W/Z PRODUCTION AND PROPERTIES

Anton Kapliy (University of Chicago)on behalf of the ATLAS collaboration

PHENO-2012

2ATLAS 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.

3W/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

4Monte-Carlo samples

Di-electron invariant mass W transverse mass (muon channel)

• 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.

5Fiducial 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.

6Lepton universalityRatio 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.

RW (ATLAS) World Average1.006±0.024 1.0017±0.019

RZ (ATLAS) World Average1.018±0.031 0.9991±0.0024

7Cross-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.

Differential cross-sections 8

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|

W charge asymmetry 9

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)

10Strange 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 Q0

2=1.9 GeV2

• ATLAS measurements can improve this!– Parameterize qi(x) and g(x) at Q0

2=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 Q0

2

• To visualize the results, rs is reported for existing and fitted PDFs at Q2=Q0

2 , x=0.023 and Q2=MZ2

, x=0.013

11Strange 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

12Incorporation 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

13W 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

14W polarization - resultsResults are averaged over lepton flavors and charges.Good agreement with theoretical predictions:• Low pT

W 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

15W/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

Jet multiplicity 16

• 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

W+jets Z+jets

Kinematic properties 17

• Well reproduced NLO and LO(ME+PS) predictions

pT of first jet, W+jets Dijet mass of two leading jets, Z+jets

18Conclusions

• 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.

BACKUP SLIDES

19

Inclusive cross-section values 20

Common fiducial region Extrapolation to total phase space

Inclusive cross-section systematics 21

Electron channel: Muon channel:

22W polarization

cos(θ3D) vs cos(θ2D) in MC:

Systematics for pTW>50 GeV bin

23W+jets: backgroundsUncorrected Njets distributions in muon and electron channels

24W/Z+jets: systematics

Jet energy scale systematic dominates for Njets > 0

Electron channel, at least one jet