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EWK Cross-sections and Widths

EWK Cross-sections and Widths. Aidan Robson Glasgow University for the CDF and D0 Collaborations. ICHEP, Philadelphia, 30 July 2008. CDF Z  ee. (from Stirling, ICHEP04). 2004, using < 100 pb –1. A high- p T physics programme. Now:.  More challenging t channels

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EWK Cross-sections and Widths

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  1. EWK Cross-sections and Widths Aidan Robson Glasgow University for the CDF and D0 Collaborations ICHEP, Philadelphia, 30 July 2008

  2. CDF Zee (from Stirling, ICHEP04) 2004, using < 100 pb–1

  3. A high-pT physics programme

  4. Now: More challenging t channels  Differential cross-sections  High-precision measurements of Standard Model parameters

  5. Outline s (ppZ) . Br(Ztt) ds / dy ds / dpT GW GZ(invisible) More challenging t channels Differential cross-sections High-precision measurements of Standard Model parameters

  6. h = 0.6 h= 1.0 pre-radiator shower max h = 2.0 muon chambers hadronic cal  had cal EM cal solenoid EM cal had cal tracker silicon CDF and D0 D0 CDF h=1 2 1 0 Fibre tracker to |h|<1.8 Calorimeter to |h|<4 Muon system to |h|<2 h=2 Drift chamber to |h|<1 Further tracking from Si Calorimeter to |h|<3 Muon system to |h|<1.5 h=3 0 1 2 3 m

  7. W and Z selection Electrons: good EM shower shape small hadronic energy isolated in calorimeter well-matching good track (except far forward) Muons: MIP in calorimeter isolated hits in muon chamber well-matching good track Z selection: 2 oppositely-charged electrons or muons invariant mass consistent with mZ W selection: exactly one electron or muon energy imbalance in reconstructed event, associated with neutrino

  8. Taus at D0 The elements:  calorimeter cluster (cone R<0.5)  energy concentrated in inner cone R<0.3  tracks in cone R<0.3, mass<1.8GeV  EM sub-clusters in finely segmented shower-max layer of calorimeter ET measurement: For E<100GeV, track pT & calorimeter ET are combined for Types 2&3, and single p± energy corrections derived from special hadronic calorimeter simulation. Otherwise track pT used. Three D0 tau categories: Type 1: 1 track, no EM sub-cluster Type 2: 1 track, ≥1 EM sub-clusters Type 3: ≥2 tracks, ≥0 EM sub-clusters Neural net separator trained on variables measuring: isolation shower shape calorimeter–track correlations Type 1 Type 2 Type 3 0 NN output score 1 0 NN output score 1 0 NN output score 1

  9. Ztt Select Ztmth from inclusive muon trigger. Additionally:  pTm > 15 GeV  pTt > 15 GeV  opposite charge  scalar sum pT of t tracks > 15GeV or 5 GeV (types 1&3/2)  NN > 0.9 or 0.95 (types 1&3/2) Backgrounds: corrected for SS–OS ratio in QCD-enhanced sample  QCD (bb) data-driven from same-sign events  EWK backgrounds from MC W+jets corrected for component accounted for in same-sign events mvis = √(Pm + Pt + PT)2 D0 1fb–1 preliminary D0 1fb–1 preliminary D0 1fb–1 preliminary Type 1 Type 2 Type 3 visible mass / GeV visible mass / GeV visible mass / GeV

  10. Ztt Total 1527 events observed, ~20% background s (ppZ) . Br(Ztt) = 247 ± 8(stat) ± 13(sys) ± 15(lumi) pb 2.5% 3.5% 2.3% Values from previous published analysis (PRD 71 072004 (2005))

  11. P1 fq(x1) E+pz E–pz 1 2 x1P1 e+ */Z m √s x1,2 = e±y e– x2P2 P2 fq(x2) Events / 0.1 x - e+ e– CDF CDF CDF y Z rapidity Z production: rapidity y= ln (LO) closely related to parton x:

  12. Z rapidity: shapes Fraction Si tracking efficiency energy scaling factor Tracking |h’| Calorimetry |h|<0.4 0.4<|h|<0.8 Total Acceptance x efficiency 10 ET 60 10 ET 60 |h|<0.8 All h 10 ET 60 10 ET 60

  13. Z rapidity Events Isolation E • Systematics: • Detector material modeling • Background estimates • Electron identification efficiencies • Silicon tracking efficiency • Acceptance

  14. Z rapidity fractional systematic uncertainty (%)

  15. Z pT Measurement of Z pT tests QCD predictions for initial state gluon radiation resummation / parton shower with non-perturbative model pQCD reliable resummation required 0 30 pT(Z) pT(Z) multiple soft gluon radiation RESBOS event generator implements NLO QCD and CSS resummation (with BNLY form-factor) 3-parameter function, global data fit Recent global fits suggest extra small-x form-factor

  16. ppZ0Xe+e–X, LHC:√s=14TeV ppW+Xe+n, LHC:√s=14TeV ppZ0X, Tevatron:√s=1.96TeV ppZ0X, Tevatron:√s=1.96TeV ds /dpT / pb/GeV ds /dpT / nb/GeV |ye| < 2.5 pTe > 25 GeV ET> 25 GeV |y|< 2.5 pTe > 25 GeV pT (Z) / GeV Z pT No small-x broadening (BLNY) With small-x broadening No small-x broadening (BLNY) With small-x broadening ds /dpT / nb/GeV ds /dpT / pb/GeV all y |y|> 2 35 35 0 0 pT(Z) / GeV pT(Z) / GeV e+  Recent global fits suggest extra small-x form-factor – impliespT(Z) broadened at high y e– ATLAS e+ e– D0 35 35 0 0 pT (W) / GeV

  17. Z pT Selection: pT(e1), pT(e2) > 25 GeV |h|<1.1 or 1.5<|h|<3.2 70 < mee < 110 64k events, > 5k for |yZ|>2 Backgrounds: fit mee with templates 1.3–8.5% Regularized unfolding Unfolding method (input MC) Smearing parameters PDF Unfolding method (parameters) QCD background pT dependence of eff. x acc.

  18. Z pT (curves normalised) PRL 100 102002 (2008) without small-x effect: c2/dof = 11.1/11 including small-x effect: c2/dof = 31.9/11 Data does not favour small-x broadening (but no refit of usual 3 parameters has been done)

  19. Z pT The complete spectrum: PRL 100 102002 (2008) Compare 4 models: Resbos with default parameters Resbos with additional NLO–NNLO K-factor NNLO (Melnikov and Petriello) NNLO rescaled at to data at 30GeV/c

  20. W width GW predicted in Standard Model: GWSM = 2091±2 MeV (PDG) Experimentally have access to transverse quantities:

  21. W width  Generator: LO MC matched with Resbos (QCD ISR) and Berends/Kleiss (QED FSR) c2/dof=27.1/22  Fast simulation for templates: electron conversions + showering muon energy loss parametric model of recoil energy (QCD, underlying event + brem)  Tracking scale/resn DG= 17 MeV, 26 MeV mmm (GeV) DG= 54 MeV (ele), 49 MeV (mu)  Backgrounds c2/dof=18/22  Calorimeter scale/resn DG= 21 MeV, 31 MeV DG= 33 MeV DG= 32 MeV mee (GeV) mT (GeV) mT (GeV)

  22. W width PRL 100 071801 (2008) World most precise single measurement GW = 2032 ± 73 (stat+sys) MeV Compare to CDF indirect measurement: (GWSM = 2091 ± 2 MeV) SM value GW (indirect) = 2092 ± 42 MeV NNLO calc From LEP J Phys G 34 2457

  23. Z invisible width GZ(invisible) measured very precisely indirectly from LEP: GZ(invis) = 500.8 ± 2.6 MeV However combined direct LEP measurement: GZ(invis) = 503 ± 16 MeV Tevatron measurement uncorrelated, relies less strongly on theoretical inputs Select single-jet events (ET trigger) Independently measure s(Z+1jet) . Br (Z  ll) from high-pT lepton trigger Selection:  ET > 80 GeV  ETjet > 80 GeV second jet ET<30 allowed but ≥ 3 jets ET>20 rejected GZ(inv) GZ(ll) s(Z+1jet) . Br (Z  inv) s(Z+1jet) . Br (Z  ll) = Nobs – Nbck s(Zll+1jet) . L = Must be corrected for different acceptance of ‘1-jet’ selection in Znn versus Zll (applied after lepton removal)

  24. Z invisible width EWK ‘background’ includes Znn prediction Measureds(Zll+1jet) = 0.555 ± 0.024 pb GZ(inv) GZ(ll) = 5.546 ± 0.506 ~ equal contributions from EWK bck, QCD bck and s(Zll+1jet) GZ(inv) = 466 ± 42 MeV Nn = 2.79 ± 0.25

  25. Summary W and Z cross-section measurements underpin the Tevatron high-pT physics programme Dedicated measurements are harnessing the high statistics datasets:  improving tau identification  testing higher-order calculations and PDFs and probing QCD  making precision measurements of SM parameters

  26. Backup Backup

  27. longer Q2 extrapolation smaller x (W James Stirling)

  28. D0 Z rapidity

  29. CDF ds/dy

  30. ppZ0X, Tevatron:√s=1.96TeV ppZ0X, Tevatron:√s=1.96TeV ds /dpT / nb/GeV all y |y|> 2 ds /dpT / pb/GeV pT(Z) / GeV Z pT • Nadolsky et al: • global fits to HERA and fixed-target data suggest increased intrinsic pT carried by proton constituents, for interactions involving only a small fraction of proton’s momentum  they insert extra factor in differential cross-section ; pT(Z) broadened at high y e+ e– CDF

  31. ppZ0Xe+e–X, LHC:√s=14TeV ppW+Xe+n, LHC:√s=14TeV ds /dpT / nb/GeV ds /dpT / pb/GeV |ye| < 2.5 pTe > 25 GeV ET> 25 GeV |ye| < 2.5 pTe > 25 GeV pT (Z) / GeV e+ e– ATLAS Z pT • LHC: beam energies ~7x higher than Tevatron • Probing new part of phase space • Tevatron forward detectors map on to LHC central detectors • Problem! W/Z production is a benchmark

  32. Z pT C-P Yuan

  33. Indirect GW

  34. without small-x effect: pT < 5GeV/c: c2/dof = 0.8/1 pT<30GeV/c: c2/dof = 11.1/11 including small-x effect: pT < 5GeV/c: c2/dof = 5.7/1 pT<30GeV/c: c2/dof = 31.9/11

  35. u u p d d u p u W± l± n

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