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Higgs to tt at ATLAS

Learn about the search for the Higgs boson decaying to tau lepton pairs by Professor Peter Higgs at the ATLAS seminar. Understand the mechanism that gives mass to particles and the methods used to investigate the Higgs boson at the LHC.

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Higgs to tt at ATLAS

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  1. Higgs to tt at ATLAS Sinéad Farrington 8th December 2014

  2. The Higgs Boson • Professor Peter Higgs • Emeritus Professor at Edinburgh • Also Brout, Englert, Kibble, Guralnik, Hagen • Devised a mechanism to account for the generation of mass • Predicts one new particle, the Higgs boson • Specifically to give mass to W/Z bosons • Yukawa couplings allow the same particle to give mass to up- and down-type fermions • Unseen until 2012

  3. How to look for Higgs at the LHC? • We didn’t know the Higgs Boson’s mass (not predicted directly by the theory) • Very different composition of PRODUCTION and DECAY mechanisms depending on mass

  4. Were there any clues? Yes! Most likely Higgs mass: 95+30-24GeV (from indirect evidence) Mass > 115 GeV (direct evidence until 2012)

  5. Many ways to search for the Higgs PRODUCTION DECAY Most likely mass ranges

  6. Standard Model Higgs Production

  7. New Boson: Status until Nov 2013 • Observed by its decay to ZZ*, gg, WW* bosons (CMS and ATLAS) • Combined mass from ZZ, gg: 125.5±0.2+0.5-0.6 GeV • Spin/CP measurements agree with SM expectation of JP=0+

  8. New Boson: Status until Nov 2013 • Evidence for Vector Boson Fusion and gg fusion production • Signal strength m=s/sSM • All consistent with 1 • CMS data gives the same picture • Properties are compatible with SM Higgs Boson

  9. New Boson

  10. Nobel Prize • Prize motivation: "for the theoretical discovery of a mechanism that contributes to our understanding of the origin of mass of subatomic particles, and which recently was confirmed through the discovery of the predicted fundamental particle, by the ATLAS and CMS experiments at CERN's Large Hadron Collider” • Today’s seminar is about the search for the Higgs boson decaying to tau lepton pairs • Another step in this “confirmation”

  11. New Boson: Decay to Fermions • Status until Nov 2013 (evaluated at 125 GeV) • Tevatron H to bb: 2.8 s • CMS H to bb: 2.1 s • CMS H to tt: 2.85 s • ATLAS H to tt: 1.1 s • Search for Higgs to fermions decay important part of knowing whether we have seen the SM Higgs • Does the New Boson couple to fermions? • Indirect evidence from gg fusion through top loop • Furthermore: Couple to leptons? • If yes, are we sure the same particle is responsible for boson and all fermion decays? • Yukawa PRL 109, 071804 (2012) CMS-HIG-13-012-003 CMS-HIG-13-004

  12. Standard Model Higgs (at 125 GeV)

  13. Htt nt h+ h- h+ (K/p) e+/m+ t+ H ne/m nt t- e-/m- nt ne/m h+ nt h- h- (K/p) • Perform the search in all combinations of decays • Involves all lepton identification methods • Additionally for the Vector Boson Fusion mechanism, require jets • Neutrinos lead to missing energy (MET) • Complex signatures! Lepton-lepton 12.4% Lepton-hadron 45.6% Hadron-hadron 42.0%

  14. ATLAS dataset • High pile-up conditions, challenging environment • Analysed • 20.3 fb-1 of 8 TeV data • 4.3 fb-1 of 7 TeV data

  15. Stability of electron ID • Efficiency of electron identification quite stable versus number of primary vertices

  16. Stability of Hadronic tau ID • Hadronic tau’s identified by multivariate method (boosted decision tree) • Shower shape, decay length, etc • Tau Energy Scale (TES) derived from Z tt mass distribution

  17. Missing Transverse Energy • Multiple neutrinos in ditau decays • MET resolution is an important aspect of mass reconstruction

  18. ATLAS H to tt Analysis • Does the same boson observed to decay to WW*, ZZ*, gg, couple to t leptons? • Try to answer this with a multivariate analysis (BDT) • Data blinded • BDT trained to distinguish SM Higgs signal samples from backgrounds

  19. Triggers and preselection • Lepton-lepton • Single and di-lepton triggers • N(lepton)=2, N(jet pt>40GeV)≥1 • Mll and MET cuts to suppress Drell-Yan and multijet • Lepton-hadron • Single lepton triggers • N(lepton)=1, N(tau)=1 • MT<70 GeV cut to suppress W+jets • Hadron-hadron • Di-tau triggers • N(tau)=2 • MET>20GeV, DR(tt) and Dh(tt) cuts suppress multijets • Apply preselection • Train BDT on remaining events • Validate background modelling on these events

  20. Analysis Categories • Vector Boson Fusion (54-63% of signal, rest is gg) • Two forward jets with leading pt>40-50 (30-35) GeV, Dh(jj)> 2 • Boosted (gg fusion is ~ 71-74% of the signal, rest is gg,VH) • Pt(H)>100 GeV • Veto events with b-tags in lep-lep and lep-had • Suppress top background • In had-had use “rest” of events to constrain backgrounds

  21. Backgrounds • Backgrounds estimated using data directly or MC normalised to control regions Z tt: dominant background, modelled by data Others: MC for Dibosons, H WW Data normalisation for Z ee/mm and top Fake e/m/t: W+jets, top, QCD multijet modelled by data

  22. Z tt Background • Embedding method • Harvest Z mm events from data • Replace the muons with simulated taus • Gives a hybrid Z tt event • Advantages • Take from data: MET resolution, pile-up, jets, Z kinematics, VBF W/Z backgrounds modelled in data

  23. Backgrounds from “fakes” • Estimated from data • e or m fakes estimated from sample of anti-isolated leptons • Hadronic tau fakes estimated • In lep-had channel from sample with hadronic tau failing ID • In had-had channel from events which do not have opposite sign t’s

  24. Top Background • Shape from MC; normalisation from b-tagged control region • Normalisation performed separately for boosted/VBF categories • Validation regions defined to check shapes • Mll>100 GeV (lep-lep) • HT>350 GeV (lep-had)

  25. BDT Input variables

  26. Pre-fit steps • Check modelling of all input variables • And the modelling of the correlations among them • Control regions are fitted simultaneously with signal regions to constrain • Z ee/mm + jets in lep-lep, lep-had • Top in lep-lep, lep-had • W+jets in lep-had • Fakes in lep-lep • QCD(multijet) in had-had • Fit performed in 60-100 and 140+ GeV sidebands • Provides check of background model, especially Z tt

  27. Di-tau mass • Mass reconstruction not straightforward, owing to neutrinos in the final state • Use likelihood method (Missing Mass Calculator, MMC) using all measured kinematics and their resolutions and tau mass constraint • This variable is included in the BDT, mass resolution:

  28. Control regions

  29. The Fit

  30. Post-fit distributions VBF lep-lep lep-had had-had BOOSTED

  31. Systematic Uncertainties • Signal strength m=s/sSM • Dominant theory uncertainty: matching, t and b quark treatment • Dominant expt uncertainty: background normalisations

  32. Results • ATLAS observes significant excess of data events in high S/B region • Expected significance at 125 GeV is 3.5 s • Observed significance at 125 GeV is 4.5 s

  33. Results • ATLAS observes significant excess of data events in high S/B region • Expected significance at 125 GeV is 3.5 s • Observed significance at 125 GeV is 4.5 s

  34. Compatibility with 125 GeV • Weight each event by ln(1+S/B) for corresponding bin in BDT score • Excess is consistent with SM Higgs at 125 GeV • Signals at 110, 125, 150 are shown for the best fit m at 125 GeV

  35. Crosscheck Analysis • Cut based analysis performed as a crosscheck (8 TeV data only) • Expected significance: 2.5σ • Observed significance 3.2σ

  36. Couplings • Signal seen in all channels and both production mechanisms

  37. Couplings • Consistent with SM within one sigma

  38. ATLAS Channels • Combine this picture with the ATLAS H mm result • Expected limit 8.2xSM • Observed 9.8xSM • If the Higgs coupled universally to leptons, we would have already observed H mm ! • So we know that Higgs couples to fermions, but not universally

  39. Summary • ATLAS has observed evidence for decay of a particle consistent with the SM Higgs boson • 4.5 standard deviation significance • CMS also produced evidence at a similar time (3.4 s)

  40. Outlook • Run 2 will yield • Higher luminosity and energy • Higgs cross section increases: • But Z cross section only increases by ~1.8x • Challenges for triggering • Spin/CP measurements in fermions • H bb observation? • H mm observation?

  41. VBF Higgs to tt?

  42. H tott • H to tt is the newest of the evidence modes at ATLAS and CMS • Projections have been made by both experiments extrapolating analyses to the future • CMS evaluate two scenarios: • 1: leave systematic uncertainties the same • 2: Halve theory uncertainty; scale others by luminosity

  43. ATLAS-CONF-2013-108 Htt • ATLAS recent result uses Boosted Decision Tree • Perform projections from a simple cut based analysis • Assume no improvement in theory uncertainty(!) • Assume experimental challenges (pile-up, trigger) compensate for increased signal:background cross section • Pessimistic?

  44. Higgs seen at CERN

  45. Many ways to search for the Higgs PRODUCTION DECAY Most likely mass ranges

  46. Htt nt h+ h- h+ (K/p) e+/m+ t+ H ne/m nt t- e-/m- nt ne/m h+ nt h- h- (K/p)

  47. Htt • Experimental signature • Electron or muonwith neutrinos (missing energy) • Electron or muon identified fairly cleanly • Hadrons • Large rate for tau leptons to decay this way • Experimental challenges (significant) • Difficult to differentiate these signatures from backgrounds • Production of generic jets of hadrons • Z+jet production, W+jet production, pairs of top quarks

  48. Httchallenges • Background sources calibrated with several control regions

  49. Future • Key properties of this new boson will take some time to ascertain • This was always anticipated • In fact we are fortuitous in nature’s choice for the Higgs mass – all decay modes are accessible at this point • Key to characterising this particle are • Production and decay rates • Spin: first measurements made public last week! • Mass (to greater precision) • Switch from search mode to precision physics

  50. nt h+ h- h+ (K/p) e+/m+ t+ H ne/m nt t- e-/m- nt ne/m h+ nt h- h- (K/p) What does a Higgs event look like? t ET t Distinctive signature Reconstruct each element

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