Available on the CERN CDS information server CMS PAS SMP-16-001

CMS Physics Analysis Summary

Contact: cms-pag-conveners-smp@cern.ch 2016/04/26

Measurement of the ZZ production cross section andZ→ ```′`′ branching fraction in pp collisions at√

s = 13 TeV

The CMS Collaboration

Abstract

Four-lepton production in proton-proton collisions, pp → Z/γ∗Z/γ∗ → ```′`′,where (`, `′ = e or µ), is studied at a center-of-mass energy of 13 TeV with the CMSdetector at the LHC. The data sample corresponds to an integrated luminosity of2.6 fb−1. The ZZ production cross section, σ(pp → ZZ) = 14.6+1.9

−1.8(stat)+0.5−0.3(syst)±

0.2(theo) ± 0.4(lum) pb, is measured for both Z bosons produced in the mass re-gion 60 < mZ < 120 GeV. The Z branching fraction to four leptons is measured tobe B(Z → ```′`′) = 4.9+0.8

−0.7(stat)+0.3−0.2(syst)+0.2

−0.1(theo)± 0.1(lum)× 10−6 for the four-lepton mass in the range 80 < m```′`′ < 100 GeV and dilepton mass m``′ > 4 GeV forall opposite-charge lepton pairs. The results are compared to standard model predic-tions.

1

1 IntroductionMeasurements of diboson production at the LHC allow precision studies of the electroweaksector of the standard model (SM). The measurements are important for testing the predictionsthat are recently available up to next-next-to-leading order (NNLO) in QCD [1], and deviationsfrom expected values could be an early indication of physics beyond the SM.

Previous measurements of the ZZ production cross section with CMS were performed in theZZ→ ```′`′ and ZZ→ ``νν final states, where ` = e, µ and `′ = e, µ, τ [2–4] for both Z bosonsproduced on-shell, in the mass region 60–120 GeV. These measurements, carried out withdatasets corresponding to integrated luminosities of 5.1 fb−1at

√s = 7 TeV, and 19.6 fb−1at√

s = 8 TeV, agree with SM predictions. The ATLAS collaboration produced similar results [5–7], which agree with those from CMS.

Extending the mass window for the dilepton candidates to lower values allows measurementsof Z/γ∗Z/γ∗ production, which includes Higgs boson events in the “golden channel” H→ZZ∗→ ```′`′ and rare Z boson decays to four leptons. The Z→ ```′`′ decay has been studiedin detail at LEP [8], and was observed in pp collisions by CMS [9] and later by ATLAS [10].Though its branching fraction is orders of magnitude lower than the branching fraction intodileptons (Z→ ``), the well-known mass of the Z boson makes this decay important for cali-brating mass measurements of the nearby Higgs resonance.

This Summary reports a study of four-lepton production (pp→ ```′`′, where ` and `′ indicateelectrons or muons) at

√s = 13 TeV, with a dataset corresponding to an integrated luminosity

of 2.62± 0.07fb−1. From this study, the cross sections for non-resonant production of pairs ofon-shell Z bosons (pp→ZZ) and resonant pp→Z→ ```′`′ production, are inferred, along withthe Z→ ```′`′ branching fraction. Resonant Higgs boson production will be discussed in detailin a separate publication.

2 Signal and background simulationSeveral Monte Carlo (MC) event generators are used to simulate the signals and backgrounds.

Signal events are generated with POWHEG 2.0 [11–13] at next-to-leading order (NLO) in QCDfor quark-antiquark processes and leading order (LO) for quark-gluon processes. This includesZZ, Zγ∗, Z, and γ∗γ∗ production with m``′ > 4 GeV between all pairs of oppositely-chargedleptons. The gg→ ZZ process is simulated at LO with MCFM [14]. These samples are scaledto correspond to cross sections calculated at NNLO for qq→ZZ [1] (k-factor 1.1) and at NLOfor gg→ZZ [15] (k-factor 1.7). The gg→ZZ process is formally calculated to a higher order inQCD (N3LO) than the qq→ZZ process; this correction is included because the contribution isknown to be large.

A sample of Higgs boson events is produced in the gluon-gluon fusion process with POWHEG 2.0in the NLO QCD approximation. The Higgs boson decay is modeled with JHUGEN 3.1.8 [16–18]. The qq→WZ process is generated with POWHEG 2.0.

The PYTHIA 8 [19–21] package is used for parton showering, hadronization, and the underly-ing event simulation. The NNPDF3.0 [22] set is used as the default set of parton distributionfunctions.

The detector response is simulated with a detailed description of the CMS detector, imple-mented with the GEANT4 package [23], and event reconstruction is performed with the samealgorithms as used for data. The simulated samples include additional interactions per bunch

2 3 Event reconstruction

crossing (“pileup”). The simulated events are weighted so that the pileup distribution matchesthat of the data, with an average pileup of about 11 interactions per bunch crossing.

3 Event reconstructionAll stable particles in each collision event—electrons, muons, photons, and charged and neu-tral hadrons—are identified and reconstructed by the CMS particle-flow (PF) algorithm [24]from a combination of the signals from all sub-detectors. Reconstructed electrons [25] andmuons [26] are considered candidates for inclusion in four-lepton final states (“signal leptons”)if they have transverse momentum p`T > 7(5)GeV and pseudorapidity

∣∣η`∣∣ < 2.5(2.4) for elec-

trons (muons).

Signal leptons are required to originate from the event vertex, defined as the proton-protoninteraction vertex whose associated charged particles have the highest sum of pT

2. The distanceof closest approach between each lepton track and the event vertex is required to be less than0.5 cm in the plane transverse to the beam axis, and less than 1 cm in the direction along thebeam axis. Furthermore, the significance of the three-dimensional impact parameter relativeto the event vertex, SIP3D, is required to satisfy SIP3D ≡ | IP

σIP| < 4 for each lepton, where IP is

the distance of closest approach of each lepton track to the event vertex and σIP is its associateduncertainty.

Signal leptons are required to be isolated from other particles in the event. The relative isolationis defined as

RIso =

(∑

chargedpT + max

[0, ∑

neutralpT + ∑

photonspT − pPU

T (`)

])/p`T (1)

where the sums run over the charged hadrons, neutral hadrons, and photons in a cone defined

by ∆R ≡√(∆η)2 + (∆φ)2 < 0.3 around the lepton trajectory. To mitigate the contribution of

charged particles from pileup to the isolation calculation, charged hadrons are included only ifthey originate from the event vertex. The contribution of neutral particles from pileup, pPU

T (`),is subtracted. For electrons, pPU

T (e) is evaluated with a “jet area” method described in Ref. [27].For muons, pPU

T (µ) is taken to be half the sum of the pT of all charged particles in the coneoriginating from pileup vertices. The factor one-half accounts for the expected ratio of chargedto neutral energy in hadronic interactions. Leptons are considered isolated if their relativeisolations are less than 0.35.

Final-state radiation (FSR) photons emitted by the signal leptons are expected, if not properlytaken into account, to degrade both the performance of the isolation requirements and the Zboson mass reconstruction. Photons are considered FSR candidates if pγ

T > 2 GeV, |ηγ| < 2.4,their relative isolation (defined as in Eq. 1 with pPU

T (γ) = 0) is less than 1.8, and ∆R (`, γ) < 0.5with respect to the nearest signal lepton. Photons are not considered if there is any signalelectron within ∆R (γ, e) < 0.15 or within |∆φ (γ, e)| < 2 and |∆η (γ, e)| < 0.05. Because FSRhas a higher energy spectrum than photons from pileup and is expected to be quasi-collinearwith the emitting lepton, a signal photon is accepted as FSR if ∆R(`,γ)

ET2γ

< 0.012 GeV−2. SelectedFSR photons are omitted from the isolation determination for signal leptons and are includedin dilepton kinematic calculations. In simulated ZZ → ```′`′ events, the efficiency to selectgenerated FSR photons is around 55%, and roughly 85% of selected photons are matched togenerated FSR photons. At least one FSR photon is identified in approximately 2% of simulated

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events in the 4e channel, 5% in the 2e2µ channel and 8% in the 4µ channel. In events with twoon-shell Z bosons, at least one FSR photon is selected in five data events in the 4µ decay channeland three in the 2e2µ decay channel. In the following, the inclusion of FSR photons in `` and```′`′ reconstruction is implicit.

The signal lepton reconstruction, identification, and isolation efficiencies are measured in data,with a tag-and-probe technique [28] based on a sample of Z→ `` events. The measurements areperformed in several bins of p`T and |η`|. The electron selection efficiency in the ECAL barrel(endcaps) varies from about 85% (77%) at pe

T ≈ 10 GeV, to about 95% (89%) for peT ≥ 20 GeV,

and is about 85% in the barrel-endcap transition region. The muons are reconstructed andidentified with efficiencies above ∼98% within |ηµ| < 2.4.

4 Event selectionThe primary triggers for this analysis require the presence of a pair of loosely isolated leptonsof the same or different flavors. The highest-pT lepton must have pT > 17 GeV, and the second-highest-pT lepton must have pT > 12 GeV if it is an electron or pT > 8 GeV if it is a muon. Thedi-electron and di-muon triggers require that the tracks corresponding to the leptons originatefrom within 2 mm of each other in the plane transverse to the beam axis. Triggers requiring atriplet of lower-pT leptons with no isolation criterion, or a single high-pT electron, are also used.The total trigger efficiency for events within the acceptance of this analysis is greater than 98%.

To be considered, an event must contain at least two Z/γ∗ candidates, each formed from anoppositely-charged pair of isolated signal electrons or muons. The highest-pT lepton musthave pT > 20 GeV, and the second-highest-pT lepton must have pT > 10 GeV. All leptons arerequired to be separated by ∆R (`1, `2) > 0.02, and electrons are required to be separated frommuons by ∆R (e, µ) > 0.05.

Within each event, all permutations of leptons giving a valid pair of Z/γ∗ candidates are con-sidered separately. Within each ```′`′ candidate, the dilepton candidate with invariant massclosest to nominal Z boson mass is designated Z1, and is required to have mass greater than40 GeV. The other dilepton candidate is designated Z2. Both mZ1 and mZ2 are required to beless than 120 GeV. All pairs of oppositely-charged leptons in the candidate are required to havem``′ > 4 GeV regardless of flavor.

If multiple ```′`′ candidates within an event pass all selections, the passing candidate with mZ1

closest to the nominal Z boson mass is chosen. In the rare case of further ambiguity, which mayarise in events with five or more signal leptons, the Z2 candidate that maximizes the scalar sumof the transverse momenta of the four leptons is chosen.

For measurements related to individual processes, further requirements are applied to selectevents in appropriate phase spaces. The pp→ ZZ cross section is measured in events wheremZ1 and mZ2 are required to be greater than 60 GeV. The Z → ```′`′ branching fraction ismeasured in events in which 80 < m```′`′ < 100 GeV.

5 Background estimateThe major background contributions arise from Z boson and WZ diboson production in asso-ciation with jets, as well as from tt production. In all these cases, particles from jets satisfy bothlepton identification and isolation criteria and are thus misidentified as a signal lepton.

The probability for such an object to be selected is measured from a sample of Z + `candidate

4 6 Systematic uncertainties

events, where Z is a pair of oppositely-charged same-flavor leptons which pass all analysisrequirements and satisfy |m`` −mZ| < 10 GeV, where mZ is the nominal Z boson mass 91.2 GeV,and `candidate passes relaxed identification requirements with no isolation requirements applied.The misidentification probability for each lepton flavor is defined as a ratio of the number ofcandidates that pass the final isolation and identification requirements to the total numberin the sample, measured in bins of lepton candidate pT and η. The contamination from WZevents is suppressed by requiring that the missing transverse energy Emiss

T is below 25 GeV,

and that the transverse mass mT ≡√(

E`T + Emiss

T

)2 −(~p`T + ~pmiss

T

)2 of `candidate and the missingtransverse momentum vector be less than 30 GeV. The residual contribution of WZ and ZZevents is estimated from simulation and subtracted.

To estimate the number of background events in the signal regions, control regions are exam-ined, defined to contain events with a dilepton candidate satisfying all requirements (Z1) andtwo additional lepton candidates `′`′ to which relaxed isolation and identification criteria areapplied. The additional leptons must have opposite charge and matching flavor (e±e∓, µ±µ∓).From this set of events, the expected number of background events in the signal region is ob-tained by scaling the number of observed Z1 + `′`′ events by the misidentification probability.To take into account all possible backgrounds, events in which both additional leptons fail thefull selection requirements are treated separately from events in which one of them passes. Acorrection is applied for cross-contamination between these two control regions.

Low-mass dileptons may be sufficiently collinear that their isolation cones overlap, and theirmisidentification probabilities must be treated as correlated. To mitigate the effect of thesecorrelations, only the control region in which both additional leptons fail the full selection isused if ∆R (`′1, `′2) < 0.6

The background contributions to the signal regions of Z→ ```′`′ and ZZ→ ```′`′ are summa-rized in Section 7.

6 Systematic uncertaintiesSystematic uncertainties are summarized in Table 1. Uncertainties related to the determinationof trigger efficiency are estimated to be the difference between trigger efficiencies in data and insimulated events. This difference is found to be around 2% of the final event yield. For Z→ 4eevents, the difference increases to 4%. In both data and simulated samples, trigger efficienciesare found with a tag-and-probe technique performed on events from Z→ `` production.

The lepton identification and isolation efficiencies in simulation are corrected with scaling fac-tors derived with the tag-and-probe method and applied as a function of lepton pT and η. Toestimate the uncertainties associated with these corrections, the total yield is recomputed withthe scaling factors varied up and down by the tag-and-probe fit uncertainties. The uncertaintyassociated with identification efficiency in the ZZ→ ```′`′ (Z→ ```′`′) signal regions is foundto be 0.9% (6%) in the 4e final state, 0.7% (4%) in the 2e2µ final state, and 0.4% (2%) in the 4µfinal state. The corresponding uncertainty associated with isolation efficiency is 1.1% (6%) inthe 4e final state, 0.7% (3%) in the 2e2µ final state, and 0.3% (1%) in the 4µ final state. These un-certainties are higher for Z→ ```′`′ events because the leptons generally have lower pT, and thesamples used in the tag-and-probe method have fewer events and more contamination fromnon-prompt leptons in this low-pT region.

The uncertainty on the LHC integrated luminosity of the data sample is 2.7%.

Uncertainties from the effect of QCD scales on the ZZ → ```′`′ acceptance are evaluated with

5

POWHEG and MCFM, by varying the QCD scales up and down by a factor of two with respectto the default µR = µF = mZZ. The variations in the acceptance are much less than 1% andare neglected. Parametric uncertainties (PDF+αs) are evaluated from the difference betweenthe CT10 and NNPDF sets, found to be less than 1%. The largest difference between predictionsfrom POWHEG and MCFM with different scales and PDF sets, 1.5%, is considered to be the the-oretical uncertainty on the acceptance calculation. An additional theoretical uncertainty arisesfrom scaling the qq→ZZ and gg→ZZ simulated samples from their NLO for POWHEG and LOfor MCFM cross sections, to their NNLO and NLO predicted cross sections. The change in theacceptance corresponding to these k-factors is found to be 1.1%. All theoretical uncertaintiesare added in quadrature.

A systematic uncertainty of 40% is applied to the estimated background yields, to account forthe statistical uncertainty arising from the limited number of events in the Z + ` control regionused to calculate the lepton misidentification probability and systematic uncertainties arisingfrom the difference in sample composition between this control region and the Z + `` controlregions. This uncertainty varies by channel but is less than 1% of the total expected yield.

Table 1: The contributions of each signal systematic uncertainty source on the cross sectionmeasurements. Luminosity uncertainty, and PDF and scale uncertainties, are considered sepa-rately. Uncertainties that vary by decay channel are listed as a range.

Uncertainty Z→4` ZZ→4`ID efficiency 2–6% 0.4–0.9%

Isolation efficiency 1–6% 0.3–1.1%Trigger efficiency 2–4% 2%

MC statistics 1–2% 1%Background 0.7–1.4% 0.7–2%

Pileup 0.4–0.8% 0.2%PDF 1% 1%Scale 1% 1%

Luminosity 2.7% 2.7%

7 Cross section measurementsThe distributions of the four-lepton mass and the masses of the Z1 and Z2 candidates are shownin Fig. 1. The SM preditions include the non-resonant ZZ predictions normalized to the NNLOcross section, production of the standard model Higgs boson with mass 125 GeV, and resonantZ → ```′`′ production. The reducible background estimated from data is also shown. Thereconstructed invariant mass of the Z1 candidates, and a scatter plot showing the correlationbetween mZ2 and mZ1 in data events, are shown in Fig. 2.

The four-lepton invariant mass distribution below 110 GeV is shown in Fig. 3a. Figure 3b showsmZ2 plotted against mZ1 for events with m```′`′ between 80 and 100 GeV, and the observed andexpected event yields in this mass region are given in Table 2.

The reconstructed four-lepton invariant mass is shown for events with two on-shell Z bosonsin Fig. 4a. Figure 4b shows the invariant mass distribution for all Z candidates in these events.The corresponding observed and expected yields are given in Table 3.

The observed yields are used to evaluate the pp→Z→ ```′`′ and pp→ZZ→ ```′`′ productioncross sections from a combined fit to the number of observed events in all the final states. Thelikelihood is a combination of individual channel likelihoods for the signal and backgroundhypotheses with the statistical and systematic uncertainties in the form of scaling nuisance

6 7 Cross section measurements

(a) (b)

Figure 1: Distributions of (a) the four-lepton invariant mass m```′`′ and (b) the invariant massof the dilepton candidates in all selected four-lepton events, including both Z1 and Z2 in eachevent. Points represent the data, shaded histograms represent the standard model predictionand reducible background estimate. Hatched regions around the predicted yield representcombined statistical, systematic, theoretical, and luminosity uncertainties.

(a) (b)

Figure 2: (a) The distribution of the reconstructed mass of Z1. Points represent the data,shaded histograms represent the standard model prediction and reducible background esti-mate. Hatched regions around the predicted yield represent combined statistical, systematic,theoretical, and luminosity uncertainties. (b) The reconstructed mZ2 plotted against the recon-structed mZ1 in data events, with distinctive markers for each final state.

7

Table 2: The observed and expected yields of four-lepton events in the mass region 80 <m```′`′ < 100 GeV, and estimated yields of background events evaluated from data, are shownfor each final state and are summed in the total expected yield. The first uncertainty is statisti-cal, the second systematic.

Final Nexp```′`′ Background Total Observed

state expected4µ 16.88± 0.14± 0.62 0.31± 0.30± 0.12 17.19± 0.33± 0.63 17

2e2µ 15.93± 0.14± 0.87 0.37± 0.27± 0.15 16.31± 0.31± 0.88 164e 5.62± 0.08± 0.53 0.21± 0.10± 0.08 5.83± 0.13± 0.53 6

Total 38.43± 0.21± 1.19 0.89± 0.42± 0.22 39.32± 0.47± 1.21 39

(a) (b)

Figure 3: (a) The distribution of the reconstructed four-lepton mass m```′`′ for events selectedwith m```′`′ < 110 GeV. Points represent the data, shaded histograms represent the standardmodel prediction and reducible background estimate. Hatched regions around the predictedyield represent combined statistical, systematic, theoretical, and luminosity uncertainties. (b)The reconstructed mZ2 plotted against the reconstructed mZ1 in data events selected with m```′`′

between 80 and 100 GeV, with distinctive markers for each final state.

Table 3: The observed and expected yields of ZZ events, and estimated yields of backgroundevents evaluated from data, are shown for each final state and are summed in the total expectedyield. The first uncertainty is statistical, the second systematic.

Final Nexp```′`′ Background Total Observed

state expected

4µ 21.80± 0.15± 0.46 0.00+0.24−0.00

+0.10−0.00 21.80± 0.28± 0.47 26

2e2µ 36.15± 0.20± 0.81 0.60± 0.34± 0.24 36.75± 0.34± 0.85 304e 14.87± 0.12± 0.36 0.81± 0.26± 0.33 15.68± 0.26± 0.48 8

Total 72.82± 0.27± 1.00 1.42± 0.49± 0.42 74.23± 0.56± 1.08 64

8 7 Cross section measurements

(a) (b)

Figure 4: Distributions of (a) the four-lepton invariant mass m```′`′ and (b) dilepton candidatemass for four-lepton events selected with both Z bosons on-shell. Points represent the data,shaded histograms represent the standard model prediction and reducible background esti-mate. Hatched regions around the predicted yield represent combined statistical, systematic,theoretical, and luminosity uncertainties.

parameters. The signal strength given by this fit across all channels is scaled by the cross sectionused in the simulation to find the measured fiducial cross section.

The definitions for the fiducial phase spaces for the Z→ ```′`′ and ZZ→ ```′`′ cross sectionmeasurements are given in Table 4.

Table 4: Fiducial definitions for reported cross sections. The common requirements are appliedfor both measurements.

Cross section measurement Fiducial cuts

Common requirements p`1T > 20 GeV, p`2

T > 10 GeV, p`3,4T > 5 GeV,∣∣∣η`

∣∣∣ < 2.5 GeV, m`+`− > 4 GeV (any `` pair)Z→ ```′`′ mZ1 > 40 GeV

80 < m```′`′ < 100 GeVZZ→ ```′`′ 60 < mZ1 , mZ2 < 120 GeV

The measured cross sections are

σfid(pp→ Z→ ```′`′) = 30.5+5.2−4.7 (stat)+1.8

−1.4 (syst)± 0.8 (lum) fb, (2)

σfid(pp→ ZZ→ ```′`′) = 34.8+4.6−4.2 (stat)+1.2

−0.8 (syst)± 0.9 (lum) fb. (3)

The pp → Z → ```′`′ fiducial cross section is scaled to σ(pp → Z) × B(Z → 4`) using theacceptance correction factorA = 0.122± 0.002, estimated with POWHEG. It corrects the fiducialZ→ ```′`′ cross section to the phase space with only the 80–100 GeV mass window and m`` >4 GeV requirements, and also includes a correction, 0.96 ± 0.01, for the contribution of non-resonant four-lepton production to the signal region. The uncertainty includes effects arising

9

from variation of the factorization and renormalization scales, PDF and αs uncertainties, andpossible effects of interference between resonant and non-resonant production, estimated withMADGRAPH5 AMG@NLO [29]. The measured cross section is

σ(pp→ Z)×B(Z→ ```′`′) = 250+43−39 (stat)+15

−11 (syst)± 4 (theo)± 7 (lum) fb. (4)

The branching fraction of the Z → ```′`′ decay, B(Z → ```′`′), is measured by comparing thecross section given in Eq. 4 with the Z→ `` cross section, and is computed as

B(Z→ ```′`′) =σ(pp→ Z)×B(Z→ ```′`′)

σ(pp→ Z)×B(Z→ ``)/B(Z→ ``) · C60−12080−100

(5)

where σ(pp → Z) × B(Z → ``) = 1870+50−40 pb is the Z → `` cross section times branch-

ing fraction calculated at NNLO with FEWZ [30] in the mass range 60–120 GeV. The uncer-tainty takes into consideration uncertainties on the strong coupling constant αs, the charmand bottom quark masses, and the effect of neglected higher-order corrections to the calcu-lation. The factor C60−120

80−100 = 0.926 ± 0.001 corrects for the difference in Z boson mass win-dows and is estimated using POWHEG. Its uncertainty includes scale and pdf variations. Thenominal Z to dilepton branching fraction B(Z→ ``) is 0.03366 [31]. The measured value isB(Z → ```′`′) = (4.9+0.8

−0.7 (stat)+0.3−0.2 (syst)+0.2

−0.1 (theo) ± 0.1 (lum)) × 10−6, where the theoreticaluncertainty includes the uncertainties on A, C60−120

80−100 , and σ(pp→ Z)×B(Z→ ``). This can becompared with 4.6× 10−6, computed with MCFM or MADGRAPH5 AMC@NLO and is con-sistent with CMS and ATLAS measurements at 7 and 8 TeV [9, 10].

The total ZZ production cross section for both Z bosons produced in the mass region 60–120GeV and m`` > 4 GeV is found to be

σ(pp→ ZZ) = 14.6+1.9−1.8 (stat)+0.5

−0.3 (syst)± 0.2 (theo)± 0.4 (lum)pb. (6)

The measured total cross section can be compared to the theoretical value of 16.5+0.7−0.5 pb, calcu-

lated via MATRIX Ref. [1, 32], or 15.0+0.8−0.6 pb, calculated with MCFM. Both values are calculated

with NNPDF3.0 PDF at NNLO and NLO respectively, and fixed renormalization and factoriza-tion scales set to µR = µF = mZ.

The total ZZ cross section is shown in Fig. 5 as a function of the proton-proton center-of-massenergy. Results from the CMS [2–4] and ATLAS [5–7] experiments are compared to predictionsfrom MATRIX and MCFM with NNPDF3.0 PDF sets and fixed scales µF = µR = mZ. The MA-TRIX prediction uses PDFs calculated at NNLO, while the MCFM prediction uses NLO PDFs;using the NNLO PDFs increases the MCFM prediction by around 3%. The uncertainties on thedata are statistical (inner bars) and statistical and systematic added in quadrature (outer bars).The band around the MATRIX predictions reflects scale uncertainties, while the band aroundthe MCFM predictions reflects both scale and PDF uncertainties. The theoretical predictionsand all CMS measurements are performed in the Z boson mass range 60–120 GeV. All ATLASmeasurements are in the mass window 66–116 GeV. The smaller mass window is estimated tocause a 1.6% reduction in measured cross section.

10 7 Cross section measurements

(TeV)s6 8 10 12 14

(pb

) Z

Z→

pp

σ

5

10

15

20

Z= MR

µ= F

µ NNPDF3.0, fixed

MATRIX NNLO (qq+qg+gg)

Z= MR

µ= F

µNNPDF3.0, fixed

MCFM NLO+gg

CMS 4l channel

channelνCMS 2l2

ATLAS 4l channel

νATLAS 4l+2l2

CMSPreliminary

Figure 5: The total ZZ cross section as a function of the proton-proton center-of-mass energy.Results from the CMS and ATLAS experiments are compared to predictions from MATRIX andMCFM with NNPDF3.0 PDF sets and fixed scales µF = µR = mZ.

11

8 SummaryResults have been presented for a study of four-lepton final states in proton-proton collisionsat√

s = 13 TeV with the CMS detector at the LHC. In particular, the pp→ ZZ cross sectionwas measured to be σ(pp→ZZ) = 14.6+1.9

−1.8 (stat)+0.5−0.3 (syst)± 0.2 (theo)± 0.4 (lum) pb for both

Z bosons produced in the mass range 60 < m`` < 120 GeV, and the branching fraction for Zdecays to four leptons was measured to be B(Z→ ```′`′) = (4.9+0.8

−0.7 (stat)+0.3−0.2 (syst)+0.2

−0.1 (theo)±0.1 (lum))× 10−6, for four-lepton mass in the range 80 < m```′`′ < 100 GeV and dilepton massm``′ > 4 GeV for all opposite-charge lepton pairs. The results are consistent with standardmodel predictions.

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