1. W/Z and Di-Boson Results from ATLAS Srivas Prasad Harvard University On behalf of the ATLAS Collaboration
Pheno Madison, Wisconsin
May 09, 2011

2. Outline W, Z Measurements with Electrons and Muons
W and Z Inclusive Cross-section
Forward Z Cross-section
W Charge Asymmetry
W, Z Observation with Taus
W and Z + jets
Diboson Results
Wg and Zg
WW Cross-section
Most results for full 2010 dataset
LINT pb-1 depending on analysis
Uncertainty brought down to 3.4%
09 May 2011
Srivas Prasad, Harvard University

3. W and Z/g* Inclusive Cross-Section
ATLAS-CONF
Compare to theory at NNLO in QCD
Vital for detector performance
High pT leptons: reconstruction / trigger / identification efficiencies, resolution, scale…
ETmiss resolution and scale
Update 320 nb-1 result with x100 data
Improved analysis e.g. tag-and-probe, binning
Systematics reduced by factor of three
ETmiss scale/resolution largest experimental uncertainty (2%)
Excellent agreement between electron and muon results
09 May 2011
Srivas Prasad, Harvard University

4. W and Z/g* Inclusive Cross-Section
ATLAS-CONF
Define fiducial cross-sections using analysis kinematic requirements
All detector corrections included
Most precise experimental measurements for theory comparison
Extend Z cross-section measurement using forward electrons (2.5<|h|<4.9)
Expands range of sensitivity to parton-x
Reduces extrapolation uncertainty
sFID = ± (sta) ± (sys) ± (lum)
Z→mm
FIDUCIAL CROSS-SECTION [nb]
W→mn
4.959 ± (sta) ± (sys) ± (lum)
W→en
4.877 ± (sta) ± (sys) ± (lum)
Z→mm
0.456 ± (sta) ± (sys) ± (lum)
Z→ee
0.433 ± (sta) ± (sys) ± (lum)
Z→ee: central-forward
09 May 2011
Srivas Prasad, Harvard University

5. W and Z/g* Inclusive Cross-Section
ATLAS-CONF
Uncertainty from extrapolation to full phase space, using Pythia / MRST LO*
RESULTS AGREE (∼5% UNCERTAINTY) WITH NNLO PREDICTIONS
09 May 2011
Srivas Prasad, Harvard University

6. W Charge Asymmetry More valence u in proton: W+ favored
arxiv:
Submitted to Phys. Lett. B
More valence u in proton: W+ favored
Rapidity dependence probes u and d quark parton distribution functions
Measurement versus muon rapidity
Luminosity and systematics cancel
valence
sea
valence
sea
Dominant systematic from statistics on tag-and-probe studies
ALREADY CONSTRAINS LIGHT QUARK PDFS FOR MEDIUM AND LOW-X
09 May 2011
Srivas Prasad, Harvard University

7. W/Z Observation with Taus
ATLAS-CONF
ATLAS-CONF
Impressive control over t reconstruction
W→thadronicn observation with 546 nb-1
Z observed with emnn, en thadronic and mn thadronic modes
Important stepping stone toward H→tt
Yields compatible with SM expectations
09 May 2011
Srivas Prasad, Harvard University

8. W/Z+Jets Test of QCD and Monte Carlo predictions
ATLAS-CONF
ATLAS-CONF
Test of QCD and Monte Carlo predictions
NLO predictions
Matrix element and parton shower, matching
Dependence on QCD scale
Background to top, di-bosons, new physics
Jet defined with anti-kT algorithm (R=0.4)
|yjet|<2.8
pT>20 GeV for W+jet, 30 GeV for Z+jet
Jets from pileup or near leptons (DR≤0.5) removed
Background:
Mainly top at higher multiplicities
QCD especially in electron channel (estimated from data)
Bin-by-bin unfolding for detector and acceptance
Systematics: jet scale (dominant, 10-20%), pileup removal
09 May 2011
Srivas Prasad, Harvard University

9. W/Z+Jets Cross-section vs Njet
ATLAS-CONF
ATLAS-CONF
Z+jet, ee
W+jet, mn
Similar results in electron and muon channels
Good agreement with MCFM, Alpgen and Sherpa
Pythia not expected to describe distributions for more than one jet
09 May 2011
Srivas Prasad, Harvard University

10. W/Z+Jets pT spectra, multiplicities useful in background estimation
ATLAS-CONF
ATLAS-CONF
pT spectra, multiplicities useful in background estimation
Ratios constrain as, PDFs
W+jet, en
Z+jet, mm
09 May 2011
Srivas Prasad, Harvard University

11. Wg and Zg Cross-Sections
ATLAS-CONF
Highest s di-boson processes
Study triple gauge couplings (TGC)
Probe ZZg and Zgg (forbidden at tree level in Standard Model)
Signature: add isolated g to W/Z signature (isolation energy, distance from lepton)
192 (48) Wg (Zg) candidates
Signal extracted from 2D sideband using photon isolation: 80% purity g Z
Excellent agreement with SM predictions
09 May 2011
Srivas Prasad, Harvard University

12. WW Cross-section Probe electroweak theory at high energy
arXiv:
Submitted to PRL
Probe electroweak theory at high energy
Study triple gauge couplings
BSM expectations of anomalous values
Important background to H→WW
sWWNLO = [44±3] pb
Signature has opposite sign leptons, large ETmiss, reduced jet activity
Backgrounds: 1.7±0.6 events
Veto on Z mass, high pT jet
W+jets from loose lepton control
Eight candidate events seen (5 in em)
3s signal signal significance
09 May 2011
Srivas Prasad, Harvard University

13. Conclusion and Outlook
Reviewed W/Z results from ATLAS with 2010 dataset
Powerful tests of pQCD theoretical predictions; constraining PDFs
Excellent understanding of detector performance demonstrated
W/Z Observations with t made; demonstrate reconstruction
More differential measurements upcoming
First Diboson results for WW, Wg and Zg
In agreement with Standard Model expectations
Currently limited statistically
2011 data will dramatically sharpen results
Allow WZ and ZZ measurements
TGC will become interesting!
Differential W/Z measurements will further constrain PDFs
09 May 2011
Srivas Prasad, Harvard University

14. WZ Candidate Event
09 May 2011
Srivas Prasad, Harvard University

15. BACKUP
09 May 2011
Srivas Prasad, Harvard University

16. 2011 Luminosity and Collected Data
Maximum instantaneous luminosity of 8.8 × 1032 cm-2 s-1!
09 May 2011
Srivas Prasad, Harvard University

17. W→thn Candidate
09 May 2011
Srivas Prasad, Harvard University

18. Z→thadtm Candidate
09 May 2011
Srivas Prasad, Harvard University

19. Wg Event h
09 May 2011
Srivas Prasad, Harvard University