Search for a SM Higgs Boson in semi-leptonic di-tau final states with ATLAS at the LHC

Transcription

1 Search for a SM Higgs Boson in semi-leptonic di-tau final states with ATLAS at the LHC ET miss = 9 GeV μ (pt=63 GeV) τhad (pt=96 GeV) mττ =9 GeV Nils Ruthmann mjj =65 GeV Nils Ruthmann Universität Freiburg International School of Subnuclear Physics ERICE - Italy 6 June 3

2 L EW µh@ µ H + g 4 (v + H) (W + µ W µ + Introduction Higgs mechanism major ingredient of the SM Spontaneous breaking of the U()Y SU()L symmetry leading to massive W,Z states cos Z µ Z µ )+ W ( µ )H vh 3 4 H Lepton masses generated via independent Yukawa couplings G f v p f f G f p f fh V ( )=µ + ( ) Nils Ruthmann - ISSP 3 Erice 6.6.3

3 L EW µh@ µ H + g 4 (v + H) (W + µ W µ + Introduction Higgs mechanism major ingredient of the SM Spontaneous breaking of the U()Y SU()L symmetry leading to massive W,Z states cos Z µ Z µ )+ W ( µ )H vh 3 4 H Lepton masses generated via,independent Yukawa couplings G f v p f f G f p f fh V ( )=µ + ( ) Higgs Boson mass hardly constrained by EW data and theory (log. loop corr., stability bounds) mh = 94 (+ 9-4) GeV - favouring low mass range LHC direct searches exclude broad range leaving a small window 95% CL Limit on / SM ATLAS Obs. Exp. ± ± Ldt = fb s = 7 TeV Data ATLAS: GeV CMS: < 7.5 GeV CLs Limits m H [GeV] Phys. Rev. D86 () 95% C.L. June Nils Ruthmann - ISSP 3 Erice

4 L EW µh@ µ H + g 4 (v + H) (W + µ W µ + Introduction Higgs mechanism major ingredient of the SM Spontaneous breaking of the U()Y SU()L symmetry leading to massive W,Z states cos Z µ Z µ )+ W ( µ )H vh 3 4 H Lepton masses generated via,independent Yukawa couplings G f v p f f G f p f fh V ( )=µ + ( ) Huge progress in the past months Discovery Mass measurement Coupling measurements Spin determination ATLAS Preliminary W,Z H bb s = 7 TeV: Ldt = 4.7 fb : Ldt = 3 fb H s = 7 TeV: Ldt = 4.6 fb : Ldt = 3 fb (*) H WW H s = 7 TeV: Ldt = 4.8 fb : Ldt =.7 fb (*) H ZZ Combined ll s = 7 TeV: Ldt = 4.6 fb : Ldt =.7 fb 4l s = 7 TeV: Ldt = 4.6 fb : Ldt =.7 fb s = 7 TeV: Ldt = fb : Ldt = fb Direct observation in fermionic decays ( ) Its up to ττ / bb µ =.3 ±. m H = 5.5 GeV + Signal strength (µ) p ATLAS-CONF-3-34 ATLAS Preliminary Observed SM expected June 3 s = 7 TeV, Ldt = fb, Ldt = 3-.7 fb m H [GeV] Nils Ruthmann - ISSP 3 Erice

5 SM Higgs branching ratios (HDECAY) bb _ WW. ττ gg ZZ cc _ - Branching ratios of a SM Higgs Boson Low mass Higgs Boson branching ratios diverse γγ : Clean signature - High mass resolution => Discovery, mass measurement ZZ: Low background - High mass resolution => Discovery, mass measurement WW: Experimentally challenging - Statistically powerful (Spin determination) bb: Challenging due to multijet bkd - Accessible in association with W/Z (tt) ττ: Large BR - Probes leptonic Yukawa coupling - Large irreducible Z bkd γγ Zγ m (GeV/c ) H Nils Ruthmann - ISSP 3 Erice

6 . - SM Higgs branching ratios (HDECAY) bb _ WW ττ gg ZZ cc _ Branching ratios of a SM Higgs Boson Low mass Higgs Boson branching ratios diverse γγ : Clean signature - High mass resolution => Discovery, mass measurement ZZ: Low background - High mass resolution => Discovery, mass measurement WW: Experimentally challenging - Statistically powerful (Spin determination) bb: Challenging due to multijet bkd - Accessible in association with W/Z (tt) ττ: Large BR - Probes leptonic Yukawa coupling - Large irreducible Z bkd γγ Zγ m (GeV/c ) H Three τ decay channels: Leptonic ( light leptons + 4 ν) ee/µµ Large bkd, statistically limited ll % lh 46% hh 4% Hadronic ( hadronic τ + ν) Large QCD background - Trigger complicated High statistics Semi-leptonic ( light lepton + 3 ν ) Presence of trigger lepton - Statistically powerful Nils Ruthmann - ISSP 3 Erice

7 Production mechanisms and backgrounds Mainly two production mechanisms important for H ττ searches. Gluon-Fusion Largest cross-section Jets at LO Additional partons due QCD radiation Categorisation to account for different bkd composition ( Jets low/hig MET, Jet) Main Backgrounds Z->ττ + n jets W + (n+) jets QCD.Vector-Boson-Fusion nd largest cross-section Jets at LO Additional partons due to QCD + EW radiation Topology allows strong bkd suppression Z->ττ + jets W + 3 jets tt-> bbww QCD Nils Ruthmann - ISSP 3 Erice

8 Hadronic τ-decay reconstruction Hadronic τ decays are identified by : low track multiplicity A narrow shower profile A specific EM/HAD energy mix Nils Ruthmann - ISSP 3 Erice

9 Hadronic τ-decay reconstruction Hadronic barrel/ extended barrel/ endcap (, )= (.,.) LAr EM barrel/endcap 3 () layers, middle layer: (, )= (.5,.5) Hadronic τ decays are identified by : multiplicity low track A narrow shower profile A specific EM/HAD energy mix R Reece, KG Tan, P Malecki, F Nils Ruthmann - ISSP 3 Erice Friedrich, N Dawe, K Nakamura + ; " classifier Identification based on multivariate Using pile-up corrected input variables /, # Typical background rejection of ~ at signal efficiencies of 6% & scale Energy calibrated to the visible τh!! : Typical scale uncertainties ~3% " " ' "

10 Preselection Events ATLAS L dt =.3 fb work in progress Data SS Data t t +single-top Z ll WW/WZ/ZZ W +jets Z H(5) x 5 Events selected by: e/μ OR l+τ trigger Events ATLAS L dt =.3 fb swork = 8 TeV in progress Data SS Data t t +single-top Z ll WW/WZ/ZZ W +jets Z H(5) x 5 Data/Bkd [GeV] m T [GeV] m T νe/μ e/μ ντ E/ T ντ τhad Jet topology separates VBF like events Data/Bkd N Jet N Jet Data/Bkd Fake τ s: - W+Jets - QCD ATLAS L dt =.3 fb work in progress Data SS Data t t +single-top Z ll WW/WZ/ZZ W +jets Z H(5) x MET [GeV] Tight lepton ID criteria Crucial to reduce multijet background before actual τ-id MET [GeV] MET resolution crucial for mass reconstruction Jet(s) ν 4-momenta fitted exploiting known τ-decay kinematic properties MET resolution dominates Real τ s: σmass - Z ττ - WW/WZ - top Poor mττ resolution Events Data/Bkd ATLAS L dt =.3 fb work in progress Data SS Data t t +single-top Z ll WW/WZ/ZZ W +jets Z H(5) x MMC m [GeV] Nils Ruthmann - ISSP 3 Erice MMC m [GeV]

11 Background estimation Fake τ-background needs data-driven estimation! Same sign events offer control region dominated by fake contributions QCD: Mainly gluon initiated fakes, no charge correlation OS SS W + Jets: Quark enriched jet sample with charge correlation to lepton OS>SS Fake rates depend on shower variables => Tough to simulate n Bkd OS = n All SS + n QCD OS-SS + nw+jets OS-SS + nz! OS-SS + n other OS-SS Nils Ruthmann - ISSP 3 Erice 6.6.3

12 Background estimation Fake τ background needs data-driven estimation Same sign events offer control region dominated by fake contributions QCD: Mainly gluon initiated fakes, no charge correlation OS SS W + Jets: Quark enriched jet sample with charge correlation to lepton OS>SS n Bkd OS = n All SS + n QCD OS-SS + nw+jets OS-SS + nz! OS-SS + n other OS-SS W normalisation measured in W control region Corrects Jet τ fake rate Depends on Quark/Gluon composition in the jet sample Charge Correlation and Jet Multiplicity dependent Additional control regions for cross-checks or normalisation corrections: tt Z->ll Quark/Gluon ratio ATLAS Simulation work in progress OS SS Events / 5 GeV ATLAS-CONF-6 e had Preselection Data 5 x H(5) Z (OS-SS) Others (OS-SS) Same Sign Data Stat. uncert. L dt = 3. fb ATLAS Preliminary m T [GeV] Nils Ruthmann - ISSP 3 Erice 6.6.3

13 Background rejection : Real τ s Events L dt =.3 fb Real τ s: - Z ττ - WW/WZ - top Data SS Data t t +single-top Z ll WW/WZ/ZZ W +jets Z H(5) x 5 5 ATLAS work in progress Data/Bkd MMC m MMC m [GeV] [GeV] Nils Ruthmann - ISSP 3 Erice

14 Background rejection : Z ττ Irreducible Z ττ background hard to separate Same final state including resonance event topology Real τ s: - Z ττ - WW/WZ - top VBF Production: Focus on specific Jet topology: Z: Mainly Drell-Yan production (+radiation) H: Jets at leading order (large angular separation, high pt) Events Data/Bkd µ had H+-jet VBF + e had L dt =.3 fb ATLAS work in progress Data t t +single-top Z ll WW/WZ/ZZ Z Fake H(5) x lep_centrality Tagging jets bracket the leptons lep_centrality Events Data/Bkd µ had H+-jet VBF + e had L dt =.3 fb ATLAS work in progress Data t t +single-top Z ll WW/WZ/ZZ Z Fake H(5) x jj Large rapidity separation of two tagging jets jj No additional jet activity Nils Ruthmann - ISSP 3 Erice

15 Background rejection : Fake τ s Events L dt =.3 fb Fake τ s: - W+Jets - QCD Data SS Data t t +single-top Z ll WW/WZ/ZZ W +jets Z H(5) x 5 5 ATLAS work in progress Data/Bkd MMC m MMC m [GeV] [GeV] Nils Ruthmann - ISSP 3 Erice

16 Events Fake τ s: - W+Jets - QCD µ + e Boosted had had 6 L dt =.3 fb Data ATLAS work in progress Background rejection : Fake τ s Rejection of non-resonant backgrounds: Does the event look like a resonant ττ-system? t t +single-top Z ll WW/WZ/ZZ Z Fake H(5) x MET centered between the τ s? e/μ Δ R MET τhad e/μ Δ R MET Jet faking τhad Data/Bkd Events Data/Bkd µ + e Boosted had had L dt =.3 fb metphi_centrality metphi_centrality Is the : neutrino share reflected in the lepton/τ pt? ATLAS work in progress Data t t +single-top Z ll WW/WZ/ZZ Z Fake H(5) x ptl_over_ptt ptl_over_ptt Nils Ruthmann - ISSP 3 Erice Jet(s) mττ : Mass consistent with 5 GeV? Events Data/Bkd µ + e Boosted had had L dt =.3 fb ATLAS work in progress Data t t +single-top Z ll WW/WZ/ZZ Z Fake H(5) x R Small opening angle between two taus in case of high pt recoil system R

17 Categorisation Jets VBF like topology Not VBF Not Boosted NJet>= Not VBF Not Boosted NJet= VBF - jet + jets inclusive Boosted H Not VBF High pt ττ Good mass resolution Analyses categories aiming at: Best possible background rejection based on specific event topologies Partial enhancement of specific production modes allowing for dedicated Higgs coupling fits Split the phase space - Not restrict it Based on Jet and Resonance quantities Nils Ruthmann - ISSP 3 Erice

18 Results Result based on 3 fb combined with 4.6fb of 7 TeV data No excess observed => Extract upper limit on cross-section Expected exclusion limit ~.7 σsm Events / GeV Boosted + e had µ had Events / GeV Data 5 x H(5) Z (OS-SS) Others (OS-SS) Same Sign Data Bkg. uncert. L dt = 3. fb ATLAS Preliminary 5 5 MMC mass m [GeV] H+-jet VBF + e had µ had Data H(5) Z Others Fake Bkg. uncert. L dt = 3. fb ATLAS Preliminary MMC mass m [GeV] 95% CL Limit on / SM 8 H lep had Observed CL s Expected CL s ± ± ATLAS Preliminary L dt = 4.6 fb, s = 7 TeV L dt = 3. fb, m H [GeV] ATLAS-CONF-6 Nils Ruthmann - ISSP 3 Erice

19 Results Result based on 3 fb combined with 4.6fb of 7 TeV data No excess observed => Extract upper limit on cross-section Expected exclusion limit ~. σsm when combined with other subchannels 95% CL Limit on / SM H Observed CL s Expected CL s ± ± ATLAS Preliminary L dt = 4.6 fb, s = 7 TeV L dt = 3. fb, µ VBF+VH B/B SM ATLAS Preliminary L = 4.6 fb, L = 3. fb, s = 7 TeV best fit best fit (µ>) 95% Contour 68% Contour SM prediction Background only m H = 5 GeV m H [GeV] ATLAS-CONF-6 µ B/B ggf SM Nils Ruthmann - ISSP 3 Erice

20 Outlook Searching for leptonic Higgs Boson decays now more interesting than ever H τ τ plays a crucial role in this challenge A preliminary analysis of a partial dataset shows no significant excess, but.. our collaboration is currently reviewing a improved analysis of the full fb dataset Aiming at a 3 σ sensitivity - in the presence of a signal Stay tuned! Nils Ruthmann - ISSP 3 Erice 6.6.3

21 Outlook Thanks for your attention Nils Ruthmann - ISSP 3 Erice 6.6.3

22 BACKUP Nils Ruthmann - ISSP 3 Erice 6.6.3

23 E/p h ±, pt < GeV Gaussian PDFs Mean: Data/MC Width: Uncertainty Testbeam h ±, pt > GeV TES Uncertainty Single particle uncertainties propagated to composite objects (Jets, Taus) Bottom-up: From single particles to a more complex object Pseudo-experiment approach Sampling over various uncertainties: Toy experiments randomly shifted energy scales Fractional uncertainty ATLAS Preliminary prong η <.3 <E/p> corr. Data/MC Data <.8 Simulation p [GeV] Single particle resp. Underlying event Pile-Up ATLAS Preliminary Data Data + Simulation Material modeling Non-closure Total uncertainty p [GeV] τ p [GeV] T Nils Ruthmann 3.4.3

24 Multivariate analysis approach Based on the HCP results last year: continuing the same analysis just adding data would not allow for a 3 σ evidence within Run Refined analysis techniques are needed to accomplish this goal H ττ fully focussed on a BDT analysis from the beginning 95% CL Limit on / SM 8 6 had H lep Observed CL s Expected CL s ± ± ATLAS Preliminary L dt = 4.6 fb, s = 7 TeV L dt = 3. fb, Furthermore: Fully focussed on a Higgs Boson mass of 5 GeV 4 V<y V<x V5<z m H [GeV] Boosted Decision Trees widely used in ATLAS Iterative training procedure on a signal and background sample Mis-Classified events in one iteration will be amplified for the next one (boosting) Final classification score averages the decision of all trees V3<v V4<k V6<q V7<p Classified as Signal Classified as Background Nils Ruthmann - ISSP 3 Erice

25 φ for the two methods. In contrast to the nce of long tails toward large masses in the cant improvement for low-mass Higgs boel by significantly reducing a large Z ττ Mass estimation mτ << mz => Tau decay products heavily boosted Collinear approximation assumes θ(ν,vis) = Calculate visible energy fraction: x, (,) Deviations from θ(ν,vis) = broaden resolution leading to unphysical solutions => efficiency loss Parametrized PDFs of ν-angular distributions - pt and decay dependent Fit the best solution in multi-dimensional space of opening angles and MET x-y resolution METx, METy, + angles => 5 variables floating in the fit ise Improves completely mass overwhelm resolution majorly the: Higgs search ΔRMS/RMS = 33% for Z + Jets Has high efficiency compared to collinear approximation 35 ) Arbitrary units H τ τ, lep-had channel, M h =5 GeV/c Missing Mass Calculator Collinear Approximation arxiv:.4686 [hep-ex] (GeV/c ) M τ τ arxiv:.4686 [hep-ex] ΔR Nils Ruthmann - ISSP 3 Erice Events 4 Z τ τ 3 -prong τ decays