W and Z physics at CMS

W and Z physics at CMSG. Franzoni

University of Minnesota

On behalf of the CMS collaboration

LHC Days - SplitOct 5th, 2010

1

outline

• Physics motivation • Leptons and mET at CMS

• W and Z signal extraction

• Inclusive W and Z cross sections with 0.198 pb-1

• Differential measurements:– Lepton charge asymmetry– W + jets

Oct 5th, 2010 G. Franzoni UMN - CMS 2

Why are W and Z interesting• Discovered nearly three decades ago…• Tools for commissioning:

– Calibration and alignment– Source of isolated high PT leptons

• Production theoretically well understood

• Early physics probes:– Cross sections at 7 TeV– Constrain PDF at LHC– Understand background for many physics

searches


Electromagnetic

calorimeter in CM

SECAL+ PbWO4 Crystals+Si/Pb Preshower

Tracker: Si

strips and

pixels

Muon chamber

: Drift Tubes,

CSC and RPC’s

HCAL

Iron yoke

3.8T solenoi

d

CMSWeight 14000tDiameter 15mLength 21.6mMagnetic field 3.8T

Beam pipe

CMS

Trigger- L1: hardware w/ output 100 kHz- HLT: farm of pc’s w/ same framework offline, O(102) Hz

Datasets• CMS recorded now 10 pb-1, with

efficiency in excess of 90% (and growing)

• Data : – Analysis presented here: 198 nb-1– Ever growing dataset included in this

analysis. Update with 2.9pb-1

• MC:– NLO Monte Carlo (POWHEG) for EWK

processes– PYTHIA for QCD and t tbar

– PYTHIA for hadronization– Detector simulation with GEANT4


€

Ldt∫

Muon reconstructions and selection• HLT trigger: pT > 9 GeV/c

• Kinematics: – pT > 20 GeV/c

– μ1 :|η|<2.1 μ2 for Z: |η|<2.4

• Track quality:– Matching hits in tracker (10) pixel (1)

and at least 2 muon stations– χ2/ndf<10; d0 cut reject cosmics


• Isolation: in cone of ΔR<0.3• W: relative isolation from tracker and calorimeters a < 0.15• Z: absolute tracker isolation < 3 GeV/c

Electron selection• HLT trigger: cluster ET>15 GeV

• Kinematics:– ET > 20 GeV

– 0.<||<1.442 or 1.566<||<2.5

• Clustering and tracking customized to recover bremsstrahlung losses

• Identification: cluster-track matching, e.m. shower shape and hadron leakage H/E

• Isolation:– Isolation from tracker, ECAL and HCAL

relative to electron PT

– Sum PT within cone of ΔR<0.3, removing electron ‘footprint’


Background enriched

Signalenriched

Missing ET • Cleaning of instrumental and beam-

induced background. Remove hits:– with unphysical pulse shapes or energy

sharing across cells in calorimeters– matching parallel-to- beam trajectories

• Two ways of exploiting excellent CMS tracking to reconstruct mET: – Track-corrected mET

• sum calorimeter towers and replace towers ET with pT of matched charged tracks

– Particle Flow mET

• vector sum of of all particles identified in the event

• PF mET used in this analysis, track-corrected for cross-check


min bias

di-jets

W cross section

Oct 5th, 2010 9G. Franzoni UMN - CMS

Wμν candidate

W eν candidate

Yield Wμν • Veto presence of second μ

with pT>10 GeV/c to remove Z and DY

• Wμν yield determined from simultaneous MT(W) fit to signal and background contributions with fixed shape– QCD background shape from data,

by inversion of isolation cut > 0.2• Fair agreement of data-driven shape with

MC inverted and non-inverted.• Difference treated as systematic

– Signal Wμν and EWB background (DY, τ, ttbar) shapes from MonteCarlo


NW=818±27

W μν cross section


NW=818±27 NW-=289±13NW+=529±24

• Total Wμν cross section:

• Separate positively and negatively charged W:

Yield W eν


• Veto presence of second electron with ET>20 GeV

• Weν yields determined from simultaneous mET(W) fits to signal and background contributions with fixed shape:

• QCD background shape:– modelled with Rayleigh

distribution– Constrained to background-

enriched sample obtained by tack-cluster matching cut inversion

• Shapes of signal Weν and EWB background obtained from MonteCarlo (POWHEG)

W eν cross section


• Total Weν cross section:

• Separate positively and negatively charged W:

systematic uncertainties on W lν • Main uncertainty from luminosity• Other uncertainties of statistical origin, will decrease in the 2.9pb-1:

– efficiencies: measured from MC and corrected with data-drive scale factors (Z events). Uncertainties on factors enter systematics

– momentum scale (<1%) and cluster energy scale (<3%)– Scale of mET: estimate <10% from data-MC comparison of W recoil

– Impact of PDF on acceptance (CTEQ66, MSTW08NLO, NNPDF2.0)


Source Wμν (%) Weν (%)

Reconstruction/Id 3.0 6.1

Trigger efficiency 3.2 0.6

Isolation efficiency 0.5 1.1

Momentum scale/resolution 1.0 2.7

mET scale/resolution 1.0 1.4

Background subtraction 3.5 2.2

PDF on acceptance 2.0 2.0

Other theoretical uncertainty 1.4 1.3

Total (w/o luminosity) 6.3 7.7

Luminosity 11.0 11

Z cross section


Z μμ candidate

Z ee candidate

Zμμ cross section• Selection:

– Opposite-charge μ’s; relaxed selection for μ2 (10 TK matching hits)

– 60< mμμ <120 GeV

• Predicted background QCD+EWK negligible (≈0.3%)• Cross section:


NZ=77

Zee cross section• Selection:

– Opposite-charge e’s; same cuts as W, but at looser working point (ε: 80->95%)– 60< mee <120 GeV

• Predicted background from di-jets less than one event• Systematic assigned to the impact of energy scale

– Update with 2.9pb-1: scale recalibrated (consistent with π0 and η in ECAL barrel)• Cross section:


NZ=61

systematic uncertainties on Z ll


Source Zμμ (%) Zee (%)

Reconstruction/Id 2.5 7.2Trigger efficiency 0.7 -

Isolation efficiency 1.0 1.2Momentum scale/resolution 0.5 -

PDF on acceptance 2.0 2.0Other theoretical uncertainty 1.6 1.3

Total (w/o luminosity) 3.8 7.7Luminosity 11.0 11

• Main uncertainty from luminosity• Other uncertainties of statistical origin, will decrease in the 2.9pb-1:

– efficiencies: measured from MC and corrected with data-drive scale factors (Z events). Uncertainties on factors enter systematics

– momentum scale (<1%) and cluster energy scale (<3%)– Impact of PDF on acceptance (CTEQ66, MSTW08NLO, NNPDF2.0)

W and Z cross sections

• Lepton channels combined w/ likelihood accounting errors and their correlations

• Cross sections compatible with Standard Model Oct 5th, 2010 G. Franzoni UMN - CMS 19

Differential results with 198 nb-1


W+ and W- separately• The prevalence of u quarks in protons yields larger rate of W+ than W- • R(W+/W-) relevant to constrain PDF at low-x


W+lν W-lν

R(W+/W-)

W charge asymmetry• Slicing the asymmetry in bins of lepton pseudo-rapidity:


• With 198nb-1 dominated by statistical uncertainties • 0.05-0.08

• Systematic of 0.03 estimated for both leptons (~10pb-1):– Efficiencies and

background subtraction – Different kinematics

results in small acceptance differences for W+ and W-

W+Jets associated production• Important background to many

analyzes and searches• Powerful tool to test perturbative QCD


• Reconstruction of jets:– Particle flow objects– Anti-Kt algorithm (ΔR<0.5)

C. Lazaridis – Poster: “W/Z + Jets production measurements with 7 TeV pp collisions data at CMS experiment”

2.9pb-1 W and Z cross section update


2.9pb-1 update for W μν


2.9pb-1 update for Z ll


• Update to electromagnetic energy scale in ECAL

Conclusions and outlook

• Electroweak physics program in full swing at CMS!• Reported:

– W and Z cross section at 7 TeV– First differential measurements

• LHC delivering ever growing luminosities: we’ve entered the phase of precision measurements.– Soon an update on the Z/W inclusive cross section

measurement– 2010 full dataset for differential measurements


References• “Measurement of the W and Z inclusive production cross

sections at sqrts=7 TeV with the CMS experiment at the LHC”, CMS-PAS-EWK-10-002

• “Missing Transverse Energy Performance in Minimum-Bias and Jet Events from Proton-Proton Collisions at √s = 7 TeV”, CMS-PAS-JME-10-004

• “Electron Reconstruction and Identification at √s = 7 TeV”, CMS DP-2010/032


http://cdsweb.cern.ch/record/1279615?ln=en

http://cdsweb.cern.ch/record/1279142/

http://cms.cern.ch/iCMS/jsp/openfile.jsp?type=DP&year=2010&files=DP2010_032.pdf

W and Z physics at CMS

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