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Vector Boson Fusion: Analyzing Higgs Decay to WW in ATLAS at CERN

This report discusses the detailed analysis of Higgs boson production via vector boson fusion (VBF) and its decay into WW final states within the ATLAS experiment at CERN. Focusing on event selection criteria, tracking of high-energy jets, and background estimation techniques, we outline the methods used to identify clean events indicative of Higgs decay. Key aspects include the analysis of charged leptons' correlations, the significance of the no-lose theorem in WW scattering, and background dominance by ttbar processes. Our findings emphasize the importance of effective background extrapolation for precise measurements.

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Vector Boson Fusion: Analyzing Higgs Decay to WW in ATLAS at CERN

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  1. Top background inVBF H  WW (ll) Max Baak, CERN Atlas CAT top meeting 8 August ‘08

  2. Vector boson fusion: H  W+W-  ll VBF H  WW (ll) • Clean events: color-coherence between initial and final state W-radiating quarks  suppressed hadronic activity in central region • Spin zero Higgs: charged leptons prefer to point in same direction. • Two forward, high-Pt jets from WW fusion process (“tagging jets”).

  3. Event selection • Good muon: • Muid, pT > 15 GeV/c, ||<2.5 • Good electron • pT > 15 GeV/c, ||<2.5, isEM=0, overlap removal with good muons, R>0.2 • Good jet: • C4Tower, pT > 20 GeV/c, ||<4.8, overlap removal with good muons/electrons, R>0.4 • Tagjets: • 1: Highest pT good jet, 2: Highest p good jet + require jj>2.5, m(jj) > 520 GeV/c2 • Higgs: • 2 good leptons between tagjets, opposite charge • mT(H) > 30 GeV/c2, m(ll) < 300 GeV/c2 • Basic cuts around the Z mass • Event selection • MET>20 GeV, 2 good leptons, 2 or 3 good jets • No higgs mass dependent cuts • Selection not optimized. Regular cuts ...

  4. Higgs production x-sections (NLO) requirement: lepton → e / mu • HWW: signifcant discovery potential over wide mass range (>130) • VBF: second significant production mechanism for Higgs at LHC • Expect ~ 40 reconstructed Higgs events / fb (@ 170 GeV/c2) ggH  WW ll VBF H  WW ll

  5. Some interesting facts • “No-lose” theorem applies to W-W scattering: something must show up below mWW < 500 GeV/c2 to avoid unitarity violation. • Main background components • 90% ttbar production, mostly dilepton channel • Some W(W) + jets, QCD & EW  • Great synergy with top reconstruction

  6. Higgs transverse mass • mT(H) : calculated like normal transverse mass • Assume that : m() = m(ll) m(H)TRUE = 170 GeV/c2 tauL : missing momentum in z tauR : m(ll) = m() sigmaC: missing Et

  7. Higgs (ll), (ll) Higgs W+ W–  l+ l–  (ll) • WW comes from spin zero Higgs: charged leptons prefer to point in same direction. Define angle in transverse plane ll . • Significant fraction of various backgrounds does not have (anti-)correlated W spins. (ll)

  8. Control sample: ttbar background ttbar background • Dominant background contribution in VBF H→WW analysis • Extrapolation of di-lepton ttbar background into signal box using b-tag information. • Extrapolation of di-lepton background from semi-leptonic bkg.

  9. Sample categories BVeto sample BTag sample • Fit variables: (ll), (ll), mT(H) • Higgs events mostly end up in BVeto-sigbox • Use other boxes to extrapolate bkg description into BVeto-sigbox. • Both shapes and normalization • Four possible background sample approximations: • 1  3, or 1  2 • 1  3  correction_factor(2/4), or 1  2  correction_factor(3/4) (ll) (ll) 4 2 3 1 sigbox sigbox sideband sideband (ll) (ll)

  10. Ttbar bkg (ll), (ll) • Use BVeto-sigbox for complete background estimate. • 1  3 4 2 • BVeto-sigbox • – Projection from BTag-sigbox – 1 /fb 3 1

  11. Bkg mT(H) 4 2 • Leptons not affected by b-tag. • Use BVeto-sideband for 1st order background estimate. • 1  3  correction_factor(2/4) 3 1 Smoothed correction factor

  12. Bkg mT(H) • Projection onto BVeto-sigbox ... • BVeto-sigbox – Projection from BTag-sigbox – Projection from BTag-sigbox, with correction factor

  13. Ttbar bkg discrimination • Ttbar bkg dominates signal when Bweight > 5 ttbar Higgs

  14. Extrapolation • Reconstructed transverse mass for selected ttbar events High Bweight Medium Bweight Low Bweight

  15. Extrapolation Method • Divide Bweight into N domains • Fit purest background region, and extrapolate to signal box. Bweight Signalbox bkg Background

  16. Extrapolation into signal box • Purest ttbar sample: fit with distribution f0 Signalbox p1, q1 p2, q2 p3, q3 Background p4, q4 Bweight

  17. Visual impression of results

  18. Polynomial Fit results • (Note: statistically independent points.) • No clear extrapolation curve. • Too little statistics for proper extrapolation. B weight bin

  19. Conclusion / Plan • Interest in di-lepton ttbar channel as background in VBF H->WW (ll) process. • Plan: acquire better understanding of ttbar di-lepton channel. • Probably get involved in X-sec measurement. • Involvement in new Top reconstruction group • Bkg extrapolation techniques • B-tagging performance / validation.

  20. Backup

  21. Keys pdf • Kernal estimation pdf : provides unbinned, unbiased estimate pdf for arbitrary set of data • K. Cranmer, hep-ex/0011057 • E.g. 1-dim keys pdf heavily used in BaBar. • I extended this to n-dim keys pdf to model any bkg distribtion. • To be included in HEAD of RooFit • Automatically includes correct correlations between all observables

  22. One fit example ATLAS CSC BOOK • background • signal + bkg 1/fb mH(true) = 170 GeV/c2 Transverse Higgs mass (GeV/c2) 27

  23. Significance determination Bkg-only samples Entries per bin Bkg-only samples faking signal Sig+bkg samples m(higgs)=180 GeV 2 x 2 • Generate many pseudo-experiments (using grid): • Background-only samples • Background + signal samples, for various Higgs mass. • Fit each sample with background-only and signal+bkg hypothesis • Plot 2 between the fits. • Extrapolate fraction of bkg-only sample to fake average signal sample.

  24. Significance results ATLAS CSC BOOK Background shapes and normalization obtained fully from data control samples. • Results with 1/fb of data: • If higgs mass = 170 GeV/c2 : • Close to 2.5 sigma signal sensitivity • 9 GeV/c2 mass resolution. • For mH < 140 GeV/c2, similar sensitivity to gluon fusion analysis.

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