ttbar Background Estimation in the Search for b-associated MSSM Higgs Bosons Decaying to Tau-Pairs with ATLAS DPG Bonn, T 45. Higgs

Transcription

1 ttbar Background Estimation in the Search for b-associated MSSM Higgs Bosons Decaying to Tau-Pairs with ATLAS DPG Bonn, T 45. Higgs Jana Schaarschmidt Supervised by Michael Kobel, Wolfgang Mader (TU Dresden) 1 / 15

2 Outline 1. Introduction 2. Idea of ttbar Estimation Procedure 3. Application to 14 TeV and 30 fb Application to 10 TeV and 168 pb Conclusions ttbar background Estimation in the Search for b-associated h/a/h ττ 2 / 15

3 Introduction MSSM Higgs sector in a tiny nutshell Minimal Supersymmetric Standard Model 2 Higgs doublets 5 Higgs bosons: h 0 (CP = +1), H 0 (CP = +1), A 0 (CP = -1), H ± Tree level described by two parameters: Mass degeneracy h, H h, H m A, tanβ = v u /v d Enhanced Couplings to down type fermions σ bbh/a tan 2 β Only one Higgs boson signal generated wih m A and sum of σ b-quark associated production Harlander, Kilgore Phys. Rev. D 68 (2003) Real b-jets in signal process b-tag applied Reduces dijet, Z and W backgrounds Enhances ttbar background (dominant background for m A > 150 GeV) Large uncertainties on ttbar σ ( 10 %) demand for data driven approaches ttbar background Estimation in the Search for b-associated h/a/h ττ 3 / 15

4 Idea of ttbar Estimation Procedure ttbar events show a significantly higher jet multiplicity than all other processes ttbar -> bb WW -> bb leplep + MET or bb qqbar lep + MET 14 TeV Basic object reconstruction applied: Normalized to unity one tau one lepton at least one b-tag Jet p T > 20 GeV Cone 0.4 algorithm ttbar MC: MC@NLO m t = GeV The high multiplicity bins (#jets > 3) contain mostly ttbar events (without further optimization) The low multiplicity bins (#jets=1 2) contain the Higgs boson signal among other processes and ttbar A ttbar control sample is used to estimate the low jet multiplicity shape The high multiplicity tail is used for normalization ttbar background Estimation in the Search for b-associated h/a/h ττ 4 / 15

5 Idea of ttbar Estimation Procedure N ttbar Signal region, njet=1 2 = Nttbar Control,njet=1 2 N ttbar Control,njet high No MC input needed. Therefore applicable to first data. In typical SM or MSSM Higgs analysis cut on the number of jets applied to reduce ttbar background #jets < 3, with jet p T > 20 GeV (Statistical significance is better if 2-jet bin kept in analysis) N ttbar Signal region,njet high Gives only the number of ttbar events in a mass range, not a mass line shape (eg. m ττ ) Independent of ttbar cross section uncertainty Independent of luminosity uncertainty Any systematic effect in signal region will be visible in control region The other backgrounds are agressively reduced by kinematical cuts (p T, m, Φ,...) Method fully applicable if analysis is split in two analysis (1-jet, 2-jet) and combined later. Application to 0-jet bin probably also possible but not done yet. ttbar background Estimation in the Search for b-associated h/a/h ττ 5 / 15

6 Application to 14 TeV and 30 fb -1 Part of new effort to update discovery potential of MSSM H/A ττ in semi leptonic final state using high statistic AtlFast II samples and Athena Release 14. ATLAS Collaboration, Discovery Potential of H/A ττ lephad in ATLAS, note in preparation. ATL-COM-PHYS Construction of ttbar Control Regions: Selection of signal regions m A dependent m A dependent control regions Start from signal region cuts, apply trigger efficiencies, apply tight tau ID (esp. against QCD) Drop tight lepton selection (to reduce W eν, W µν), drop cut on m ττ window, drop coll. approximation requirements, drop cut on number of jets Cut on m R,τ, p T,miss, p T,b-jet to increase ttbar purity Tune cut on m T for best match between jet multiplicity shapes in signal and control region in low multiplicity range if being normalized to high multiplicity range Evaluated single top contribution (t channel and Wt) by using cut factorisation ttbar background Estimation in the Search for b-associated h/a/h ττ 6 / 15 Normalized to Unity Normalized to Unity Atlas work in progress Atlas work in progress

7 Application to 14 TeV and 30 fb -1 - m A = 150 GeV Signal Region m A =150 GeV Control Region m A =150 GeV Comparison only ttbar Signal region m ττ / GeV Normalization ttbar purity: % 5210 ttbar, 157 single top events Result of estimation at m A =150 GeV: N ttbar MC = / (23.0) N ttbar Data = / (23.7) Expected statistical uncertainty includes contributions from all 3 regions (in brackets the real MC uncertainty) ttbar background Estimation in the Search for b-associated h/a/h ττ 7 / 15

8 Application to 14 TeV and 30 fb -1 - m A = 200 GeV Signal Region m A =200 GeV Control Region m A =200 GeV Comparison only ttbar Atlas work in progress m ττ / GeV ttbar purity: % 1726 ttbar, 52 single top events Result of estimation at m A =200 GeV: N ttbar MC = / (23.4) N ttbar Data = / (31.6) ttbar background Estimation in the Search for b-associated h/a/h ττ 8 / 15

9 Application to 14 TeV and 30 fb -1 - m A = 300 GeV Signal Region m A =300 GeV Control Region m A =300 GeV Comparison only ttbar Atlas work in progress m ττ / GeV ttbar purity: % 837 ttbar, 4 single top events Result of estimation at m A =300 GeV: N ttbar MC = / (19.5) N ttbar Data = / (27.0) ttbar background Estimation in the Search for b-associated h/a/h ττ 9 / 15

10 Application to 14 TeV and 30 fb -1 - m A = 450 GeV Signal Region m A =450 GeV Control Region m A =450 GeV Comparison only ttbar Atlas work in progress m ττ / GeV Atlas work in progress ttbar purity: % 340 ttbar, 1 single top events Result of estimation at m A =450 GeV: N ttbar MC = /- 3.8 (6.0) N ttbar Data = /- 3.8 (5.9) ttbar background Estimation in the Search for b-associated h/a/h ττ 10 / 15

11 Application to 14 TeV and 30 fb -1 - Systematic Uncertainties Systematic uncertainties may arise from: - Contaminations of control region and high multiplicity tail of signal region - Influence of detector uncertainties (E scales and resolutions, efficiencies) - Different factorisation scale, influence of Pile-up (both will require cut retuning) Total uncertainties under typical experimental systematic variations: m A 150 GeV 200 GeV 300 GeV 450 GeV Control purity 0.89 % 0.99 % 0.24 % 0.34 % High multiplicity signal purity 5.4 % 3.0 % 1.1 % 1.1 % Total exp. uncertainty 11.9 % % % % (N Data - N MC ) w/o variation 4.4 % 7.8 % 1.9 % % Numbers too conservative due to statistics and pessimistic detector uncertainties Control regions not optimized for best stability against systematic variations Biggest impact from light jet rejection and tau E scale ttbar background Estimation in the Search for b-associated h/a/h ττ 11 / 15

12 Application to 10 TeV and 168 pb -1 The Topmixing excercise All kind of SM backgrounds merged into one sample and MC truth info removed Cross sections scaled by secret factors E = 10 TeV, L = 168 pb -1, Trigger menu according to cm -2 s -1 Data content: W eν, W µν, W τν, Z ee, Z µµ, Z ττ, Wbb, WW, ZZ, tt, single t, Wt Merged into egamma and muon stream QCD events simply added to data and MC (no secret scaling) A loose Higgs boson signal region Looser cuts than in 14 TeV analysis (otherwise no data events expected with 168 pb -1 ) No tight tau ID tau LLH > -10 >= 1 btag #jets=1 2 tight lepton isolation p Tmiss > 35 GeV m T < 40 GeV p T,tau > 20 GeV Φ > 2.3, coll. approx. 1 3 tracks per tau cand. Signalregion tanβ=45 m A =120 GeV Number of events Used for normalization 168 pb TeV Atlas work in progress ttbar background Estimation in the Search for b-associated h/a/h ττ 12 / 15

13 Application to 10 TeV and 168 pb -1 A loose ttbar control region p Tmiss > 100 GeV m T > 50 GeV p T,lep > 40 GeV Number of events 168 pb TeV p T,tau > 50 GeV taullh > -10 (no tight tau ID) >= 1 btag Atlas work in progress ttbar % W lν/τν Wbb Z ττ/ll % 1.1 % 0 % ttbar purity: 83 % ttbar yield: 32 Events WW/ZZ Wt/single t J0-J5 bbh 120 GeV % 5.7 % 0.7 % 0.6 % Factorized single top and QCD σ (10 TeV) / σ (14 TeV): ttbar 49% W 57 % Z 70% ttbar background Estimation in the Search for b-associated h/a/h ττ 13 / 15

14 Application to 10 TeV and 168 pb -1 Results of ttbar estimation Method applicable but statistics on the limit to be used. Exploring different cuts might help. Scaling factors in Topmix data > 1 Tight tau ID is not usable for such little data, instead cut on MET to reduce dijet background Arbitrary Units Comparison of ttbar MC only MC uncertainty Atlas work in progress N MC = N data = Number of events Topmix MC: Expected uncertainty tanβ=45 m A =120 GeV 168 pb TeV Number of events Topmix Data Streams: N data = Atlas work in progress ttbar background Estimation in the Search for b-associated h/a/h ττ 14 / 15

15 Conclusions Studied possibility of data-driven ttbar estimation in the context of the b-associated MSSM h/a/h ττ lephad Use jet multiplicity distribution of a ttbar control region and normalized it to the high jet multiplicity tail in the signal region to estimate the number of ttbar events in a m ττ mass window Jet multiplicity typical instrument for ttbar cross section measurements (eg. CDF publication hep-ex/ ) Method totally independent of the ttbar cross section and therefore its systematic uncertainty Applicable not only to well understood data but also to first data (300 pb -1 with 7 TeV will be sufficient to start this study) ttbar background Estimation in the Search for b-associated h/a/h ττ 15 / 15

16 Back up

17 The ATLAS Detector 46 m length, 24 m diameter 7000 t weight 3000 km cables Luminosity: cm -2 s -1 (10 fb -1 per 14 TeV) Trigger: 10 9 Hz 75 khz 100 MB / s storage ttbar background Estimation in the Search for b-associated h/a/h ττ Backup 1 / 7

18 MSSM 2 Higgs doublets 5 Higgs bosons: h 0, H 0 (CP = +1), A 0 (CP = -1), H ± Tree level decribed by only two parameters: m A, tanβ = v u /v d v u2 +v d2 = v 2 m h < m Z but large loop corrections increase this limit! Couplings: g MSSM = ξ g SM Additional parameters: X t Stop mixing parameter M SUSY Energy scale of SUSY breaking M 2 Gaugino mass at EW scale M~ g Gluino mass at EW scale µ Strength of SUSY Higgs mixing ξ h H A t cosα/sinβ sinα/sinβ cotβ All parameters except tanβ, m A fixed in benchmark scenarios: α = mixing angle between h and H b / τ -sinα/cosβ cosα/cosβ tanβ h/a/h ττ enhanced if tanβ large W / Z sin(α-β) cos(α β) h/h ZZ * suppressed A does not couple to W/Z - m max h : m h < 133 GeV, maximum allowed mass for h nomixing: m h < 116 GeV, no mixing in stop sector gluphobic: m h < 119 GeV, suppressed gg fusion small α: m h < 123 GeV, suppressed ttbar h, h bb m h max considered here Carena, Heinemeyer, Wagner, Weiglein Eur. Phys. J. C26 (2003) pp ttbar background Estimation in the Search for b-associated h/a/h ττ Backup 2 / 7

19 Tau Decay: Tau Reconstruction 35% leptonic (τ e νν / τ µ νν) 65% hadronic (τ π ν / τ ππ ν /...) Probability that tau-pair decays fully leptonic only 12 % (leplep) In 45% of the cases one leptonic and one hadronic decay (lephad) (Plot from Artur Kalinowski) In the remaining 42 % both taus decay hadronically (hadhad) Hadronic taus in the Atlas detector: Collimated calorimeter cluster, low charged tracks multiplicty, Displaced secondary vertex Tau reconstruction is combination from tracker and calorimeter based algorithms Sources for fake taus: QCD jets Electrons Muons ttbar background Estimation in the Search for b-associated h/a/h ττ Backup 3 / 7

20 Collinear Approximation Conditions: Higgs mass large compared to τ mass Higgs boson has non-zero p T p T,miss in the detector due to neutrinos only x = p T,R / p T,τ 0 < x < 1 m m ll ττ = x 1 x 2 No solution if Φ=π cut Φ < 3.0 applied (VBF: cos( Φ)< 0.9 cut Φ < 2.7 applied) leplep Collinearity ttbar background Estimation in the Search for b-associated h/a/h ττ Backup 4 / 7

21 Signal and Control Region Cuts (14 TeV) m A 150 GeV 200 GeV 300 GeV 450 GeV p T,miss > 20 GeV > 20 GeV > 25 GeV > 60 GeV p T,tau GeV > 30 GeV > 50 GeV > 90 GeV Φ Φ p T,tau GeV > 30 GeV > 50 GeV > 90 GeV p T,b-jet > 50 GeV > 60 GeV > 20 GeV > 20 GeV m l,τ > 100 GeV GeV GeV GeV p T,miss > 100 GeV > 100 GeV > 100 GeV > 100 GeV m T - > 10 GeV > 80 GeV > 80 GeV Signal Control ttbar background Estimation in the Search for b-associated h/a/h ττ Backup 5 / 7

22 Experimental Systematic Uncertainties Systematic uncertainties assumed for 10 fb -1 (commonly used numbers in ATLAS): Electron efficiency ± 0.2 % Electron E scale ± 0.2 % Electron resolution E T Muon efficiency ± 1 % Muon pt scale ± 1 % Muon resolution 0.001/p T r Tau efficiency ± 5 % Tau E scale ± 5 % Tau resolution 0.45 E Jet E scale Jet E resolution ± 3 % (10 %, h >3.2) 0.45(0.63) E B-tagging efficiency ± 5 % Light jet rejection ± 10 % Only relevant in lephad ttbar background Estimation in the Search for b-associated h/a/h ττ Backup 6 / 7

23 Systematic Uncertainties - Numbers for 14 TeV, m A =150 GeV ttbar background Estimation in the Search for b-associated h/a/h ττ Backup 7 / 7