Higgs Decays to Bottom and Tau PairsWith the ATLAS and CMS Experiments

Ulrike Schnooron behalf of the ATLAS and CMS Collaborations

ulrike.schnoor@cern.ch

Physikalisches Institut, Universitat Freiburg

May 3, 2017

Introduction H → bb H → τ+τ− Interpretations Summary

Higgs decays to Bottom and Tau pairs

...directly probe fermion mass generation:

LEWSBf = mf (fLfR + fR fL)︸ ︷︷ ︸

mass term

+gf√

2(fLfR + fR fL)H︸ ︷︷ ︸

Higgs−fermion interaction

Yukawa coupling gf =√

2 mfv

Experimental access to Higgs–fermion coupling:Access Channel Sector Type

Indirect H → γγ quark up/downGGF quark up/down

DirectH → ττ lepton downH → bb quark downttH quark up

[GeV]HM80 100 120 140 160 180 200

Hig

gs B

R +

Tota

l U

ncert

­410

­310

­210

­110

1

LH

C H

IGG

S X

S W

G 2

01

3

bb

ττ

µµ

cc

gg

γγ γZ

WW

ZZ

• H → bb, H → ττ :large branching ratios

• Predictions for GGF,VBF:POWHEG at NLO+ PYTHIA,renormalized toNNLO QCD

Ulrike Schnoor (Freiburg) Higgs Decays to Bottom and Tau Pairs 2/27

Introduction H → bb H → τ+τ− Interpretations Summary

Measurements

• Measure cross sections• Express as signal strength µ

µ =σmeasured

prod+decay

(σprod × BR)SM

• Interpretation: coupling modifier κifor production or decay process i :

κ2i = σi/σ

SMi or κ2

i = Γi/ΓSMi

⇒ κi =gi

gSMi

[JHEP 08 (2016) 045]

Ulrike Schnoor (Freiburg) Higgs Decays to Bottom and Tau Pairs 3/27

Introduction H → bb H → τ+τ− Interpretations Summary

Measurements of H → ττ and H → bbH → bb- Experimental challenges- ATLAS and CMS MeasurementsH → τ+τ−

- Experimental challenges- ATLAS and CMS MeasurementsInterpretationsSummary

H → bb

• VBF H → bb:ATLAS: 8 TeV, CMS: 8, 13 TeV

• VH H → bb:ATLAS: 8, 13 TeV, CMS: 7, 8 TeV

• VBF+γ H → bb: ATLAS: 13 TeV

• t(t)H: ATLAS: 8, 13 TeV, CMS: 7, 8 , 13 TeV

H → ττ

• GGF, VBF H → ττ :ATLAS: 7, 8 TeV, CMS: 7, 8 TeV

• VH,H → ττ :ATLAS 8 TeV, CMS: 7, 8 TeV

• CP measurement in VBF:ATLAS: 8 TeV

Ulrike Schnoor (Freiburg) Higgs Decays to Bottom and Tau Pairs 4/27

Introduction H → bb H → τ+τ− Interpretations Summary

H → bb

Ulrike Schnoor (Freiburg) Higgs Decays to Bottom and Tau Pairs 5/27

Introduction H → bb H → τ+τ− Interpretations Summary

H → bb... probe Yukawa couplings to (down-type) quarks• Largest BR (∼58 %)• Challenging: large multi-jet and b-jet background

• Profits from good b-tagging• Makes use of leptonic decay of particles produced in association• Uses kinematic properties of the production process

Z(→ ``)H(→ bb) candidate eventRun 2 so far: ATLAS + CMS > 1 M H → bb events

Ulrike Schnoor (Freiburg) Higgs Decays to Bottom and Tau Pairs 6/27

Introduction H → bb H → τ+τ− Interpretations Summary

Experimental access to H → bb

GGF overwhelming background, difficult to trigger

VBF large background, trigger on jets, make use ofVBF kinematics

VBF+γ clean signature, small σprod, trigger on photon

VH trigger on decay products of W/Z (e, µ, EmissT )

t(t)H trigger on decay products of top quark(s)

Ulrike Schnoor (Freiburg) Higgs Decays to Bottom and Tau Pairs 7/27

Introduction H → bb H → τ+τ− Interpretations Summary

Searches in VBF H → bbATLAS: JHEP 11 (2016) 112 (8 TeV, 20.2 fb−1)CMS: HIG-16-003 (13 TeV, 2.3 fb−1) [JHEP 11 (2016) 112]

• Signal: VBF H (→ bb) + jj, GGF H → bb• Background:

• Resonant: Z (→ bb) + jj• Non-resonant: dominantly multi-jet

(estimated from data in mbb side-band)• Triggers:

• 4 jets incl. 2 b-tagged in HLT• 3 (2) jets incl. 1 (2) forward + 1 b-tagged

jet• Signal efficiency ∼ 6 %

• Selection:4 high-pT jets incl. 2 b-tagged central jets;pT(bb) > 100 GeV

Multivariate analysis: Boosted decision tree (BDT) to discriminate VBFsignal from non-resonant background; fit in mbb in 4 BDT categories

Ulrike Schnoor (Freiburg) Higgs Decays to Bottom and Tau Pairs 8/27

Introduction H → bb H → τ+τ− Interpretations Summary

Results for VBF H → bb

ATLAS: JHEP 11 (2016) 112: 8 TeV, 20.2 fb−1

Signal strength: µ = −0.8± 1.3(stat.)+1.8−1.9(syst.) , upper limit µ = 4.4 at 95 %C.L.

CMS: HIG-16-003: 2.3 fb−1(13 TeV)⊕

19.8 fb−1(8 TeV)

• Triggers on 1 forward jet and 1 (2)b-tagged jets (total signal efficiency6.2 %)

• Further selection based on jetkinematics and VBF topology

• Fit in Mbb

• Combined with 8 TeV result:µ = 1.3+1.2

−1.1

• Dominant: statistical uncertainty,QCD background estimate

Ulrike Schnoor (Freiburg) Higgs Decays to Bottom and Tau Pairs 9/27

Introduction H → bb H → τ+τ− Interpretations Summary

Searches in VHATLAS-CONF-2016-091: 13 TeV, 13.2 fb−1

CMS: PRD 89 (2014) 012003: 7 TeV (5.1 fb−1), 8 TeV (18.9 fb−1)

[ATLAS-CONF-2016-091]

• Large and complex backgrounds• Trigger on decay products of W/Z• Preselection: ≥ 2 jets, 2 b-tags• Categorization in

0 leptons Z → ννEmiss

T > 150 GeV; 2 or 3 jets1 lepton W → `ν

pT(W ) > 150 GeV;Emiss

T > 30 GeV; 2 or 3 jets2 leptons Z → `+`−

71 < m`` < 121 GeV;pT(Z ) < or > 150 GeV;2 or ≥ 3 jets

• BDT in each category→ simultaneous fit

µ = 0.21+0.36−0.35(stat.)± 0.36(syst.)

Ulrike Schnoor (Freiburg) Higgs Decays to Bottom and Tau Pairs 10/27

Introduction H → bb H → τ+τ− Interpretations Summary

Searches in VHATLAS-CONF-2016-091: 13 TeV, 13.2 fb−1

CMS: PRD 89 (2014) 012003: 7 TeV (5.1 fb−1), 8 TeV (18.9 fb−1)

CMS: PRD 89 (2014) 012003: 5.1 fb−1(7 TeV)⊕

18.9 fb−1(8 TeV)

• Channels: W (µν)H, W (eν)H, W (τν)H, Z (µµ)H, Z (ee)H,Z (νν)H

• Single-lepton, tau, EmissT or b-tag triggers

• BDT regression based on b-jet kinematicsand properties of VH system to correct jetenergy

• BDT classification in signal regions⇒ Combined signal strength including gg → ZH

(@NLO) contribution: µ = 0.89± 0.43[PoS ICHEP2016 (2016) 777, Eur. Phys. J. C 75 (2015)212]

Ulrike Schnoor (Freiburg) Higgs Decays to Bottom and Tau Pairs 11/27

Introduction H → bb H → τ+τ− Interpretations Summary

Search in VBF+γATLAS-CONF-2016-063: 13 TeV, 12.6 fb−1 [ATLAS-CONF-2016-091]

• Trigger on photon, 4 jets, and maximum dijetmass Mjj > 700 GeV

• Backgrounds: Non-resonant QCD productionof photon and 4 jets, Z (→ bb)+jets

• Event selection:• Suppress non-resonant QCD background:

isolated and high-pT photon• 2 b-jets and 2 jets with high Mjj

• BDT against multi-jet background (jetproperties and kinematics, e.g. photoncentrality)

⇒ Fit to mbb in 3 regions of BDT score:

µ = −3.9+2.8−2.7 and µ < 4.0 at 95 % C.L.

• Statistically limitedUlrike Schnoor (Freiburg) Higgs Decays to Bottom and Tau Pairs 12/27

Introduction H → bb H → τ+τ− Interpretations Summary

Searches in association with single top quarksCMS: CMS-PAS-HIG-16-019 (2.3 fb−1, 13 TeV)

tH production

Interference betweenproduction with HWW and Httvertices→ sensitive tomagnitude and sign of κt

• Several kinematic categories according to top and Higgs kinematics• BDT for jet-quark assignment• BDT for discrimination from

single top, t t , t tH

• For SM: upper limit 113.7× σSM

• Limits on Higgs boson coupling to topquarks (κt ): −→(κV = κb = 1)

Ulrike Schnoor (Freiburg) Higgs Decays to Bottom and Tau Pairs 13/27

Introduction H → bb H → τ+τ− Interpretations Summary

Searches in association with top quark pairsATLAS: ATLAS-CONF-2016-080 (13.2 fb−1, 13 TeV);CMS: CMS-PAS-HIG-16-038 (12.9 fb−1, 13 TeV)

ttH production

• One or both top quarks decayingleptonically

• Backgrounds: t t+light jets,t t + bb, t t + b and t t + cc

• Combine:• BDT based on object

kinematics, event shape,b-tagger⇒ discriminationagainst inclusive t t

• Matrix-Element Method (MEM):M(t tH)

M(t t+bb)⇒ discrimination

against t t + bb• MEM = final discriminant µ = −0.19+0.45

−0.44(stat.)+0.66−0.68(syst.)

Ulrike Schnoor (Freiburg) Higgs Decays to Bottom and Tau Pairs 14/27

Introduction H → bb H → τ+τ− Interpretations Summary

Searches in association with top quark pairsATLAS: ATLAS-CONF-2016-080 (13.2 fb−1, 13 TeV);CMS: CMS: CMS-PAS-HIG-16-038 (12.9 fb−1, 13 TeV)

ttH production

• One or both top quarks decaying leptonically• Backgrounds: t t+light jets, t t+ ≥ 1b and

t t+ ≥ 1c

• Categories defined according to numberof leptons, jets, b-tags

• Two step multivariate analysis:• Jet assignment• Signal/background classification

• Fit to the discriminants in each category

→ combined µ = 2.1+1.0−0.9

Ulrike Schnoor (Freiburg) Higgs Decays to Bottom and Tau Pairs 15/27

Introduction H → bb H → τ+τ− Interpretations Summary

H → ττ

Ulrike Schnoor (Freiburg) Higgs Decays to Bottom and Tau Pairs 16/27

Introduction H → bb H → τ+τ− Interpretations Summary

H → τ+τ−

... probe Yukawa coupling to down-type leptons• large BR (∼6.3 %)• multi-jets or Z+jets background• profits from good hadronic τ trigger and identification⇒ trigger on Higgs decay products

Accessible production channels

GGF boosted topology

VBF VBF topology

VH associated gauge bosonUlrike Schnoor (Freiburg) Higgs Decays to Bottom and Tau Pairs 17/27

Introduction H → bb H → τ+τ− Interpretations Summary

VBF and GGFATLAS: JHEP 1504 (2015) 117: 7 TeV (4.5 fb−1), 8 TeV (20.3 fb−1);CMS: JHEP 1405 (2014) 104: 7 TeV (4.9 fb−1), 8 TeV (19.7 fb−1)

[JHEP 1504 (2015) 117]

• Signal: H → ττ• Channels:

• τ`τ`, τ`τhad , τhadτhad• leptonic decays: isolated leptons• hadronic decays: tau identification through MVA→ different background composition

• Main background Z → τ+τ−: estimatedwith data-driven “embedding” technique

• Reconstruction of di-tau mass:Missing Mass Calculator (MMC) based onlikelihood scan

• 2 preselection regions:• Boosted topology (GGF dominated)• VBF topology (VBF dominated)

• Maximum likelihood fit to BoostedDecision Tree outputs

Ulrike Schnoor (Freiburg) Higgs Decays to Bottom and Tau Pairs 18/27

Introduction H → bb H → τ+τ− Interpretations Summary

VBF and GGFATLAS: JHEP 1504 (2015) 117: 7 TeV (4.5 fb−1), 8 TeV (20.3 fb−1);CMS: JHEP 1405 (2014) 104: 7 TeV (4.9 fb−1), 8 TeV (19.7 fb−1)

• Mass reconstruction using likelihood-based algorithm (mττ )• Several BDTs to suppress backgrounds• Including also VH channel

[JHEP 1405 (2014)104]

• QCD background from same-chargecontrol region

• Z → τ+τ− background from Z → µµdata with simulated τs embedded

• Extract signal from binnedmaximum-likelihood fit to mττ

Ulrike Schnoor (Freiburg) Higgs Decays to Bottom and Tau Pairs 19/27

Introduction H → bb H → τ+τ− Interpretations Summary

VBF and GGFATLAS: JHEP 1504 (2015) 117:

7 TeV (4.5 fb−1), 8 TeV (20.3 fb−1)

CMS: JHEP 1405 (2014) 104:

7 TeV (4.9 fb−1), 8 TeV (19.7 fb−1)

Combination of all channels,regions, center-of-massenergies, ATLAS and CMS:

µ = 1.11+0.24−0.22

[JHEP 08 (2016) 045]• Dominated by statistics• Normalization of Z , W ,

multijets backgrounds

Ulrike Schnoor (Freiburg) Higgs Decays to Bottom and Tau Pairs 20/27

Introduction H → bb H → τ+τ− Interpretations Summary

Searches in VHATLAS: Phys. Rev. D 93, 092005 (2016): 8 TeV (20.3 fb−1),CMS: CMS-PAS-HIG-12-053: 7 TeV (5.0 fb−1) + 8 TeV (19.5 fb−1)

Signal:Selection: multiple objects defining the final stateBackground: W+jets, Z+jets,t t with 1 prompt and 1 fake lepton

• WH channels: `±`±τh and `τhτh

• ZH channels: ``LL with LL = eµ,eτh, µτh, τhτh

• Background in hadronic channels dominated by fake τh CMS-PAS-HIG-12-053

Ulrike Schnoor (Freiburg) Higgs Decays to Bottom and Tau Pairs 21/27

Introduction H → bb H → τ+τ− Interpretations Summary

Searches in VH[Phys. Rev. D 93, 092005 (2016)]

Selection:• Isolation and quality of leptons• Only same-charge same-flavor lepton pairs• b-veto• Requirement on scalar pT sum of leptons and

taus

Exclusionlimits:

Ulrike Schnoor (Freiburg) Higgs Decays to Bottom and Tau Pairs 22/27

Introduction H → bb H → τ+τ− Interpretations Summary

Properties (CP nature)ATLAS: Eur. Phys. J. C76 (2016) 658 (8 TeV, 20.2 fb−1)

[Eur. Phys. J. C76 (2016) 658]

• Measurement based on 8 TeV H → ττ couplings analysis• Use Optimal Observable (OO) as CP-odd observable:

if 〈OO〉 6= 0⇒ CP violation• Matrix element for OO calculation and event reweighting extracted fromHAWK [Denner, Dittmaier, Kallweit, Muck, Comput. Phys. Commun. 195 (2015)]

• OO =2<(M∗SMMCP−odd)

|MSM |2

• Set limits on CP-mixing couplingd (EFT parameter):Leff = LSM + d e

2mW sin θwHW +

µνW−µν

• No signal strength information• Only shape information• Fit to full OO distribution to

extract exclusion limits on d

Ulrike Schnoor (Freiburg) Higgs Decays to Bottom and Tau Pairs 23/27

Introduction H → bb H → τ+τ− Interpretations Summary

Exclusion limits on CP odd contribution

Expected 68 % confidence interval: [-0.08,0.08] for µ = 1.55

Observed confidence interval at 68 % C.L.:[-0.11,0.05]

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Introduction H → bb H → τ+τ− Interpretations Summary

Up-type vs. Down-type fermions

JHEP 08 (2016) 045

λdu = κd/κu

λVu = κV/κu

κuu = κu · κu/κH

• Coupling modifiers

κ =g

gSM

⇒ σi × BFf =σi (~κ)× Γf (~κ)

ΓH

• Many BSM extensions (e.g.2HDMs) predict λdu 6= 1

• No sensitivity on sign of λdu

• Precision: |λdu| ∈ [0.80,1.04]

Ulrike Schnoor (Freiburg) Higgs Decays to Bottom and Tau Pairs 25/27

Introduction H → bb H → τ+τ− Interpretations Summary

Leptons vs. Quarks

• Many BSM extensions (e.g.2HDMs) predict λ`q 6= 1

• Quark couplings mainly probed byGGF, H → γγ, H → bb

• Lepton couplings probed byH → ττ

• Precision: |λ`q | = 1.06+0.15−0.18 JHEP 08 (2016) 045

λ`q = κ`/κq

λVq = κV/κq

κqq = κq · κq/κHUlrike Schnoor (Freiburg) Higgs Decays to Bottom and Tau Pairs 26/27

Introduction H → bb H → τ+τ− Interpretations Summary

Summary

• Evidence for H → bb still not reached (measured significance in Run-1combination 2.6σ [exp. 3.7σ])

→ Quite challenging; search over many production modes• Evidence for H → ττ established at 7, 8 TeV (measured significance in

Run-1 combination 5.5σ [exp. 5.0σ], coupling precision ∼ 14 %)→ Rediscovery at 13 TeV ongoing• Challenging decay channels with state-of-the art techniques• Results interpreted in terms of coupling modifiers for up-type vs.

down-type fermions as well as leptons vs. quarks: in good agreementwith SM predictions

• Results being updated with full 2015+16 datasets

Ulrike Schnoor (Freiburg) Higgs Decays to Bottom and Tau Pairs 27/27

Introduction H → bb H → τ+τ− Interpretations Summary

Discussion points

• Mostly still statistics dominated (also via background estimates), but:Theory uncertainties (QCD scale, jet bin migration) start to play a role

• Measuring “Higgs properties” in a model-independent way: limitations ofκ-framework, limited validity of EFT, Templified Cross Sections, ...

• Multivariate Analyses: Matrix-Element/Optimal Observable Methods vs.Machine Learning

Ulrike Schnoor (Freiburg) Higgs Decays to Bottom and Tau Pairs 28/27

Introduction H → bb H → τ+τ− Interpretations Summary

BACKUP

Backup

Ulrike Schnoor (Freiburg) Higgs Decays to Bottom and Tau Pairs 29/27

Introduction H → bb H → τ+τ− Interpretations Summary

Coupling modifiers

Fermionic Higgs coupling:

• From SM Lagrangian: Mass term ∼ mFv f f for fermion-Higgs coupling

• From definition of coupling modifiers: κ2F =

ΓmeasiΓSM

i∼ g2

v (meas)g2

v (SM)⇒ κF =

gmeasFgSM

F

• Thus, the quantity that is proportional to the particle’s mass in the SM is

κFmF

v(SM)∼ gF ∼ mF

→ probes the proportionality of the coupling to the mass

Bosonic Higgs coupling:• From SM Lagrangian: Mass term ∼ m2

V VµVµ

• Thus, in the SM, this is proportional to the particle’s mass:

√κv

mV

v(SM)=

mV

v∼ mV

Ulrike Schnoor (Freiburg) Higgs Decays to Bottom and Tau Pairs 30/27

Introduction H → bb H → τ+τ− Interpretations Summary

B-Tagging in ATLAS

• possible offline and in HLT• generally: exploiting relatively long lifetime and large mass of b-hadrons• require presence of tracks with large impact parameter w.r.t. primary

vertex, plus secondary decay vertices• this information is combined into a single neural-network discriminant• several working-points in terms of efficiency/purity and mis-tag rates are

provided• Reference: JINST 11 (2016) P04008, arXiv:1512.01094 [hep-ex].

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Introduction H → bb H → τ+τ− Interpretations Summary

VBF H → bb+ jj

ATLAS AnalysisOffline selection:• events with exactly 4 jets with pT > 50 GeV, |η| < 4.5• jets ordered in eta: η1 < η2 < η3 < η4

• jets with smallest and largest etas are the tagging jets: J1, J4• the other two jets (J2, J3) are associated with Higgs decay, required to be

b-tagged and |η| < 2.5• trigger matching• pT(bb) > 100GeV (due to the 50 GeV cut on the jet pT, the Mbb

distribution of non-resonant backgrounds is shaped in such a way that itgets a peak around 130GeV. This is removed by requiring the pT of thebb system to be pT(bb) > 100GeV )

Ulrike Schnoor (Freiburg) Higgs Decays to Bottom and Tau Pairs 32/27

Introduction H → bb H → τ+τ− Interpretations Summary

VBF H → bb+ jjATLAS AnalysisMVA analysis: BDT• trained to discriminate between VBF H → bb signal and non-resonant

backgrounds using data in the side-band regions (Mbb: 70..90 GeV and150..190GeV)

• chose input variables which exploit the difference in topologies betweensignal and bg while keeping them as uncorrelated as possible with Mbb.Most important discriminating variables:

• jet width (for jets with |η| < 2.1) to distinguish between quark and gluonjets

• scalar sum of pT of additional jets with pT > 20 GeV in the region|η| < 2.5 (central jet activity)

• Mjj, |∆η(jj)|• max( |η1|, |η2| )• separation between tagging jets’ average |η| and that of the Higgs jets• polar angle of the cross product of the tagging jets momenta (∆φ(jj))

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Introduction H → bb H → τ+τ− Interpretations Summary

H → ττ

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Introduction H → bb H → τ+τ− Interpretations Summary

CP-odd observables

CP-odd observables have non-zero mean in presence of CP violation (modelindependent→ EFT used for quantification)

Signed azimuthal difference of jets Sgnd∆φ(jj)

• Sgnd∆φ(jj) = φ(jη>0)− φ(jη<0)

• non-signed ∆φ(jj) is CP-even

Ulrike Schnoor (Freiburg) Higgs Decays to Bottom and Tau Pairs 35/27

Introduction H → bb H → τ+τ− Interpretations Summary

CP-odd observables

Optimal Observable OO

OO =2<(M∗SMMCP−odd)

|MSM |2

• defined with the reconstructed four-momenta of particles in the events(tagging jets, Higgs reconstructed with MMC)

• Why optimal?→ combines the information on the entire phase space in one scalarvariable (for small d)[D. Atwood, A. Soni; Phys. Rev. D45 (1992)] , [M. Davier et al.; Phys. Lett. B306 (1993)] ,[M. Diehl, O. Nachtmann; Z. Phys. C62 (1994)]

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