integrated luminosity: ∫Ldt = 34pb-1 (ATLAS) 36pb-1 (CMS)

* ATLAS /arXiv:1104.5225 / Submitted to PRL* CMS / arXiv:1102.5429 / Phys. Lett. B699 (2011) 25

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2010 20112009 t

Measurement of the W+W- cross section in √s = 7 TeV pp collisions at LHC—

* nonresonant part of WW is an important background to SM Higgs searches* sensitive to the triple gauge couplings of the W boson (NP search ?)* gluon fusion is NLO (3%) but enhanced by large gg parton luminosity (fourth family of fermions in the triangle loop ?)

Motivation

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γ/Z

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SM prediction for pp → W+(→lν) W-(→lν)☼ J.M. Campbell, R.K. Ellis. An Update on vector boson pair production at hadron colliders. Phys.Rev. D60 (1999) 113006

MSFM is a parton-level event integrator which gives results for a series of processes, especially those containing the bosons W, Z and H and heavy quarks, c, b and t. Most processes are included at next-to-leading order (NLO) and include spin correlations in the decay.

☼ T. Binoth et al, Gluon-induced W-boson pair production at the LHC, JHEP 12 (2006) 046.

GG2WW, includes all background and signal contributions, full spin correlations, off-shell and interference effects, as well as finite top and bottom quark mass effects.

Singly resonant contribution should also be included

☼ J.M. Campbell, R.K. Ellis. An Update on vector boson pair production at hadron colliders. Phys.Rev. D60 (1999) 113006

MSFM is a parton-level event integrator which gives results for a series of processes, especially those containing the bosons W, Z and H and heavy quarks, c, b and t. Most processes are included at next-to-leading order (NLO) and include spin correlations in the decay.

☼ T. Binoth et al, Gluon-induced W-boson pair production at the LHC, JHEP 12 (2006) 046.

GG2WW, includes all background and signal contributions, full spin correlations, off-shell and interference effects, as well as finite top and bottom quark mass effects.

(NLO) QCD predictionσ = 43.0 ± 2.0 pb

SM prediction for pp → W+(→lν) W-(→lν)

Events are selected by requiring two reconstructed oppositely-charged leptons and a large transverse momentum imbalance due to the neutrinos, which escape detection

Backgrounds

* W+jets production with a jet misidentied as a lepton

* Drell-Yan production:● Z/γ*→ll where the observed momentum imbalance is due to mismeasurements● Z/γ*→ττ→llνν

* top production (tt and Wt), which also produces two W bosons, but is not considered signal and is suppressed by vetoing candidates with jets

* other diboson processes, which include ● WZ with Z→ll and W→lν where one charged lepton is lost ● ZZ with Z→ll and Z→νν● Wγ with the photon misidentied as an electron.

signal acceptance and backgrounds (except W+jets) are derived from simulations

Monte-Carlo

* signal qq→WW, WZ, tt, tW

* ZZ, W→lν, PS, hadronization

* Wγ

* W+jets (data driven technique or MC)

* Drell-Yan

* PDFs

* gg→WW

* QCD jets

* Underlying event

* Detector simulation

MC@NLO

HERWIG

MADGRAPH+PYTHIA

(ALP+HERW+JIMMY)

ALPGEN, PYTHIA

CTEQ6.6, CTEQ6M

GG2WW

PYTHIA

JIMMY

GEANT4

ATLAS

Monte-Carlo

* signal qq→WW, tt, tW

* W+jets

* H→WW and Drell-Yan

* PDFs

* gg→WW

* W→lν, PS, hadronization

* Underlying event

* Detector simulation

MADGRAPH

POWHEG

CTEQ6L

GG2WW

PYTHIA

GEANT4

CMS

The luminosity in a single bunch-crossing was sufficient to produce multiple collisions, observed as multiple vertices, in the same recorded event. The vertex with the largest sum (pT)2 is primary vertex. Inclusive pp collisions are simulated to reproduce the vertex multiplicity observed in data.

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inner detector

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calorimeters

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muon spectrometer

ATLAS detector

CalorimetersThere is the transition region between the barrel and the end-cap EMC

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Electron identification (general)ATLAS, JHEP12(2010)060

“Medium”: full information from EM + some from the inner tracking detector (ID) (track quality variables + cluster-track matching variable)

“Tight”: exploits the full electron identication potential of the ATLAS detector:● shower shape ● ratio of energy deposited in the hadronic to electromagnetic calorimeters ● inner-detector track quality ● track-to-shower matching ● ratio of calorimeter energy measurement to track momentum ● and transition radiationin the straw tube tracker.

starts in the high-granularity liquid-argon sampling electromagnetic (EM) calorimeters. Further, there are three reference sets of requirements:

“Loose”: uses EM shower shape information and discriminant variables from hadronic calorimeters

Electron identification (arXiv:1104.5225)

* pass “tight” electron selection criteria

* have pT>20GeV and 1.57<|η|<2.47 or |η|<1.52(transition region between the barrel and the end-cap EMC)

* pass isolation criteria: total ET within a cone around the electron is less than 6GeV

* The overall electron reconstruction and identification efficiency is measured from data using W→eν and Z→ee candidates. It varies from 78% for the central region (η<0.8) to 64% in the forward region (2.0<η<2.47) with a statistical uncertainty of less than 0.4% and a systematic uncertainty of 5% averaged over rapidity.

ΔR=√(Δη)2+(Δφ)2 = 0.3

Pseudorapidity:η =-lnTan(θ/2)

Muon identification* have |η|<2.4

* to reject muons from charged π or K decays and charged particles from the beam-induced backgrounds, the MS muon pT must exceed 10 GeV and be consistent with the ID measurement, |pTMS-pTID|/pTID<0:5

* to suppress muons from hadronic jets, the ∑pT for the of other tracks with pT<1GeV in a cone of ∆R=0.2 is required to be less than 0.1 of the muon pT

* the muon reconstruction and isolation efficiencies are measured in data using Z→μμ candidates to obtain a combined efficiency of 92±1(stat)±1(syst)%

Jet identification

* Jets used to discriminate top from W+W- production

* reconstruction using the anti-kT-algorithm with a distance parameter R=0.4

* Jets within a ΔR<0.3 of an electron are not used because the electrons are in general also reconstructed as jets

* Jets are corrected for calorimeter non-compensation, material and other effects using pT- and η-dependent calibration factors obtained from Monte Carlo and validated with test-beam and collision-data studies

No missing transverse energy (ET)miss=-∑ET→

Non zero missing transverse energy

Relative missing transverse energy

small ∆φ

(ET)miss cut should be implemented in order to suppress Drell-Yan bkg. But (ET)miss

is sensitive to the mismeasurement of an individual lepton or jet

big ∆φ

ET , relmiss ={ETmiss×sin (Δϕ ) if Δϕ<π /2

ETmiss if Δϕ>π /2∣

where Δφ is the difference in the azimuthal angle between (ET)miss

and the nearest lepton or jet.

Two important cases ● are high-mass muonic Drell-Yan events, where the momentum resolution can be comparable to (ET)miss in W+W- events, and Z→ττ, where the real (ET)miss from leptonic τ decays is parallel to the momenta of the leptons. (ET)rel cut provides the higher signal to bkg ratio than (ET)miss

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Selection ATLAS (CMS)* select two opposite-sign charged leptons with pT > 20 GeV

* the leptons are required to be consistent with coming from a primary vertex with at least three associated tracks (to remove cosmics)

* for ee and μμ events, remove all events with |mll-mZ|<10(15) GeV, or mll<15 GeV and (ET,miss)rel < 40 GeV, in order to suppress Z/γ*→ee or μμ.

* for eμ events, a less stringent requirement (ET,miss)rel >20 GeV is made

* remove candidates containing jets with pT > 20(25) GeV and |η|< 3(5), in order to suppress the tt contribution

Demonstration of signal region

WW→eνeν+μνμν WW→eνμν

Demonstration of signal region

Signal acceptance (selection efficiency)

channels ee μμ eμ

lepton acceptance and identification

mll

(ET,miss)rel

jet-veto

18%

85%

41%

64%

4.1±0.1%

41%

84%

43%

59%

8.6±0.1%

27%

100%

69%

61%

11.5±0.6%

is derived from simulation and is corrected with scale factors based on measurements in independent data samples. The scale factors correct for the difference in trigger, lepton reconstruction and identification, and jet-veto efficiencies between data and simulation.

Uncertainties* both the signal acceptance and the background estimates have uncertainties due to the trigger, lepton reconstruction and identification, and jet-veto efficiencies, in addition to the uncertainties on the integrated luminosity and theoretical cross sections

* two major sources of systematic uncertainty in the jet-veto efficiency: ISR+FSR (radiation in t-channel for signal) and jet-energy scale

* Most of the Drell-Yan events are removed by the dilepton invariant mass and (ET,miss)rel requirements, but because of the large cross section some remain as background. The uncertainty on this background due to the simulation of (ET,miss)rel is assessed using a control sample of Z/γ*→ee and Z/γ*→μμ events in the Z mass peak region, |mll-mZ|< 10 GeV, passing a relaxed requirement of (ET,miss)rel > 30 GeV. Despite the (ET,miss)rel requirement,this sample is still dominated by Z→ll events in which the observed momentum imbalance is due to a combination of detector resolution, limited detector coverage, and neutrinos from heavy flavor decays. A 64% systematic uncertainty is assigned based on the difference betweenthe observed yield in data and the Monte Carlo prediction, which are statistically consistent.

Observed vs expected events (ATLAS)Final State

Observed Events

Expected WW

Backgrounds

Drell-Yan

WZ, ZZ, W

W+jets

TopTotal Background

ee (ET,miss)rel μμ (ET,miss)rel eμ (ET,miss)rel Combined

1 2 5 8

0.79±0.02±0.09 1.61±0.04±0.14 4.45±0.06±0.44 6.85±0.07±0.66

0.00±0.10±0.07 0.01±0.10±0.07 0.22±0.06±0.15 0.23±0.15±0.17

0.05±0.01±0.01 0.10±0.01±0.01 0.23±0.05±0.02 0.38±0.04±0.04

0.08±0.05±0.03 0.00±0.29±0.10 0.46±0.12±0.17 0.54±0.32±0.21

0.04±0.02±0.02 0.14±0.06±0.07 0.35±0.10±0.19 0.53±0.12±0.28

0.17±0.11±0.08 0.25±0.31±0.15 1.26±0.17±0.31 1.68±0.37±0.42

* To estimate the statistical significance of the signal, Poisson-distributed pseudoexperiments are generated, varying the expected background according to its uncertainty. The probability to observe 8 or more events in the absence of a signal is 1.2·10-3, which corresponds to a significance of 3σ standard deviations.

Observed vs expected events (CMS)Final State

Observed Events

All expected

Backgrounds

Z+WZ+ZZ→ll

WZ+ZZ, lept. not from the same bos.

W+jets

tt+tWWγZ/γ*→ττTotal Background

Combined ee (ET,miss)rel μμ (ET,miss)rel eμ (ET,miss)rel

13 2 1 10 13.5±0.3 2.7±0.1 2.3±0.2 8.5±0.3

0.20±0.20±0.3

0.22±0.01±0.04

1.70±0.40±0.70

0.77±0.05±0.77

0.31±0.04±0.05

0.09±0.05±0.09

3.29±0.45±1.09

Kinematic properties

Distributions of the leading lepton pT (left), transverse momentum of the dilepton system (center), and azimuthal angle between the leptons (right). The gray band indicates the combined statistical and systematic uncertainty on the sum of the signal and background expectations.

Kinematic properties

Cross section

σobs=41.1±15.3(stat)±5.8(sys)±4.5(lumi) pb

σSM=43.0 ± 2.0 pb

σobs=41 (stat)±5(sys)±1(lumi) pb+20-16

ATLAS

CMS

Standard Model

Comparison with inclusive W production

σWW/σW=(4.46±1.66(stat)±0.64)·10-4

CMS

Standard Model

σWW/σW=(4.45±0.30)·10-4

* CMS Collaboration Collaboration, “Measurements of Inclusive W and Z cross sections in pp collisions at √s= 7TeV”, JHEP 1101 (2011) 080

Conclusion

* The measured W+W- production cross section is in good agreement between CMS and ATLAS experiments and with the standard model prediction calculated at next-to-leading order in QCD.

* With the significantly larger integrated luminosities expected to be provided by the LHC, this signal will form the basis of a research program that will include searches for the standard model Higgs boson, anomalous triple gauge couplings, and other processes beyond the standard model.