We investigate the CMS potential for the observation and study of the pp -> ZZ -> e+e-e+e- and pp -> WZ -> 3l (l=e,mu) reactions, using fully simulated signal and background samples. It is shown that these processes are potentially accessible with early CMS data, thanks to relatively high production cross sections at LHC energies, together with clear signatures of the multi-lepton final states. The main systematic effects relevant for cross section measurements with 1 and 10 inverse femtobarns of data are addressed. We demonstrate that multiple gauge-boson production in pp collisions at LHC energies can be observed in the early phase of the experiment, with an integrated luminosity of 1 inverse femtobarn or less

Brigljevic, Vuko

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Available on CMS information server CMS CR 2006/063CMS Conference Report21 September 2006Measurement of Diboson Production with CMSVuko Brigljevi·cRudjer Boskovi·c Institute, Zagreb, CroatiaAbstractWe investigate the CMS potential for the observation and study of the   	and   	ﬁﬀﬂﬃ "!$#&% reactions, using fully simulated signal and background samples.It is shown that these processes are potentially accessible with early CMS data, thanks to relativelyhigh production cross sections at LHC energies, together with clear signatures of the multi-lepton nalstates. The main systematic effects relevant for cross section measurements with ' and ')( fb +* of dataare addressed. We demonstrate that multiple gauge-boson production in    collisions at LHC energiescan be observed in the early phase of the experiment, with an integrated luminosity of 1 fb +* or less.Presented at Physics at LHC,Cracow, Poland, 3-8 July 20061 IntroductionThe study of multiple gauge-boson production at the TeV scale constitutes a unique opportunity to test the StandardModel of Electroweak interactions at the highest possible energies. The production of -,.	 and /,10 eventsat the LHC probes triple gauge-boson couplings and therefore the non-Abelian gauge symmetry of the StandardModel. On the other hand, no neutral gauge-boson couplings exist in the Standard Model thus anomalies in 2	 and 	30 production, hinting at large 4 -channel contributions, could be the rst indirect manifestation of NewPhysics. In the following, the selections of  ,   and     events are described, their signal-over-backgroundratio discussed and the outlook for an early measurement of multiple gauge-boson production is assessed. Furtherdetails are given in Reference [1].The 	5	 production represents an irreducible background to the most-awaited discovery at the LHC: the Stan-dard Model Higgs boson. Its early measurement is therefore important. In general, the multi-lepton nal states ofmultiple gauge-boson production are an important background in the search for New Physics, in particular Super-symmetry. A sound understanding of the production process of multiple gauge-bosons is therefore needed in therst phase of LHC data-taking before any discovery can be claimed.The cross sections for multiple gauge-boson production at the LHC are of about 50 pb for the 6,78 channel and20 pb for the 59	 channel [2]. These large cross sections and the clean signature of fully-leptonic nal statesmake /,:8 and 	58 production observable in the rst LHC data. Final states where the gauge bosons decayinto electrons and muons are considered in these analyses: , , #&,;< , =,1#># and #&,#># for /,?	production and =1< for the 	58 channel. The competing background processes are the Standard Modelproduction of gauge bosons and top quarks, which yield leptonic nal states.2 Signal denition and modelingBoth the /,@	 and 		8 analyses focus on on-shell gauge bosons.On-shell production of the ,8 nal state proceeds mainly through the 4 -channel, involving a / triplegauge-boson coupling. Additional contributions from the A,10>B nal state through a /C0 coupling are effec-tively suppressed by constraining the mass of the observed lepton pair to be compatible with a 9 boson. ThePYTHIA Monte Carlo generator [3] is used to model  ,   production and subsequent decay into fully-leptonicnal states. All leptonic nal states are allowed, including D , and no constraints are imposed on the D decays. Inthe generated sample, 95.5% of all events are classied as A,E8 signal while the small remaining fraction isassigned to the ,10&B process.Four-electron nal-states can originate from 5F	 production and via either 	G0>B or 0&BH0>B production, all largelydominated by the I -channel. The requirement of on-shell boson is enforced by considering only electron-positronpairs with a mass between 70 and 110 JKML . The PYTHIA Monte Carlo is used to generate events of this process,with the additional requirement that the electrons have a rapidity N O>NFPRQTSVU and a transverse momentum  >W-XYJKML . Of all generated events, 72% are classied as Z	 signal while 26% are ascribed to the 530&B processand 2% to the 0BH0&B process.Taking into account the branching fraction into leptons, [ , and the kinematic requirements, \)]8^H_ , the relevantNLO cross sections times branching fraction are calculated using the MCFM [4] Monte Carlo to be`_baTcd[edf\g]8^H_ﬀh  ij%= 1034 fb`_baTcd[kd\H]8^H_ﬀl  mA	Zj=>G1=%= 630 fb`_baTcEd[nd\H]8^H_ﬀl  o	<	Zp%= 18.7 fbThe NLO corrections correspond to overall q -factors of 1.9 and 1.4 for A,8 and 	r8 production, respectively.The NNLO box-diagram contribution to 5	8 production is not taken into account.3 Background processesThe background to the selection of s,:8 and 	Z	 events comprises other processes with multiple leptons inthe nal states, some of which might be due to fake signals. The most copious sources of multiple leptons at theLHC are I I and 	8t t production. The cross section of these processes is large: 830 pb and 1492 pb, respectively,as calculated with MCFM at NLO. These processes may have two leptons in the nal states from leptonic decaysof the  bosons arising from I m/t decays or of the 5 boson, respectively. The other leptons can be produced2in the direct or cascade decays of the t quarks. The ZZt t process is modeled with the COMPHEP Monte Carlogenerator [8] and the I I process with the TOPREX Monte Carlo program [7]. Both I and uI quarks in the I I sampleare forced to decay semi-leptonically. In addition, the special case in which four electrons are produced in I I eventsis considered in detail and modeled with PYTHIA.Events from 58 production can constitute a background to the s,@8 selection. Events from the 530&B and0&Bg0&B processes consitute a background for both the s,v	 and 888 analyses.4 Event selectionThree-lepton and four-lepton nal-states from  ,   and     production are collected with high efciencyby the L1 and HLT electron and muon triggers. The L1 and HLT efciencies for events retained by the selectionsdiscussed below is 100%.4.1 wmxEyz selectionEvents with three charged leptons, either electrons or muons, with  +W{X '<(rJKML and N O>N|PeQ|S Y , are considered bythe /,}8 selection. Electrons are identied from superclusters in the electromagnetic calorimeter matched witha charged track, while muons are reconstructed by matching information from the muon chambers with tracks inthe central tracker.All possible 	 -boson candidates from same-avour opposite-charge lepton pairs are formed. Events are retainedif a 	 candidate is found with a mass within 20 JKML of the 5 -boson mass, ~7 . The background from 5b	nal states is reduced by rejecting events with a second Z candidate with a mass within 40 JKL of ~  . Theremaining lepton is associated to the s, -boson decay; its transverse momentum must be larger than 20 JKL .This requirement effectively suppresses background from   t t events. The highest- W lepton associated to the	 boson must satisfy  WX ' Y JKML . If the event contains more than three leptons, the lepton with highest  >Wis chosen as originating from the s, . The signal efciency after these cuts is 9.5% while the I I , 1t t and#>#1t t efciencies are 0.8%, 0.4% and 0.6%, respectively.Leptons from the decay of t quarks in the background processes are produced in a higher-multiplicity environmentand isolation criteria suppress the background contamination. All muon candidates must have an energy measuredin the calorimeters within a  ﬃ (TS  cone around their direction smaller than 5 JKL and the sum of the   W oftracks within a  ﬃ (S Q Y cone smaller than 2 JKL . The isolation criteria for electrons are the same as thosedescribed in the 58 selection below. Besides fullling those requirements, electrons associated to the -,boson must have no other charged track with   W X Q;JKML within a  ﬃ (TS  cone around their direction.The signicance of the lepton impact parameter in the plane transverse to the beam, ^g , discriminates againstleptons from heavy-quark decays. This variable is dened as the ratio between the measured impact parameter inthe transverse plane and its uncertainty. It is required to satisfy ^HeP  . The signal efciency after these cuts is7.5% while the I I , 1t t and #>#1t t efciencies are 0.08%, 0.009% and 0.03%, respectively.The I I and 	;t t nal states are associated with one or more hard jets and their contribution is reduced by removingevents containing at least a jet with Ł WX Q(rJKML . Jets are reconstructed using a Cone algorithm of size  ﬃ (S Yand requiring its component calorimeter towers to have Ł$W X (TS Y JKML and Ł2 X (TS rJKML . Only jets outsidecones of  ﬃ (S  around the three leptons are considered.Finally, the reconstructed mass of the 5 boson is required to be within 10 JKML of ~7 , leading to the totalefciencies presented in Table 1.4.2 yzyz selectionThe 828 selection is based on events with four electrons. The transverse momenta of the electron candidates,ordered from the largest to the smallest, have to be above 30 JKL , 20 JKL , 15 JKML and 10 JKL , respectively.This cut suppresses the contribution from the ZG0>B and 0&B0>B nal states and reduces by 30% and 60% the I Iand   t t backgrounds, respectively. Leptons from t quarks decays in the I I and   t t background processesare produced in association with hadrons. Their contribution is reduced by requiring the electrons to be isolated:the ratio between the energy deposited in the hadronic and the electromagnetic calorimeters must be below 8%;no more than two other charged track with  WX QrJKL must be within a  ﬃ (S  cone around the electron;32), GeV/c-,l+M(l70 75 80 85 90 95 100 105 110 1152Entries / 0.9 GeV/c02468101214162Entries / 0.9 GeV/c WZ(3l)ZbbttZZCMS-1 Ldt = 1 fb∫Figure 1: Distribution of the mass of the Z candidates for events retained by the s,	 selection, for anintegrated luminosity of 'G >* .Fﬀh W1Ł%MŁP-(TS, where Łis the transverse energy of the electron candidate and the sum runs on alltracks with  W:X Q;JKML within a  ﬃ (TS  cone around the electron.Electron-positron pairs are combined to form Z candidates. Pairs with reconstructed masses between 50 and120 JKML are retained. Of the two possible     pairings, the one where the   candidate masses are closest to~? is chosen. This pairing is correct for almost all events with two on-shell  bosons. For 2.5% of the events,more than four electrons are present and only the Z	 pairing which contains the highest-  W electron is retained.Table 2 presents the signal and background selection efciencies.5 Systematic uncertaintiesFor the rst '3 +* of integrated luminosity, the total systematic uncertainties on the A,n	 and 		 crosssection determinations are 17.4% and 12.9%, respectively. These gures include a 10% uncertainty on the deter-mination of the integrated luminosity.The most important sources of systematic uncertainties are lepton identication and isolation, and the backgroundsubtraction. A 2% uncertainty on each lepton propagates to an uncertainty on the cross section between 2.6% and7.8%, according to the channel. Background subtraction dominates the -,:8 sytematics with an uncertainty of12%, while it accounts for a 1.3% uncertainty in the Z28 channel. An additional uncertainty of 5% on the jetenergy scale affects the  ,   channel, while an uncertainty of 1% on the trigger efciency affects both channels.The signicance of the observation of the s,k	 and 	 8 signals in the rst '3 +* is not sensitive to theluminosity uncertainty. It is affected by all other sources of systematic uncertainty listed above, with a total effectof 14.8% and 14.2% on the two channels, respectively. These uncertainties include additional PDF and QCDuncertainties in the Monte Carlo modeling, contributing 3.7% and 6.4% for the -,E	 and 	ﬁ8 selections,respectively.42), GeV/c-e+M(e50 60 70 80 90 100 110 1202Entries/3.5 GeV/c010203040500Z0Z*γ0Zttbb0ZCMS-1 Ldt = 10 fb∫Figure 2: Distribution of the mass of the Z candidates, two entries per event, retained by the Zb	 selection, foran integrated luminosity of '<(= >* .6 ResultsFigure 1 presents the mass distribution of the Z candidates in the ,@	 channel for an integrated luminosityof 'G +* before the last requirement of a ¡'<(rJKML window is applied. A large signal-over-background ratio isobserved, as shown in Table 1.Figure 2 shows the mass distribution of the 5 candidates, two entries per event, selected by the Z		 selectionfor an integrated luminosity of ')( >* . Table 2 lists the selection yield for 'G +* and '<(= +* . The selection resultsinto an almost background-free signal sample, which will constitute a valuable input to assess the background inthe search for the Higgs boson.Both the ,@	 and 	8	 nal states can be selected with high purity and a signicance of 13.1 and 4.8, whichinclude systematic effects, is expected in the rst '3 +* of integrated luminosity making these process suitable forearly observations. The ,@	 channel can be observed with a signicance of 5, including systematic effects, inan integrated luminosity of ' Y (¢ +* .In conclusion, the large signal-over-background ratios achieved by the  ,   and     selections suggest thatearly observation of these channels will take place at the LHC start up. In addition, precise investigations of triplegauge-boson couplings will be possible with the rst ')( >* of LHC data.Table 1: Yield of the ,@	 selection for an integrated luminosity of '3 >* .,#,,###,## Total Efciency/,r	Zj),G1= 14.8 26.9 28.1 27.0 96.8 6.2%		 0.63 1.54 1.50  3.68 4.2%I I 0.93 1.55  0.31 2.79 0.02%#>#1t t  6.54 4.9 11.4 0.005%t t 1.21 1.82   3.03 0.007%5Table 2: Yield of the 	5	 selection for integrated luminosities of 'G +* and ')( >* . The last row indicates thesignal signicance, which include systematic effects.Efciency £ﬁ¤M¥g¤§¦H¨© /1fb >* £¤M¥H¤§¦H¨© /10fb +*8	8 38% 7.1 71.18G0>B 4.5% 0.16 1.608t t 0.07% 0.08 0.84I I 0.06% 0.12 1.22+a 4.8 13.1References[1] V. Brigljevi·c et al., Study of Di-Boson Production with the CMS Detector at the LHC, CMS Note 2006/108(2006).[2] S. Haywood et al., Electroweak physics, hep-ph/0003275.[3] T. Sjostrand, L. Lonnblad, S. Mrenna and P. Skands, PYTHIA 6.3: Physics and manual, hep-ph/0308153.[4] J. M. Campbell and R. K. Ellis, MCFM v4.1, A Monte Carlo for FeMtobarn processes at hadron colliders,(2005).[5] S. Frixione and B. R. Webber, Matching NLO QCD computations and parton shower simulations, JHEP06, 029 (2002), [hep-ph/0204244].[6] Particle Data Group, S. Eidelman et al., Review of particle physics, Phys. Lett. B592, 1 (2004).[7] S. R Slabospitsky and L. Sonnenschein,TopReX generator (version 3.25): Short manual , Comput. Phys.Commun. 148, 87 (2002).[8] A. Pukhov et al., CompHEP: A package for evaluation of Feynman diagrams and integration over multi-particle phase space. User’s manual for version 33, hep-ph/9908288.6

Measurement of Diboson Production with CMS

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