10th Anniversary of the Higgs
Boson Discovery-What we learned in a decadeTatsuya Masubuchi
University of Tokyo (ICEPP)

2022/9/28

Outline• The role of the Higgs boson in Standard Model• Short history of discovery for Higgs boson at LHC

 Mass measurement
 Coupling and cross section measurement
 Width measurement
 Higgs self-coupling measurement• Summary and Future
Credit: Abigail Malate for Physics TodayTGSW 2022• What we learned in a decade since discovery

22022/9/28

A Higgs boson and the Standard Model

19 unpredictable parameters in the SM
15/19 parameters are related to the Higgs
boson!!
Higgs boson has “special role” in the SMTGSW 2022

Higgs boson was the lastundiscovered particle in the SM3

2022/9/28

Higgs Sector

TGSW 2022

SM Lagrangian42022/9/28

SM Lagrangian
Higgs potential
𝑉 𝜙 = 𝜇! 𝜙 " 𝜙 + 𝜆 𝜙 " 𝜙!

+ ⋯ (µ! < 0, λ > 0)

usly noentan y brokeopSetrm
symv=

Expand around new vacuum

𝜙=𝜇!
−2𝜆10
2 𝑣+ℎ%

Higgs mass term
𝑚! =2𝜆𝑣 " =Higgs self-coupling−2𝜇"Potential parameters are unpredictable⇔ Higgs mass is unpredictableè Need to measure experimentally!!

TGSW 2022$𝑉 𝜙 = 𝜆𝜐 ! ℎ! + 𝜆𝜐ℎ# + 𝜆ℎ% + ⋯5

2022/9/28

SM LagrangianInteraction between Higgs andGauge bosons
Describe Gauge boson mass term and interactions(Electroweak symmetry breaking)

𝑚&!
∝𝑣

𝜙=𝑔𝑣
1
"𝐷# 𝜙 = ( )"𝑊#$𝑊# % +22

W boson mass𝑔" + 𝑔&"𝑣
2"

𝑍# 𝑍#Z boson mass

W/Z bosons acquire mass by vacuum shift 0àv(γ keeps massless)TGSW 2022

mass term

1 02 𝑣

6

2022/9/28

SM LagrangianInteraction between Higgs andFermions (Yukawa sector)
Describe fermion mass term and Yukawa interactions

𝑚'𝑣

Fermion (charged lepton/quark) mass is given by
proportional of Yukawa coupling in the SM
1𝑚' =𝑦' 𝑣
2However, Yukawa couplings are just parameters in SM
è Can’t predict fermion mass
è Different Yukawa coupling makes ”generation” offermionsTGSW 2022∝7

2022/9/28

LHC-ATLAS experiment• LHC (Large Hadron Collider) is proton-proton collider

located underground in suburban of Geneva~140/fb
è ~8M Higgs!!

~10/fb (Just started!!)ATLAS detector

TGSW 2022~25/fbè ~0.6M Higgs!

Multi-purpose 4π detector82022/9/28

LHC-ATLAS experimental conditions

ATLAS Event display in2018 data
28 pileup on top of Zàμμ(red lines) eventsTGSW 2022

In Run2 13 TeV condition~30 Pile-up events on averageè Experimentally severe
condition to identify interestingevents
è Many efforts to mitigate pileup effect9

2022/9/28

Higgs decay branching ratio• Higgs boson decays to other SMparticles
 Observed Higgs mass(~125 GeV)
is experimentally really good Higgs boson is able to decay
various particles
è Property measurement withdifferent decay modes
 Hàbb decay mode is dominantWW gg

ττ

58%

21% 8.2% 6.3% 2.9% 2.6%γγZγ

μμ0.23% 0.15% 0.022%

ccZZ

TGSW 2022bb10

2022/9/28

Higgs Production at LHC• Gluon-fusion process is dominant at LHC (Gluon collider!!)

VBF ~7%

VH ~4.5%

ttH ~1.4%• Higgs physics strategy is built by the combination of production and decay (cannot observe all Higgs events experimentally!!)Ø Lots of QCD background, not triggerable, detector coverage...

TGSW 2022

ggF ~88%112022/9/28

Summary of Higgs physics strategy• Clean (bosonic decay) modes contributed to the discovery!!

(Discovered by ggF process) Contribute to various property measurements as well
 Fermionic decay modes are particularly important for Yukawa measurement
mH=125 GeV

Hàbb

HàWWàlvlv

HàZZà4l

Hàγγ

Hàττ

Hàμμ

Hàcc

ggF

△

★

★ Discovery★

Discovery◎△✕◎◎◎△◎MassCross section
CouplingSpin/CPmeasurementVBF○VH(WH/ZH)◎○

Cross section
Coupling
Spin/CP, widthmeasurementttH○ y measurement ○yt measurement

bbH✕✕✕tH△△✕ybmeasurement

◎t△

Mass
Cross section
Coupling
Spin/CP, widthmeasurement○△ytauCrosssection
CP△
△ymu✕
△△△

✕

✕

✕

✕

✕

△

✕✕

✕

For example:
σ(ggF)*BR(Hàbb)*Lumi(Run2) = 55.7pb*0.58*139000/pb = ~4.5M event (not triggerable)σ(ggF)*BR(Hàγγ)*Lumi(Run2) = 55.7pb*0.0023*139000/pb = ~18K eventsσ(ggF)*BR(Hàμμ)*Lumi(Run2) = 55.7pb*0.00022*139000/pb = ~1.7K events

ycTGSW 2022

Discovery12• A Higgs boson was discovered in 2012 in ATLAS and CMS experiments

HàWWà2l2ν
HàZZà4l• Excellent mass resolution ~1-2% • Higher event yield but large• Branching fraction is very low.
variety of difficult backgrounds~ 0.02%• mass resolution ~10 GeV due• Clean background (good S/B)
to neutrino in the final state

TGSW 2022

Hàγγ• Excellent mass resolutionσ(mγγ)~1% è Narrow mass peakcan be observed on top of largebackground2022/9/28

Higgs Discovery at Run1 (2012)

Significance (local) 4.7σ
@126.5 GeV

Significance(local) 3.4σ@125 GeV

Significance (local) 2.8σ
(2.3σ exp.)@125 GeV13

• A Higgs boson
th was discovered in 2012

July 4 2012
Hàγγ Observed ~ 5σ excess
HàZZà4l• Excellent mass resolution ~1• Branching fraction is very low
around 125 GeVσ(mγγ)~1%• BR(HàZZà4l) ~ 0.02%• Background is huge but narrow
peak can be observed• Excellent mass resolution~1-2%σ(mγγ)~ a few GeV and clean
background2022/9/28

Higgs Discovery at Run1 (2012)
HàWWà2l2ν
• Higher Event yield but large
variety of difficult background• mass resolution is ~10 GeV due
to neutrino in the final state

TGSW 2022

2013 Nobel Prize

Significance (local) 4.7σ
@126.5 GeV

Significance (local) 2.8σ
(2.3σ exp.)
Significance(local)
3.4σ
http://www.nobelprize.org/nobel_prizes/physics/laureates/2013/@125 GeV@125 GeV14

• Higgs discovery provides rich physics topics

2022/9/28

Higgs Measurements in 10 years• Higgs boson properties have been measured for ~past 10 years• Show highlights of the resent results up to LHC-Run2

WidthSpin/CP

MassHiggs Potential,
Self coupling

BSM Higgs Search
Rare/Exotic decay

TGSW 2022

Coupling to the SM(boson and fermion)

Cross-Section(Differential)

15/80

2022/9/28

Higgs Mass measurement• Higgs mass is one of unpredictable parameters• Measure using excellent mass resolution channel(Hàγγ, HàZZà4l)• ATLAS recently measured mass in 4l channel

Verwit y niceh fupll R eakun2
data!!

124.99±0.18(stat)±0.04(sys) GeV
0.15% precision!! still statistically dominatedc.f. CMS (Run1+partial Run2) 125.38±0.14 GeVTGSW 2022

 Use Neutral Network to discriminate signal frombackground Predict uncertainties (detector region, e or μ,kinematics) event-by-event
è Multi-dimensional fit (mH vs NN score, uncertainty)

16

2022/9/28

Hàττ measurement• Provide an unique opportunity to accesstau-Yukawa coupling• Observed Hàττ signal b. > 5σ in Run1
• Events are categorized by the tau decay mode and Higgs

production channel×Higgs production
ggF(pTττ>100 GeV)
VBF, VH, ttH

 Measured VBF cross section
0.197±0.041pb (SM pred. 0.220±0.005pb), 5.3σ (6.2σexp.)
 Measured ggF cross section
2.65±0.85pb (SM pred. 2.77±0.09pb), 3.9σ (4.6σ exp.)
è Consistent with SMTGSW 2022

Tau decay mode
τlepτhad, τhadτhad, τlepτlep

17• Hàμμ is an unique opportunity to study 2ndgeneration lepton Yukawa coupling

HighLow

2022/9/28

Hàμμ measurement
S/B• BR is extremely small (0.022%)• Mass resolution is good (~3 GeV), but background ishuge (DY Zàμμ tail)• Crucial to extract better S/B region in each production

mode using Machine learning technique (BDT)

2.0σ (1.7σ exp.)• Upper limit on BR(Hàμμ) < 0.047%

c.f. CMS: 3.0σ (2.5σ exp.)

TGSW 2022• Observed significance182022/9/28

Hàbb measurement• Measurement of bottom Yukawa coupling• Dominant branching ratio (58%)• VH channels are most sensitive (difficult to trigger ggF/VBF Triggered by Zàνν(MET), Wàlν, Zàll
 Require 2 b-tagged jets
 Select high pTV to enhance S/B
0-lep
1-lep
2-lep

×2 b-jet
2 b-jet+1jets

×75 < pTV < 150
150 < pTV < 250
250 < pTV

Main background
Z+jets(0/2 lep), W+jets(0/1 lep)
ttbar, single top(0/1/2 lep)
diboson(0/1/2 lepton)è Utilize machine leaning todiscriminate signal

TGSW 2022

and background is huge)192022/9/28

Hàbb measurement• Measurement of bottom Yukawa coupling• Dominantbranching ratio(58%)
Final discriminant(BDToutput)• VH channels are most sensitive (difficult to trigger ggF/VBF

and background is huge)
2lepton1lepton Triggered by Zàνν(MET), Wàlν, Zàll
 Require 2 b-tagged jetsV
75 < pTV < 150 Select high2pb-jet
T to enhance S/B

×
0-lep
1-lep ×2-lep

Main background
Z+jets(0/2 lep), W+jets(0/1 lep)
ttbar,singletop(0/1/2lep)
150 < pTVin< 250
Signalcanbeenhanced
thehighBDT
outputscore
region×
2 b-jet+1jets
250
diboson(0/1/2 lepton)
75highest< pT<V pTV
< 150
S/B>
1in2leptonscorebin2 b-jet
150 < pTV < 250×è Utilize machine leaning to2 b-jet+1jets
V250 < pT
discriminate signal

TGSW 20220lepton20• Signal strength μ = (σ×BR)measure/ (σ×BR)SMSignificance
WH: 4.0σ (4.1σ exp.)
ZH: 5.3σ (5.1σ exp.)
VH(WH+ZH): 6.7σ (6.7σ exp.)

Cross-check in mbb fit
5.5σ (4.9σ exp.)

TGSW 2022

2022/9/28

Hàbb measurement21

2022/9/28

Hàcc measurement• Charm Yukawa measurement• Small branching ratio (2.9%), huge SM backgroundèSuper difficult measurement at LHCèSimilar analysis strategy to VH(àbb), but dedicated c-tagging has been developed

WHàlvcc
ZHàllccTGSW 2022

ZHàvvcc

No significant Hàcc signal is observed222022/9/28

Hàcc measurement• Charm Yukawa measurement• Small branching ratio (2.9%), huge SM backgroundèSuper difficult measurement at LHCèSimilar analysis strategy to VH(àbb), but dedicated c-tagging has been developed

WHàlvcc
ZHàllcc

Upper limit: 26*SM (31*SM exp.)
CMS recently developed boosted Hàcc tagger: 14.4*SM (7.6*SM)No significant Hàcc signal is observed

TGSW 2022

ZHàvvcc23

2022/9/28

Higgs Combination• Analyzed many production and decay modes

Input channels

Hàbb

HàWW

HàZZ

Hàγγ/Zγ

Hàττ

HàμμHàcc

ggF

✔

✔

✔

✔

✔

✔

-VBF

✔

✔

✔

✔

✔

✔

-VH(WH/ZH)✔

△(partial data) ✔

✔

✔

-

✔

ttH

✔

△(partial data) △(partial data)✔

△(partial data)

--tH

✔

△(partial data) △(partial data)✔

△(partial data)--

TGSW 2022

è Statistical combination of all channels to obtain the most precise
measurements24• Main Production channels and decay modes are already2022/9/28

Higgs combined Results ~Production/Decay~
observed in Run1 and Run2 data

5σ5σ5σ5σ5σggF

VBFWH

ZH

ttH

7%

13%23%22%24%

5σ

5σ

5σ

5σ

5σ

tH

Hàbb

HàWW

Hàττ

HàZZ

Hàγγ

HàZγ

~70%

15%

12%

12%

11%

10%~100% ~60%

HàμμNo any significant deviation from SM (10-20% precision for main channels)

TGSW 2022

Start accessing
rare productionand decay mode25

2022/9/28

Higgs combined results ~Coupling~• Measured couplings between Higgsboson and SM particles

κ-framework: 𝜿 = 𝒈𝒎𝒆𝒂𝒔𝒖𝒓𝒆/𝒈𝑺𝑴𝒙𝒙
𝜎 𝑝𝑝 → 𝑉𝐻 & 𝐵𝑅 𝐻 → 𝑏𝑏!'( "!)(= ( 𝜎#$ & 𝐵𝑅%&!*• Coupling modifier κt, κb, κτ, κμ, κW, κZ, (kc)(measured coupling normalized to SM)• Precision is 7-11% for top, W/Z,

 Yukawa coupling works well in 103 different

scale (O(100 MeV) ~ O(100 GeV)!!
 Higgs boson builds generation of quark andleptonYμ≪ Yτ, Yc < Yb ≪Yt~30%~7%~11%~6%~7%TGSW 2022bottom, τ, ~30% for μ10326

2022/9/28

Higgs combined results ~Coupling~• Measured effective photon, gluon

TGSW 2022

couplings as free parameters (loopdiagram)

BR(Hàinv) < 0.13% @95% CL27

• Inclusive measurement

2022/9/28

Cross Section Measurement

In high statistics Run2 data,
Differential cross section corner ofphase space (high-pTH)
è Sensitive to BSM (even higher
energy scale than LHC)è Effective field theory may reveal
BSM model if observe deviationfrom SMTGSW 2022

à differential measurement28• Differential cross section measurement in Hàγγ, HàZZà4l

2022/9/28

Cross Section Measurement• gluon fusion pTH shape is sensitive to κc, κbè Constrain themStronger constraintTGSW 2022

Combine direct VHbb/cc andindirect diff Xsec measurement292022/9/28

Higgs Width measurement• Higgs boson natural width : ΓHSM ~4.07 MeV

 impossible to measure by direct measurement!
• Measure Higgs width indirectly from on-shell/off-shellmeasurement

𝑩𝑹(𝑯 → 𝒁𝒁)
𝜞𝑺𝑴𝑯𝝁𝒐𝒏)𝒔𝒉𝒆𝒍𝒍 = 𝝁𝒈𝒈×= 𝝁𝒈𝒈𝝁𝒁𝒁
𝑩𝑹𝑺𝑴(𝑯 → 𝒁𝒁)
𝜞𝑯𝝁𝒐𝒇𝒇)𝒔𝒉𝒆𝒍𝒍 = 𝝁𝒈𝒈𝝁𝒁𝒁

𝝁𝒐𝒇𝒇Y𝒔𝒉𝒆𝒍𝒍𝜞𝑯= 𝑺𝑴𝝁𝒐𝒏Y𝒔𝒉𝒆𝒍𝒍𝜞𝑯

off-shell upper limit 3.4×SM(3.8×SM)è ΓH < 14.4 (15.2 exp.) MeV @ 95%CL(to be updated with full Run2 data)

TGSW 2022

Interfere30

2022/9/28

Higgs Self-coupling• Is Higgs potential shape really SM-like?
• What’s the mechanism of EW Phase transition?

è Still remaining mystery in Higgs properties
è Anomaly in the Higgs self-coupling?
• Need to observe Higgs pair production to measure

Higgs self-coupling(λ) and understand Higgs potential• DiHiggs production cross section is ~31fb at 13 TeV(1000 times smaller than single Higgs production)• 4b, 2b2τ, 2b2γ final states are sensitive

TGSW 2022

Interfere31• At the beginning of LHC, we thought the observation of Higgs pair production is2022/9/28

Higgs Pair Production Search
impossible in LHC (even HL-LHC)• Many improvements on the analysis (b-tagging algorithm and multi-variate analysis

2.4×SM

HH combination: -0.6 < κλ < 6.6 @ 95% CL
HH+H combination: -0.4 < κλ < 6.3 @ 95% CL

TGSW 2022

using machine learning) è Not a dream to observe Higgs pair production in Run3!!

32

2022/9/28

Summary and Future• After Higgs boson discovery in 2012, ATLAS has measured the Higgs

boson properties as precisely as possible in various ways
 Major production and decay modes are observed in Run1+2 data
 Established the Higgs boson coupling to 3rd generation fermion• There are many many interesting Higgs measurements can’t show today

https://twiki.cern.ch/twiki/bin/view/AtlasPublic/WebHome#Recent_Results
Today!

Hàμμ
Observation?

HH production
Evidence?

HHobservation?

We analyzed only 5% data of total data!! Higgs physics may open BSM door!!

TGSW 2022

Hàcc?

33

TGSW 2022

Backup34

2022/9/28

2022/9/28

Tevatron Flying?

TGSW 2022• 2012/7/3

35

2022/9/28
TGSW 2022

Improvement of analysis

36

2022/9/28

ttH Production• Direct measurement of Higgs interaction withtop quark• ttHàγγ (cleanest channel)• ttHàbb(High yield but difficult tt+bb

background)• ttHàWW/ZZ/ττ (relatively clean, butcomplicated final states)

ttH(àγγ)Observed significance: 5.2σ(4.4σ exp.)
Measured Cross-section
μttH=1.43±0.33(stat)±0.21(syst)

TGSW 2022

ttH(àbb)
Observed significance: 1.0σ(2.7σ exp.)
Signal strength μ = 0.35+0.36-0.34

37

31.05fb• σ(VBFàHH):1.73fb
Cross section
enhancement
depends on κλMinimum at kλ=2.45
SM

TGSW 2022• σ(ggFàHH):2022/9/28

Higgs pair production382022/9/28

HHà4b analysis• Highest branching ratio BR(HHà4l) ~34%
• Background is QCD multi b-jets è difficult to model in MC• Extrapolate from 2b+2jet CR to 4b SR using reweighting factor estimatedfrom NN in data-driven way

4b data

TGSW 2022

2b data39

2022/9/28

HHà2b2γ analysis• Select diphoton and 2 b-jets• Event categorization based on mbbγγ* andBDT score mbbγγ* = mbbγγ – mbb – mγγ+250 GeVTGSW 2022• mγγ distribution is used as final discriminant40

2022/9/28

HHà2b2τ analysis• Require 2τ and 2 b-jets• τhadτhad and τlepτhad channels are used
• Further subdivided by the trigger• MVA scores are used as finalTGSW 2022

discrimenant412022/9/28Single Higgs contribution• Single Higgs production modes also have self-coupling contribution via NLO EW correction• κλ dependence by a function of Higgs pT

 ttH has highest dependence• Perform combined fit with single Higgs and HHTGSW 2022

analysis è Possible to constrain other coupling
parameters (κt) simultaneously

422022/9/28

Comparison with CMS• ATLAS upper limit at 95% CL

 Observed < 2.4*SM Expected < 2.9*SM

CMS upper limit at 95% CL
 Observed < 3.4*SM Expected < 2.5*SMTGSW 2022•43