm{J}/\psi $ (left) and $\psi \text{(2S)} $ (right) cases. |
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Figure 7:
Dimuon decay length distributions measured for the $ \mathrm{J}/\psi $ (left) and $\psi \text{(2S)} $ (right) cases, in their mass signal windows, in the mentioned $ p_{\mathrm{T}} $ bins. The vertical dashed lines mark the limits of the PR and NP ranges. The total fit function, as well as the individual contributions, are also shown. |
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Figure 7-a:
Dimuon decay length distributions measured for the $ \mathrm{J}/\psi $ (left) and $\psi \text{(2S)} $ (right) cases, in their mass signal windows, in the mentioned $ p_{\mathrm{T}} $ bins. The vertical dashed lines mark the limits of the PR and NP ranges. The total fit function, as well as the individual contributions, are also shown. |
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Figure 7-b:
Dimuon decay length distributions measured for the $ \mathrm{J}/\psi $ (left) and $\psi \text{(2S)} $ (right) cases, in their mass signal windows, in the mentioned $ p_{\mathrm{T}} $ bins. The vertical dashed lines mark the limits of the PR and NP ranges. The total fit function, as well as the individual contributions, are also shown. |
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Figure 8:
Variation with $ p_{\mathrm{T}} $ of the fraction of events in the PRS region from non-prompt $ \mathrm{J}/\psi $ and $\psi \text{(2S)} $ mesons. |
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Figure 9:
$ |\cos\vartheta_\mathrm{HX}| $ distributions measured in the mass sidebands (shifted horizontally for better visibility), and their weighted average, for the prompt $ \mathrm{J}/\psi $ (left) and non-prompt $\psi \text{(2S)} $ (right) samples, in the mentioned $ p_{\mathrm{T}} $ bins. |
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Figure 9-a:
$ |\cos\vartheta_\mathrm{HX}| $ distributions measured in the mass sidebands (shifted horizontally for better visibility), and their weighted average, for the prompt $ \mathrm{J}/\psi $ (left) and non-prompt $\psi \text{(2S)} $ (right) samples, in the mentioned $ p_{\mathrm{T}} $ bins. |
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Figure 9-b:
$ |\cos\vartheta_\mathrm{HX}| $ distributions measured in the mass sidebands (shifted horizontally for better visibility), and their weighted average, for the prompt $ \mathrm{J}/\psi $ (left) and non-prompt $\psi \text{(2S)} $ (right) samples, in the mentioned $ p_{\mathrm{T}} $ bins. |
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Figure 10:
$ |\cos\vartheta_\mathrm{HX}| $ distributions measured in the PRS $ \mathrm{J}/\psi $ (left) and NPS $\psi \text{(2S)} $ (right) samples, of the terms of Eqs. (4) and (3), respectively, in the mentioned $ p_{\mathrm{T}} $ bins. |
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Figure 10-a:
$ |\cos\vartheta_\mathrm{HX}| $ distributions measured in the PRS $ \mathrm{J}/\psi $ (left) and NPS $\psi \text{(2S)} $ (right) samples, of the terms of Eqs. (4) and (3), respectively, in the mentioned $ p_{\mathrm{T}} $ bins. |
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Figure 10-b:
$ |\cos\vartheta_\mathrm{HX}| $ distributions measured in the PRS $ \mathrm{J}/\psi $ (left) and NPS $\psi \text{(2S)} $ (right) samples, of the terms of Eqs. (4) and (3), respectively, in the mentioned $ p_{\mathrm{T}} $ bins. |
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Figure 11:
Ratios between the measured and simulated $ |\cos\vartheta_\mathrm{HX}| $ distributions for the prompt and non-prompt $ \mathrm{J}/\psi $ (left) and $\psi \text{(2S)} $ (right) events, in the $ p_{\mathrm{T}} $ bins mentioned in the legends. The curves represent fits using Eq. (2) and excluding the largest $ |\cos\vartheta_\mathrm{HX}| $ bins. |
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Figure 11-a:
Ratios between the measured and simulated $ |\cos\vartheta_\mathrm{HX}| $ distributions for the prompt and non-prompt $ \mathrm{J}/\psi $ (left) and $\psi \text{(2S)} $ (right) events, in the $ p_{\mathrm{T}} $ bins mentioned in the legends. The curves represent fits using Eq. (2) and excluding the largest $ |\cos\vartheta_\mathrm{HX}| $ bins. |
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Figure 11-b:
Ratios between the measured and simulated $ |\cos\vartheta_\mathrm{HX}| $ distributions for the prompt and non-prompt $ \mathrm{J}/\psi $ (left) and $\psi \text{(2S)} $ (right) events, in the $ p_{\mathrm{T}} $ bins mentioned in the legends. The curves represent fits using Eq. (2) and excluding the largest $ |\cos\vartheta_\mathrm{HX}| $ bins. |
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Figure 12:
The $ \lambda_\vartheta $ parameter measured, as a function of $ p_{\mathrm{T}} $, for non-prompt $ \mathrm{J}/\psi $ and $\psi \text{(2S)} $ mesons. The vertical bars represent the total uncertainties. Predicted polarizations of $ \mathrm{J}/\psi $ mesons produced in $ {\mathrm{B}} \to {\mathrm{J}/\psi} \, X $ decays are shown for three calculations [48,49], discussed in the text. The low-$ p_{\mathrm{T}} $ CDF measurement [50] is also shown. |
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Figure 13:
The $ \lambda_\vartheta $ parameter measured, as a function of $ p_{\mathrm{T}} $, for prompt $ \mathrm{J}/\psi $ (left) and $\psi \text{(2S)} $ (right) mesons, in pp collisions at $ \sqrt{s} = $ 13 TeV, compared to measurements made at 7 TeV by CMS [10] and LHCb [11,12]. The vertical bars represent the total uncertainties. The curves on the left panel are described in the text. |
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Figure 13-a:
The $ \lambda_\vartheta $ parameter measured, as a function of $ p_{\mathrm{T}} $, for prompt $ \mathrm{J}/\psi $ (left) and $\psi \text{(2S)} $ (right) mesons, in pp collisions at $ \sqrt{s} = $ 13 TeV, compared to measurements made at 7 TeV by CMS [10] and LHCb [11,12]. The vertical bars represent the total uncertainties. The curves on the left panel are described in the text. |
png pdf |
Figure 13-b:
The $ \lambda_\vartheta $ parameter measured, as a function of $ p_{\mathrm{T}} $, for prompt $ \mathrm{J}/\psi $ (left) and $\psi \text{(2S)} $ (right) mesons, in pp collisions at $ \sqrt{s} = $ 13 TeV, compared to measurements made at 7 TeV by CMS [10] and LHCb [11,12]. The vertical bars represent the total uncertainties. The curves on the left panel are described in the text. |
| Summary |
| The prompt and non-prompt $ \mathrm{J}/\psi $ and $ \psi\mathrm{(2S)}$ $\lambda_\vartheta $ polarization parameters have been measured, in the helicity frame and for the $ |y| < $ 1.25 interval, using a sample of pp collisions at $ \sqrt{s} = $ 13 TeV collected in 2017 and 2018, corresponding to an integrated luminosity of 103.3 fb$ ^{-1} $. The results cover $ p_{\mathrm{T}} $ ranges significantly broader than previous measurements: 25-120 and 20-100 GeV, for the $ \mathrm{J}/\psi $ and $\psi \text{(2S)} $, respectively. The non-prompt $ \mathrm{J}/\psi $ and $\psi \text{(2S)} $ polarization measurements are compatible with each other, regarding the $ p_{\mathrm{T}} $ dependence and the overall magnitude, plateauing at $ \lambda_\vartheta \approx - $0.2 for $ p_{\mathrm{T}} > $ 30 GeV. The measured trends agree with predictions based on the hypothesis that these charmonia are predominantly produced by two-body B meson decays, through colour-singlet processes. Regarding the prompt results, we see no evidence of strong transverse polarizations ($ \lambda_\vartheta $ approaching $ + $1), even at $ p_{\mathrm{T}} $ values exceeding 30 times the $ \mathrm{J}/\psi $ mass. Using NRQCD concepts, there is no evidence that, at very high $ p_{\mathrm{T}} $, the transversely polarized $ ^3 S_1^{[8]} $ and $ ^3 P_J^{[8]} $ octet terms become dominant with respect to the unpolarized $ ^1 S_0^{[8]} $ octet. Taken together with previous CMS and LHCb measurements, covering a lower $ p_{\mathrm{T}} $ domain, we see a significant variation of the prompt polarizations with $ p_{\mathrm{T}} $, at low $ p_{\mathrm{T}} $. These results will significantly constrain phenomenological analyses of charmonium production, so far mostly focused on $ p_{\mathrm{T}} $-differential cross sections. |
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Compact Muon Solenoid LHC, CERN |
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