Measurement of diboson production with CMS

in early LHC data:the example of WZ production

Measurement of diboson production with CMS

in early LHC data:the example of WZ production

Vuko BrigljevićRuđer Bošković Institute, Zagreb

on behalf of the CMS Collaboration

Physics @ LHCSplit, 29 September – 4 October 2008

V. Brigljevic Dibosons with CMS Physics@LHC 2008, Split 2

Motivation

• Prediction of the non-abelian SM gauge structure:Couplings between gauge bosons

• Measuring the coupling between the gauge bosons tests a central part of the SM

• Deviations could hint to new physics• Complementary to direct search for new physics

Manifestation of gauge boson couplings at the LHC:

production of final states with boson pairs (W,Z,)

V. Brigljevic Dibosons with CMS Physics@LHC 2008, Split 3

Gauge boson couplings

Triple gauge couplings (W,Z,)– Charged couplings WWZ and WW

Allowed in the SM

– Neutral couplings ZZZ, ZZForbidden in the SM

V1

V2

V3

V. Brigljevic Dibosons with CMS Physics@LHC 2008, Split 4

C even

even

even

odd oddeven

even

P even

even

even

even

odd odd odd

CP even

even

even

oddeven

odd odd

SM 1 1 0 0 0 0 0

gV

1 κV λVgV

4gV

5κ~ V λ~ V

K. Hagiwara et al. PRD 41, 2113

WWZ : λZ

WWγ : λ γ

gγ

1

gZ

1Δ κZΔ

κ γΔ

( = 1 : EM gauge invariance)

κΔ= κ -1

gZ

1Δ = -1gZ

1

V=Z,γ

Most general description of the TGC vertex by a Lorentz invariant effective Lagrangian

Triple Gauge boson couplings

V. Brigljevic Dibosons with CMS Physics@LHC 2008, Split 5

Diboson production at the LHC

Production Processes at the LHC– Leading order Feynman diagrams:– Only s-channel has three boson

vertex– Anomalous couplings tend to

manifest in:• Cross section enhancement

• Enhancement at high pT of V1,2.

• Production angle.

q

q

q’

V2

V1

q

q

q’

V2

V1

q

q V2

V1

V0

V. Brigljevic Dibosons with CMS Physics@LHC 2008, Split 6

Diboson processes at the LHC (@14 TeV)

• WZ– Cross section: =51.5 pb (MCFM,

NLO)– s-channel dominated, sensitive to

TGC• WW

– Cross section: =117 pb (MCFM NLO)

– s-channel dominated, sensitive to TGC

• ZZ*– Cross section: = 18 pb (MCFM,

NLO, m(Z*) = 91 +/- 45 GeV )– t-channel dominated only at tree level

• W– Cross section: = 140 pb (Baur

NLO, pt()>10 GeV)– s-channel, sensitive to TGC

• Z– Cross section: = 74 pb (Baur NLO,

pt()>10 GeV)

• Large cross sections• Expect tens to hundreds

of events in first fb-1

• Part of the SM rediscovery we can do with luminosity from ~50-100 pb-1

• CMS working on all these channels

Todaypresent prospects for

WZ measurement with CMS

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Sensitivity to anomalous couplings

Atlas-Phys-Pub-2006-011

σ(fb)

λ

Δκ

enhancement of cross section enhancement at high PT

e.g. WW cross section

Signatures of anomalous couplings

V. Brigljevic Dibosons with CMS Physics@LHC 2008, Split 8

Measurement of pp → WZ →3l

in early LHC dataConsidering 4 channels:

“3e” : Z →ee, W →e“2e1” : Z →ee, W →“21e” : Z →, W →e“3” : Z →, W →

• Background for searches with 3 leptons (+ MET)• Background for searches with WZ in final state (W’, Techni-Rho)

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WZ production at the LHCProduction

• s-channel dominates• NLO cross section (MCFM)

NLO (pp → W+Z) = 31.9 pb

NLO (pp → W-Z) = 19.6 pb

• Apply pt(Z)-dependent k-factor to Pythia– Accounts for dependence of k-factor on PT(Z): affects Z

reconstruction efficiency

k-factor vs Pt(Z)

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Background samples

Sample Generator

Z + jets Alpgen

W + jets Alpgen

ttbar + jets Alpgen

Z Pythia

Zbb CompHEP

ZZ Pythia

• Constant k-factors applied to all backgrounds• All samples fully simulated with conditions

(calibration, alignment) corresponding to 100 pb-1

V. Brigljevic Dibosons with CMS Physics@LHC 2008, Split 11

WZ Preselection:Trigger and lepton ID

Electrons• Good match in position and energy

between calorimeter and tracker• Narrow shower in ECAL• Tracker isolation• Additional requirement for W →e

calorimetric (ECAL + HCAL) isolation

Muons• Combine central tracker and

muon chamber information• Require calorimetric and

tracker isolation• Require impact parameter

significance consistent with primary vertex

1.Trigger requirements

Single electron trigger for Z →ee channels

Single muon trigger for Z → channelsMinimize trigger bias on W lepton

97-100% efficient for selected events

2. 3 leptons (e or ) with pt>15 GeV, ||<2.5 (2.4) for e ()

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WZ Candidate selection

• Z selection:– Look for e+e- or +- pair with Mass in [50-120]

GeV• Keep large mass window for background estimation

– Veto if more than one Z candidate

• W reconstruction:– Associate 3rd lepton to W-decay, – require pt(lW)>20 GeV– Use neutrino presence (MET):

MT(W) > 50 GeV

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W Selection: using the neutrino

• Exploit MET to discriminate WZ from Z+jets:

MT(W) > 50 GeV

3e 2e

2e 3

CMS Preliminary300 pb-1

CMS Preliminary300 pb-1

CMS Preliminary300 pb-1

CMS Preliminary300 pb-1

V. Brigljevic Dibosons with CMS Physics@LHC 2008, Split 14

Event yields after all cutsExpected number of events for 300 pb-1

• Dominant background: Z+jets (including bbll)• More background in W →e channels: jets much more likely to fake electrons than muons

V. Brigljevic Dibosons with CMS Physics@LHC 2008, Split 15

Z mass distribution

Z →ee

Z →

W →e W →

3e 2e

2e 3

CMS Preliminary300 pb-1

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Signal extraction strategy

3 categories of background1. Non-genuine Z background:

ttbar+jets, W+jets• Sideband fit• For low luminosity (no events

in sideband): subtract from MC

2. Genuine Z Physics background (irreducible):

ZZ, Z• Estimate from MC

3. Genuine Z instrumental background (fake leptons):

Z+jets• Data-driven background

estimation (“Matrix method”)

Question: how do we extract WZ signal (cross section) from accepted events?

1

2 3

All channels, 300 pb-1

CMS Preliminary300 pb-1

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Data-Driven background estimation

Procedure known as “Matrix method”, e.g. used in D0:

Define 2 samples– “tight”: final selection– “loose”: relaxed requirement on W lepton

Nloose = Nl + Njet

Ntight = tight Nl + pfake Njet

Nl : # isolated true leptons

Njet : # fake or non-isolated leptons

tight• Efficiency for true isolated lepton to

pass from loose to tight• Determine from data with Tag &

Probe with Z →ee /

pfake• Efficiency for fake or non-isolated

lepton to pass from loose to tight• Determine from data control sample:

W+jets, QCD, bbbar

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Data-driven method: pfake

• Loose sample definition:– Electrons: relax calorimetric isolation– Muons: relax isolation and impact parameter significance

requirement

• Pfake determination for electrons:1. with W+jets:

– Standard W → selection, trigger on muon– Select loose electron with same charge as (reject ttbar bg)– Count number passing tight requirement

2. With multi-jet events:– Start with jet trigger– Select loose electron separated from triggering jet– Count number of electrons passing tight requirement

Methods 1 and 2 can cross-check each other! (in agreement on MC)

• Pfake determination for muons: as for electrons, can also use bb-bar sample

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Matrix method results

• Apply matrix method to our MC sample as if it were data

• Pfake and tight determined from independent samples

• Numbers and errors correspond to what is expected with 300 pb-1

• Method gives correct signal estimation• Powerful: can be applied to any distribution bin by bin

• If statistics available, can correct as a function of pt,

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Systematic uncertainties

Systematic uncertainties estimated for a scenario of 300 pb-1 of integrated luminosity

V. Brigljevic Dibosons with CMS Physics@LHC 2008, Split 21

Observation potential

• Estimate expected signal significance with toy MC:– Vary expected number of

events within systematics

– Dice signal and background events and compute significance for each try and estimate 68% and 95% C.L. regions

• Can achieve 5 observation with less than ~350 pb-1 at 95% C.L.

CMS Preliminary

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Summary

• Diboson production test a central area of the electroweak theory, and their measurement at the LHC is an important part of the Standard Model rediscovery

• Observation of all diboson processes expected with luminosity smaller than 1 fb-1

• Diboson production is sensitive to new physics and is important background for many searches

• Prospects for WZ observation with CMS:– 5 observation with less than 350 pb-1

– Validated data-driven background estimation