W/Z PRODUCTION AND PROPERTIES

1. W/Z PRODUCTION AND PROPERTIES Anton Kapliy (University of Chicago) on behalf of the ATLAS collaboration PHENO-2012

2. ATLAS measurements 2010 dataset • Results shown in this talk use the 2010 dataset (~35 pb-1). • Work ongoing to update precision EWK measurements to the full 2011 dataset.

3. W/Z cross-sections: why and how • Precision tests of pQCD at NNLO • Sensitivity to Parton Distribution Functions (PDFs) • Clean signature: leptons and missing energy • 2010 dataset provides a rich sample of W’s and Z’s: • ~270k W candidates • ~24k Z candidates • 2010 measurement has a total uncertainty on W and Z cross-sections on the order of a few percent. • At NLO, variation of renormalization and factorization scales alone introduces a ~3% uncertainty on the theoretical predictions (down to ~0.5% at NNLO) • FEWZ and DYNNLO programs are used with several NNLO PDF sets

4. Monte-Carlo samples • In general, Monte-Carlo shows remarkable agreement with data. • It is corrected for pileup, reconstruction/identification efficiency, and energy scale and resolution • Electroweak backgrounds are taken from MC; QCD is data-driven. Di-electron invariant mass W transverse mass (muon channel)

5. Fiducial cross-sections • Electron and muon channels combined in common fiducial region • Include correlations between all three channels: W+, W-, Z • Cross-sections are also extrapolated to the full phase space: • This introduces a sizeable acceptance uncertainty • Precision measurements are more sensitive in the fiducial region.

6. Lepton universality RW (ATLAS) World Average 1.006±0.024 1.0017±0.019 RZ(ATLAS) World Average 1.018±0.031 0.9991±0.0024 • Ratio of electron and muon cross-sections is performed in a common fiducial region. • Many uncertainties are correlated between electron and muon channels and cancel in the ratio: • Luminosity • Theory extrapolation (acceptance) • Electroweak backgrounds • Pileup effects, etc • e-µ universality is confirmed in both W and Z ratios.

7. Cross-section ratios Some systematics (including luminosity uncertainty) cancel in the ratio of W±/Z and W+/W- cross-sections. Overall, there is good agreement with NNLO predictions. We also see sensitivity to different PDF sets, especially in the W+ to W- ratio.

8. Differential cross-sections Vector boson rapidity is sensitive to PDFs: For W’s, it’s easier to work with lepton pseudorapidity (ηl). • Overall, there is good agreement with NNLO predictions, although there is tension with some PDF choices: • JR09 predicts lower Z cross-section at central rapidities • ABKM09 overshoots W cross-section at larger |ηl|

9. W charge asymmetry • Motivation: • Alternative probe of PDF structure • Sensitive to valence quarks at Bjorken x ≈ 0.001 – 0.1 • This region is not strongly constrained by HERA. • Many systematics cancel! • Measurement: • Largely statistically limited • ABKM09 and HERAPDF give the best agreement. • ATLAS+CMS+LHCb combination: • First useful PDF constraint from LHC (→NNPDF2.2)

10. Strange quark sea density • Flavor SU(3) suggests the three light sea quark distributions are equal. This implies: • Because strange is heavy, it may be suppressed • Current PDFs assume rs≈ 0.5 at initial scale Q02=1.9 GeV2 • ATLAS measurements can improve this! • Parameterize qi(x) and g(x) at Q02=1.9 GeV2 (15 pars) • Perform NNLO fit to HERA DIS and ATLAS W+, W-, Z data • Unlike existing PDFs, don’t assume s(x)/d(x) ≈ 0.5 at Q02 • To visualize the results, rsis reported for existing and fitted PDFs at Q2=Q02 , x=0.023 and Q2=MZ2, x=0.013

11. Strange quark sea density: results pQCD evolution • Existing NNLO and NLO PDFs show tension with the new ATLAS prediction for strange quark density at Q2=1.9 GeV2 • Uncertainties are smaller at Q2=MZ2 because gluon splitting probability is flavor independent, reducing any initial flavor asymmetries. • However, the same tension remains

12. Incorporation of strange density results • As a consequence of enhanced strange quark density at low x, the total light sea is enhanced by about 8% (left figure) • NNLO prediction for the ratio of W to Z cross-sections is in nearly perfect agreement with data when using the PDFs that incorporate the new strange sea density results. Total light sea with suppressed strange density (s/u fixed at 0.5), versus best-fit s/u = 1.0

13. W polarization • W boson has 3 helicity states, with fractions fL, fR, f0 (longitudinal) • Different angular kinematics of W decay products → template fit • cos(θ3D) – angle between W (lab frame) and lepton (W rest frame) • To avoid neutrino pz ambiguity, we measure transverse cos(θ2D) • Unfolding to “full helicity” using Monte-Carlo information • Separate measurements in two bins of pTW: • pTW>50 GeV • pTW=35..50 GeV Definition of θ2D J-B. Blanchard

14. W polarization - results • Results are averaged over lepton flavors and charges. • Good agreement with theoretical predictions: • Low pTW bin: compatible with both MC@NLO and POWHEG • Also cross-checked with Sherpa (not shown) • High pTW bin: data favors somewhat lower values of f0

15. W/Z + jets • Precise tests of pQCD calculations and MC programs: • Novel Blackhat-Sherpa NNLO calculation • General-purpose Parton Shower (PS): Pythia • Matrix Element + Parton Shower (ME+PS): Alpgen, Sherpa • Validation of detector performance and our understanding of a wide range of backgrounds • Important irreducible background to new physics

16. Jet multiplicity • Pythia predictions (PS) fail at high jet multiplicity • Alpgen (ME+PS) describes data well • Sherpa (ME+PS) has some tension with data • NLO calculation (Blackhat-Sherpa) is superbly accurate Z+jets W+jets

17. Kinematic properties • Well reproduced NLO and LO(ME+PS) predictions pT of first jet, W+jets Dijet mass of two leading jets, Z+jets

18. Conclusions • Inclusive and differential measurements of W/Z cross-sections provide stringent tests for electroweak theory predictions • Already with 2010 data, we are able to provide useful constraints for Parton Distribution Functions • W/Z production in association with jets show remarkable agreement with NLO and LO (ME+PS) predictions for events with up to 4 jets. • Much larger 2011 dataset will explore more differential distributions and substantially expand kinematic reach of the measurements.

19. Backup slides

20. Inclusive cross-section values Common fiducial region Extrapolation to total phase space

21. Inclusive cross-section systematics Electron channel: Muon channel:

22. W polarization cos(θ3D) vscos(θ2D) in MC: Systematics for pTW>50 GeV bin

23. W+jets: backgrounds Uncorrected Njets distributions in muon and electron channels

24. W/Z+jets: systematics Jet energy scale systematic dominates for Njets > 0 Electron channel, at least one jet