Top background in VBF H  WW (ll)

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

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”).

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 ...

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

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

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

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)

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.

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)

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

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

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

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

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

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

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

Visual impression of results

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

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.

Backup

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

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

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.

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.

Top background in VBF H  WW (ll)