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W Boson production

W Boson production . and properties at CDF. Emily Nurse. University College London. EPS, Manchester, July 2007. W production at the Tevatron. C.O.M energy = 1.96 TeV. W events can be used to: constrain the QCD part of the production mechanism: W charge asymmetry .

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W Boson production

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  1. W Boson production and properties at CDF Emily Nurse University College London EPS, Manchester, July 2007 W production and properties at CDF

  2. W production at the Tevatron C.O.M energy = 1.96 TeV W events can be used to: • constrain the QCD part of the production mechanism: W charge asymmetry. • measure W properties: W mass(see talk by Oliver Steltzer-Chilton) and W width. The electron and muon channels are used to measure W properties, due to their clean experimental signature. The large mass (~80 GeV ) of W bosons gives their decay products large pT. W production and properties at CDF

  3. Detecting W decay particles at CDF Electrons:detected in central trackers (p measurement) and EM calorimeter (E measurement). Muons:detected in central trackers (p measurement), calorimeter (MIP signal) and muon chambers. Neutrinos:not detected! Vector sum the total transverse energy (ET=Esin) in calorimeters. “Missing ET” inferred as neutrino pT. S I L I C O N DRIFT CHAMBER S O L E N O I D EM HADRONIC MUON CHAMBERS e   TRACKERS CALORIMETERS W production and properties at CDF

  4. W charge asymmetry:introduction W+ W- y antiproton proton Parton Distribution Functions describe the momentum distribution of partons in the (anti-)proton. They are constrained by fits to data. Q2 = 100 GeV2 MRST2004NLO xf(x,Q2) u Fraction of proton momentum carried by quark d x 10-3 10-2 10-1 1 e+ u d W+ p p d u u u e (helps constrain PDFs!) W production and properties at CDF W production and properties at CDF

  5. W charge asymmetry: from e+(-) asymmetry • Since pL is not known the W rapidity cannot be reconstructed  traditionally the electron charge asymmetry is measured. • The V-A structure of the W+(-) decay favours a backward (forward) e+(-) “diluting” the W charge asymmetry. W production and properties at CDF

  6. W charge asymmetry: direct measurement • This CDF measurement (performed in We channel): • pL determined by constraining MW = 80.4 GeV two possible yW solutions. Each solution receives a weight probability according to: • V-A decay structure • W cross-section:  (yW) (Process iterated since  (yW) depends on asymmetry) W production and properties at CDF

  7. W width :introduction • Predicted very precisely within the Standard Model by summing the leptonic and hadronic partial widths: WSM = 2091 ± 2 MeV [PDG: J. Phys. G 33, 1] • Deviations from this prediction suggest non-SM decay modes. • W is an input to the W mass measurement: MW ~ W / 7 Ideally we would reconstruct the invariant mass of the decay products to measure W, since  isn’t detected we reconstruct the transverse mass: W MW W production and properties at CDF

  8. W width :Analysis strategy • Simulate mT distribution with a dedicated fast parameterised MC (using a Breit-Wigner lineshape). • MC simulates QCD (RESBOS Balazs et.al. PRD56, 5558) and QED (Berends & Kleiss Berends et.al. ZPhys. C27, 155) corrections. • Utilise Zll(and W l) data to calibrate the detector to a high precision. • Fit mT templates (withWvarying) to the data, fit range: 90-200 GeV optimised to reduce total uncertainty W production and properties at CDF

  9. W width :Lepton pT e energy measured in EM calorimeter - scale and resolution calibrated using Zee resonance and E/p in We data.  momentum measured in central tracker - scale and resolution calibrated using Z resonance. W production and properties at CDF

  10. W width :Neutrino pT • pT inferred from ETmiss • U=(Ux, Uy)=towersEsin (cos,sin) vector sum over calorimeter towers Excluding those surrounding lepton • pT = ETmiss = -(U + pTlep) • U mostly comes from gluon radiation from incoming quarks (also Underlying Event (UE), photons from bremsstrahlung). • Accurate predictions of U (QCD radiation, UE, hadronisation and detector response to hadrons) is difficult (and slow) from first principles. • Devise an ad-hoc parameterised model of the recoil in terms of the boson pT, tuned to the recoil in Zll events (where the Z pT can be reconstructed). W production and properties at CDF

  11. W width: results W = 2032  71 (stat + syst) MeV W production and properties at CDF

  12. W width: systematic uncertainties W production and properties at CDF

  13. W width:world average World’s most precise single measurement! Central value decreases by 44 MeV: 2139  2095 MeV Uncertainty reduced by 22%: 60  47 MeV W production and properties at CDF

  14. Indirect Width Measurement CDF Run II INDIRECT width (72 pb-1): 2092 ± 42 MeV CDF Run II DIRECT width (350 pb-1): 2032 ± 71 MeV preliminary PRL 94, 091803   BR (Wl) Rexp =   BR (Zll) Precision LEP Measurements SM Calculation NNLO Calculation W (Wl) (Z) R = X X Z (W) (Zll) W production and properties at CDF

  15. Summary • W charge asymmetry measurement - will help constrain PDFs (due to be updated this summer). • W width measurement (world’s most precise measurement!) - consistent with the Standard Model prediction and indirect measurement. W production and properties at CDF

  16. Back-up slides W production and properties at CDF

  17. e asymmetry || || W production and properties at CDF

  18. Hadronic Recoil: U • Accurate predictions of U is difficult (and slow) from first principles. • U simulated with ad-hoc parameterised model, tuned on Zll data. • U split into components parallel (U1) and • perpendicular (U2) to Z pT • 7 parameter model describes the response and resolution in the U1 and U2 directions as a function of the Z pT. • Systematic comes from parameter uncertainties (limited Z stats). U2 U1 response resolution Data MC Zee Zee W production and properties at CDF

  19. Hadronic Recoil: U (pT) U|| U • U split into components parallel (U||) and • perpendicular (U) to charged lepton. • Many distributions used to cross check the model in Wl data: We We Data MC W production and properties at CDF

  20. Backgrounds x x x x K, fake high-pT track x x x x electron and muon channel: muon channel only: Zll W Decay In-Flight • Kaon/pion decays “In-Flight” to a . • Kink in track gives high-pT measurement. • ETmiss from mis-measured track pT. • One lepton lost • ETmiss from missing lepton • decays to e/ • intrinsic ETmiss multijet • Jet fakes/contains a lepton • ETmiss from misconstruction Need the mT distributions and the normalisations! W production and properties at CDF

  21. Backgrounds W production and properties at CDF

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