Search for leptophobic Z' bosons decaying into four-lepton final states in proton-proton collisions at sqrt(s) = 8 TeV

0.38 MB·

by  CMS Collaboration

EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH (CERN)
CERN-EP/2016-295
2017/01/06
CMS-EXO-14-006
(cid:48)
Search for leptophobic Z bosons decaying into four-lepton
√
final states in proton-proton collisions at s = 8TeV
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1 The CMS Collaboration
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J

5 Abstract

]
x
e Asearchforheavynarrowresonancesdecayingintofour-leptonfinalstatesfromcas-
p- c√ade decays of a Z(cid:48) boson has been performed using proton-proton collision data at
e s = 8TeV collected by the CMS experiment, corresponding to an integrated lumi-
h
[ nosityof19.7fb−1. Noexcessofeventsoverthestandardmodelbackgroundexpecta-

tionisobserved. Upperlimitsforabenchmarkmodelontheproductofcrosssection
1
v andbranchingfractionfortheproductionoftheseheavynarrowresonancesarepre-
5 sented. The limit excludes leptophobic Z(cid:48) bosons with masses below 2.5TeV within
4 thebenchmarkmodel. ThisisthefirstresulttoconstrainaleptophobicZ(cid:48) resonance
3
1 inthefour-leptonchannel.
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0 SubmittedtoPhysicsLettersB
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(cid:13)c 2017CERNforthebenefitoftheCMSCollaboration.CC-BY-3.0license
∗SeeAppendixAforthelistofcollaborationmembers

1
1 Introduction
Extensions of the standard model (SM) that incorporate one or more extra Abelian gauge
groups predict the existence of one or more neutral gauge bosons [1, 2]. These occur natu-
rally in most grand unified theories. Heavy neutral bosons are also predicted in models with
extra spatial dimensions [3, 4], e.g. Randall–Sundrum models [5, 6], where these resonances
may arise from Kaluza–Klein excitations of a graviton. Searches for heavy neutral resonances
at hadron colliders, and most recently at the CERN LHC, are typically performed using the
dijet [7–10], dilepton [11–14] diphoton [15–17], and tt [18–21] final states. The dilepton chan-
nel provides a clean signal compared with the dijet and tt channels. However, in leptophobic
Z(cid:48) models, where the Z(cid:48) does not couple to SM leptons, the dilepton limits are not applicable.
Although searches based on the dijet final state remain applicable, they suffer from large di-
jet backgrounds produced by quantum chromodynamics (QCD) subprocesses. We extend the
search for heavy neutral vector bosons by considering possible Z(cid:48) decays into new particles
predictedbyvarioustheoreticalextensionsoftheSM.
InthisLetter,wereportonasearchforaleptophobicZ(cid:48) resonancethatdecaysintofourleptons
via cascade decays as described in Ref. [22]. In this model, the Z(cid:48) is coupled to quark pairs
butnottoleptonpairs,andcanbeproducedwithalargecrosssectionattheLHC.Thesenon-
standardZ(cid:48)resonancesalsodecaytopairsofnewscalarbosons(ϕ)eachofwhichsubsequently
decaystopairsofleptons(ϕ → (cid:96)(cid:96)(cid:48),where(cid:96)and(cid:96)(cid:48) = eorµ). Figure1showstheleading-order
Feynmandiagramfortheproductionoffour-leptonfinalstatesviaaZ(cid:48) resonanceatahadron
collider. The reconstruction of the ϕ bosons in the dilepton channel is inefficient because the
twodaughterleptonsarehighlycollimated. Inthefollowingsectionswedescribeatechnique
toincreasetheselectionefficiency.
‘
−
q¯
ϕ
Z0 ‘+
‘
−
ϕ
q
‘+
Figure 1: Leading order Feynman diagram for the production and cascade decay of a Z(cid:48) reso-
nancetoafour-leptonfinalstate.
The analysis is a search for heavy narrow resonances decaying into four isolated final state
leptons. The benchmark model [22] assumes (Γ/M < 1%), corresponding to a natural width
oftheZ(cid:48) resonancethatismuchsmallerthanthedetectorresolution. Thefollowingfinalstates
areconsidered: µµµµ,µµµe,µµee,µeee,andeeee. Inparticular,µµee,µµµeandµeeechannels
are included to allow for the possibility of lepton flavor violation (LFV) [23–25] in the decays
of the new scalar bosons. In this Letter, we set limits on the product of the cross section and
branching fraction for production and decay to four leptons, and interpret the results in the
contextofthebenchmarkmodeldescribedabove[22].
2 3 TheMonteCarlosamples
2 The CMS detector and signal simulation
ThecentralfeatureoftheCMSapparatusisasuperconductingsolenoidof6minternaldiame-
ter,providingamagneticfieldof3.8T. Withinthesolenoidvolumeareasiliconpixelandstrip
tracker, aleadtungstatecrystalelectromagneticcalorimeter(ECAL),andabrassandscintilla-
torhadroncalorimeter(HCAL).Eachdetectoriscomposedofabarrelandtwoendcapsections.
Muons are measured in gas-ionization detectors embedded in the steel flux-return yoke out-
side the solenoid. Extensive forward calorimetry complements the coverage provided by the
barrelandendcapdetectors.
Muons are measured in the range |η| < 2.4 with detection planes made using three tech-
nologies: drift tubes, cathode strip chambers, and resistive-plate chambers. Matching muons
to tracks measured in the silicon tracker results in a relative p resolution for muons with
T
20 < p < 100GeVof1.3–2.0%inthebarrelandbetterthan6%intheendcaps. The p resolu-
T T
tioninthebarrelisbetterthan10%formuonswith p upto1TeV[26].
T
The ECAL consists of 75848 crystals that provide coverage in pseudorapidity |η| < 1.48 in a
barrel region (EB) and 1.48 < |η| < 3.00 in two endcap regions (EE). The electron momentum
is estimated by combining the energy measurement in the ECAL with the momentum mea-
surement in the tracker. The momentum resolution for electrons with transverse momentum
p ≈ 45GeVfromZ → e+e− decaysrangesfrom1.7%fornonshoweringelectronsinthebarrel
T
regionto4.5%forshoweringelectronsintheendcaps[27].
A more detailed description of the CMS detector, together with a definition of the coordinate
systemusedandtherelevantkinematicvariables,canbefoundinRef.[28].
3 The Monte Carlo samples
The Monte Carlo (MC) samples for the benchmark model are produced using the CALCHEP
3.4.1generator[29]interfacedwith PYTHIA 6.4.24[30]. Thesesamplesaredividedintofivede-
cay channels (µµµµ,µµµe,µµee, µeee,eeee) for different Z(cid:48) boson masses (mZ(cid:48)) ranging from
250 to 3000GeV in increments of 250GeV. The benchmark model assumes that new particles
other than Z(cid:48) and ϕ are heavy enough not to affect the production and decay of the Z(cid:48) boson.
SignalMCsamplesareproducedwithsixdifferentvaluesoftheϕmass(m ),withm =50GeV
ϕ ϕ
used as the reference mass value in the interpretation of the results. An important feature of
thisanalysisisthepresenceofa“boostedsignature”associatedwiththecollimationofthetwo
leptonscomingfromthesameparentparticleandresultingfromthelargedifferencebetween
mZ(cid:48) andmϕ. Inaddition,samplesaregeneratedwithmϕ massesof5,10,20,30and40%ofthe
mZ(cid:48),forwhich,inmostcases,thecontributionfromtheboostedsignatureislessimportant. The
product of the leading order (LO) signal cross section and branching fraction in each channel
varieswithmZ(cid:48) (from250to3000GeV)asfollows: µµµµandeeeefrom0.8pbto3.0×10−6pb,
µµeefrom12.3pbto4.7×10−5pb,andµµµeandµeeefrom3.1pbto1.2×10−5pb. Thebranch-
ingfractionof ϕ → (cid:96)(cid:96)(cid:48) issetto1andthereforeonlytheleptonicdecaychannelsareconsidered.
ThesesignalMCsamplesareusedtooptimizeeventselection,evaluatesignalefficienciesand
calculateexclusionlimits.
The dominant SM background is the production of ZZ decaying into four leptons. The qq-
induced ZZ production is generated using the PYTHIA event generator and the gg-induced
production using the GG2ZZ program [31]. Additional backgrounds from diboson produc-
tion (WW and WZ) are generated with PYTHIA, and from top quark production (tt, tW, and
tW) are generated with POWHEG 1.0 [32]. Other processes, such as ttZ and triboson produc-
3
tion (WWγ, WWZ, WZZ, and ZZZ), are generated with MADGRAPH 5.1.3.30, rescaled by the
next-to-leadingorder(NLO)K-factors[33]. Simulatedeventsamplesarenormalizedusingthe
integratedluminosityandhigherordertheoreticalcrosssections: next-to-next-to-leadingorder
fortt[34]andNLOforZZ[35]andtheotherbackgrounds.
The MC samples are generated using the CTEQ6L [36] set of parton distribution functions
(PDFs)andthe PYTHIA Z2*tune[37,38]inordertomodeltheprotonstructureandtheunder-
lying event. The samples are then processed with the full CMS detector simulation software,
basedon GEANT4 [39,40],whichincludestriggersimulationandeventreconstruction.
4 Event selection
√
The 2012 data set of proton-proton collisions at s = 8TeV, corresponding to an integrated
luminosity of 19.7fb−1, is used for the analysis. Data are collected with lepton triggers with
various p thresholds. Thetriggerusedforthemuon-enrichedchannels(µµµµ,µµµe)requires
T
thepresenceofatleastonemuoncandidatewith p > 40GeVand |η| < 2.1. Thetriggerused
T
for the electron-enriched channels (µeee,eeee) requires two clusters of energy deposits in the
ECAL with transverse energy E > 33GeV each. For the µµee channel, the trigger requires
T
p > 22GeVforboththemuonandtheelectron.
T
Inthesubsequentanalysis,eventsarerequiredtocontainareconstructedprimaryvertex(PV)
withatleastfourassociatedtracks,anditsr (z)coordinatesarerequiredtobewithin2(24)cm
ofthenominalinteractionpoint. ThePVisdefinedasthevertexwiththehighestsumof p2 for
T
theassociatedtracks. Weselecttheeventswithfourleptonsinthefinalstate,wheretheleptons
areidentifiedbytheselectioncriteriadescribedbelow. Thetwoleadingleptonsarerequiredto
have p > 45GeVandthetwosubleadingleptonstohave p > 30GeV. Allfourleptonsmust
T T
satisfy|η| < 2.4. Nochargerequirementisappliedtotheleptonselection.
Muoncandidatesarereconstructedbyacombinedfitincludinghitsinbothtrackerandmuon
detectors (“global muons”) [26]. Global muons are required to pass the following criteria: at
leastonepixeldetectorhit,atleastsixstriptrackerlayerswithhits,atleastonemuonchamber
hit, at least two muon detector planes with muon segments, a transverse impact parameter of
the tracker track |d | < 0.2cm with respect to the PV, a longitudinal distance of the tracker
xy
track |d | < 0.5cm with respect to the PV, and δp /p < 0.3 where δp is the uncertainty in
z T T T
the measured p of the track. All muon candidates are required to be isolated. A muon is
T
consideredisolatedifthescalar p sumofalltracks,excludingtheidentifiedmuoncandidates,
T
within a cone of ∆R < 0.3 around the muon does not exceed 10% of the muon p , where
√ T
∆R = (∆φ)2+(∆η)2. We remove the contribution of the second lepton candidate if it is
withinaconeof∆R < 0.3.
An electron candidate is identified by matching a cluster in the ECAL to a track in the silicon
tracker[27]. Identificationcriteriaareappliedtosuppressjetsmisidentifiedaselectrons. Elec-
trons are required to pass the following criteria: the profile of energy deposition in the ECAL
should be consistent with an electron, the sum of HCAL energy deposits behind the ECAL
cluster should be less than 10% of the associated ECAL deposit, the track associated with the
clustershouldhavenomorethanonehitmissinginthepixeldetectorlayersand |d | should
xy
be less than 0.02cm with respect to the selected PV. All electron candidates are required to be
isolated using the following definition: the p sum of all other tracks in a cone of ∆R < 0.3
T
around the track of the electron candidate is required to be less than 5GeV and the E sum of
T
the energies of the calorimeter deposits that are not associated with the candidate is required
to be less than 5% of the candidate’s E . This differs from the isolation requirement of 3% in
T
4 5 Backgroundestimation
Ref. [13], because of the inefficiency (of approximately 6% at electron E = 1TeV) caused by
T
overlapping electrons due to the high Lorentz boost of the ϕ boson (m = 50GeV). In addi-
ϕ
tion, if the direction of the second lepton candidate falls within the isolation cone of the first
(∆R < 0.3), the contributions it makes to both p and E are subtracted when imposing the
T T
isolationrequirements.
Thekinematicdistributionsofthefinal-stateparticlesaresimilarforallfivechannels. Thefinal
state consists of two leading leptons with high p and two subleading leptons with relatively
T
low p . The two leptons from the same parent ϕ boson can be highly Lorentz boosted if m
T ϕ
is significantly smaller than mZ(cid:48). This feature is generally found for high-mass (mZ(cid:48) > 1TeV)
samplesinthecaseofm = 50GeV. Thisboostedsignatureintroducesasignificantinefficiency
ϕ
for the event selection except for the LFV case (ϕ decaying into eµ). To take into account the
boosted signature for ϕ decaying into µµ, one of the muon candidates selected by the above
criteria is allowed to be reconstructed only as a tracker muon, a track in the tracker matched
totracksegmentsinthemuonsystem(“trackermuons”)[26],ifthetwomuonsareascloseas
∆R < 0.4. In such exceptional cases, the requirement of at least one muon chamber hit and at
leasttwomuondetectorplaneswiththemuonsegmentsarenotappliedtothetrackermuon.
The boosted signature for a ϕ decaying into ee is much more complicated since the electrons
can easily merge into a single cluster in the ECAL. In this case, only one electron candidate is
reconstructed from the two original electrons. The probability for having a merged candidate
isabout50%with mZ(cid:48) = 3TeVand mϕ = 50GeV. Theseeventswouldberejectedbythefour-
lepton requirement, introducing a large signal inefficiency. To select such events, an electron
candidate having a ratio of ECAL cluster energy to track momentum larger than 1.5 and a
second track with p > 30GeV within the cone of ∆R(electron,track) < 0.25, is considered
T
asa“mergedelectron”. Eventsareacceptedwiththree(two)leptonsiftheycontainone(two)
merged electron(s), since each merged electron is considered to contribute to two electrons to
thetotal. Inorder toavoidsignificant misidentification, mergedelectronsare onlyconsidered
iftheECALclusterenergyisbiggerthan500GeV.
The dominant background in this analysis arises from ZZ events decaying into four leptons.
Tosuppressthisbackground,eventswithtwooppositelychargedsame-flavorleptonpairsare
rejected if the mass of the lepton pair, m , is in the range 89–93GeV. The Z mass window is
(cid:96)(cid:96)
madeasnarrowaspossibleinordertominimisedegradationofthesignalefficiencyinthecase
of mϕ ≈ mZ. This requirement results in negligible signal efficiency loss for mZ(cid:48) > 500GeV.
Morethan70%(30%)oftheZZbackgroundisrejectedbythemasswindowvetorequirement
in the muon (electron) channel. This requirement is not applied to the merged electron case,
thusaccountingforthedifferenceinrejectionefficiencyforthetwochannels.
A typical event selection efficiency is 50–70% (µµµµ), 55–65% (µµµe and µµee) and 45–65%
(µeee and eeee) throughout the entire mZ(cid:48) range at mϕ = 50GeV. The heavier mϕ values cor-
respond to a less boosted signature and therefore are selected with a higher efficiency. For
mZ(cid:48) > 2GeV, the efficiency for the other mϕ samples is approximately 10–15% (1–5%) higher
intheelectron(muon)channelsthanforthe m = 50GeVscenario, wheretherangeofvalues
ϕ
reflectsthevariationwithmZ(cid:48).
5 Background estimation
Most of the SM backgrounds are suppressed by requiring four isolated high-quality lepton
candidates. As discussed above, the dominant backgrounds are ZZ events decaying into four
leptons. Other background originates from top quark events with two genuine leptons and
5
two lepton candidates arising from misidentified jets. The WW (WZ) events can also pass the
signal selection if they contain two (one) misidentified or nonprompt leptons from jets. In the
caseoftribosonproduction,theremaybefourgenuineleptonsassociatedwithjetsormissing
transverseenergy. ThesebackgroundsareestimatedusingMCsimulation.
Thecontributionfromeventswithmorethantwoleptonsarisingfrommisidentifiedjetsisex-
pected to be small because this analysis requires four isolated leptons in the final state. This
backgroundisestimatedusingthe“misidentificationrate”methoddescribedinRef.[13]. The
misidentification rate measured as a function of electron E in the barrel and endcap is ap-
T
plied to events with electron candidates passing the trigger but failing the full selection. The
contributionfromjetbackgroundsestimatedusingthisprocedureisfoundtobenegligible.
Figure 2 shows the four-lepton invariant mass (m ) distribution for selected events. The ob-
4(cid:96)
served events and estimated backgrounds are summarized in Table 1. As shown in the figure
and table, the observed events are in agreement with the expected backgrounds. The table
shows two different mass ranges. In the region m > 1TeV, the backgrounds from SM pro-
4(cid:96)
cessesareverysmall,typicallylessthanoneevent.
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19.7 fb (8 TeV)
V
e CMS Data
G 18
ZZ
0
5 Top quark
16
/
s EW
t
n 14 Z' (2.5TeV) signal
e
v
E
12 V
e
G
10 50 1.5
s /
nt
8 ve 1
E
0.5
6
0
4 2000 2200 2400 2600 2800 3000
m [GeV]
4l
2
0
200 400 600 800 1000 1200
m [GeV]
4l
Figure 2: The m spectrum for the combination of the five studied channels. The points with
4(cid:96)
verticalbarsrepresentthedatawiththeirstatisticaluncertainties;thehistogramsrepresentthe
expectations from SM processes; “Top quark” denotes the sum of the events for tt, tW, tW,
ttZ processes; “EW” denotes the sum of the events from WW,WZ,WWγ,WWZ,WZZ, and
ZZZprocesses. TheinsetshowstheexpectationfromthebenchmarkmodelforasignalatmZ(cid:48)
=2.5TeVwithm = 50GeV.
ϕ
6 Results
NoexcessofeventsisobservedinthedatasamplecomparedtotheSMexpectations. Exclusion
limits at 95% confidence level (CL) are calculated in the context of the benchmark model with
6 6 Results
Table 1: Summary of the observed yield and expected backgrounds for all channels, where
N isthenumberofobservedeventsindata. Thetotalbackground(N )isthesumofthree
obs tot
differentbackgroundsthatareestimatedusingMCsimulations; N referstothebackground
ZZ
fromZZevents; N isthebackgroundfromtt,singletopquark,andttZproduction; N isthe
t EW
backgroundfromWWandWZ, andtriplegaugebosonproduction. Thequoteduncertainties
arestatisticalonly.
0.1 < m < 1.0TeV m > 1.0TeV
4(cid:96) 4(cid:96)
Channel SMbackgrounds
N N N
obs N N N N obs tot
ZZ t EW tot
Z(cid:48) → µµµµ 3 4.9±0.3 0.9±0.5 —- 5.9±0.6 0 —-
Z(cid:48) → µµµe 6 0.4±0.1 1.3±0.6 1.2±0.3 2.9±0.7 0 —-
Z(cid:48) → µµee 12 9.3±0.4 3.0±1.5 1.2±0.3 13.5±1.6 0 0.1±0.1
Z(cid:48) → µeee 2 0.2±0.1 0.4±0.1 0.6±0.2 1.2±0.2 0 0.1±0.1
Z(cid:48) → eeee 9 15.0±0.5 0.2±0.1 0.2±0.1 15.4±0.5 0 0.2±0.1
Combined 32 29.9±0.7 5.7±1.9 3.3±0.5 38.9±2.1 0 0.4±0.2
a Bayesian approach. The signal region (0.1 < m < 3.0TeV) consists of four leptons (e or
4(cid:96)
µ) with |η| < 2.4: two leading (subleading) leptons are required to have p > 45(30)GeV.
T
Thelikelihoodfunctionisdefinedwithasignalstrengthmodifier,apriorprobability,andaset
of nuisance parameters that are used to incorporate systematic uncertainties. This method is
basedoninterpretingthelikelihoodasaprobabilitydistribution,withlog-normaldistributions
used for nuisance parameters [41]. Integrating over the latter we obtain a limit on the signal
contribution.
The systematic uncertainties are dominated by the uncertainty in the background estimates
and in the lepton selection efficiencies. A 30% uncertainty in the total background cross sec-
tion (ZZ and tt) is used to account for uncertainties arising from PDFs and higher-order QCD
corrections in the measured 8TeV cross sections. The systematic uncertainty in the muon se-
lection including reconstruction, identification, and isolation is 0.5% [26]. The uncertainties in
the electron selection are 0.7% (0.6%) for electrons below 100GeV in EB (EE) and 1.4% (0.4%)
for electrons above 100GeV in EB(EE) [13]. The uncertainties due to the lepton efficiency in
bothsignalandbackgroundyieldsvarybetween2.2%and2.7%asafunctionofm . Including
4(cid:96)
theeffectofthemergedleptonsignature, atotaluncertaintyof10%isassignedforeachchan-
nel. The impact of the uncertainty in the electron energy scale on signal (background) yield is
1% (0.5%) [13]. Uncertainties in the muon momentum scale and mass resolutions are below
0.1%[26]. Theuncertaintyintheintegratedluminosityisassignedtobe2.6%[42]. Inthisanal-
ysis, the statistical uncertainties are dominant and the systematic uncertainties have a small
impact on the results. We tested the robustness of the limits by doubling the values assumed
forthesystematicuncertainties. Weobservedanegligiblechangeinthecalculatedlimits, and
concludethatthelimitsareinsensitivetoanyunderestimationofthesystematicuncertainties.
Limitsontheproductofcrosssectionandbranchingfractionaresetinthecontextofthebench-
mark model as a function of m . The mass resolution of the detector is assumed to be larger
4(cid:96)
than the natural width of the Z(cid:48) resonance in all channels. In the limit calculation, we set the
masswindowtobesixtimesthemassresolutioncentredaroundthesignalmasspointconsid-
ered. A counting experiment is performed for the limit calculation. Figure 3 shows the upper
limitontheproductofthecrosssectionandbranchingfraction,forthecombinationofallfive
channels. UsingthebenchmarkmodelofRef.[22]wetranslatethesecrosssectionupperlimits
intolowerlimitsontheZ(cid:48) bosonmass. Forthecombinationofthefivechannels,thevalueob-
tainedforthislowermasslimitis2.5TeV. Theblacksolid(dashed)lineindicatestheobserved
7
(expected)95%CLupperlimits,theinner(outer)bandindicatesthe±1(2)standarddeviation
uncertaintyintheexpectedlimits,andthebluedashedlineshowsthetheoreticalZ(cid:48) crosssec-
tion for m = 50GeV. This theoretical cross section is calculated under the benchmark model
ϕ
assumptionthatthebranchingfractionB(ϕ → (cid:96)(cid:96)(cid:48)) = 100%. Intheregionabovethe1–1.5TeV,
thebandsarenotvisiblesincebackgroundsarenegligiblehere.
- 1
19.7 fb (8 TeV)
] 60
b 50
[f 40 CMS Observed

B 30 Expected
x
20 Expected – 1 std. dev.
s
Expected – 2 std. dev.
10
Z' (LO): m = 50 GeV
j
2
1
0.3
500 1000 1500 2000 2500 3000
m [GeV]
4l
Figure3: The95%CLupperlimitonthecrosssectiontimesbranchingfractionasafunctionof
m forthecombinationofthefivechannels. Theshadedgreen(yellow)bandindicatestheone
4(cid:96)
(two)sigmauncertaintyintheexpectedlimits. Thebluedashedlinerepresentsthetheoretical
predictionsforthebenchmarkmodel[22]form = 50GeV.
ϕ
Table2showstheexclusionlimitonmZ(cid:48) forthefiveseparatechannelsandforthecombination.
Results are presented for the benchmark assumption m = 50GeV, and for the five different
ϕ
values of the ratio m /m assumed for the generated signal samples, taking into account the
ϕ 4(cid:96)
event selection efficiencies described above. The predicted cross sections decrease as the ratio
m /m increases. Thecontributionofthemergedleptonsignaturealsodecreases,resultingin
ϕ 4(cid:96)
an overall efficiency increase. Therefore the scenarios with m /m = 5, 10, 20, 30 and 40% of
ϕ 4(cid:96)
mZ(cid:48),giveslightlyhigherlimitsthanthemϕ = 50GeVscenario.
7 Summary
Results have been presented from a search for heavy narrow resonances decaying into four-
lepton final states via intermediate scalar particles ϕ, where the branching fraction of ϕ → (cid:96)(cid:96)
((cid:96) = e or µ) is set to 1. These results are based on a sample of proton-proton collision data at
√
s = 8TeV, correspondingtoanintegratedluminosityof19.7fb−1. Thefour-leptoninvariant
mass spectra are consistent with the standard model predictions. Masses of Z’ bosons have
beenexcludedat95%confidencelevelforaspecificbenchmarkmodelwithm = 50GeV,and
ϕ
for five different assumptions for the ratio mϕ/mZ(cid:48) (mϕ/mZ(cid:48) = 5, 10, 20, 30 and 40%). Five
decay channels (µµµµ,µµµe,µµee, µeee,eeee) are considered in this analysis. Combining all
8 7 Summary
Table 2: The 95% CL lower limits (in TeV) on mZ(cid:48) for the five separate channels and for their
combination. Results are presented for the benchmark assumption m = 50GeV, and for the
ϕ
fivedifferentvaluesoftheratiomϕ/mZ(cid:48).
mϕ 50GeV 0.05mZ(cid:48) 0.1mZ(cid:48) 0.2mZ(cid:48) 0.3mZ(cid:48) 0.4mZ(cid:48)
µµµµ 1.7 1.6 1.7 1.7 1.7 1.7
µµµe 2.0 2.0 2.1 2.1 2.1 2.1
µµee 2.4 2.4 2.5 2.5 2.5 2.5
µeee 2.0 2.0 2.1 2.1 2.1 2.1
eeee 1.7 1.7 1.7 1.7 1.7 1.7
Combined 2.5 2.6 2.6 2.6 2.6 2.6
channels, a lower limit on the Z(cid:48) mass of 2.5TeV is obtained for the benchmark model, and
2.6TeV for each of the models assuming a fixed ratio between mϕ and mZ(cid:48). This is the first
resulttoconstrainaleptophobicZ(cid:48) resonanceinthefour-leptonchannel.
Acknowledgments
We congratulate our colleagues in the CERN accelerator departments for the excellent perfor-
mance of the LHC and thank the technical and administrative staffs at CERN and at other
CMS institutes for their contributions to the success of the CMS effort. In addition, we grate-
fully acknowledge the computing centres and personnel of the Worldwide LHC Computing
Grid for delivering so effectively the computing infrastructure essential to our analyses. Fi-
nally, we acknowledge the enduring support for the construction and operation of the LHC
and the CMS detector provided by the following funding agencies: BMWFW and FWF (Aus-
tria);FNRSandFWO(Belgium);CNPq,CAPES,FAPERJ,andFAPESP(Brazil);MES(Bulgaria);
CERN; CAS, MoST, and NSFC (China); COLCIENCIAS (Colombia); MSES and CSF (Croatia);
RPF (Cyprus); SENESCYT (Ecuador); MoER, ERC IUT, and ERDF (Estonia); Academy of Fin-
land, MEC, and HIP (Finland); CEA and CNRS/IN2P3 (France); BMBF, DFG, and HGF (Ger-
many); GSRT (Greece); OTKA and NIH (Hungary); DAE and DST (India); IPM (Iran); SFI
(Ireland); INFN (Italy); MSIP and NRF (Republic of Korea); LAS (Lithuania); MOE and UM
(Malaysia);BUAP,CINVESTAV,CONACYT,LNS,SEP,andUASLP-FAI(Mexico);MBIE(New
Zealand); PAEC (Pakistan); MSHE and NSC (Poland); FCT (Portugal); JINR (Dubna); MON,
RosAtom, RAS, RFBR and RAEP (Russia); MESTD (Serbia); SEIDI and CPAN (Spain); Swiss
Funding Agencies (Switzerland); MST (Taipei); ThEPCenter, IPST, STAR, and NSTDA (Thai-
land); TUBITAK and TAEK (Turkey); NASU and SFFR (Ukraine); STFC (United Kingdom);
DOEandNSF(USA).
Individuals have received support from the Marie-Curie programme and the European Re-
search Council and EPLANET (European Union); the Leventis Foundation; the A. P. Sloan
Foundation; theAlexandervonHumboldtFoundation; theBelgianFederalSciencePolicyOf-
fice; the Fonds pour la Formation a` la Recherche dans l’Industrie et dans l’Agriculture (FRIA-
Belgium);theAgentschapvoorInnovatiedoorWetenschapenTechnologie(IWT-Belgium);the
Ministry of Education, Youth and Sports (MEYS) of the Czech Republic; the Council of Sci-
ence and Industrial Research, India; the HOMING PLUS programme of the Foundation for
Polish Science, cofinanced from European Union, Regional Development Fund, the Mobil-
ity Plus programme of the Ministry of Science and Higher Education, the National Science
Center (Poland), contracts Harmonia 2014/14/M/ST2/00428, Opus 2014/13/B/ST2/02543,
2014/15/B/ST2/03998,and2015/19/B/ST2/02861,Sonata-bis2012/07/E/ST2/01406;theThalis
and Aristeia programmes cofinanced by EU-ESF and the Greek NSRF; the National Priorities