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CMS-SMP-21-006 ; CERN-EP-2022-144
Measurements of jet multiplicity and jet transverse momentum in multijet events in proton-proton collisions at $ \sqrt{s}= $ 13 TeV
Eur. Phys. J. C 83 (2023) 742
Abstract: Multijet events at large transverse momentum ($ p_{\mathrm{T}} $) are measured at $ \sqrt{s}= $ 13 TeV using data recorded with the CMS detector at the LHC, corresponding to an integrated luminosity of 36.3 fb$^{-1}$. The multiplicity of jets with $ p_{\mathrm{T}} > $ 50 GeV that are produced in association with a high-$ p_{\mathrm{T}} $ dijet system is measured in various ranges of the $ p_{\mathrm{T}} $ of the jet with the highest transverse momentum and as a function of the azimuthal angle difference $ \Delta\phi_{1,2} $ between the two highest $ p_{\mathrm{T}} $ jets in the dijet system. The differential production cross sections are measured as a function of the transverse momenta of the four highest $ p_{\mathrm{T}} $ jets. The measurements are compared with leading and next-to-leading order matrix element calculations supplemented with simulations of parton shower, hadronization, and multiparton interactions. In addition, the measurements are compared with next-to-leading order matrix element calculations combined with transverse-momentum dependent parton densities and transverse-momentum dependent parton shower.
Figures & Tables Summary References CMS Publications
Figures

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Figure 1:
Distribution of $ E_{\mathrm{T}}^{\text{miss}}/\sum{E_{\mathrm{T}}} $ for data and simulated jet production for three regions of $ \Delta\phi_{1,2} $. Shown are the contributions from QCD, $ \mathrm{W}/\mathrm{Z} $ and $ {\mathrm{t}\overline{\mathrm{t}}} $ events. The main contributions of events with large $ E_{\mathrm{T}}^{\text{miss}} $ in the final state come from processes like $ \mathrm{Z} \to \nu \overline{\nu} $ and $ \mathrm{W} \to \ell\nu $. The data (MC prediction) statistical uncertainty is shown as a vertical line (grey shaded bar in the ratio).

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Figure 2:
Probability matrix (condition number: 3.0) for the jet multiplicity distribution constructed with the MADGRAPH+PY8 sample. The global 3$ \times $3 sectors (separated by the thick black lines) correspond to the $ p_{\mathrm{T1}} $ bins, indicated by the labels in the x (lower) and y (left) axes; the smaller 3$ \times $3 structures correspond to the $ \Delta\phi_{1,2} $ bins, indicated in the leftmost row and lowest column, the x(y) axis of these $ \Delta\phi_{1,2} $ cells corresponds to the jet multiplicity at particle (detector) level. The z axis covers a range from 10$^{-6} $ to 1 indicating the probability of migrations from the particle-level bin to the corresponding detector-level bin.

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Figure 3:
Probability matrix (condition number: 4.9) for the $ p_{\mathrm{T}} $ of the four leading jets constructed with the MADGRAPH+PY8 sample. The global 4$ \times $4 sectors correspond to the $ p_{\mathrm{T}} $ distributions each of the first four jets, the x axis corresponds to the particle (gen) level and y axis corresponds to the detector (rec) level. The z axis covers a range from 10$^{-6} $ to 1 indicating the probability of migrations from the particle-level bin to the corresponding detector-level bin.

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Figure 4:
Correlation matrix at the particle-level for the jet multiplicity distribution. It contains contributions from the data and from the limited-size MADGRAPH+PY8 sample. The global 3$ \times $3 sectors (separated by the thick black lines) correspond to the $ p_{\mathrm{T1}} $ bins, indicated by the labels next to the x (lower) and y (left) axes; the smaller 3$ \times $3 structures correspond to the $ \Delta\phi_{1,2} $ bins, indicated in the leftmost row and lowest column, the x and y axes correspond to the jet multiplicity. The z axis covers a range from-1 to 1 indicating the correlations in blue shades and anticorrelations in red shades, the values between-0.1 and 0.1 are represented in white.

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Figure 5:
Correlation matrix for the particle-level $ p_{\mathrm{T}} $ of the four leading jets. It contains contributions from the data and from the limited-size MADGRAPH+PY8 sample. Here each one of the 4$ \times $4 sectors corresponds to one of the $ p_{\mathrm{T}} $ spectra measured, indicated by the x and y axis labels. The z axis covers a range from-1 to 1 indicating the correlations in blue shades and anticorrelations in red shades, the values between-0.1 and 0.1 are represented in white.

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Figure 6:
Relative uncertainties for JES, JER, ``Other'' and total statistical uncertainty for the jet multiplicity distribution in bins of $ p_{\mathrm{T1}} $ and $ \Delta\phi_{1,2} $. Here ``Other'' indicates luminosity, pileup, prefiring, and model uncertainty.

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Figure 7:
Relative uncertainties for JES, JER, ``Other'' and total statistical uncertainty for the $ p_{\mathrm{T}} $ distributions of the four leading jets. Here ``Other'' indicates luminosity, pileup, prefiring, and model uncertainty.

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Figure 8:
Differential cross section as a function of the exclusive jet multiplicity (inclusive for 7 jets) in bins of $ p_{\mathrm{T1}} $ and $ \Delta\phi_{1,2} $. The data are compared with LO predictions of PYTHIA 8, HERWIG++, MADGRAPH+PY8 and MADGRAPH+CA3. The predictions are normalized to the measured dijet cross section using the scaling factors shown in the legend. The vertical error bars correspond to the statistical uncertainty, the yellow band shows the total experimental uncertainty.

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Figure 8-a:
Differential cross section as a function of the exclusive jet multiplicity (inclusive for 7 jets) for 200 $ < p_{\mathrm{T1}} < $ 400 GeV and 0 $ < \Delta\phi_{1,2} < $ 150$^{\text{o}}$ (left), 150 $ < \Delta\phi_{1,2} < $ 170$^{\text{o}}$ (middle), 170 $ < \Delta\phi_{1,2} < $ 180$^{\text{o}}$ (right). The data are compared with LO predictions of PYTHIA 8, HERWIG++, MADGRAPH+PY8 and MADGRAPH+CA3. The predictions are normalized to the measured dijet cross section using the scaling factors shown in the legend. The vertical error bars correspond to the statistical uncertainty, the yellow band shows the total experimental uncertainty.

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Figure 8-b:
Differential cross section as a function of the exclusive jet multiplicity (inclusive for 7 jets) for 400 $ < p_{\mathrm{T1}} < $ 800 GeV and 0 $ < \Delta\phi_{1,2} < $ 150$^{\text{o}}$ (left), 150 $ < \Delta\phi_{1,2} < $ 170$^{\text{o}}$ (middle), 170 $ < \Delta\phi_{1,2} < $ 180$^{\text{o}}$ (right). The data are compared with LO predictions of PYTHIA 8, HERWIG++, MADGRAPH+PY8 and MADGRAPH+CA3. The predictions are normalized to the measured dijet cross section using the scaling factors shown in the legend. The vertical error bars correspond to the statistical uncertainty, the yellow band shows the total experimental uncertainty.

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Figure 8-c:
Differential cross section as a function of the exclusive jet multiplicity (inclusive for 7 jets) for $ p_{\mathrm{T1}} > $ 800 GeV and 0 $ < \Delta\phi_{1,2} < $ 150$^{\text{o}}$ (left), 150 $ < \Delta\phi_{1,2} < $ 170$^{\text{o}}$ (middle), 170 $ < \Delta\phi_{1,2} < $ 180$^{\text{o}}$ (right). The data are compared with LO predictions of PYTHIA 8, HERWIG++, MADGRAPH+PY8 and MADGRAPH+CA3. The predictions are normalized to the measured dijet cross section using the scaling factors shown in the legend. The vertical error bars correspond to the statistical uncertainty, the yellow band shows the total experimental uncertainty.

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Figure 9:
Differential cross section as a function of the exclusive jet multiplicity (inclusive for 7 jets) in bins of $ p_{\mathrm{T1}} $ and $ \Delta\phi_{1,2} $. The data are compared with NLO dijet predictions MG5_aMC+Py8 (jj) and MG5_aMC+CA3 (jj) as well as the NLO three-jet prediction of MG5_aMC+CA3 (jjj). The vertical error bars correspond to the statistical uncertainty, the yellow band shows the total experimental uncertainty. The shaded bands show the uncertainty from a variation of the renormalization and factorization scales. The predictions are normalized to the measured inclusive dijet cross section using the scaling factors shown in the legend.

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Figure 9-a:
Differential cross section as a function of the exclusive jet multiplicity (inclusive for 7 jets) for 200 $ < p_{\mathrm{T1}} < $ 400 GeV and 0 $ < \Delta\phi_{1,2} < $ 150$^{\text{o}}$ (left), 150 $ < \Delta\phi_{1,2} < $ 170$^{\text{o}}$ (middle), 170 $ < \Delta\phi_{1,2} < $ 180$^{\text{o}}$ (right). The data are compared with NLO dijet predictions MG5_aMC+Py8 (jj) and MG5_aMC+CA3 (jj) as well as the NLO three-jet prediction of MG5_aMC+CA3 (jjj). The vertical error bars correspond to the statistical uncertainty, the yellow band shows the total experimental uncertainty. The shaded bands show the uncertainty from a variation of the renormalization and factorization scales. The predictions are normalized to the measured inclusive dijet cross section using the scaling factors shown in the legend.

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Figure 9-b:
Differential cross section as a function of the exclusive jet multiplicity (inclusive for 7 jets) for 400 $ < p_{\mathrm{T1}} < $ 800 GeV and 0 $ < \Delta\phi_{1,2} < $ 150$^{\text{o}}$ (left), 150 $ < \Delta\phi_{1,2} < $ 170$^{\text{o}}$ (middle), 170 $ < \Delta\phi_{1,2} < $ 180$^{\text{o}}$ (right). The data are compared with NLO dijet predictions MG5_aMC+Py8 (jj) and MG5_aMC+CA3 (jj) as well as the NLO three-jet prediction of MG5_aMC+CA3 (jjj). The vertical error bars correspond to the statistical uncertainty, the yellow band shows the total experimental uncertainty. The shaded bands show the uncertainty from a variation of the renormalization and factorization scales. The predictions are normalized to the measured inclusive dijet cross section using the scaling factors shown in the legend.

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Figure 9-c:
Differential cross section as a function of the exclusive jet multiplicity (inclusive for 7 jets) for $ p_{\mathrm{T1}} > $ 800 GeV and 0 $ < \Delta\phi_{1,2} < $ 150$^{\text{o}}$ (left), 150 $ < \Delta\phi_{1,2} < $ 170$^{\text{o}}$ (middle), 170 $ < \Delta\phi_{1,2} < $ 180$^{\text{o}}$ (right). The data are compared with NLO dijet predictions MG5_aMC+Py8 (jj) and MG5_aMC+CA3 (jj) as well as the NLO three-jet prediction of MG5_aMC+CA3 (jjj). The vertical error bars correspond to the statistical uncertainty, the yellow band shows the total experimental uncertainty. The shaded bands show the uncertainty from a variation of the renormalization and factorization scales. The predictions are normalized to the measured inclusive dijet cross section using the scaling factors shown in the legend.

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Figure 10:
Transverse momenta of the four leading jets, with the yellow band representing the total experimental uncertainty. The data are compared with LO (PYTHIA 8) and NLO (MG5_aMC+Py8 ) predictions. The red band in the NLO prediction represents the renormalization and factorization scale uncertainty.

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Figure 11:
Transverse momentum distributions of the four leading jets. The transverse momentum of the leading and subleading (third and fourth leading) $ p_{\mathrm{T}} $ jets from left to right is shown in the upper (lower) figure. The data are compared with LO predictions of PYTHIA 8, HERWIG++, MADGRAPH+PY8 and MADGRAPH+CA3. The predictions are normalized to the measured dijet cross section using the scaling factors shown in the legend. The vertical error bars correspond to the statistical uncertainty, the yellow band shows the total experimental uncertainty.

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Figure 11-a:
The transverse momentum of the leading (left) and subleading (right) leading $ p_{\mathrm{T}} $ jets is shown. The data are compared with LO predictions of PYTHIA 8, HERWIG++, MADGRAPH+PY8 and MADGRAPH+CA3. The predictions are normalized to the measured dijet cross section using the scaling factors shown in the legend. The vertical error bars correspond to the statistical uncertainty, the yellow band shows the total experimental uncertainty.

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Figure 11-b:
The transverse momentum of the third (left) and fourth (right) leading $ p_{\mathrm{T}} $ jets is shown. The data are compared with LO predictions of PYTHIA 8, HERWIG++, MADGRAPH+PY8 and MADGRAPH+CA3. The predictions are normalized to the measured dijet cross section using the scaling factors shown in the legend. The vertical error bars correspond to the statistical uncertainty, the yellow band shows the total experimental uncertainty.

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Figure 12:
Transverse momentum distributions of the four leading jets. The transverse momentum of the leading and subleading (third and fourth leading) $ p_{\mathrm{T}} $ jets from left to right is shown in the upper (lower) figure. The data are compared with NLO predictions MG5_aMC+Py8 (jj) and MG5_aMC+CA3 (jj) as well as the NLO three-jet prediction of MG5_aMC+CA3 (jjj). The vertical error bars correspond to the statistical uncertainty, and the yellow band to total uncertainty of the measurement. The bands show the uncertainty from a variation of the renormalization and factorization scales. The predictions are normalized to the measured dijet cross section using the scaling factors shown in the legend.

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Figure 12-a:
he transverse momentum of the leading (left) and subleading (right) leading $ p_{\mathrm{T}} $ jets is shown. The data are compared with NLO predictions MG5_aMC+Py8 (jj) and MG5_aMC+CA3 (jj) as well as the NLO three-jet prediction of MG5_aMC+CA3 (jjj). The vertical error bars correspond to the statistical uncertainty, and the yellow band to total uncertainty of the measurement. The bands show the uncertainty from a variation of the renormalization and factorization scales. The predictions are normalized to the measured dijet cross section using the scaling factors shown in the legend.

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Figure 12-b:
The transverse momentum of the third (left) and fourth (right) leading $ p_{\mathrm{T}} $ jets is shown. The data are compared with NLO predictions MG5_aMC+Py8 (jj) and MG5_aMC+CA3 (jj) as well as the NLO three-jet prediction of MG5_aMC+CA3 (jjj). The vertical error bars correspond to the statistical uncertainty, and the yellow band to total uncertainty of the measurement. The bands show the uncertainty from a variation of the renormalization and factorization scales. The predictions are normalized to the measured dijet cross section using the scaling factors shown in the legend.
Tables

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Table 1:
Description of the simulated samples used in the analysis.

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Table 2:
The integrated luminosity for each trigger sample considered in this analysis with the $ p_{\mathrm{T}} $ thresholds for HLT (PF) reconstruction.
Summary
A study of multijet events has been performed in proton-proton collisions at a center-of-mass energy of 13 TeV using data collected with the CMS detector corresponding to an integrated luminosity of 36.3 fb$^{-1}$. The measurements are performed by selecting a dijet system containing a jet with $ p_{\mathrm{T}} > $ 200 GeV and a subleading jet with $ p_{\mathrm{T}} > $ 100 GeV within $ |y| < $ 2.5. For the first time, the jet multiplicity in bins of the leading jet $ p_{\mathrm{T}} $ and the azimuthal angle difference between the two leading jets, $ \Delta\phi_{1,2} $, is measured. The jet multiplicity distributions show that even in the back-to-back region of the dijet system, up to seven jets are measurable. The differential production cross sections are measured for the highest $ p_{\mathrm{T}} $ jets up to the TeV scale. The measurement of the differential cross section as a function of the jet $ p_{\mathrm{T}} $ for the four highest $ p_{\mathrm{T}} $ jets is an important reference for standard model multijet cross section calculations, and especially for the simulations including parton showers for higher jet multiplicity. The measured multiplicity distribution of jets with $ p_{\mathrm{T}} > $ 50 GeV and $ |y| < $ 2.5 is not well described by the leading order MADGRAPH+PYTHIA 8 simulation. However, in the back-to-back region HERWIG++ and MADGRAPH+CASCADE3 provide a better description of the shape of the jet multiplicity. The measured differential cross section as a function of the transverse momentum of the four leading $ p_{\mathrm{T}} $ jets is not described by any of the LO predictions either in normalization or in shape. However, MADGRAPH+CASCADE3 describes the shape of the distribution better than MADGRAPH+PYTHIA 8. The predictions using dijet NLO matrix elements, MG5_AMC+PYTHIA 8(jj) and MG5_AMC+CASCADE3(jj) describe the lower multiplicity regions, as well as the transverse momenta of the leading jets, reasonably well. The three-jet NLO calculation MG5_AMC+CASCADE3(jjj) describes very well the cross section of the third and fourth jets. The measurements presented here provide stringent tests of theoretical predictions in the perturbative high-$ p_{\mathrm{T}} $ and high-jet multiplicity regions. Although the higher jet multiplicities are not described with either parton shower approach, it is interesting that the lower jet multiplicity cross section is described satisfactorily with NLO dijet calculations supplemented with PB -TMDs and TMD parton shower with fewer tunable parameters than in the case with conventional parton showers. The measured observables and its statistical correlations are provided in HEPData [41] as tabulated results.
References
1 A. Bermudez Martinez et al. Collinear and TMD parton densities from fits to precision DIS measurements in the parton branching method PRD 99 (2019) 074008 1804.11152
2 F. Hautmann et al. Soft-gluon resolution scale in QCD evolution equations PLB 772 (2017) 446 1704.01757
3 F. Hautmann et al. Collinear and TMD quark and gluon densities from parton branching solution of QCD evolution equations JHEP 01 (2018) 070 1708.03279
4 J. Alwall et al. The automated computation of tree-level and next-to-leading order differential cross sections, and their matching to parton shower simulations JHEP 07 (2014) 079 1405.0301
5 S. Baranov et al. CASCADE3 A Monte Carlo event generator based on TMDs EPJC 81 (2021) 425 2101.10221
6 A. Bermudez Martinez et al. Production of Z-bosons in the parton branching method PRD 100 (2019) 074027 1906.00919
7 D0 Collaboration Measurement of dijet azimuthal decorrelations at central rapidities in $ \mathrm{p}\overline{\mathrm{p}} $ collisions at $ \sqrt{s}=$ 1.96 TeV PRL 94 (2005) 221801 hep-ex/0409040
8 D0 Collaboration Measurement of the combined rapidity and $ p_{\mathrm{T}} $ dependence of dijet azimuthal decorrelations in $ \mathrm{p}\overline{\mathrm{p}} $ collisions at $ \sqrt{s}=$ 1.96 TeV PLB 721 (2013) 212 1212.1842
9 ATLAS Collaboration Measurement of dijet azimuthal decorrelations in pp collisions at $ \sqrt{s}=$ 7 TeV PRL 106 (2011) 172002 1102.2696
10 CMS Collaboration Dijet azimuthal decorrelations in pp collisions at $ \sqrt{s}=$ 7 TeV PRL 106 (2011) 122003 CMS-QCD-10-026
1101.5029
11 CMS Collaboration Measurement of dijet azimuthal decorrelation in pp collisions at $ \sqrt{s}=$ 8 TeV EPJC 76 (2016) 536 CMS-SMP-14-015
1602.04384
12 CMS Collaboration Azimuthal correlations for inclusive 2-jet, 3-jet, and 4-jet events in pp collisions at $ \sqrt{s}= $ 13 TeV EPJC 78 (2018) 566 CMS-SMP-16-014
1712.05471
13 CMS Collaboration Azimuthal separation in nearly back-to-back jet topologies in inclusive 2- and 3-jet events in pp collisions at $ \sqrt{s}= $ 13 TeV EPJC 79 (2019) 773 CMS-SMP-17-009
1902.04374
14 CMS Collaboration Performance of the CMS Level-1 trigger in proton-proton collisions at $ \sqrt{s} = $ 13 TeV JINST 15 (2020) P10017 CMS-TRG-17-001
2006.10165
15 CMS Collaboration The CMS trigger system JINST 12 (2017) P01020 CMS-TRG-12-001
1609.02366
16 CMS Collaboration Particle-flow reconstruction and global event description with the CMS detector JINST 12 (2017) P10003 CMS-PRF-14-001
1706.04965
17 CMS Collaboration Technical proposal for the Phase-II upgrade of the Compact Muon Solenoid CMS Technical Proposal CERN-LHCC-2015-010, CMS-TDR-15-02, 2015
CDS
18 M. Cacciari, G. P. Salam, and G. Soyez The anti-$ k_{\mathrm{T}} $ jet clustering algorithm JHEP 04 (2008) 063 0802.1189
19 M. Cacciari, G. P. Salam, and G. Soyez FastJet user manual EPJC 72 (2012) 1896 1111.6097
20 CMS Collaboration Pileup Removal Algorithms CMS Physics Analysis Summary, 2014
CMS-PAS-JME-14-001
CMS-PAS-JME-14-001
21 CMS Collaboration Jet energy scale and resolution in the CMS experiment in pp collisions at 8 TeV JINST 12 (2017) P02014 CMS-JME-13-004
1607.03663
22 CMS Collaboration Performance of missing transverse momentum reconstruction in proton-proton collisions at $ \sqrt{s} = $ 13 TeV using the CMS detector JINST 14 (2019) P07004 CMS-JME-17-001
1903.06078
23 CMS Collaboration The CMS experiment at the CERN LHC JINST 3 (2008) S08004
24 T. Sjöstrand et al. An introduction to PYTHIA 8.2 Comput. Phys. Commun. 191 (2015) 159 1410.3012
25 CMS Collaboration Event generator tunes obtained from underlying event and multiparton scattering measurements EPJC 76 (2016) 155 CMS-GEN-14-001
1512.00815
26 NNPDF Collaboration Unbiased global determination of parton distributions and their uncertainties at NNLO and at LO NPB 855 (2012) 153 1107.2652
27 M. Bahr et al. Herwig++ physics and manual EPJC 58 (2008) 639 0803.0883
28 J. Pumplin et al. New generation of parton distributions with uncertainties from global QCD analysis JHEP 07 (2002) 012 hep-ph/0201195
29 J. Alwall et al. Comparative study of various algorithms for the merging of parton showers and matrix elements in hadronic collisions EPJC 53 (2008) 473 0706.2569
30 A. Bermudez Martinez , F. Hautmann, and M. L. Mangano TMD evolution and multi-jet merging PLB 822 (2021) 136700 2107.01224
31 T. Sjöstrand, S. Mrenna, and P. Skands PYTHIA 6.4 physics and manual JHEP 05 (2006) 026 hep-ph/0603175
32 NNPDF Collaboration Parton distributions for the LHC Run II JHEP 04 (2015) 040 1410.8849
33 G. Corcella et al. HERWIG 6: An event generator for hadron emission reactions with interfering gluons (including supersymmetric processes) JHEP 01 (2001) 010 hep-ph/0011363
34 S. Frixione, P. Nason, and B. R. Webber Matching NLO QCD and parton showers in heavy flavor production JHEP 08 (2003) 007 hep-ph/0305252
35 S. Frixione and B. R. Webber Matching NLO QCD computations and parton shower simulations JHEP 06 (2002) 029 hep-ph/0204244
36 GEANT4 Collaboration GEANT 4 -- a simulation toolkit NIM A 506 (2003) 250
37 CMS Collaboration Measurement and QCD analysis of double-differential inclusive jet cross sections in proton-proton collisions at $ \sqrt{s} $ = 13 TeV JHEP 02 (2022) 142 CMS-SMP-20-011
2111.10431
38 CMS Collaboration Performance of the CMS Hadron Calorimeter with Cosmic Ray Muons and LHC Beam Data JINST 5 (2010) T03012 CMS-CFT-09-009
0911.4991
39 CMS Collaboration Jet algorithms performance in 13 TeV data CMS Physics Analysis Summary , CERN, Geneva, 2017
CMS-PAS-JME-16-003
CMS-PAS-JME-16-003
40 S. Schmitt TUnfold: an algorithm for correcting migration effects in high energy physics JINST 7 (2012) T10003 1205.6201
41 CMS Collaboration Precision luminosity measurement in proton-proton collisions at $ \sqrt{s} = $ 13 TeV in 2015 and 2016 at CMS EPJC 81 (2021) 800 CMS-LUM-17-003
2104.01927
42 CMS Collaboration Measurement of the inelastic proton-proton cross section at $ \sqrt{s}= $ 13 TeV JHEP 07 (2018) 161 CMS-FSQ-15-005
1802.02613
43 CMS Collaboration HEPData record for this analysis link
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