Presentation on theme: "W/Z PRODUCTION AND PROPERTIES Anton Kapliy (University of Chicago) on behalf of the ATLAS collaboration PHENO-2012."— Presentation transcript:
W/Z PRODUCTION AND PROPERTIES Anton Kapliy (University of Chicago) on behalf of the ATLAS collaboration PHENO-2012
ATLAS measurements 2 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.
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 3
Monte-Carlo samples 4 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.
Fiducial cross-sections 5 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.
Lepton universality 6 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. R W (ATLAS) World Average 1.006±0.024 1.0017±0.019 R Z (ATLAS) World Average 1.018±0.031 0.9991±0.0024
Cross-section ratios 7 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 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 |η l |
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)
Strange quark sea density: results 11 pQCD evolution Existing NNLO and NLO PDFs show tension with the new ATLAS prediction for strange quark density at Q 2 =1.9 GeV 2 Uncertainties are smaller at Q 2 =M Z 2 because gluon splitting probability is flavor independent, reducing any initial flavor asymmetries. However, the same tension remains
Incorporation of strange density results 12 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
W polarization 13 W boson has 3 helicity states, with fractions f L, f R, f 0 (longitudinal) Different angular kinematics of W decay products → template fit – cos(θ 3D ) – angle between W (lab frame) and lepton (W rest frame) To avoid neutrino p z ambiguity, we measure transverse cos(θ 2D ) – Unfolding to “full helicity” using Monte-Carlo information Separate measurements in two bins of p T W : – p T W >50 GeV – p T W =35..50 GeV Definition of θ 2D J-B. Blanchard
W polarization - results 14 Results are averaged over lepton flavors and charges. Good agreement with theoretical predictions: Low p T W bin: compatible with both MC@NLO and POWHEG Also cross-checked with Sherpa (not shown) High p T W bin: data favors somewhat lower values of f 0
W/Z + jets 15 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 p T of first jet, W+jetsDijet mass of two leading jets, Z+jets
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. 18