Measurement of dijet azimuthal decorrelation in pp collisions at sqrt

32 Pages·2016·0.64 MB·English

EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH (CERN)
CERN-PH-EP/2016-013
2016/02/16
CMS-SMP-14-015
Measurement of dijet azimuthal decorrelation in
√
pp collisions at s = 8TeV
6
1 ∗
The CMS Collaboration
0
2

b
e
F

3
Abstract
1

]
x
A measurement of the decorrelation of azimuthal angles between the two jets with
e
- the largest transverse momenta is presented for seven regions of leading jet trans-
p
verse momentum up to 2.2TeV. The analysis is based on the proton-proton collision
e
h data collected with the CMS experiment at a centre-of-mass energy of 8TeV corre-
[ sponding to an integrated luminosity of 19.7fb−1. The dijet azimuthal decorrelation

1 iscausedbytheradiationofadditionaljetsandprobesthedynamicsofmultijetpro-
v
duction. Theresultsarecomparedtofixed-orderpredictionsofperturbativequantum
4
8 chromodynamics(QCD),andtosimulationsusingMonteCarloeventgeneratorsthat
3 include parton showers, hadronization, and multiparton interactions. Event genera-
4
tors with only two outgoing high transverse momentum partons fail to describe the
0
. measurement,evenwhensupplementedwithnext-to-leading-orderQCDcorrections
2
0 andpartonshowers. Muchbetteragreementisachievedwhenatleastthreeoutgoing
6 partonsarecomplementedthrougheithernext-to-leading-orderpredictionsorparton
1
showers. This observation emphasizes the need to improve predictions for multijet
:
v production.
i
X
r
a
SubmittedtotheEuropeanPhysicalJournalC
(cid:13)c 2016CERNforthebenefitoftheCMSCollaboration.CC-BY-3.0license
∗SeeAppendixAforthelistofcollaborationmembers

1
1 Introduction
Hadronic jets with large transverse momenta p are produced in high-energy proton-proton
T
collisions when two partons interact with high momentum transfer via the strong force. At
leadingorder(LO)inperturbativequantumchromodynamics(pQCD),twofinal-statepartons
are produced back-to-back in the transverse plane. For this case, the azimuthal angular sepa-
rationbetweenthetwoleading p jetsinthetransverseplane,∆φ = |φ −φ |,equalsπ.
T dijet jet1 jet2
The nonperturbative effects of multiparton interactions or hadronization disturb this corre-
lation only mildly, and ∆φ ≈ π still holds. However, the production of a third high-p jet
dijet T
leadstoadecorrelationinazimuthalangle. Thesmallestachievablevalueof∆φ = 2π/3oc-
dijet
cursinasymmetricstar-shaped3-jetconfiguration. Fixed-ordercalculationsinpQCDfor3-jet
productionwithuptofouroutgoingpartonsprovidenext-to-leading-order(NLO)predictions
for the region of 2π/3 ≤ ∆φ < π. If more than three jets are produced, the azimuthal an-
dijet
gle between the two leading jets can approach zero, although very small angular separations
are suppressed because of the finite jet sizes for a particular jet algorithm. The measurement
of the dijet azimuthal angular decorrelation is an interesting tool to gain insight into multijet
productionprocesseswithoutmeasuringjetsbeyondtheleadingtwo.
This paper reports the measurement of the dijet differential cross section as a function of the
dijetazimuthalangularseparation,
1 dσdijet
, (1)
∆
σ d φ
dijet dijet
for seven regions of the leading jet p , pmax, within a rapidity region of |y| < 2.5. Experi-
T T
∆
mental and theoretical uncertainties are reduced by normalizing the φ distribution to the
dijet
total dijet cross section σ within each region of pmax. For the first time, azimuthal angular
dijet T
∆
separations φ over the full phase space from 0 to π are covered. Comparisons are made
dijet
tofixed-orderpredictionsuptoNLOfor3-jetproduction, andtoNLOandLOdijetaswellas
to tree-level multijet production, each matched with parton showers and complemented with
multipartoninteractionsandhadronization.
The measurement is performed using data collected during 2012 with the CMS experiment at
theCERNLHC,correspondingtoanintegratedluminosityof19.7fb−1 ofproton-protoncolli-
√
sionsat s = 8TeV. Previousmeasurementsofdijetazimuthaldecorrelationwerereportedby
√
the D0 Collaboration in pp collisions at s = 1.96TeV at the Tevatron [1, 2], and by the CMS
√
andATLASCollaborationsinppcollisionsat s = 7TeVattheLHC[3,4].
2 The CMS Detector
AdetaileddescriptionoftheCMSdetectorcanbefoundinRef. [5]. Thecentralfeatureofthe
CMSdetectorisasuperconductingsolenoid,13minlengthand6mininnerdiameter,provid-
inganaxialmagneticfieldof3.8T. Withinthefieldvolumeareasiliconpixelandstriptracker,
aleadtungstatecrystalelectromagneticcalorimeter(ECAL)andabrassandscintillatorhadron
calorimeter (HCAL), each composed of a barrel and two endcap sections. Charged particle
trajectories are measured by the tracker with full azimuthal coverage within pseudorapidities
|η| < 2.5. The ECAL, which is equipped with a preshower detector in the endcaps, and the
HCAL cover the region |η| < 3. In addition to the barrel and endcap detectors, CMS has ex-
tensive forward calorimetry, which extends the coverage up to |η| < 5. Finally, muons are
measured up to |η| < 2.4 by gas-ionization detectors embedded in the steel flux-return yoke
outsidethesolenoid.
2 3 Eventreconstructionandselection
3 Event reconstruction and selection
This measurement uses data samples that were collected with single-jet high-level triggers
(HLT) [6]. Four such single-jet HLTs were considered that require at least one jet in the event
to have p > 140, 200, 260, and 320GeV, respectively. All triggers were prescaled during
T
the 2012 run except the highest-threshold trigger. The integrated luminosity L for the four
trigger samples is shown in Table 1. The trigger efficiency is estimated using triggers with
lower p thresholds. Using these four jet-energy thresholds gives 100% trigger efficiencies in
T
the corresponding four momentum regions 200 < pmax < 300GeV, 300 < pmax < 400GeV,
T T
400 < pmax < 500GeV,and pmax > 500GeV.
T T
Table1: Theintegratedluminosityforeachtriggersampleconsideredinthisanalysis.
HLT p threshold(GeV) 140 200 260 320
T
L(fb−1) 0.06 0.26 1.06 19.7
Particlesarereconstructedandidentifiedusingaparticle-flow(PF)algorithm,whichcombines
theinformationfromtheindividualsubdetectors[7,8]. Thefour-vectorsofparticlecandidates,
reconstructedbytheabovetechnique,areusedasinputtothejet-clusteringalgorithm. Jetsare
reconstructedusingtheinfrared-andcollinear-safeanti-k clusteringalgorithmwithadistance
T
parameter R = 0.7[9]. Theclusteringisperformedwiththe FASTJET package[10]usingfour-
momentumsummation.
Thereconstructedjetsrequiresmalladditionalenergycorrectionstoaccountforvariousrecon-
struction inefficiencies in tracks and clusters in the PF algorithm. These jet energy corrections
[11] are derived using (1) simulated events, generated with PYTHIA 6.4.22 [12] with tune Z2*
[13, 14] and processed through the CMS detector simulation based on GEANT4 [15], and (2)
measurementscontainingdijet,photon+jet,andZ+jetevents. Thejetenergycorrections,which
dependontheη and p ofthejet,areappliedtothejetfour-momentumvectorsasmultiplica-
T
tivefactors[16]. Theoverallfactoristypically1.2orsmaller,approximatelyuniformin η,and
is 1.05 or smaller for jets having p > 100GeV. An offset correction is applied to take into
T
account the extra energy clustered into jets from additional proton-proton interactions within
thesameorneighbouringbunchcrossings(in-timeandout-of-timepileup)[11]. Pileupeffects
areimportantonlyforjetswithlow p andbecomenegligibleforjetswith p > 200GeV. The
T T
currentmeasurementis,therefore,insensitivetopileupeffects.
Eacheventisrequiredtohaveatleastonevertexreconstructedoffline[17]withapositionalong
the beam line that is within 24cm of the nominal interaction point. To suppress nonphysical
jets,i.e.jetsresultingfromnoiseintheECALand/orHCALcalorimeters,stringentcriteria[18]
are applied for identifying jets: each jet should contain at least two particles, one of which is
a charged hadron, and the jet energy fraction carried by neutral hadrons and photons should
be less than 90%. The efficiency for identifying physical jets using these criteria is greater
than99%.
∆
Thetwoleadingjets,whichdefine φ ,areselectedbyconsideringalljetsintheeventwith
dijet
p > 100GeVandanabsoluterapidity |y| < 5. Eventsareselectedinwhichtheleadingjet p
T T
exceeds200GeVandthetwoleadingjetsliewithinthetrackercoverageof|y| < 2.5.
To reduce the background from tt and heavy vector boson production, the variable E/ /∑E
T T
is used. The sum of the transverse energies is ∑E = ∑ E sinθ , and the missing transverse
T i i i
(cid:113)
energy E/ = [∑ (E sinθ cosφ)]2+[∑ (E sinθ sinφ)]2, where θ isthepolarangleandthe
T i i i i i i i i
sum runs over all PF candidates in the event. A noticeable fraction of high-p jet events with
T
large E/ emerges from tt production with semileptonically decaying b quarks. In addition,
T
3
Z/W+jet(s) events with Z decays to neutrinos and W decays into charged leptons with neu-
trinos have high E/ values. The distributions of the variable E/ /∑E are shown in Fig. 1 for
T T T
thetworegions ∆φ < π/2(left)and π/2 < ∆φ < π (right). Thedata(points)arecom-
dijet dijet
pared to simulated events (stacked), using MADGRAPH 5.1.3.30 [19] matched to PYTHIA6 [12]
for eventgeneration. Although some deviationsof the simulationwith respect tothe data are
visibleinFig.1(cf.Ref. [20]),thedistributionsallowaselectioncriteriontobeoptimizedwith
respecttotheratioofsignaloverbackground. Eventswith E/ /∑E > 0.1arerejectedinboth
T T
∆
regions of φ considered in Fig. 1, which corresponds to about 0.7% of the data sample.
dijet
Negligiblebackgroundfractionsof ≈1%and ≈0.1%remainforthetwo regions ∆φ < π/2
dijet
andπ/2 < ∆φ < π,respectively.
dijet
19.7 fb-1 (8 TeV) 19.7 fb-1 (8 TeV)
h h
Widt1011 CMS DQaCtDa Widt1011 CMS DQaCtDa
n 1010 n 1010
Bi tt Bi tt
s/ W fi ln s/ W fi ln
ent Z fi n n ent Z fi n n
Ev 108 Ev 108
106 106
D f < p /2 p /2 < D f < p
dijet dijet
104 104
102 102
0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1
E/(cid:229) E E/(cid:229) E
T T T T
Figure1: DistributionofE/ /∑E fordata(points)incomparisonwithsimulatedjetproduction
T T
andotherprocesseswithlarge E/ (stacked)separatelyforthetworegions ∆φ < π/2(left)
T dijet
andπ/2 < ∆φ < π (right). Themaincontributionofeventswithlarge E/ inthefinalstate
dijet T
iscausedbyprocessessuchasZ/W+jet(s)withZ → ννandW → (cid:96)ν.
∆
4 Measurement of the dijet cross section differential in φ
dijet
∆
Thenormalizeddijetcrosssectiondifferentialin φ (Eq.1)iscorrectedfordetectorsmear-
dijet
ing effects and unfolded to the level of stable (decay length cτ > 1cm) final-state particles.
In this way, a direct comparison of the measurement with corresponding results from other
experimentsandwithQCDpredictionscanbemade.
Theunfoldingmethodisbasedonthematrixinversionalgorithmimplementedinthesoftware
package ROOUNFOLD [21]. Unfolding uses a response matrix that maps the distribution at
particle-level onto the measured one. The response matrix is derived from a simulation that
uses the true dijet cross section distribution from PYTHIA6 with tune Z2* [13] as input, and
∆
introduces the smearing effects by taking into account the φ resolution. The unfolded
dijet
distributions differ from the raw distributions by 3 to 4% for ∆φ < π/2 and by less than
dijet
3% for π/2 < ∆φ < π. A two-dimensional unfolding based on the iterative D’Agostini
dijet
∆
algorithm [22], which corrects for the smearing effects by taking into account both φ and
dijet
p resolutions,givesalmostidenticalresults.
T
The main systematic uncertainties arise from the estimation of the jet energy scale (JES) cali-
4 5 Comparisontotheoreticalpredictions
bration,thejet p resolution,andtheunfoldingcorrection. TheJESuncertaintyisestimatedto
T
be 1.0–2.5% for PF jets, depending on the jet p and η [11, 16, 23]. The resulting uncertainties
T
inthenormalized∆φ distributionsrangefrom7%at∆φ ≈ 0via3%atπ/2to1%atπ.
dijet dijet
The jet p resolution is determined from a full detector simulation using events generated by
T
PYTHIA6 withtuneZ2*,andisscaledbyfactorsderivedfromdata[11]. Theeffectofthejet pT
resolutionuncertaintyisestimatedbyvaryingitbyonestandarddeviationupanddown,and
∆
comparing the φ distributions before and after the changes. This results in a variation in
dijet
thenormalized∆φ distributionsrangingfrom5%at∆φ ≈ 0via3%atπ/2to0.5%atπ.
dijet dijet
The uncertainty in the unfolding correction factors is estimated by checking the dependence
of the response matrix on the choice of the Monte Carlo (MC) generator. An alternative re-
sponse matrix is built using the HERWIG++ 2.5.0 [24] event generator with the default tune of
version2.3. Theobservedeffectislessthan1%. Anadditionalsystematicuncertaintyobtained
by varying the ∆φ resolution by ±10% to determine the unfolding correction factors is es-
dijet
timatedtobeoftheorderof1%. Thisvariationofthe ∆φ resolutionby ±10%ismotivated
dijet
∆
by the observed difference between data and simulation in the φ resolution. A total sys-
dijet
tematic unfolding uncertainty of 1% accounts for the choice of the MC generator in building
∆
theresponsematrixandthe φ resolution.
dijet
∆
The unfolded dijet cross section differential in φ and normalized by the dijet cross sec-
dijet
tion integrated over the entire phase space is shown in Fig. 2 for seven pmax regions. Each
T
∆
region is scaled by a multiplicative factor for presentation purposes. The φ distributions
dijet
are strongly peaked at π and become steeper with increasing pmax. Overlaid on the data for
T
∆φ > π/2 are predictions from pQCD, presented in more detail in the next section, using
dijet
partondistributionfunctions(PDF)oftheCT10PDFset.
5 Comparison to theoretical predictions
5.1 Predictions from fixed-order calculations in pQCD
∆
Thetheoreticalpredictionsforthenormalizeddijetcrosssectiondifferentialin φ arebased
dijet
ona3-jetcalculationatNLO. Thecorrectionofnonperturbative(NP)effects,whichaccountfor
multiparton interactions (MPI) and hadronization, is studied using event samples simulated
with the PYTHIA6 (tune Z2*) and HERWIG++ (tune 2.3) event generators. Small NP effects are
expected, since this measurement deals with a normalized distribution. These corrections are
found to be of the order of 1%, roughly at the limit of the accuracy of the MC simulations.
ThereforeNPcorrectionsareconsideredtobenegligibleandarenotapplied.
The fixed-order calculations are performed using the NLOJET++ program version 4.1.3 [25,
26] within the framework of the FASTNLO package version 2.3.1 [27]. The differential cross
sectioniscalculatedfor3-jetproductionatNLO,i.e.uptotermsoforderα4,withthreeorfour
S
partonsinthefinalstate. ThiscalculationhasLOprecisionintheregionπ/2 ≤ ∆φ < 2π/3
dijet
and NLO precision for 2π/3 ≤ ∆φ < π. The region near ∆φ = π is excluded as the
dijet dijet
required higher-order calculations for two jets are not yet available. For each region in pmax,
T
thedifferentialcrosssectionisnormalizedtothedijetcrosssectioncalculatedatLOforπ/2 ≤
∆φ < 2π/3 and at NLO, i.e. up to terms proportional to α3, for 2π/3 ≤ ∆φ < π. The
dijet S dijet
useoftheLOdijetcrosssectionforthenormalizationintheregionπ/2 ≤ ∆φ < 2π/3leads
dijet
to an improved description of the data and avoids artificially increased scale uncertainties as
described in Refs. [28, 29]. Of course, this difference in normalization leads to a discontinuity
proportionaltoσNLO/σLO at∆φ = 2π/3.
dijet dijet dijet
5.1 Predictionsfromfixed-ordercalculationsinpQCD 5
19.7 fb-1 (8 TeV)
1) 1018
dijet- (rad dijet11001175 CMS CNTLO10 ( 2NpL/O3 <P DD fFdijet < p )
sd fDd LO (p /2 < D f < 2p /3)
1 dijet 1013 Anti-kt R = 0.7 dijet
pmax > 1100 GeV (x1012)
s T
900 < pmax < 1100 GeV (x1010)
1011 700 < pTmax < 900 GeV (x108)
T
500 < pmax < 700 GeV (x106)
T
109 400 < pmax < 500 GeV (x104)
T
300 < pmax < 400 GeV (x102)
T
200 < pmax < 300 GeV (x100)
107 T
105
103
10
10-1
10-3
10-5
0 p /6 p /3 p /2 2p /3 5p /6 p
D f (rad)
dijet
Figure 2: Normalized dijet cross section differential in ∆φ for seven pmax regions, scaled
dijet T
by multiplicative factors for presentation purposes. The error bars on the data points include
statistical and systematic uncertainties. Overlaid on the data (points) are predictions from LO
(dashed line; π/2 ≤ ∆φ < 2π/3) and NLO (solid line; 2π/3 ≤ ∆φ < π) calculations
dijet dijet
usingtheCT10NLOPDFsetandexcludingthebinat ∆φ = π. ThePDF, α , andscaleun-
dijet S
certaintiesareaddedinquadraturetogivethetotaltheoreticaluncertainty,whichisindicated
by the downwards-diagonally (LO) and upwards-diagonally (NLO) hatched regions around
thetheorylines.
6 5 Comparisontotheoreticalpredictions
Thenumberofquarkflavoursthatareassumedtobemasslessissettofive,andtherenormal-
ization and factorization scales, µ and µ , are chosen to be equal to pmax. The PDF sets with
r f T
NLOevolutionsusedinthecalculationsaretabulatedinTable2. TheABM11PDFsetutilizes
afixedflavournumberscheme,whereastherestofthePDFsetsuseavariableflavournumber
scheme. Themaximumnumberofflavoursisdenotedby N .
f
Table 2: The PDF sets used to compare the data with expectations, together with the corre-
spondingmaximumnumberofflavours N andthedefaultvaluesofα (M ).
f S Z
Baseset Refs. N α (M )
f S Z
ABM11 [30] 5 0.1180
CT10 [31] ≤5 0.1180
HERAPDF1.5 [32] ≤5 0.1176
MSTW2008 [33] ≤5 0.1202
NNPDF21 [34] ≤6 0.1190
The uncertainties due to the renormalization and factorization scales are evaluated by vary-
ing the default choice of µ = µ = pmax between pmax/2 and 2pmax, simultaneously in
r f T T T
the differential cross section and in the total cross section, in the following six combinations:
(µ /pmax,µ /pmax) = (1/2,1/2), (1/2,1), (1,1/2), (1,2), (2,1), and (2,2). The PDF uncer-
r T f T
tainties are evaluated according to the prescriptions for the CT10 PDF set in Ref. [35]. The
CT10 PDF set employs the eigenvector method with upward and downward variations for
eacheigenvector. Toevaluatetheuncertaintyduetothevalueofthestrongcouplingconstant
at68%confidencelevel,α (M )isvariedby±0.001asrecommendedinRef. [36].
S Z
The results of fixed-order calculations with the CT10 PDF set are overlaid on the data for
∆φ > π/2inFig.2. Figure3showstheratioofthenormalizeddijetcrosssectiondifferen-
dijet
∆
tialin φ totheorycalculatedusingtheCT10PDFset,togetherwiththecombinedPDFand
dijet
α uncertainty(innerband), andthescaleuncertainty(outerband). Alsoshownaretheratios
S
of theory derived with the alternative PDF sets ABM11 (dashed line), HERAPDF1.5 (dashed–
three-dottedline), MSTW2008(dashed-dottedline), andNNPDF2.1(dottedline)comparedto
thepredictionwiththeCT10PDFs.
Thefixed-ordercalculationsagreewiththedataforazimuthalangularseparationslargerthan
5π/6exceptforthehighest pmax region,wheretheyexceedthedata. Forsmaller∆φ values
T dijet
between 2π/3 and 5π/6, in particular where the estimate of the theoretical uncertainties be-
comessmall,systematicdiscrepanciesareexhibitedthatdiminishwithincreasing pmax. Inthe
T
4-jetLOregionwith∆φ < 2π/3,thepatternofincreasingdeviationstowardssmaller∆φ
dijet dijet
and decreasing deviations towards larger pmax is repeated, but with less significance because
T
ofthelargerscaleuncertainty. SimilarobservationsweremadeinthepreviousCMSmeasure-
ment [3], which exhibited larger discrepancies in the 4-jet region due to the normalization to
theNLOdijetcrosssectioninsteadofaLOone.
5.2 Predictions from fixed-order calculations matched to parton shower simu-
lations
The PYTHIA6 [12], PYTHIA8 [37], and HERWIG++ [24] event generators complement LO dijet
matrix elements with parton showers to simulate higher-order processes. Both PYTHIA ver-
sions, PYTHIA6 with the Z2* tune [13] and PYTHIA8 with the CUETM1 tune [14], employ pT-
ordered parton showers [38, 39], while HERWIG++ with the default tune of version 2.3 uses a
coherent-branchingalgorithmwithangularorderingoftheshowers[40].
The MADGRAPH programversion5.1.5.7[19]suppliestheresultsofLOmatrixelementcalcu-
5.2 Predictionsfromfixed-ordercalculationsmatchedtopartonshowersimulations 7
CMS 19.7 fb-1
1.8 Anti-kt R = 0.7
Data/NLO
1.6 pmax > 1100 GeV TLEGpmax > 1100 GeV
T Data/LO T
1.4 Theory CT10-NLO PDF
Scale uncertainty
1.2
DF) 1 PADBMF 1&1 a s uncertainty
P0.8 NNPDF2.1
CT10 0.6 MHESRTWAP2D00F81.5
y (1.8
heor1.6 700 < pmTax < 900 GeV 900 < pmTax < 1100 GeV
T
o 1.4
o t1.2
Rati 1
0.8
0.6
1.8
1.6 400 < pmax < 500 GeV 500 < pmax < 700 GeV
T T
1.4
1.2
1
0.8
0.6
1.8
1.6 200 < pmax < 300 GeV 300 < pmax < 400 GeV
T T
1.4
1.2
1
0.8
0.6
p /2 2p /3 5p /6 p p /2 2p /3 5p /6 p
D f (rad) D f (rad)
dijet dijet
∆
Figure3: Ratiosofthenormalizeddijetcrosssectiondifferentialin φ toLO(triangles)and
dijet
NLO(squares)pQCDpredictionsusingtheCT10PDFsetatnext-to-leadingevolutionorderfor
all pmax regions. Theerrorbarsonthedatapointsrepresentthetotalexperimentaluncertainty,
T
which is the quadratic sum of the statistical and systematic uncertainties. The uncertainties
of the theoretical predictions are shown as inner band (PDF & α ) and outer band (scales).
S
ThepredictionsusingvariousotherPDFsetsrelativetoCT10areindicatedwithdifferentline
styles.
8 6 Summary
lationswithtwotofouroutgoingpartonsthatcanbematchedtotheimplementationsofparton
showers,hadronization,andMPIoftheeventgenerators. Inthisanalysis,itisinterfacedwith
PYTHIA6withtuneZ2*usingtheMLMmatchingprocedure[41]toavoidanydoublecounting
betweentree-levelandpartonshowergeneratedpartonconfigurations.
ThePOWHEGframework[42–44]providesanNLOdijetcalculation[45]thatcanalsobematched
via the parton showers to event generators. Here, POWHEG is interfaced to PYTHIA8 with the
CUET[14]tuneandtheCTEQ6L1[35]PDFset.
∆
In Fig. 4 the normalized dijet cross section differential in φ is compared to the predic-
dijet
tions from fixed-order calculations supplemented with parton showers, hadronization, and
MPI. The error bars on the data points represent the total experimental uncertainty, which is
the quadratic sum of the statistical and systematic uncertainties. Figure 5 shows the ratios of
thesepredictionstothenormalizeddijetcrosssectiondifferentialin ∆φ ,fortheseven pmax
dijet T
regions. The solid band indicates the total experimental uncertainty and the error bars on the
MCpointsrepresentthestatisticaluncertaintiesinthesimulateddata.
Among the LO dijet event generators PYTHIA6, PYTHIA8, and HERWIG++, PYTHIA8 exhibits
thesmallestdeviationsfromthemeasurements. PYTHIA6 and HERWIG++ systematicallyover-
shoot the data, particular around ∆φ = 5π/6. The best description of the measurement is
dijet
given by the tree-level multiparton event generator MADGRAPH interfaced with PYTHIA6 for
showering,hadronization,andMPI. The POWHEG generator,inspiteofprovidingdijetevents
at NLO that are matched to PYTHIA8, shows deviations from the data similar to the LO dijet
eventgenerators.
6 Summary
Ameasurementispresentedofthenormalizeddijetcrosssectiondifferentialintheazimuthal
angularseparation∆φ ofthetwojetsleadingin p forsevenregionsintheleading-jettrans-
dijet T
verse momentum pmax. The data set of pp collisions at 8TeV centre-of-mass energy collected
T
in 2012 by the CMS experiment and corresponding to an integrated luminosity of 19.7fb−1 is
analysed.
∆
The measured distributions in φ are compared to calculations in perturbative QCD for 3-
dijet
jet production with up to four outgoing partons that provide NLO predictions for the range
of 2π/3 ≤ ∆φ < π and LO predictions for π/2 ≤ ∆φ < 2π/3. The NLO predictions
dijet dijet
describethedatadowntovaluesof∆φ ≈ 5π/6,butdeviateincreasinglywhenapproaching
dijet
the 4-jet region, starting at ∆φ = 2π/3, particularly at low pmax. The pattern of increasing
dijet T
deviations towards smaller ∆φ and decreasing deviations towards larger pmax is repeated
dijet T
inthe4-jetLOregionwith∆φ < 2π/3,butwithlesssignificancebecauseofthelargerscale
dijet
uncertainty.
∆
Inacomparisonofthenormalized φdijetdistributionstotheLOdijeteventgeneratorsPYTHIA6,
PYTHIA8,andHERWIG++,PYTHIA8givesthebestagreement. PYTHIA6andHERWIG++system-
atically overshoot the data, particularly for ∆φ ≈ 5π/6. A good overall description of the
dijet
measurement is provided by the tree-level multijet event generator MADGRAPH in combina-
tionwith PYTHIA6 forshowering,hadronization,andmultipartoninteractions. ThedijetNLO
calculationsfromPOWHEGmatchedtoPYTHIA8exhibitdeviationssimilartotheLOdijetevent
generators,emphasizingtherebytheneedtoimprovethemultijetpredictions.
SimilarobservationswerereportedpreviouslybyCMS[3]andATLAS[4],butwithlesssignifi-
∆
cancebecauseofthesmallerdatasets. Theextensionto φ valuesbelowπ/2,theimproved
dijet