Measurement of the underlying event in the Drell-Yan process in proton-proton collisions at sqrt(s) = 7 TeV

0.97 MB·English

by  CMS Collaboration

EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH (CERN)
CERN-PH-EP/2012-085
2013/01/24
CMS-QCD-11-012
Measurement of the underlying event in the Drell–Yan
√
process in proton-proton collisions at s = 7TeV
3
∗
1 The CMS Collaboration
0
2

n
a
J

3
2

] Abstract
x
e
-
p Ameasurementoftheunderlyingevent(UE)activityinproton-protoncollisionsata
e
h centre-of-massenergyof7TeVisperformedusingDrell–Yaneventsinadatasample
[ corresponding to an integrated luminosity of 2.2fb−1, collected by the CMS experi-

2 ment at the LHC. The activity measured in the muonic final state (qq → µ+µ−) is
v corrected to the particle level and compared with the predictions of various Monte
1
Carlo generators and hadronization models. The dependence of the UE activity on
1
4 the dimuon invariant mass is well described by PYTHIA and HERWIG++ tunes de-
1 rived from the leading jet/track approach, illustrating the universality of the UE ac-
.
4 tivity. TheUEactivityisobservedtobeindependentofthedimuoninvariantmassin
0 the region above 40GeV/c2, while a slow increase is observed with increasing trans-
2
1 verse momentum of the dimuon system. The dependence of the UE activity on the
: transversemomentumofthedimuonsystemisaccuratelydescribedbyMADGRAPH,
v
i whichsimulatesmultiplehardemissions.
X
r
a
SubmittedtotheEuropeanPhysicalJournalC
∗SeeAppendixAforthelistofcollaborationmembers

1
1 Introduction
In hadron-hadron scattering, the “underlying event” (UE) is defined as any hadronic activity
thatcannotbeattributedtotheparticlesoriginatingfromthehardscattering,whichischarac-
terized by a large momentum transfer, or to the hadronization of initial- and final-state radi-
ation. The UE activity is thus due to the hadronization of partonic constituents, not involved
in the hard scattering, that have undergone multiple-parton interactions (MPIs) and to the
hadronization of beam remnants that did not participate in other scatterings. These semihard
interactionscannotbecompletelydescribedbyperturbativequantumchromodynamics(QCD)
andrequireaphenomenologicaldescriptioninvolvingparametersthatmustbetunedwiththe
helpofdata[1].
TheexperimentalstudyoftheUEprobesvariousaspectsofhadronproductioninhighenergy
hadron-hadron collisions. In particular it is sensitive to the interplay of perturbative methods
describingthehardprocessandphenomenologicalmodelsofthesoftinteractionsthatattempt
tosimultaneouslydescribeMPIs,initial-andfinal-stateradiation,thecolourflowbetweenfinal
state partons, and the hadronisation process. Understanding the UE in terms of particle and
energydensitieswillleadtobettermodellingbyMonteCarloprogramsthatareusedinprecise
measurementsofstandardmodelprocessesandsearchesfornewphysicsathighenergies. The
UE affects the estimation of the efficiency of isolation criteria applied to photons and charged
leptons, and the energy scale in jet identification. It also affects the reconstruction efficiency
for processes like H→ γγ, where the primary vertex is partly determined from the charged
particles originating from the UE. Hard MPIs are an important background for new physics
searches,e.g. same-signWproductionfromMPIs[2]isapossiblebackgroundtothesame-sign
doubleleptonSUSYsearches[3].
The Compact Muon Solenoid (CMS) [4], ATLAS, and ALICE experiments have carried out
√
UE measurements at centre-of-mass energies ( s) of 0.9TeV and 7TeV using hadronic events
(minimum-biasandsingle-jettriggered)containingaleadingtrack-jet[5,6]oraleadingtrack[7,
8]. The analysis of the central charged particles and forward energy flow correlations in hard
processes, e.g. pp → W(Z)X → (cid:96)ν((cid:96)(cid:96))X [9], provides supplementary insights into the nature
of MPIs. In this paper, we use the Drell–Yan (DY) process [10] with the muonic final state at a
centre-of-massenergyof7TeVtoperformacomplementaryUEmeasurement. TheDYprocess
withmuonicfinalstateisexperimentallycleanandtheoreticallywellunderstood,allowingthe
particles from the UE to be reliably identified. The absence of QCD final-state radiation (FSR)
permits a study of different kinematic regions with varying transverse momentum of γ∗/Z
duetoharderorsofterinitial-stateradiation(ISR).ThecomparisonoftheUEmeasurementin
DY events with QCD events having a leading track-jet is useful for probing the UE activity in
differentprocesses. UEmeasurementsusingtheDYprocesshavebeenreportedpreviouslyin
√
proton-antiprotoncollisionsat s=1.96TeV[11].
The UE activity at a given centre-of-mass energy is expected to increase with the momentum
transfer of the interaction. Events with a harder scale are expected to correspond, on average,
to interactions with a smaller impact parameter and, in some models, to more MPIs [12, 13].
This increased activity is observed to reach a plateau for high energy scales corresponding to
small impact parameter. In this paper we investigate some aspects of the UE modelling in
detailbymeasuringtheinvariantmassdependenceoftheUEactivityforDYeventswithsmall
transverse momentum of the DY system. This measurement separates the scale dependence
of the UE activity from the ISR effect. The universality of the model parameters, denoted as
tunes,implementedinthevariousMCprogramsistestedbycomparingtheirpredictionswith
our measurements. The portability of the UE parameters across different event generators,
2 2 Observables
combinedinsomecaseswithdifferentpartondistributionfunctions(PDFs),isinvestigatedas
well. The modelling of the ISR is studied by measuring the UE activity as a function of the
transversemomentumoftheDYsystem. Finally,thedependenceoftheUEactivityonISRand
FSRisdeterminedbycomparingthemeasurementsfromDYeventswithpreviousresultsfrom
hadroniceventscontainingaleadingjetwhereFSRalsoplaysarole.
The outline of the paper is as follows. Section 2 describes the various observables used in the
presentstudy. Section3summarizesthedifferentMCmodelsusedandcorrespondingUEpa-
rameters. Section 4 presents experimental details: a brief detector description, data samples,
eventandtrackselectioncriteria,correctionprocedure,andsystematicuncertainties. Section5
presents the results on UE activity measured in DY events and the comparison with the mea-
surementsbasedonaleadingtrack-jet. ThemainresultsaresummarizedinSection6.
2 Observables
The UE activity is measured in terms of particle and energy densities. The particle density
(1/[∆η∆(∆φ)](cid:104)N (cid:105)) is computed as the average number of primary charged particles per
ch
∆
unit pseudorapidity η and per unit azimuthal separation φ (in radians) between a track
and the transverse momentum of the dimuon system. The pseudorapidity is defined as η =
−ln(tan(θ/2)),whereθisthepolaranglemeasuredwithrespecttotheanticlockwisebeamdi-
rection. Theazimuthalangleφismeasuredintheplaneperpendiculartothebeamaxis. Theen-
ergydensity(1/[∆η∆(∆φ)](cid:104)Σp (cid:105))isexpressedintermsoftheaverageofthescalarsumofthe
T
transverse momenta of primary charged particles per unit pseudorapidity per unit azimuthal
separation. The ratio of the energy and particle densities, as well as the total charged-particle
multiplicity N and the transverse momentum spectrum are also computed. The charged-
ch
particle multiplicity and transverse momentum distributions are normalized to unit area and
to the average number of charged particles per event, respectively. Particles are considered
as primary if they originate from the initial proton-proton interaction and are not the decay
products of long-lived hadrons with a lifetime exceeding 10−10s. Apart from the muons from
the DY process, all charged particles in the central region of the detector with pseudorapidity
|η| < 2andwithtransversemomentum p > 0.5GeV/careconsidered.
T
∆
The spatial distribution of the tracks is categorized by the azimuthal separation φ. Particle
production in the away region (|∆φ| > 120◦) is expected to be dominated by the hardest ISR
emissions, which balance the dimuon system. The transverse region (60◦ < |∆φ| < 120◦) and
towards region (|∆φ| < 60◦) are more sensitive to soft emissions and, in particular, those due
to MPIs. The relevant information about the hard and the soft processes is extracted from
the tracking and the muon systems of the CMS detector and thus the derived observables are
insensitivetotheuncertaintiesofthecalorimetricmeasurements. TheDYeventswithdimuon
mass M aroundtheZresonancearetheleastcontaminatedbybackgroundprocesses(heavy-
µµ
quark, tt, W+jets, and DY → ττ production) [14, 15] and best suited for the measurement of
theUEactivity.
TheUEactivityisstudiedasafunctionofthemagnitudeofthedimuontransversemomentum
(pµµ = |(cid:126)pµ +(cid:126)pµ |) and as a function of M . The dependence of the UE activity on pµµ
T T,1 T,2 µµ T
for high-mass dimuon pairs effectively probes the ISR spectrum. In order to minimize the
µµ
background contamination, the p dependence is studied only in the narrow mass window
T
81 < M < 101GeV/c2. IncontrasttothestudyoftheUEactivityinhadroniceventsusinga
µµ
leading track-jet [5, 6], this energy scale is sufficiently large to saturate the MPI contributions.
This observation is verified by studying the UE activity as a function of the dimuon mass in
a wider mass range, where the total transverse momentum of the dimuon system is kept to a
3
minimumbyrequiring pµµ < 5GeV/c.
T
3 Monte Carlo models
TheUEdynamicsarestudiedthroughthecomparisonoftheobservablesindatawithvarious
tunes of PYTHIA6 [16] and its successor PYTHIA8 [17, 18]. MADGRAPH (version 5) [19, 20],
whichsimulatesuptosixfinal-statefermions(includingthemuons),andPOWHEG[21],which
includes next-to-leading-order corrections on the hardest emission, are also compared to our
measurements. Forthesetwogenerators,softeremissionsaresimulatedby p -orderedparton
T
showersusingPYTHIA6tunesandmatchedwiththehardprocessproducedbythegenerators.
HadronizationinPYTHIA6andPYTHIA8isbasedontheLundstringfragmentationmodel[22].
The measurements are also compared to predictions of the HERWIG++ [23] angular-ordered
partonshowerandclusterhadronizationmodel[24,25].
The UE contributions from MPIs rely on modelling and tuning of the parameters in the MC
generators. TheMPImodelofPYTHIAreliesontwofundamentalassumptions[12]:
• The ratio of the 2→2 partonic cross section, integrated above a transverse momen-
tum cutoff scale, and the total of the hadronic cross section is a measure of the
amount of MPIs. The cutoff scale p is introduced to regularize an otherwise di-
0T
vergingpartoniccrosssection,
p4
σ(p ) = σ(p ) T , (1)
T 0T (p2 +p2 )2
T 0T
with
√
√ √ (cid:18) s (cid:19)(cid:101)
p ( s) = p ( s ) √ . (2)
0T 0T 0
s
0
√
Here s = 1.8TeV and (cid:101) is a parameter characterizing the energy dependence of
0
thecutoffscale.
• ThenumberofMPIsinaneventhasaPoissondistributionwithameanthatdepends
ontheoverlapofthematterdistributionofthehadronsinimpact-parameterspace.
The MPI model used here [26] includes showering of the MPI process, which is interleaved
withtheISR.
The tunes of the models vary mainly in the MPI regularization parameters, p and (cid:101), in the
0T
amount of colour reconnection, and in the PDF used. The Z1 tune [27] of PYTHIA6 adopts the
results of a global tuning performed by the ATLAS Collaboration [28] and uses the fragmen-
tationandcolourreconnectionparametersoftheATLASAMBT1tune[29]. Theparametersof
theZ1tunerelatedtotheMPIregularizationcutoffanditsenergydependenceareadjustedto
describe previous CMS measurements of the UE activity in hadronic events [6] and uses the
CTEQ5L PDF. The Z2 tune of PYTHIA6 is an update of the Z1 tune using CTEQ6L1 [30], the
defaultusedinmostCMSgenerators;theregularizationcutoffvalueatthenominalenergyof
√
s =1.8TeVisoptimizedto1.832GeV/c. Thevalueoftheenergyevolutionparameterforthe
0
Z2 tune is 0.275, as for the Z1 tune. The 4C [31] tune of PYTHIA8 follows a similar procedure
as the ATLAS AMBT1 tune, but includes ALICE multiplicity data as well. The values of the
√
p ( s )and(cid:101)parametersforthe4Ctuneare2.085GeV/cand0.19,respectively. Theeffective
0T 0 √
valueof p at s = 7TeVisabout2.7GeV/cforboththeZ2and4Ctunes.
0T
4 4 Experimentalmethods
The LHC-UE7-2 tune of HERWIG++ is based on ATLAS measurements of the UE activity in
hadronicevents[7]. Theregularizationcutoffparameterp fortheLHC-UE7-2tuneis3.36GeV/c
√ 0T
at s=7TeV. TheCTEQ6L1PDFisusedinconjunctionwith PYTHIA6Z2, PYTHIA84C, MAD-
GRAPHZ2,andHERWIG++LHC-UE7-2,whileCT10[32]isusedforPOWHEG,andCTEQ5Lfor
the PYTHIA6Z1simulations.
AcomparisonofthesemodelswiththemeasurementsispresentedinSection5.
4 Experimental methods
The present analysis is performed with a sample of proton-proton collisions corresponding to
an integrated luminosity of 2.2fb−1, collected in March–August 2011 using the CMS detec-
tor[4].
Muons are measured in the pseudorapidity range |η| < 2.4 with a detection system consist-
ing of three subsystems: Drift Tubes, Cathode Strip Chambers, and Resistive Plate Chambers.
Matchingtracksegmentsfromthemuondetectortothetracksmeasuredintheinnertrackerre-
sultsinatransversemomentumresolutionbetween1%and5%for p valuesupto1TeV/c. The
T
tracker subsystem consists of 1440 silicon-pixel and 15148 silicon-strip detector modules, and
it measures charged particle trajectories within the nominal pseudorapidity range |η| < 2.5.
The tracker is designed to provide a transverse impact parameter resolution of about 100µm
and a transverse momentum resolution of about 0.7% for 1GeV/c charged particles at normal
incidence(η =0).
The detector response is simulated in detail using the GEANT4 package [33]. The simulated
signal and background events, including heavy-quark, tt, W+jets, and DY → ττ production,
areprocessedandreconstructedinthesamemannerascollisiondata.
4.1 Event and track selection
Thetriggerrequiresthepresenceofatleasttwomuoncandidates. Inperiodsoflowerinstanta-
neousluminositybothmuonswererequiredtohave p > 7GeV/c, whileinotherperiodsthe
T
transversemomentumrequirementswere13GeV/cand8GeV/cfortheleadingandsubleading
muons,respectively. Thetriggerefficiencyisabove95%fortheofflineselectedDYeventswith
the requirement of 81 < M < 101GeV/c2. The offline selection requires exactly two muons
µµ
reconstructed in the muon detector and the silicon tracker. Muon candidates are required to
satisfy identification criteria based on the number of hits in the muon stations and tracker,
transverse impact parameter with respect to the beam axis, and normalized χ2 of the global
fit [15]. The backgrounds from jets misidentified as muons and from semileptonic decays of
heavyquarksaresuppressedbyapplyinganisolationconditiononthemuoncandidates. The
isolationvariable I formuonsisdefinedas
I = (cid:8)Σ[p (tracks)+E (EM)+E (HAD)]−π(∆R)2ρ(cid:9)/pµ, (3)
T T T T
where the sum is defined in a cone of radius ∆R = (cid:112)(∆φ)2+(∆η)2 = 0.3 around the muon
∆ ∆
direction; η and φarethepseudorapidityandazimuthalseparationbetweenthemuonand
tracksorcalorimetrictowers. Here p (tracks)isthetransversemomentumoftracks,excluding
T
muons, with p > 1GeV/c, E (EM) is the transverse energy deposited in the electromagnetic
T T
calorimeter, E (HAD) is the transverse energy deposited in the hadronic calorimeter, and ρ
T
is the average energy density [34] in the calorimeter and tracker originating from additional
inelastic pp interactions (pile-up) in the same bunch crossing as the DY interaction.The calcu-
lation of ρ takes into account the number of reconstructed primary vertices in the event; the
4.2 Correctionsandsystematicuncertainties 5
average value of ρ is 5.6GeV/c. A muon is considered to be isolated if I < 0.15. Because of
the energy density correction, the isolation efficiency is independent of the number of pile-up
interactions.
The selected muons are required to have opposite charges, transverse momenta larger than
20GeV/c, and pseudorapidity |η| < 2.4. Both muons are required to be associated with the
same vertex, which is designated as the signal vertex. The selected signal vertex is required to
be within ±18cm of the nominal interaction point as measured along the z direction. At least
fivetracksarerequiredtobeassociatedwiththesignalvertex,andthetransversedisplacement
of the signal vertex from the beam axis is required to be less than 2cm. These criteria select a
puresampleofDYeventswithatotalbackgroundcontributionoflessthan0.5%asestimated
fromsimulatedevents.
Tracks, excludingtheselectedmuons, areconsideredfortheUEmeasurementiftheyarewell
reconstructedinthesilicon-pixelandthesilicon-striptracker,have p > 0.5GeV/cand|η| < 2,
T
andoriginatefromthesignalvertex. Toreducethenumberofimproperlyreconstructedtracks,
ahighpurityreconstructionalgorithm[35]isused. Thehighpurityalgorithmrequiresstringent
cuts on the number of hits, the normalized χ2 of the track fit, and the consistency of the track
originating from a pixel vertex. To reduce the contamination of secondary tracks from decays
of long-lived particles and photon conversions, the distances of closest approach between the
trackandthesignalvertexinthetransverseplaneandinthelongitudinaldirectionarerequired
tobelessthan3timestherespectiveuncertainties. Trackswithpoorlymeasuredmomentaare
removedbyrequiringσ(p )/p < 5%,whereσ(p )istheuncertaintyonthe p measurement.
T T T T
These selection criteria reject about 10% of primary tracks and 95% of misreconstructed and
secondary tracks. The selected tracks have a contribution of about 2% from misreconstructed
andsecondarytracks.
4.2 Corrections and systematic uncertainties
The UE observables, discussed in Section 2, are corrected for detector effects and selection
efficiencies. The measured observables are corrected to reflect the activity from all primary
charged particles with transverse momentum p > 0.5GeV/c and pseudorapidity |η| < 2.
T
The particle and energy densities are corrected using a bin-by-bin technique. In the bin-by-
bin technique, the correction factor is calculated by taking the bin-by-bin ratio of the particle
level and detector level distributions for simulated events and then the measured quantity is
multiplied by this correction factor. There is a small growth in the particle and energy densi-
µµ
ties with increasing p and M in the towards and transverse regions. Because of this slow
T µµ
µµ
growth of densitiesthe bin migration in p and M has a smalleffect on the measurements,
T µµ
therefore a bin-by-bin method is considered to be sufficiently precise. There is a fast rise in
µµ
theenergyandparticledensitiesintheawayregionwiththeincreaseof p , butcorrectedre-
T
sults using a bin-by-bin method are consistent with correction obtained from a Bayesian [36]
technique. Thetransversemomentaofthechargedparticleshaveverygoodresolutionandare
corrected using a bin-by-bin method. In this analysis the average of the calculated correction
factorsfrom PYTHIA6Z2, PYTHIA6D6T,and MADGRAPH Z2isusedtocorrecttheexperimen-
tal distributions. The maximum deviation from the average correction factor is taken as the
model-dependent systematic uncertainty, estimated to be 0.7–1.4% for the particle and energy
densities. Inthecaseofcharged-particlemultiplicity,thereissubstantialbinmigrationandthe
corrected results using the Bayesian [36] and bin-by-bin techniques differ by 10–15%. There-
fore the charged-particle multiplicity is corrected using a Bayesian unfolding technique with
a response matrix obtained using the PYTHIA6 Z2 tune. The systematic uncertainty related to
the correction procedure is calculated by unfolding the data with response matrices obtained
6 4 Experimentalmethods
usingdifferenttunes.
Intheanalyzeddata,thereareonaverage6–7collisionsineachbunchcrossing. Tracksoriginat-
ing from these pile-up interactions cause the UE activity to be overestimated, so the measure-
mentsarecorrectedforthepresenceofpile-upinteractions. Thecorrectionfactoriscalculated
as the ratio of the UE activity for simulated events with and without pile-up. The uncertainty
inthemodellingofthepile-upeventsisestimatedbyvaryingthemeanoftheexpectednumber
of pile-up events by ±1. This uncertainty in pile-up modelling affects the particle and energy
densities by 0.3–1.0%. The effect due to pile-up events is small because only the tracks associ-
ated with the same vertex as the muon pair are used. The results are also cross-checked with
lowpile-up7TeVdatacollectedduring2010andthedifferencesarefoundtobenegligible.
Table1: Summaryofthesystematicuncertaintiesontheparticleandenergydensities(inpercent). The
first three rows show the systematic uncertainties for the particle density in the towards, transverse,
and away regions. The last three rows report the systematic uncertainties for the energy density. The
numbersoutsidetheparenthesesrefertothecasewherethedensitiesaremeasuredasafunctionofM
µµ
µµ
andthoseintheparenthesescorrespondtothemeasurementsasafunctionof p .
T
Observable model pile-up isolation mis-ID background total
µµ µµ µµ µµ µµ µµ
Mµµ(pT ) Mµµ(pT ) Mµµ(pT ) Mµµ(pT ) Mµµ(pT ) Mµµ(pT )
1/[∆η∆(∆φ)](cid:104)N (cid:105)(towards) 0.8(0.8) 1.0(0.9) 0.9–1.5(0.9) 1.0(1.0) 0.7(0.3) 2.0–2.3(1.8)
ch
1/[∆η∆(∆φ)](cid:104)N (cid:105)(transverse) 0.7(0.9) 0.9(0.9) 0.8–1.7(0.8) 0.9(0.9) 0.7(0.5) 1.8–2.3(1.8)
ch
1/[∆η∆(∆φ)](cid:104)N (cid:105)(away) 0.7(0.6) 0.9(0.3–0.9) 0.8–1.6(0.8) 0.9(0.9) 0.5(0.5) 1.7–2.2(1.5–1.7)
ch
1/[∆η∆(∆φ)](cid:104)Σp (cid:105)(towards) 1.2(1.2) 0.8(0.7) 1.1–2.0(1.4) 0.8(0.8) 0.8(0.7) 2.1–2.7(2.2)
T
1/[∆η∆(∆φ)](cid:104)Σp (cid:105)(transverse) 1.1(1.4) 0.7(0.7) 1.0–2.5(1.3) 0.8(0.8) 0.8(0.9) 2.0–3.0(2.4)
T
1/[∆η∆(∆φ)](cid:104)Σp (cid:105)(away) 1.0(0.8) 0.7(0.3–0.7) 1.1–2.2(1.1) 0.8(0.7) 0.7(0.2) 2.0–2.7(1.6–1.7)
T
Wealsoconsiderpossiblesystematiceffectsrelatedtotriggerrequirements,differentbeam-axis
positionsindataandsimulation,varioustrackselectioncriteria,muonisolation,andmisiden-
tificationoftracks. Thecombinedsystematicuncertaintyrelatedtotriggerconditions,thevary-
ingbeam-axisposition,andtrackselectionislessthan0.5%. Thesystematicuncertaintydueto
isolationiscalculatedbyremovingtheisolationconditioninthesimulatedeventsusedforthe
correctionandisfoundtobe0.8–2.5%fortheparticleandenergydensities.
Theyieldofsecondarytracksoriginatingfromthedecayoflong-livedparticlesisnotcorrectly
predicted by the simulation [37]. To estimate the effect of secondary tracks, a subset of simu-
latedeventsiscreatedbyrejectingtracksthatdonothaveamatchingprimarychargedparticle
at the generator level. The uncertainty is evaluated by correcting the measurements with this
subsetofthesimulatedevents,containingfewersecondarytracks,andisfoundtobe0.7–1.0%
fortheparticleandenergydensities.
Thoughthetotalcontributionofbackgroundprocessesisverysmall,itaffectsthemeasurement
at higher pµµ (50–100GeV/c) and small M (40–60GeV/c2) where the contamination from tt
T µµ
and DY→ ττ background processes is 1% and 5%, respectively. The particle and energy den-
sitiesdifferbetweenDY→ ττ andDY→ µµ (thesignalprocess)by20%. Theparticle(energy)
densityforthettbackgroundistwotimes(fourtimes)thatforthesignalprocess. Combination
ofthedifferencesinthedensitiesforbackgroundprocessesandrelativebackgroundcontribu-
tionsgivesasystematicuncertaintyof0.2–0.9%.
Table1summarizesthedominantsystematicuncertaintiesontheparticleandenergydensities.
The total systematic uncertainty on the particle and energy densities is in the range 1.5–3.0%,
whereas the uncertainties on the track multiplicity and p spectra reach 10% in the tail (not
T
7
reported in Table 1). In all figures, inner error bars represent the statistical uncertainty only,
whileoutererrorbarsaccountforthequadraticsumofstatisticalandsystematicuncertainties.
5 Results
The UE activity in DY events, for charged particles with p > 0.5GeV/c and |η| < 2.0, is
T
µµ
presented as a function of M and p . The multiplicity and the transverse momentum dis-
µµ T
tributions are also presented for two different sets of events, pµµ < 5GeV/c and 81 < M <
T µµ
101GeV/c2. Finally,theUEactivityinthetransverseregioniscomparedwiththatmeasuredin
hadroniceventsusingaleadingtrack-jet.
5.1 Underlying event in the Drell–Yan process
Theenergy-scaledependenceoftheMPIactivityisstudiedbylimitingtheISR.Toaccomplish
this we require the muons to be back-to-back in the transverse plane with pµµ < 5GeV/c and
T
measure the dependence of the UE activity on the dimuon mass, M . The resulting particle
µµ
andenergydensitiesareshowninFig.1. Becausetheactivityisalmostidenticalinthetowards
andtransverseregions,theyarecombinedas|∆φ| < 120◦. ThecontributionofISRtotheUEac-
tivityissmallafterrequiring pµµ < 5GeV/c,asshownbythepredictionof HERWIG++without
T
MPIs. ThisfigurealsoillustratesthedominantroleofMPIsinourcurrentmodelsastheygen-
eratemorethan80%oftheUEactivityintheseISR-reducedevents. Thelackofdependenceof
theUEactivityon M withintherangeunderstudy(40–140GeV/c2)indicatesthattheactivity
µµ
duetoMPIsisconstantatenergyscalesdownto40GeV. Thequantitativedescriptionbymodel
tunes based on the minimum-bias and UE observables in hadronic events is illustrated by the
MC/Data ratios in Fig. 1. In general, PYTHIA6 Z2, PYTHIA8 4C, and HERWIG++ LHC-UE7-2
describethedensitieswell,whereastheZ2tuneusedtogetherwiththePOWHEGgeneratorun-
derestimatesbothdensitiesby5–15%. Both PYTHIA and HERWIG++modeltunesderivedfrom
theUEmeasurementinhadroniceventsusingtheleadingjet/trackapproachdescribetheUE
activity in the Drell–Yan events equally well and hence illustrate a certain universality of the
underlyingeventacrossQCDandelectroweakprocessesinhadroniccollisions.
Dependence of the UE activity on the transverse momentum of the dimuon system is shown
in Fig. 2 in the towards, transverse, and away regions (top to bottom) for events having M
µµ
between 81GeV/c2 and 101GeV/c2. At this high energy scale, the pµµ dependence of the UE
T
µµ
activity is sensitive to the ISR. The slope in the p dependence of the UE activity is identical
T
for a model with and without MPIs and is therefore mainly due to ISR. The predictions of
HERWIG++withoutMPIsunderestimatethemeasurementsintheawayregionaswellbecause
the MPIs produce particles uniformly in all directions. The UE activity does not fall to zero
when pµµ → 0becauseofthepresenceofthehardscalesetby M .
T µµ
µµ
The particle and energy densities in the away region rise sharply with p and, because of
T
momentumconservationmainlysensitivetothespectrumofthehardestemission,areequally
well described by all tunes and generators considered. In the towards and transverse regions
µµ
there is a slow growth in the particle and energy densities with increasing p . The energy
T
densityincreasesmorethantheparticledensity,implyingacontinuousincreaseintheaverage
µµ
transversemomentumofthechargedparticleswith p . Thiseffectisalsoreflectedintheratio
T
oftheenergydensitytotheparticledensity. Theactivityinthetowardsregionisqualitatively
similar to that in the transverse region. Quantitatively, the activity is higher in the transverse
regionthanthetowardsregion,aneffectcausedbythespill-overcontributionsfromtherecoil
activity in the away region, which balances the dimuon system. This observation is visible in
8 5 Results
particledensity energydensity ratioofenergyandparticledensities
æÆfDD N)] ( ch1111....2468 DPPPHHyoyaeettwtrrhhawwhiiaaiiegg--g++68 ++ZZ4 2C2LLHHCC--UUEE77--22 (nCoM MSP I)s = 7 TeV æ [GeV/c] pT1111....2468 DPPPHHyoyaeettwtrrhhawwhiiaaiiegg--g++68 ++ZZ4 2C2LLHHCC--UUEE77--22 (nCoM MSP I)s = 7 TeV æ [GeV/c] Nch1111....2468 DPPPHHyoyaeettwtrrhhawwhiiaaiiegg--g++68 ++ZZ4 2C2LLHHCC--UUEE77--22 (CnoM MS P I)s = 7 TeV
hD1/[ 1 p(cphmTma >r<g 0e5.d 5G pGeaVert/Vicc/lce,s |h| < 2.0, |D f| < 120o) SÆfD )] ( 1 p(cphmTma >r<g 0e5.d 5G pGeaVert/Vicc/lce,s |h| < 2.0, |D f| < 120o) ÆæS/ p T 1
0.8 T D 0.8 T Æ 0.8
h
0.6 D1/[ 0.6 0.6
0.4 0.4 0.4 charged particles
(p > 0.5 GeV/c, |h| < 2.0, |D f| < 120o)
0.2 0.2 0.2 pmTm < 5 GeV/c
T
0 0 0
40 60 80 100 120 140 40 60 80 100 120 140 40 60 80 100 120 140
Mmm [GeV/c2] Mmm [GeV/c2] Mmm [GeV/c2]
1.4 1.4 1.4
æÆfDDhD N)] ( Data 1/[ch111...2311 SSPPPHttyoyeaattwrhhttw..hii aai+eg-- g+68S +yZZ4 s2C2LtH.C-UE7-2 CMS s = 7 TeV æSÆfDDhD p )] ( ata 1/[T111...2311 SSPPPHttyoyeaattwrhhttw..hii aai+eg-- g+68S +yZZ4 s2C2LtH.C-UE7-2 CMS s = 7 TeV æÆæSÆ N/ p MC/Data chT111...2311 SSPPPHttyoyeaattwrhhttw..hii aai+eg-- g+68S +yZZ4 s2C2LtH.C-UE7-2 CMS s = 7 TeV
MC/ 0.9 C/D 0.9 0.9
M
0.8 charged particles 0.8 charged particles 0.8 charged particles
0.7 p(pmTm >< 05. 5G GeVe/Vc/c, |h| < 2.0, |D f| < 120o) 0.7 p(pmTm >< 05. 5G GeVe/Vc/c, |h| < 2.0, |D f| < 120o) 0.7 p(pmTm >< 05. 5G GeVe/Vc/c, |h| < 2.0, |D f| < 120o)
T T T
0.6 0.6 0.6
40 60 80 100 120 140 40 60 80 100 120 140 40 60 80 100 120 140
Mmm [GeV/c2] Mmm [GeV/c2] Mmm [GeV/c2]
Figure 1: Top: The UE activity as a function of the dimuon invariant mass (M ) for events
µµ
with pµµ < 5GeV/c for charged particles having ∆φ < 120◦: (left) particle density; (centre)
T
energy density; (right) ratio of the energy and particle densities. The predictions of PYTHIA6
Z2, POWHEG Z2, PYTHIA8 4C, and HERWIG++ LHC-UE7-2 (with and without MPIs) are also
displayed. In the top right plot, the structure around 60–80GeV/c2 for HERWIG++ without
MPIsreflectstheinfluenceofphotonradiationbyfinal-statemuons,whichisenhancedbelow
theZresonance. Bottom: RatiosofthepredictionsofvariousMCmodelsandthemeasurement.
Theinnerbandshowsthestatisticaluncertainityofdatawhereastheouterbandrepresentsthe
totaluncertainty.