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The Measurement of W ’s at the CERN and FNAL hadron colliders

The Measurement of W ’s at the CERN and FNAL hadron colliders. W ’s at RHIC ! W ’s at CERN – UA2 W ’s at FNAL - CDF. W ’s at RHIC !. Measurement of W ’s in polarized-proton collisions at RHIC:

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The Measurement of W ’s at the CERN and FNAL hadron colliders

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  1. The Measurement of W’sat the CERN and FNAL hadron colliders • W’s at RHIC ! • W’s at CERN – UA2 • W’s at FNAL - CDF M. Rijssenbeek

  2. W’s at RHIC ! • Measurement of W’s in polarized-proton collisions at RHIC: • measure the u and d-quark and anti-quark contributions to the proton spin as function momentum fraction x. • use the W charge and the V–A structure of W production & decay to eν and μν to select quark flavor and quark helicity. For W–: u↔d M. Rijssenbeek

  3. Non-Hermetic Detectors • RHIC detectors are not fully hermetic… • electron and muon acceptance of the RHIC detectors varies strongly over the rapidity range… • Thus: • missing transverse energy cannot be used to clean up the W signal • Trigger has to rely exclusively on high pT electrons and muons • The backgrounds to the W from Z→e/μ and QCD/fakes may be significant • enough Z-statistics for measurement/tuning of corrections? • Detailed simulations will be crucial to determine acceptance corrections, efficiencies, and backgrounds • limited Z acceptance makes tuning of the simulations with Z→ee/μμmore difficult M. Rijssenbeek

  4. a “Non-Hermetic” Detector: UA2 – vs.1 • Central tracking + Preshower + EM Calorimetry; |η|<1 • Forward spectrometer + PS + EM Calorimetry; 1<|η|<3 a non-hermetic detector… UA2 collaboration: M Banner et al., Phys. Lett. 122B (1983) 476. UA2 collaboration: P Bagnaia et al., Phys. Lett. 129B (1983) 130. M. Rijssenbeek

  5. W and Z in UA2 • strong quality selections on electron candidates necessary: • isolation, shower shape, preshower signal, track-PS-cluster match • even so: cut on missing pT for final sample… W Z all good EM cluster pairs W + Track & PS match M. Rijssenbeek

  6. W and Z in UA2 – vs.2 • 1987: UA2 Upgrade program with hermetic calorimetry UA2 collaboration: J.Alitti et al., Z.Phys.C47 (1990) 11. QCD M. Rijssenbeek

  7. W‘s at Fermilab • the measurement of the W mass with a 0.05% accuracy (50 MeV or 100×Me) requires the ultimate understanding of the detector! •  simulations and cross checks & tuning with the data itself… M. Rijssenbeek

  8. Simulations • the state of simulations of W and Z production (and decay) has much advanced over the past decades • forced by very high statistics W/Z samples for mass determination from LEP and Tevatron • much QCD calculational progress • improved detector simulations: showering • availability of raw computing power allows more detail and increased sophistication • State-of-the-Art: • RESBOS: NLO W and Z production • CTEQ6M: NLO pdf’s with uncertainties • Note:pTe/μis quite sensitive to boson recoilMTless so, but is sensitive to yW/Z • CRUCIAL for all precision W/Z measurements M. Rijssenbeek

  9. FNAL Example: CDF W Mass(from seminar by Dr. David Waters, UC London) • CDF is a modern hermetic detector: hermetic detection of e, γ, jets; less hermetic for μ • recent MW measurement (e+μ) is single best in the world:MWCDF = 80413 ± 34 (stat) ± 34 (syst)MeV CDF Note 8665, Jan 17, 2007; CDF http://www-cdf.fnal.gov/ cfr: WA 2006: 80392±29 MeV M. Rijssenbeek

  10. pT distribution Leading Order picture:  W/Z Production in pp & Decay Modeling + Corrections: Higher orders (EW,QCD) Non-perturbative rapidity distribution angular & mass distributions : M. Rijssenbeek

  11. W Production Modeling: pT • Use the best theoretical model on the market : • RESBOS NLO QCD + resummation + non-perturabtive. • Constrain the parameters g1, g2, g3 and lineshape with the Z data: (Landry et al., 2003) W : 7 MeV MW : 3 MeV M. Rijssenbeek

  12. W→μν Backgrounds • mostly Z→μμ: easy to lose a muon (at CDF !) • But can estimate this background very reliably. M. Rijssenbeek

  13. x x x x K, fake high-pT track x x final cut value x x /NDF Decay-In-Flight Background in W→μν • difficult background: very flat in transverse mass • Use track quality: 2 and track impact parameter Zprovides the template for real muons Vary normalization & shape: W : 27 MeV High impact parameter cuts provide the DIF template MW : 5 MeV M. Rijssenbeek

  14. W→eν Backgrounds QCD background dominates Z’s ~ negligible M. Rijssenbeek

  15. QCD Background in W→eν • Multijet events: large σ Rfake(jete)  Rfake(jetET) Vary normalization & shape: W : 32 MeV MW : 7 MeV QCD template from a background-rich “anti-electron“ sample final cut value M. Rijssenbeek

  16. CDF W Mass 2007: • sample size analyzed: 200/pb • event selection: pTe,μ>30 GeV, pTν>30 GeV • Uncertainties in MW for fit to pTe/μ: M. Rijssenbeek

  17. W/Z Physics result: Van Neerven Plot: • MW/MZ with Mtopconstrain the SM Higgs range;200/pb • Current data sets:2/fb (CDF, DØ) • FNAL Expectation: δMW≈ 30 MeV/expt M. Rijssenbeek

  18. My Conclusions • A precise measurement of Δu and Δd will bring new understanding of the spin structure of the baryons… • However:in order to obtain the required precision, the measurement will need sophisticated simulations to understand and model the detector acceptance, efficiencies, and backgrounds • the physics and detector models must be tuned and checked with measurements of the Z (<10% of W statistics) • Dominant backgrounds must be measured with the data itself these simulations must be done beforehand to prove the measurement capability with RHIC’s “non-hermetic” detectors… M. Rijssenbeek

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