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| CMS-PAS-SMP-25-010 | ||
| Measurement of normalized double-differential groomed and ungroomed jet mass cross sections in Z+jet events at $ \sqrt{s}= $ 13 TeV with the CMS detector | ||
| CMS Collaboration | ||
| 2026-07-31 | ||
| Abstract: Measurements of double-differential jet production cross sections are presented as a function of the transverse momentum $ p_\mathrm{T} $ and the dimensionless jet mass $ \rho $ of the leading jet in Z+jet events. The jets are clustered with the anti-$ k_\mathrm{T} $ algorithm with distance parameter $ R= $ 0.8. This measurement provides a sample enriched in quark-initiated jets, as the dominant production mechanisms are $ \mathrm{g q} \rightarrow \mathrm{Z q} $ and $ \mathrm{q \overline{q}} \rightarrow \mathrm{Z q} $. Comparing the ungroomed and groomed jets allows for separation of the perturbative evolution of the single-gluon emission approximation of the jet mass (accurate at leading logarithmic order), from non-perturbative effects such as hadronization and contamination from the underlying event, multiple parton interactions, and interactions between multiple protons in the same bunch crossing. This provides a precise benchmark for state-of-the-art theoretical calculations and tuning of simulation parameters. This analysis utilizes the dataset of proton-proton collisions at $ \sqrt{s}= $ 13 TeV recorded by the CMS detector from 2016 to 2018, corresponding to an integrated luminosity of 138 fb$ ^{-1} $. The results are unfolded to the generated particle level to correct for detector effects, and are compared with predictions from Monte Carlo event generators. | ||
| Links: CDS record (PDF) ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Fixed-coupling, leading-logarithmic normalized jet mass distributions for quark- ($ C_i=C_F $) and gluon-initiated ($ C_i=C_A $) jets in blue and red lines, respectively, as a function of $ \log_{10}(\rho^{2}) $. The ungroomed jets are shown in dotted lines, while the distribution after the soft-drop grooming with $ \beta= $ 0 and $ z_{\text{cut}}= $ 0.1 are shown in solid (quark) and dashed (gluon) jets. The dashed vertical line marks the soft drop grooming transition at $ \rho=z_{\text{cut}} $. The larger color factor of the gluon shifts the ungroomed Sudakov peak to higher mass and raises the groomed plateau, whose height is proportional to $ \alpha_\mathrm{S} C_i $. |
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Figure 2:
Detector-level data and simulation distributions in the 200 $ < p_{\mathrm{T}} < $ 290 GeV bin for the ungroomed (left) and groomed (right) observables. The points show data, and the stacked histograms show the simulated Drell--Yan signal and the contributions from top quark, single top quark, and diboson backgrounds. The hatched bands show the total uncertainty in the simulation, i.e., the statistical and experimental systematic uncertainties added in quadrature. The lower panels show the ratios of the data to the simulation. |
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Figure 2-a:
Detector-level data and simulation distributions in the 200 $ < p_{\mathrm{T}} < $ 290 GeV bin for the ungroomed (left) and groomed (right) observables. The points show data, and the stacked histograms show the simulated Drell--Yan signal and the contributions from top quark, single top quark, and diboson backgrounds. The hatched bands show the total uncertainty in the simulation, i.e., the statistical and experimental systematic uncertainties added in quadrature. The lower panels show the ratios of the data to the simulation. |
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Figure 2-b:
Detector-level data and simulation distributions in the 200 $ < p_{\mathrm{T}} < $ 290 GeV bin for the ungroomed (left) and groomed (right) observables. The points show data, and the stacked histograms show the simulated Drell--Yan signal and the contributions from top quark, single top quark, and diboson backgrounds. The hatched bands show the total uncertainty in the simulation, i.e., the statistical and experimental systematic uncertainties added in quadrature. The lower panels show the ratios of the data to the simulation. |
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Figure 3:
Detector response matrices for the ungroomed and groomed observables, shown on the left and right, respectively. The reconstructed and particle-level observable bins are grouped by jet $ p_{\mathrm{T}} $. The 185 $ < p_{\mathrm{T}} < $ 200 GeV blocks retain migrations across the analysis threshold. |
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Figure 3-a:
Detector response matrices for the ungroomed and groomed observables, shown on the left and right, respectively. The reconstructed and particle-level observable bins are grouped by jet $ p_{\mathrm{T}} $. The 185 $ < p_{\mathrm{T}} < $ 200 GeV blocks retain migrations across the analysis threshold. |
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Figure 3-b:
Detector response matrices for the ungroomed and groomed observables, shown on the left and right, respectively. The reconstructed and particle-level observable bins are grouped by jet $ p_{\mathrm{T}} $. The 185 $ < p_{\mathrm{T}} < $ 200 GeV blocks retain migrations across the analysis threshold. |
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Figure 4:
Grouped fractional uncertainties in the normalized unfolded ungroomed (left) and groomed (right) distributions for the 200 $ < p_{\mathrm{T}} < $ 290 GeV interval. The black line shows the total uncertainty. Where the total uncertainty exceeds the axis range, its value is given as a number in the corresponding bin. |
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Figure 4-a:
Grouped fractional uncertainties in the normalized unfolded ungroomed (left) and groomed (right) distributions for the 200 $ < p_{\mathrm{T}} < $ 290 GeV interval. The black line shows the total uncertainty. Where the total uncertainty exceeds the axis range, its value is given as a number in the corresponding bin. |
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Figure 4-b:
Grouped fractional uncertainties in the normalized unfolded ungroomed (left) and groomed (right) distributions for the 200 $ < p_{\mathrm{T}} < $ 290 GeV interval. The black line shows the total uncertainty. Where the total uncertainty exceeds the axis range, its value is given as a number in the corresponding bin. |
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Figure 5:
Comparison of the unfolded data with the PYTHIA8, HERWIG 7, and Vincia predictions in the 290 $ < p_{\mathrm{T}} < $ 400 GeV interval for the ungroomed (left) and groomed (right) observables. The black markers show the unfolded data. The dark and light green bands show the statistical and total uncertainties, respectively. The lower panels show the ratios of the unfolded data to the simulation predictions. The legends quote the $ \chi^{2} $ per degree of freedom of each prediction with respect to the unfolded data, computed from the total uncertainties. |
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Figure 5-a:
Comparison of the unfolded data with the PYTHIA8, HERWIG 7, and Vincia predictions in the 290 $ < p_{\mathrm{T}} < $ 400 GeV interval for the ungroomed (left) and groomed (right) observables. The black markers show the unfolded data. The dark and light green bands show the statistical and total uncertainties, respectively. The lower panels show the ratios of the unfolded data to the simulation predictions. The legends quote the $ \chi^{2} $ per degree of freedom of each prediction with respect to the unfolded data, computed from the total uncertainties. |
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Figure 5-b:
Comparison of the unfolded data with the PYTHIA8, HERWIG 7, and Vincia predictions in the 290 $ < p_{\mathrm{T}} < $ 400 GeV interval for the ungroomed (left) and groomed (right) observables. The black markers show the unfolded data. The dark and light green bands show the statistical and total uncertainties, respectively. The lower panels show the ratios of the unfolded data to the simulation predictions. The legends quote the $ \chi^{2} $ per degree of freedom of each prediction with respect to the unfolded data, computed from the total uncertainties. |
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Figure 6:
Unfolded normalized distributions for the ungroomed (left) and groomed (right) observables. The distributions in the three jet-$ p_{\mathrm{T}} $ intervals are multiplied by the factors indicated in the legends. |
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Figure 6-a:
Unfolded normalized distributions for the ungroomed (left) and groomed (right) observables. The distributions in the three jet-$ p_{\mathrm{T}} $ intervals are multiplied by the factors indicated in the legends. |
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Figure 6-b:
Unfolded normalized distributions for the ungroomed (left) and groomed (right) observables. The distributions in the three jet-$ p_{\mathrm{T}} $ intervals are multiplied by the factors indicated in the legends. |
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Figure 7:
Ratios of the unfolded data to the PYTHIA8, HERWIG 7, and Vincia predictions for the ungroomed (left) and groomed (right) observables, shown separately for the three jet-$ p_{\mathrm{T}} $ intervals. The dark and light green bands around unity show the statistical and total uncertainties in the data, respectively, and the vertical bars show the statistical uncertainties of the predictions. |
png pdf |
Figure 7-a:
Ratios of the unfolded data to the PYTHIA8, HERWIG 7, and Vincia predictions for the ungroomed (left) and groomed (right) observables, shown separately for the three jet-$ p_{\mathrm{T}} $ intervals. The dark and light green bands around unity show the statistical and total uncertainties in the data, respectively, and the vertical bars show the statistical uncertainties of the predictions. |
png pdf |
Figure 7-b:
Ratios of the unfolded data to the PYTHIA8, HERWIG 7, and Vincia predictions for the ungroomed (left) and groomed (right) observables, shown separately for the three jet-$ p_{\mathrm{T}} $ intervals. The dark and light green bands around unity show the statistical and total uncertainties in the data, respectively, and the vertical bars show the statistical uncertainties of the predictions. |
| Summary |
| A measurement of double-differential groomed and ungroomed jet mass cross sections has been presented in $ \mathrm{Z}+\text{jet} $ events, using proton-proton collision data recorded by the CMS detector between 2016 and 2018 at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 138 fb$ ^{-1} $. The cross sections are measured as a function of the dimensionless jet mass $ \rho=m/(p_{\mathrm{T}} R) $ and the jet transverse momentum, and are unfolded to the particle level to correct for detector effects. The $ \mathrm{Z}+\text{jet} $ final state provides a sample enriched in quark-initiated jets. This is the first 13 TeV measurement of the jet mass in $ \mathrm{Z}+\text{jet} $ events. The unfolded distributions are well described by the PYTHIA8 prediction, whereas HERWIG 7 and Vincia show larger deviations, most visibly overestimating the peak and underestimating the high-mass region of the ungroomed distribution. The soft-drop grooming shifts the distribution to lower mass and removes the soft, wide-angle radiation, providing additional sensitivity to the parton-shower and hadronization modeling. These results provide a benchmark for the tuning of Monte Carlo event generators and for the understanding of the perturbative and nonperturbative QCD dynamics of jet formation. |
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