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Search for invisible Higgs in tth 0 production

Search for invisible Higgs in tth 0 production. Motivation Review of preliminary analysis Event generation: Comparison of Pythia 6.3-6.4. Ricardo Gon ç alo, RHUL. Introduction. Motivation: Several scenarios for physics BSM predict a significant invisible branching ratio:

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Search for invisible Higgs in tth 0 production

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  1. Search for invisible Higgs in tth0 production Motivation Review of preliminary analysis Event generation: Comparison of Pythia 6.3-6.4 Ricardo Gonçalo, RHUL

  2. Introduction • Motivation: • Several scenarios for physics BSM predict a significant invisible branching ratio: • The simplest is the Higgs decay to neutralinos (probably disfavoured by the MSSM) • New scenario proposed: D.Cerdeño, A.Dedes and T.Underwood (hep-ph/0607157) which includes a light, invisibly decaying Higgs) • Complementary to the Vector Boson Fusion and associated ZH production with invisible Higgs in case of suppressed coupling of Higgs to vector bosons (MSSM “intermediate regime” mA < 500 GeV; small tan) • Aim to contribute to the Invisible Higgs CSC note • Previous analyses: • Within Atlas (that I’m aware of): I must review these • Paper by E.Richter-Was, B.Kersevan and M.Malawski • M.Malawski - MSc Thesis • Dietrich Schroff - PhD Thesis • Frank Meisel – Diplomarbeit • My own preliminary analysispresented in Higgs WG meeting (3 Nov.04) and in local meetings (1 Nov.04, 22 Apr.05): overview and main results shown below ATLAS-UK - Durham Sep.06

  3. Overview of preliminary analysis Cut-based analysis Event simulation with ATLFAST Only signal and tt background Difficulties: • Two components of missing momentum (from Higgs and neutrino)  can’t reconstruct tbl without further assumptions • tt is the most significant background and is similar to signal: most other backgrounds reducible • ttZ irreducible but cross section small (well…comparable to signal, must be included!) • Signal/Background ~10-3 (*) =520 fb in reference analysis, with SM couplings ATLAS-UK - Durham Sep.06

  4. Signal and tt background ttbar • Transverse mass, ET and ETmiss are good discriminating variables • mT has sharp edge for tt background at ~mW • But background xsection ~1000 times higher than signal • Tails of background distributions very large tth mT(GeV) ETmiss (GeV) ET (GeV) ATLAS-UK - Durham Sep.06 No cuts; Lumi = 1fb-1

  5. Preliminary analysis: cuts Reference analysis cuts: • Leptons: • 1 electron (pT>25GeV; ||<2.5) or 1 muon (pT>20GeV; ||<2.5) • Veto on additional electron (pT>10GeV) or additional muon (pT>6GeV) • Jets: • 2 b-tagged jets • 2 or more un-tagged jets • tbjj reconstruction: • |mjj-mW|<15GeV; ||<2.0 for jets in Wjj • |mbjj-mt|<25GeV • tbl reconstruction: • mT > 120 GeV (using lepton and full ETmiss to calculate mT) • Other: • Missing ET > 150GeV • Scalar sum of pT of reconstructed l j j b b: ET>250GeV • In reconstructed W  jj: Rjj =(2jj+2jj) < 2.2 (to reject lep-tau decays) ATLAS-UK - Durham Sep.06

  6. Preliminary analysis: comparison with reference analysis (Richter-Was et al) Cumulative efficiency of cuts: • ~10-20% agreement with reference analysis for signal (see “cross check” column) • Slightly worse agreement wrt tt (factor 1.35 – 2.8) for cuts after mT cut Testing all tbjj combinations against mW and mt gives efficiency ~30% better wrt “cross check” (“this analysis” column) both for signal and background  (small) net gain in significance ATLAS-UK - Durham Sep.06

  7. lep-had lep-lep lep-tau tau-tau had-had Preliminary analysis: initial results • Most background comes from lep-tau and lep-lep decays of tt, as concluded by E.Richter-Was et al. •  decays increase the missing ET • Wjjreconstructed from ISR/FSR jets in lep-tau and lep-lep events • For L=30 fb-1, scaling tth cross section to SM value (as in reference analysis): • tth: = 520 fb; 44.3 events • tt: = 490x103 fb; 812 events • S/B = 1.55 ATLAS-UK - Durham Sep.06

  8. b t  mt W mW e/ Preliminary analysis: kinematic fit Tried something different: • Assume pTmiss comes from  and h0 only • Build grid of points in pT and (l,) and calculate pZ • pZ can be found from pT and plep assuming W on mass-shell • From pb , p and plep, calculate mt for each point • Propagate errors in mt from grid spacing to obtain mt and calculate 2 ATLAS-UK - Durham Sep.06

  9. Preliminary analysis: kinematic fit • Kinematic fit ~works for signal: (pTh) ~ 85GeV • Will try to use fit results for discrimination against tt background • The hope was that this would allow other cuts to be relaxed • Correlations to mT and ETmiss may be important ATLAS-UK - Durham Sep.06

  10. New variables to use in selection? Higgs pT from the kinematic fit Signal tt background ATLAS-UK - Durham Sep.06

  11. Cut on l,Etmiss • Naively expect lepton to be mainly away from ETmiss direction in signal and close to ETmiss direction in background (lepton and  from W decay) • In fact, most tt background which passes standard cuts comes from lep-tau with ETmiss from both  and W decay and one detected lepton ttbar tth Truth from angular distance between true lepton and missing ET ATLAS-UK - Durham Sep.06

  12. Preliminary analysis: optimisation • Varied several cuts over a few threshold values to optimise significance = S/(S+B) • All cut combinations considered • Cuts varied: • mT (standard cut: > 120 GeV) • ETmiss (standard cut: > 150GeV) • Scalar sum of pT of reconstructed l j j b b: ET (standard cut:>250GeV • Added pTH obtained from kinematic fit • Added  between reconstructed lepton and ETmiss • Also tried varying electron and muon pT cuts • Significance increased from ~1.6 to ~3 ATLAS-UK - Durham Sep.06

  13. Preliminary analysis: optimisation • Some cuts have more impact than others • Results point to optimal thresholds: • mTW cut ~ 115GeV • ETmiss cut ~ 220 GeV • l,Etmiss cut < 2.8 rad • No clear solution for ET or pTh,fit • But: correlations between variables are large ATLAS-UK - Durham Sep.06

  14. CSC analysis • Aim to re-do and extend preliminary analysis for Invisible Higgs CSC note • Signal simulation: • Fully simulated tth0 (h0inv.) sample requested – to be simulated with Pythia 6.4 • Using Pythia and forcing invisible decay of Higgs: h0  ZZ  4 • Background sample must be produced independently on the grid • Background rejection factor ~10-5  need O(10-100 M) events or efficient Generator level filtering • New PtmissAndOrLepton filter written and in 12.0.3 • Based on Pauline Gagnon’s PtmissPlusLeptonFilter • Request missing ET and/or one lepton (configurable) • Different pT thresholds for e and  • Expected efficiency for tt ~10% : 1 e (pT>12.5, ||<2.7) or 1  and missing ET > 100 GeV • It may have problems: will have to revisit this carefully ATLAS-UK - Durham Sep.06

  15. Study of Pythia versions/PS • Pythia parton-showers (PS) and multiple-interactions changed dramatically since Pythia 6.2 • Using validation samples (generated by Bruce Mellado, thanks!) to evaluate the differences • Single lepton filter with pT>10GeV and ||<2.7 ATLAS-UK - Durham Sep.06

  16. Comparison Pythia 6.3/6.4/new/old PS • Following plots compare Pythia 6.403 and 6.323 with “new” and “old” parton shower (PS) and multiple interactions (MI) algorithms • The label “new PS” is used for pT-ordered parton showers and “interleaved” PS-MI model (PS and MI compete for available phase space) • “old PS” refers to the Pythia < 6.3 algorithm; still available but not default in 6.3 and 6.4 • Bjorken x calculation uses first daughters of interacting protons in ev.record using barcode: not always right!! • Code in: Generators /GenAnalysisTools /CBNT_Truth/src/CBNT_SpclMC.cxx • Different parton shower algorithms clearly changed the ordering of outcoming partons ATLAS-UK - Durham Sep.06

  17. Observable quantities: Missing ET truth • Differences don’t look that huge (well...not compared to VBF!) • But the cuts on ETmiss and ET will be in poorly understood regions ATLAS-UK - Durham Sep.06

  18. tth truth: transverse momenta • pTtth probably shows the largest differences between versions • The tth system recoils against ISR and multiple interactions ATLAS-UK - Durham Sep.06

  19. Conclusions • First steps under way to generated data for CSC analysis • Signal will be produced with Pythia 6.4 (new PS) • Background (ttbar, ttZ (Z), ?…) will be done in own production • New generator filter to provide necessary ttbar background sample • Looking at differences between Pythia versions for this channel • Differences not huge, but may be significant • Could provide estimate of systematic uncertainty? • Should try other generators? • Signal sample requested • Will be produced with Pythia 6.4 (new PS) • A lot of work to be done to revive this analysis… ATLAS-UK - Durham Sep.06

  20. Backup slides ATLAS-UK - Durham Sep.06

  21. t t t t h t The truth! • At parton level: • tops back-to-back in background • more “mercedes star”-like in signal • Most missing pT from Higgs decay (especially in had-had channel) (,ETmiss)  (h,ETmiss) ATLAS-UK - Durham Sep.06

  22. Preliminary analysis: simulation • Channel: Higgs (mh = 120 GeV) decaying to neutralinos in MSSM • (tan= 5, mA= 1 TeV; M1= 44 GeV, M2= 220 GeV, M3= 1 TeV) • PYTHIA 6.203 for signal and background • Generated 60 M tt + 10 M tth • Atlfast simulation, ATLAS release 7.0.2 • Low luminosity setting • Cone jets (Rcone = 0.4) • Jet tagging: b jets 60%; c mistag 10%; u,d,s, mistag 1% • CTEQ5L PDFs • mtop = 175 GeV • Interfaced own code to Atlfast within Athena to produce dedicated ntuple ATLAS-UK - Durham Sep.06

  23. Azimuthal angle difference between 3-momenta of tops and Higgs • Small differences between Pythia 6.3/6.4 and PS versions (right) • Compare to previous plot from Pythia 6.2 (left) • Labels: • t1 = leading top (highest pT) • t2 = sub-leading top ATLAS-UK - Durham Sep.06

  24. Observable quantities: Missing ET methods • ETmiss estimated using different methods • Reconstruction/MissingET/src/CBNT_MissingETtruth.cxx • Plotting angle between ETmiss estimates…not sure I understand the differences between methods • But they are similar for all Pythia/PS versions ATLAS-UK - Durham Sep.06

  25. Observable quantities: lepton pT & isolation • Looks ok • Left plot: with isolation • Right plot: with sub-leading lepton (note lepton filter on leading lepton) ATLAS-UK - Durham Sep.06

  26. Observable quantities: transverse mass • Plot on the right: events with a single lepton (e or ) • Compare with plots below • Cut on mT at ~115GeV ATLAS-UK - Durham Sep.06

  27. tth truth: angles between true momenta • Some difference between versions; easier to see in polar angle • Pythia 6.4 seems to give an intermediate result between 6.3 new/old PS ATLAS-UK - Durham Sep.06

  28. Optimised cuts Signal… … and tt background ATLAS-UK - Durham Sep.06

  29. Sum ET Signal… … and tt background ATLAS-UK - Durham Sep.06

  30. Transverse W mass Signal… … and tt background ATLAS-UK - Durham Sep.06

  31. Lepton and jet variables Signal… … and tt background ATLAS-UK - Durham Sep.06

  32. Not using Rjj as selection variable Signal… … and tt background ATLAS-UK - Durham Sep.06

  33. Higgs pT from fit Signal… … and tt background ATLAS-UK - Durham Sep.06

  34. l,Etmiss Signal… … and tt background ATLAS-UK - Durham Sep.06

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