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CMS and a Light Higgs with t t

CMS and a Light Higgs with t t. J. R. Incandela University of California Santa Barbara. Ht t Group: The goal was to perform a realistic study of the feasibility of detecting SM Higgs in this channel. CMS Ht t study. Recently completed study for the CMS Physics TDR vol. 2

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CMS and a Light Higgs with t t

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  1. CMS and a Light Higgs withtt J. R. Incandela University of California Santa Barbara

  2. Htt Group: The goal was to perform a realistic study of the feasibility of detecting SM Higgs in this channel

  3. CMS Htt study Recently completed study for the CMS Physics TDR vol. 2 This is a publicly available note: (49 pages). UCSB (JI) Led the group through the completion of this effort and note.

  4. Overview • Analysis involved 4 subgroups • Standard top channels: Dilepton, All-hadronic, and e/m + jets • Largely independent over past two years • Goal: As realistic as possible • Include backgrounds with higher order processes (Alpgen) • Include multiple interactions (aka pile-up) • Fully simulate the detectors • Develop and use realistic algorithms: • Electron and muon identification • Jet and missing Et reconstruction • b and c jet tagging, and light quark/gluon mis-tagging • A huge amount of work!!

  5. Light H in conjunction with tt Production Modes Decay Mode

  6. H→bb ET > 3 GeV t→W-b MPI W+b←t ISR

  7. Backgrounds • Main backgrounds are ttjj, ttbb, ttZ • ttjj dominates numerically even though a mis-tagged light quark or gluon jet is required • The ttjj xsec is nearly 3 orders of magnitude higher than signal • Should one get beyond ttjj, one must still confront ttbb, and ttZ with Zbb,which are irreducible

  8. Generators and Cross sections  • What’s new: • Used a more sophisticated generation scheme for the ttNj background • PYTHIA alone under-estimates the hard radiation • CompHEP overestimates • Alpgen+MLM matching is “just right”

  9. ALPGEN v.2 & MLM

  10. Triggering • After full CMS detector simulation and digitization, trigger simulations were run. • Typically a bit lower efficiency than we ultimately expect • More complex and efficient triggers will likely be available. . Trigger offline

  11. Leptons • Likelihoods • Developed explicitly for Htt • Categorized in Monte Carlo (MC) • SIGNAL • Matched to lepton form W • use an h-f cone of radius 0.1 (0.01) for electrons (muons) • BACKGROUND • All others: • leptons from b or c hadron decays, fakes

  12. Muon Reconstruction SIP Calo Iso PT • Muon likelihood: 4 “obvious” variables: • Muon pT • Track Isolation • Calorimeter Isolation • 2-D Impact Parameter significance Track Iso

  13. Muon Reconstruction cut is –Log(L)<1.4 • 90% for signal and 1.0% for background • calculated from the semi-leptonic Htt sample

  14. Electron Reconstruction • Electron Likelihood: 5 variables: • pT • E/p • Had/EM • SpT tracks inside DR=0.3 cone and outside veto cone (DR=0.015) • DR between electron candidate and closest track outside veto cone R ≡ √( ∆η2 + ∆φ2 )

  15. Electron Reconstruction cut is –Log(L) < 1.3 • 84% for signal 1.5% for background

  16. Jets • JETS • Iterative Cone Algorithm R=0.5 (0.4 for All-had) • ET > 20 (25 All-had) • |η| < 2.5 (2.7 All-had) • Use MC calibrated jets • Remove electrons that match within R < 0.4 Raw (IC 0.5) γ-jet calibrated R ≡ √( ∆η2 + ∆φ2 ) MonteCarlo calibrated Signal (ttH, MH = 115 GeV)

  17. Jet definition: All Hadronic Case • Different Cone Sizes Tested • Signal and 3 most dangerous Bkg used for testing (ttbb, tt2j, qcd170) • 8 most energetic jets in |h|<2.7 and ET>25GeV • Jet-Parton pairing • c2 for masses of 2 W and 2 top within 3 sigma • Jets paired to b-parton have to be b-tagged Cone DR=0.4 chosen

  18. Missing Momentum Missing ET Calorimeter tower measurements / ET(ecal + hcal) − ∑ [ET(calib)−ET(raw)] − ∑ pT(μ) In semi-leptonic Htt channel Jet corrections Muon momenta

  19. bTagging b c • Combined Secondary Vertex Algorithm • Mistagging rate as a function of b-jet efficiency for signal (left) and ttjj (center) are shown for various types of jets • Gluon jets include splitting to bb or cc in center plot... • Tag Rates for b-, c- and uds-jets vs discriminator cut for ttjj sample (right) c c uds g uds uds

  20. Semi-Leptonic Selection • Preselection • HLT + Isolated Lepton + 6 or 7 Jets • ET > 20GeV • 4 bTagged jets • D>0.5 (70% bTag efficiency) • Veto events with two leptons, or wrong lepton. • Jet Pairing • Likelihood method: Levent=LmassxLbTagxLkinematics • Mass refers to likelihoods for the obtained masses for hadronic W and tops • Kinematics • Takes into account b jets from top quarks slightly more energetic than those from H or jj (rather complicated formula…) • LbtagLbsele=Dh1xDh2xDbTopHadxDbTopLep

  21. Semi-Leptonic Selection Example of performance for muon selection Chosen working points are: 0.55 and 0.72

  22. Final results 60 fb-1 e2 e1 Muon Channel = 33.8% A bit less S/ √B cause mass constraint but better S/N Electron Channel e2 e1 = 33.9% S √B = 2.35 Muon + electron, no systematics No discrepancies at e1. Just less efficiency in e channel for HLT and isolation.

  23. di-Lepton Selection • Signal • H forced to bb • One W forced to (e,m,t); the other free • Find large contribution from single lepton events (1 real + 1 fake lepton) • Preselection • 3 b-jets and D>0.7 • Selection • 2 leptons (e,m) passing Likelihood criteria • –Log(Lmu)<1.8 and –Log(Lele)<1.3 with pT>20 GeV • At least 4 jets with ET> 20 GeV • No additional tagging requirement • Corrected ETmiss > 40 GeV

  24. Full Hadron Selection • Same analysis as Jet Algo choice and same c2mass for jet pairing • 8 most energetic jets in |h|<2.7 • Centrality Cuts • c2mass for 2W and 2tops within 3 sigma from expected values • 2 working points • Low S/N  Higher Significance • ET(7th)>30GeV – ET(8th)>20GeV ordered ET jet • 3 out of Dh1Dh2DbTopHad1DTopHad2 > 0.80 • CentH>0.55 • High S/N  Lower Significance • ET(7th)>30GeV – ET(8th)>20GeV ordered ET jet • Dh1Dh2DbTopHad1DTopHad2 orderd in D; D(3th)>0.85 and D(4th)>0.70 • CentH>0.55 – CentAll>0.80

  25. Full Hadron Selection: Results 60 fb-1 An example: tables of this type are in the note for all channels

  26. Speculation on Mature Experiments • The mature CMS working point is taken to have the following systematic uncertainties: • Flat 3% JES • 10% Jet Resolution • 4% in bc-Jet tagging efficiencies • 10% in uds-Jet tagging efficiencies • 3% in luminosity

  27. Single Lepton Table • Uncertainties from JES and uds-Tagging efficiencies • Most dangerous BKGD are tt1j and tt2j

  28. Single Leptons 60 fb-1 Muon • Significances are drastically reduced once reasonable systematic uncertainties are included • Led to re-optimization with a looser selection • Still pretty grim Electron

  29. Di-Lepton Table • Again - big uncertainties from JES and uds-tagging • Most dangerous BKGDs are ttNj

  30. Hadron Table • Again big uncertainties from JES and uds-Tagging efficiencies but…Surprise! The most dangerous background is QCD (look at JES) and then tt4j, tt3j and tt2j…

  31. The message! Jet Energy Scale and light quark jet mis-tagging are major systematics Furthermore, when signal and background production systematics are included, it gets worse.

  32. Cross Section uncertainties

  33. CMS-CDF comparison • Exercise performed with the diLepton channel • CMS, CDF tag uncertainties taken to be the same (4% bc and 10% uds) • Some indication we’re not too far off the mark.

  34. Is there any hope? • A couple of possible mitigating factors: • Backgrounds from Data: 60fb-1 of integrated luminosity will provide plenty of data for which detailed studies can be performed to understand the detector and algorithms • The availability of large control samples of top events will enable b tagging of high energy jets to be very well understood. This will probably enable some further suppression of light quark and charm jet tagging relative to b tagging. Similarly, experience with real data will likely improve jet reconstruction and energy measurements. • Will they be enough? • How much work and how much data will it take?

  35. Summary • Statistical Significance for the 4 sub-channels lower than previous studies but still combine to greater than 2.0 in 60 fb-1. • But the systematic uncertainties: JES and uds-tagging are major problems • They eliminate all sensitivity! (Combined significance of less than 0.2 for all channels) • Real data will give the final answer on the feasibility of this channel, but all indications are that it will be very difficult.

  36. More Information

  37. Muon Selection: Results 60 fb-1

  38. Electron Selection: Results 60 fb-1

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