Higgs to ττ at ATLAS

Sinéad Farrington

5th March 2014 The • 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 ! ! * ! = i! 2! • Unseen until 2012 2 Sinead Farrington, 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

Sinead Farrington, University of Warwick Were there any clues?

• Yes!

Most likely Higgs mass: +30 95 -24 GeV (from indirect evidence)

Mass > 115 GeV (direct evidence until 2012)

Sinead Farrington, University of Warwick Many ways to search for the Higgs

PRODUCTION DECAY

Most likely mass ranges

Sinead Farrington, University of Warwick Standard Model Higgs Production

6 Sinead Farrington, University of Warwick New Boson: Status until Nov 2013

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

7 Sinead Farrington, University of Warwick New Boson: Status until Nov 2013

• Evidence for Vector Boson • Signal strength µ=σ/σSM • All consistent with 1 Fusion and gg fusion production

• CMS data gives the same picture • Properties are compatible with SM Higgs Boson 8 Sinead Farrington, University of Warwick New Boson

9 Sinead Farrington, University of Warwick 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 ”

• Today’s seminar is about the search for the Higgs boson decaying to tau lepton pairs • Another step in this “confirmation”

10 Sinead Farrington, University of Warwick New Boson: Decay to Fermions

• Status until Nov 2013 (evaluated at 125 GeV)

• Tevatron H to bb: 2.8 σ PRL 109, 071804 (2012) • CMS H to bb: 2.1 σ CMS-HIG-13-012-003 CMS-HIG-13-004 • CMS H to ττ: 2.85 σ • ATLAS H to ττ: 1.1 σ • 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

11 Sinead Farrington, University of Warwick Standard Model Higgs (at 125 GeV)

12 Sinead Farrington, University of Warwick Hè ττ

e+/µ+ ντ + e/µ Lepton-lepton 12.4% τ+ hν H h- τ Lepton-hadron 45.6% + e-/µ- h (K/νπ) τ- ντ Hadron-hadron 42.0% h+ e/µ h- ντ h- (K/π) • 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!

13 Sinead Farrington, University of Warwick ATLAS dataset

• High pile-up conditions, challenging environment • Analysed 20.3 fb-1 of 8 TeV data • Previous analysis 4.3 fb-1 of 7 TeV data (November 2012)

14 Sinead Farrington, University of Warwick Stability of electron ID

• Efficiency of electron identification quite stable versus number of primary vertices

15 Sinead Farrington, University of Warwick 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 ττ mass distribution 16 Sinead Farrington, University of Warwick Jets

τ →

τ →

• Jets are an important part of VBF signature

17 Sinead Farrington, University of Warwick Missing Transverse Energy

• Multiple neutrinos in ditau decays • MET resolution is an important aspect of mass reconstruction

18 Sinead Farrington, University of Warwick ATLAS H to ττ Analysis

• Does the same boson observed to decay to WW*, ZZ*, γγ, couple to τ leptons? • Try to answer this with a multivariate analysis (BDT) • Data blinded • BDT trained to distinguish SM Higgs signal samples from backgrounds

19 Sinead Farrington, University of Warwick 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, ΔR(tt) and Δη(ττ) cuts suppress multijets • Apply preselection • Train BDT on remaining events • Validate background modelling on these events 20 Sinead Farrington, University of Warwick Analysis Categories • Vector Boson Fusion (54-63% of signal, rest is gg) • Two forward jets with leading pt>40-50 (30-35) GeV, Δη(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 Sinead Farrington, University of Warwick Backgrounds

• Backgrounds estimated using data directly or MC normalised to control regions

Z ττ: dominant background, modelled by data Others: MC for Dibosons, H WW Data normalisation for Z ee/µµ and top

Fake e/µ/τ: W+jets, top, QCD multijet modelled by data

Sinead Farrington, University of Warwick 22 Z ττ Background

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

23 Sinead Farrington, University of Warwick Backgrounds from “fakes”

• Estimated from data • e or µ 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 τ’s

24 Sinead Farrington, University of Warwick 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 Sinead Farrington, University of Warwick BDT Input variables

26 Sinead Farrington, University of Warwick BDT pre-unblinding checks

• 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/µµ + 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 ττ

27 Sinead Farrington, University of Warwick 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 Sinead Farrington, University of Warwick Control regions

29 Sinead Farrington, University of Warwick The Fit

30 Sinead Farrington, University of Warwick Post-fit distributions

31 Sinead Farrington, University of Warwick Systematic Uncertainties

• Signal strength µ=σ/σSM • Dominant theory uncertainty: matching, t and b quark treatment • Dominant expt uncertainty: background normalisations

32 Sinead Farrington, University of Warwick Results

• ATLAS observes significant excess of data events in high S/B region • Expected significance at 125 GeV is 3.2 σ • Observed significance at 125 GeV is 4.1 σ

33 Sinead Farrington, University of Warwick 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 µ at 125 GeV

34 Sinead Farrington, University of Warwick Couplings

• Signal seen in all channels and both production mechanisms

35 Sinead Farrington, University of Warwick Couplings

• Consistent with SM within one sigma

36 Sinead Farrington, University of Warwick ATLAS Channels

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

37 Sinead Farrington, University of Warwick Summary

• ATLAS has observed evidence for decay of a particle consistent with the SM Higgs boson • 4.1 standard deviation significance • CMS also produced evidence at a similar time (3.4 σ)

38 Sinead Farrington, University of Warwick Outlook

• Run 2 will yield • Higher luminosity and energy • Higgs cross section increases:

Production ggF VBF VH ttH mode σ(14TeV)/ 2.6 2.6 2.1 4.7 σ(8TeV)

• But Z cross section only increases by ~1.8x • Challenges for triggering • Spin/CP measurements in fermions • H bb observation? • H µµ observation?

39 Sinead Farrington, University of Warwick VBF Higgs to ττ?

40 Sinead Farrington, University of Warwick H to ττ

• H to ττ 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

Luminosity (fb-1) Δµ (%) [scenario 1,2] 300 [8,14] 3000 [5,8]

41 Sinead Farrington, University of Warwick ATLAS-CONF-2013-108 H ττ

• 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?

300fb-1 3000fb-1 Uncertainty All No theory All No theory Δµ/µ 0.22 0.16 0.19 0.12

42 Sinead Farrington, University of Warwick Higgs seen at CERN

43 Sinead Farrington, University of Warwick Many ways to search for the Higgs

PRODUCTION DECAY

Most likely mass ranges

Sinead Farrington, University of Warwick Hè ττ

e+/µ+ ντ νe/µ τ+ h+ H h- ντ + e-/µ- h (K/νπ) τ- ντ h+ e/µ h- ντ h- (K/π)

45 Sinead Farrington, University of Warwick Hè ττ • Experimental signature • Electron or muon with 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

Sinead Farrington, University of Warwick Hè ττ challenges

• Background sources calibrated with several control regions

Sinead Farrington, University of Warwick 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

Sinead Farrington, University of Warwick What does a Higgs event look like?

τ →

ET

τ → e+/µ+ ντ νe/µ + τ+ h ν H h- τ • Distinctive signature h+ (K/πe)- /µ- - ντ τ νe/ h+ µ • Reconstruct each element h- ντ h- (K/π)

49 Sinead Farrington, University of Warwick