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Prospects for Early Measurements of W and Z Bosons with CMS at the LHC
Jeffrey Berryhill (Fermilab) The CMS Collaboration PANIC ’08, Eilat, Israel November 11, 2008 W and Z inclusive cross sections (10 pb-1) W lepton rapidity asymmetry (10, 100 pb-1) Z + bb cross section (100 pb-1)
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Conclusion “Everyone has a plan… until they get punched in the face.”
--old boxing maxim
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Electroweak Bosons as a Precision Calibration Tool
For Standard Model physics: Inclusive cross sections compared against initial 10% precision (lumi) …or estimate parton luminosity itself to ~5% (theory) Cross section ratios estimated at ~1% (PDF) …or further constrain PDFs themselves once ~% precision achieved For detector performance: Commissioning leptons for triggering, reconstruction, and identification for PT from 20 to 60 GeV measuring performance at the % level First look at efficacy of missing ET for analysis (real or instrumental) Associated production with jets an ubiquitous background for searches
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Inclusive W and Z Production Cross Sections
At TeV, initiated by an admixture of both valence-sea and sea-sea quark annihilation at HERA-like x (10-5 to 10-2) Compared to Tevatron: Cross section 4-6X larger Acceptance for |hl| < 2.5 is 2X better for W, 1X for Z 100k W’s and 10k Z’s for 10 TeV W → l n s×BR (pb) N×A/pb-1 1.96 TeV pp 2700 2*600 10 TeV pp 11800 2*6000 14 TeV pp 17200 2*9000 Z→ l l 250 2*80 1200 2*400 1800 MRST, Eur.Phys.J. C14 (2000)
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Inclusive W and Z Production Cross Sections : Z Selection
Trigger, reconstruction, and ID efficiency for single leptons can be measured directly from data with Z → ll “tag and probe” samples Electron trigger efficiency Z →ee Single electron trigger w/isolation 2 high ET electrons (ET>20.0 GeV, |h| < 2.5) 70 GeV < Μee < 110 GeV Track Isolated: Σ(PT/PTe)2<0.02 in a cone of 0.6 Electron Id: Had/EM energy (Δη, Δφ) track/cluster matching , σηη shower shape Z→ mm Single muon trigger 2 high PT muons (PT>20.0 GeV, |η|<2.0) Mμμ > 40 GeV Hits from Tracker + Muon Chambers Track Isolated: ΣPT<3 GeV in a cone of 0.3 Muon trigger efficiency NOT using precision tracking cuts (e.g. ds) in 10 pb-1 startup era
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Inclusive W and Z Production Cross Sections: Z results
4600 ee (1.5% stat), 5300 mm (1.4% stat) selected Backgrounds < sqrt(S) (i.e. any subtraction error is subleading) Mean lepton efficiencies known to 1% (stat) or better Efficiency corrected signal precision comfortably below 5% (useful for lumi)
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Inclusive W and Z Production Cross Sections: W selection
W →mn One muon (PT>25.0 GeV, |η|<2.0) Veto 2nd muon w/ PT> 20 GeV Track Isolated: ΣPT/PTμ < 0.09, ΔR<0.3 Acoplanarity and multijet vetos Background is primarily bb dijet and Z →mm Transverse mass separates W signal from bb above ~50 GeV Non-isolated muons an unbiased template for background transverse mass Z →mm sample used to describe missing ET → an unbiased template for W signal MT Use simple “four-square” (iso/noniso vs. low/high MT) calculation, or a multi-bin likelihood fit, to extract signal yield from MT.
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Inclusive W and Z Production Cross Sections: W selection
W →en One electron (ET>30 GeV, |h|<2.5) Veto 2nd electron w/ ET> 20 GeV Tighter Track Isolation: no tracks with PT>1.5 GeV in a cone of ΔR<0.6 Tighter electron Id QCD background is admixture of bb, light dijet, and photon+jet Non-isolated leptons give unbiased estimator of missing ET for isolated leptons, Z→ ee gives missing ET for W signal Four-square method (iso/noniso vs. low/high missing ET) gives marginally unbiased signal yield Need tight ID/Iso cuts to minimize S/B at expense of S/sqrt(S+B)
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Inclusive W and Z Production Cross Sections : W Results
28k e, 64k m selected, B/S ~ 25% without missing ET cut, <13% with cut Systematics from QCD background subtraction biases comparable to sqrt(S+B)/S Efficiency corrected signal yield precision ~1% for muons, ~3% for electrons Background rejection from missing ET and lepton ID crucial to performance
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W Lepton Rapidity Charge Asymmetry
W production in pp collisions is globally charge asymmetric (pp has more ud than du collisions due to valence quark content of p): (W+- W-)/(W+ + W-) = total cross section asymmetry Also differentially charge asymmetric in lepton rapidity due to d/u pdf asymmetry vs. x and V-A decay asymmetry of W For leptons in CMS W acceptance, A varies from +10 to +20% Precision at 1% or less, sampled in several rapidity bins, provides PDF constraint tighter than global fits
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W Lepton Rapidity Charge Asymmetry
Select same W →mn events as for the inclusive cross section: PT(m) > 25 GeV, |h(m)| < 2.0, MT > 50 GeV Muon triggering, reconstruction, and ID asymmetry known to <1% from tag-and-probe Major backgrounds (QCD, DY) symmetric, w/uniform dilution (20%) vs. h(m) Detector misalignment/miscalibration biases reduced to 1% by 100 pb-1
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W Lepton Rapidity Charge Asymmetry
For 20 bins in 0.1 of eta, differential asymmetry known nearly uniformly to 2%(stat) per pb-1, an interesting precision cross check <1%(stat) per pb-1 would improve the PDFs Further studies: do electrons help (esp. for |h| > 2.0)? Can W boson rapidity be measured reliably?
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Z + bb Production Cross Section
MCFM cross is 46 pb, LO diagrams via gg fusion and quark-initiated Z+split g Important background to SM heavy Higgs→ 4l and light SUSY Higgs searches Jumping off point for W+bb, Z+b, et al. Lepton selection: lepton pairs selected as in the inclusive Z analysis Jet selection: >= 2 jets with corrected ET > 30 GeV and |h| < 2.4 Missing ET > 50 GeV veto to reduce top quark background Dilepton mass, no b-tagging
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Z + bb Production Cross Section: Selection
b-tagging: 2 jets with a good value wrt a simple discriminator (3D IP significance of 3rd highest IP sig. track) Remaining background is predominantly top, which is estimated using dilepton mass sidebands b-tagging efficiency and udsg mistagging from m+jet control samples, c-mistagging from MC b jet signal Signal yield corrected for b-tag efficiency and Z+jj,cc backgrounds
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Z + bb Production Cross Section : Results
46 Z+bb events selected in 100 pb-1, with top background at 30% level Biggest systematics are b-tagging efficiency (±16%), JES (±8%), lumi (10%), MC model (-10%), missing ET model (7%) Significant but systematics limited signal with 100 pb-1
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Conclusion With first 10 pb-1 of pp collision data:
W and Z production cross section measured and dominated by luminosity error (~10%) in electron and muon channels. Z counting suitable as ~5% estimator of luminosity, dominated by theory error. W a bit more challenging due to QCD background. Lepton trigger/reconstruction/ID tuned and measured to 1% level With 100 pb-1 of pp collision data: Novel PDF constraints from W asymmetry Abundant W and Z + jets signals, including Z + heavy flavor
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