Pursuit of the top-Higgs Coupling at the LHChome.kias.re.kr/MKG/RS/ksk/talk/Chris-Neu.pdf ·...

Pursuit of the top-Higgs Coupling at the LHC

Prof. Chris Neu (크리스뉴)Department of PhysicsUniversity of Virginia

Workshop on the Search for New Physics

through the Higgs BosonGwangju, Korea18 October 2016

Discovery of the Higgs Boson

Christopher Neu 2

Celebrations on

4 July 2012

CERN and Melbourne and many other places

around the globe

Both CMS and ATLAS reporting

5 sigma evidence for a new particle with mass ~ 125 GeV


• Nobel Prize in Physics awarded to Peter Higgs and Francois Englert

• ”…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"

Christopher Neu 3


• This discovery has been billed as one of the most important scientific discoveries of the last half-century

• A great advance in our understanding of the dynamics of the fundamental world

• Yet, even now, our work is not yet done.

• Much remains to be known about this particle

Christopher Neu 4

Outline:– Characterizing the observed Higgs particle

– Missing Piece: The top-Higgs coupling

– Status of the top-Higgs coupling pursuit at CMS

– Summary and looking forward

Christopher Neu 5

The Post-Discovery Era

• Long-sought Higgs boson finally discovered – Culmination of decades-long pursuit

• LEP experiments through 2000

• Tevatron through Runs 1 and 2

– Finally, at the LHC:

• 2012: “A new particle…”

• 2013: “A Higgs boson...”

• For the remainder of LHC Run 1, the focus shifted from SM searches to measurements and BSM insights:– Is this the Higgs boson of the SM?

– Are there any other Higgs bosons to observe?

– Is this Higgs boson a window to new physics?

Christopher Neu 6

Phys.Lett. B71(2012) 1-29

Phys.Lett. B71(2012) 30-61

The Large Hadron Collider

Christopher Neu 7

• A proton-proton collider

– 27 km in circumference

– Beamlines ~100m underground

– Four active interaction regions

– Proton-proton collisions with up to 7+7 = 14 TeV

• Significant opportunity for discovery:

– Highest energies ever achieved…

– …at unprecedented collision rate

– A perfect place for searching for the Higgs

√s = 7 TeV (2010,2011)8 TeV (2012)

13 TeV (2015,2016)

LHC Performance

Christopher Neu 8

Integrated Luminosity, L:

measure of number of

collisions. Units: fb-1 “inverse

femto-barns”

Related to cross-section, σ:

“Rate” of a certain process.

Units: fb

LeventsN

SM Higgs Boson Production at the LHC

Christopher Neu 9

gluon-gluon fusion (ggF)

vector-boson fusion (VBF)

W/Z associated production (VH)

More on ttH, tH production later…

MH

=125

SM Higgs Boson Decay Channels

Christopher Neu 10

Rarer decay modes suffer from statistics but generally have lower levels of mundane processes (“background”) obscuring the signal AND have a higher resolution on the mass of the Higgs before decay.

MH

=125

H bb

H WW

H ττ

H ZZ

H γγ

Primary Decay Channels

The Experiments: CMS and ATLAS

Christopher Neu 1111

3.8T Solenoid

ECAL76k scintillating

PbWO4 crystals

HCAL Scintillator/brass

Interleaved ~7k ch

• Pixels (100x150 mm2)

~ 1 m2 ~66M ch

•Si Strips (80-180 mm)

~200 m2 ~9.6M ch

Pixels & Tracker

MUON BARREL250 Drift Tubes (DT) and

480 Resistive Plate Chambers (RPC)

473 Cathode Strip Chambers (CSC)


MUON ENDCAPS

Total weight 14000 tOverall diameter 15 mOverall length 28.7 m

IRON YOKE

PreshowerSi Strips

~16 m2

~137k ch

Foward CalSteel + quartz

Fibers ~2k ch

The Experiments: CMS and ATLAS

Christopher Neu 1212

3.8T Solenoid

ECAL76k scintillating

PbWO4 crystals

HCAL Scintillator/brass

Interleaved ~7k ch

• Pixels (100x150 mm2)

~ 1 m2 ~66M ch

•Si Strips (80-180 mm)

~200 m2 ~9.6M ch

Pixels & Tracker

MUON BARREL250 Drift Tubes (DT) and


473 Cathode Strip Chambers (CSC)


MUON ENDCAPS

Total weight 14000 tOverall diameter 15 mOverall length 28.7 m

IRON YOKE

PreshowerSi Strips

~16 m2

~137k ch

Foward CalSteel + quartz

Fibers ~2k ch

Particle Detection and Subsystems

Higgs characterization campaign

utilizes every subsystem of each detector.

Christopher Neu 13

The SM Higgs Program at the LHC

• Comprehensive Higgs pursuit campaign – Survey of each accessible production mechanism for each primary

decay mode

• All channels contribute to cumulative picture

Christopher Neu 14

MH

=125

MH

=125

Is this the Higgs Boson of the Standard Model?

Christopher Neu 15

Transition from Searches to Measurements

• In the post-discovery era, focus moves from search to precision measurements– First task:

• Need precise measurements of all of its properties

• Is this the Higgs Boson of the Standard Model?

Christopher Neu 16

gHff =m f

vgHVV = 2

mV2

v

L = Dmf( )*

Dmf( ) - m2f 2 + lf 4( ) -1

4FmnFmn

• Characteristics of the SM Higgs:– Decay modes

– Mass

– Spin and parity

– Width

– Couplings to other SM particles

– Self-coupling

mH = 2lvλ, μ unknown mH is a free parameter of the SM

Transition from Searches to Measurements

• In the post-discovery era, focus moves from search to precision measurements– First task:

• Need precise measurements of all of its properties

• Is this the Higgs Boson of the Standard Model?

Christopher Neu 17

gHff =m f

vgHVV = 2

mV2

v

L = Dmf( )*

Dmf( ) - m2f 2 + lf 4( ) -1

4FmnFmn

• Characteristics of the SM Higgs:– Decay modes

– Mass

– Spin and parity

– Width

– Couplings to other SM particles

– Self-coupling

mH = 2lvλ, μ unknown mH is a free parameter of the SM

Higgs Decays to Bosons

Christopher Neu 18

EPJC 74 (2014) 3076 Phys. Rev. D. 89, 092007 (2014) JHEP 01 (2014) 096

Phys. Rev. D. 90, 112015 (2014) Phys. Rev. D 91, 012006 (2015)

HZZl+l-l+l-

Z /

γ*

l+ l- l

+ l-

Phys. Rev. D 89, 092007 (2014)

Phys. Rev. D 92, 012006 (2015)

Hγγ HZZ HWW

Legacy 7+8 TeV Results

Higgs Decays to Fermions

Christopher Neu 19

Hττ

JHEP 05 (2014) 104

Hbb

Phys. Rev. D 89, 012003 CMS-HIG-14-004, submitted to PRD

VH modes VBF

JHEP 04 (2015) 117 JHEP01(2015)069

VH modes


Primary Higgs Decay Channels: Summary of Individual Analyses

• Significance of Observation:– “3σ significance” implies only 1 in 740 probability for background-only

data to fluctuate to produce observed excess.

– “5σ significance” = 1 in 3.5 million

• Signal Strength, μ:– How much signal is observed, in units of the amount predicted from the

Standard Model for each search channel (μ = 1.0 implies SM level) Christopher Neu 20

Decay Mode

PrimaryProduction Mechanism

Significance of Observation

(σ = standard deviation)

CMS ATLASExpected Observed Expected

Observed

Signal Strength

μ = σ/σSM

CMS ATLAS

Hbb VH, VBF, ttH 2.7σ 2.6σ 2.8σ 1.8σ 1.03 +0.44-0.42 0.52 ± 0.40

HWW ggF, VBF 5.8σ 4.3σ 5.8σ 6.1σ 0.72 +0.20-0.18 1.16 +0.24

-0.21

Hττ ggF, VBF, VH 3.7σ 3.2σ 3.4σ 4.5σ 0.78 ± 0.27 1.43 +0.43-0.37

HZZ ggF, VBF 6.7σ 6.8σ 6.2σ 8.1σ 0.93 +0.26-0.23

+0.13-0.09 1.44 +0.40

-0.33

Hγγ ggF, VBF 5.2σ 5.7σ 4.6σ 5.2σ 1.14 +0.26-0.23 1.17 ± 0.27


Primary Higgs Decay Channels: Summary of Individual Analyses

• Significance of Observation:– “3σ significance” implies only 1 in 740 probability for background-only

data to fluctuate to produce observed excess.

– “5σ significance” = 1 in 3.5 million

• Signal Strength, μ:– How much signal is observed, in units of the amount predicted from the

Standard Model for each search channel (μ = 1.0 implies SM level) Christopher Neu 21

Decay Mode

PrimaryProduction Mechanism

Significance of Observation

(σ = standard deviation)

CMS ATLASExpected Observed Expected

Observed

Signal Strength

μ = σ/σSM

CMS ATLAS

Hbb VH -- -- 1.9σ 0.42σ -- 0.21+0.51-0.50

HWW ggF, VBF -- -- -- -- -- --

Hττ ggF, VBF, VH -- -- -- -- -- --

HZZ ggF, VBF 6.5σ 6.2σ DNR DNR 0.99+0.33-0.26

DNR

Hγγ ggF, VBF 6.2σ 5.6σ 5.4σ 4.7σ 0.95+0.21-0.19 0.85+0.23

-0.20

13 TeV/2016 data sample

Results!

Grand Combination: Signal Strengths

• Combine results from every channel accounting for stat and syst uncertainties, and their correlations

• Profile likelihood fits are performed with nuisance parameters profiled

Christopher Neu 22

EPJC 75 (2015) 212

Eur. Phys. J. C76 (2016) 6

Signal strengths for primary decay modes

look very SM-like


See also JHEP08(2016)045 for CMS+ATLAS combined results

Grand Combination: Signal Strengths

• Combine results from every channel accounting for stat and syst uncertainties, and their correlations

• Profile likelihood fits are performed with nuisance parameters profiled

Christopher Neu 23

EPJC 75 (2015) 212 Signal strengths for primary production

mechanisms look very SM-like

More on ttH production in a few slides…

Eur. Phys. J. C76 (2016) 6



Grand Combination: Couplings to Particles of the SM

Christopher Neu 24

gHff =m f

vgHVV = 2

mV2

v

gHff = k f

m f

v

æ

èç

ö

ø÷gHVV = kV 2

mV2

v

æ

èç

ö

ø÷

• Look for deviations from SM predictions for couplings to vector bosons and fermions

• Here assume same κV for each vector boson, and same κf for all fermions

• (κV, κf) = (1.0, 1.0) is the SM

• Data prefers SM-like couplings – agreement within 1σ

EPJC 75 (2015) 212

Stretch

ed to

match

scale belo

w.

Eur. Phys. J. C76 (2016) 6



• Can test alternative models:– Here is considered one where only SM particles are allowed in loops and no invisible

or undetected Higgs boson decay are allowed.

– Look at scaling factors for each prominent accessible coupling

Christopher Neu 25

EPJC 75 (2015) 212


Eur. Phys. J. C76 (2016) 6


• Can test alternative models:– Here is considered one where only SM particles are allowed in loops and no invisible

or undetected Higgs boson decay are allowed.

– Look at scaling factors for each prominent accessible coupling

Christopher Neu 26

EPJC 75 (2015) 212 Eur. Phys. J. C76 (2016) 6



• Many other alternative models tested– Here shown CMS results only, but similar ones from ATLAS available

Christopher Neu 27

Test for W/Z asymmetry

Test for up-type/down-type asymmetry

Test for lepton-quark asymmetry

Test for common V and f coupling

adjustments

Test for coupling adjustments in loop-

induced processes

Test for BSM content in (kglu,kγ, BRBSM) model

Test of model with 6 free scaling parameters, one

fit at a time while profiling others.

EPJC 75 (2015) 212

• Tests of all schemes are largely consistent with expectations from the SM

• Interesting:

– Most general model tested (no assumption on width, non-SM couplings allowed in loops as well as direct interaction) sets limits on BR(HBSM) in [0.00,0.57] at 95% CL

– Still lots of room for BSM influence in Higgs couplings

See also

JHE

P08(2016)045 fo

r CM

S+A

TL

AS

com

bin

ed resu

lts

More on the Higgs Couplings

Christopher Neu 28

• So the couplings are all understood then, yes?

More on the Higgs Couplings

Christopher Neu 29

• So the couplings are all understood then, yes?

No

Higgs and Top

• Workhorse analyses already probe the top-Higgs coupling, though there are issues…

• Consider gluon fusion:

Christopher Neu 30

MH

=125

Higgs and Top

• Workhorse analyses already probe the top-Higgs coupling, though there are issues…

• Consider gluon fusion:

• But what about this?

Christopher Neu 31

MH

=125

X

X

X

Higgs and Top

Christopher Neu 32

• Similar problems on the decay side:

Ht

t

t

γ

γ

HX

X

X

γ

γ

…but what about…

The results of the multi-channel fits indicate that the couplings that “are SM-like”.

To really know what is going on, we need a direct probe of the top-Higgs coupling…

The top-Higgs Coupling

Christopher Neu 33

• Fermionic couplings:– LHC analyses have so far only been able to probe

the Yukawa couplings Yb and Yτ directly

• Within the SM, the Higgs coupling to the top quark, Yt, is predicted to be by far the largest of all the fermionic couplings

– mtop implies relatively large coupling, Yt ~ 1

• ~x30 larger than Yb

• ~x100 larger than Yτ

– Strongest coupling among all known SM particles

– Indicative of privileged role in EWSB dynamics?

• Also Yt will be the easiest (and perhaps only) up-type fermion coupling to probe

• Best channel: ttH production– tHq production also accessible – sensitivity to sign of Yt,

negative values still allowed from global coupling fits

Imperative:

Absolutely need to measure Yt directly to know the

true nature of the couplings of the new boson.

t, X

Loop-induced processes, for example:

t, X

t, X

ttH production:

Experimental Challenges

Christopher Neu 34

• Higgs production in association with a top-quark pair– Comparatively small production

cross section wrt other Higgs production channels

– Spectacular signature – rich final state

• ttH production cross section dwarfed by main background, tt+jets:

root(s) [TeV] 7 8 13

σ (ttH (125)) [fb] 90.4 130. 510.

σ (tt+jets) [fb] 177000 253000 831000

S/B 1/2000 1/1960 1/1640

MH

=125

Summary of LHC ttH Analyses

Christopher Neu 35

H bb H τττhad τhad τhad + τlep

H

WW,ZZ H γγ

7 TeVCMS-HIG-12-035

035 (NN)

ATLAS-CONF-2012-135 (NN) CMS: JHEP

1305 (2013)145 (NN)

various

8 TeV

CMS: EPJC 75 (2015) 251 (ME)

ATLAS: JHEP 05 (2016) 160

(NN)

CMS-HIG-13-020 (SS-2lep, 3lep, 4 lep)

ATLAS: Physics Letters B 749 (2015) 519-541

various

CMS-HIG-13-019(BDT)

CMS: JHEP 09(2014)087

13 TeV

CMS-HIG-16-004 CMS-HIG-

15-008

ATLAS-CONF-2016-080

CMS-HIG-16-022ATLAS-

CONF-2016-2016-058

CMS-HIG-16-020

ATLAS-CONF-2016-068

Summary of LHC ttH Analyses

Christopher Neu 36

H bb H τττhad τhad τhad + τlep

H

WW,ZZ H γγ

7 TeVCMS-HIG-12-035

035 (NN)

ATLAS-CONF-2012-135 (NN) CMS: JHEP

1305 (2013)145 (NN)

various

8 TeV

CMS: EPJC 75 (2015) 251 (ME)

ATLAS: JHEP 05 (2016) 160

(NN)

CMS-HIG-13-020 (SS-2lep, 3lep, 4 lep)

ATLAS: Physics Letters B 749 (2015) 519-541

various

CMS-HIG-13-019(BDT)

CMS: JHEP 09(2014)087

13 TeV

CMS-HIG-16-004 CMS-HIG-

15-008

ATLAS-CONF-2016-080

CMS-HIG-16-022ATLAS-

CONF-2016-2016-058

CMS-HIG-16-020

ATLAS-CONF-2016-068

H hadrons H Multileptons H γγ

ttH - Run 1 Legacy

• CMS+ATLAS final Run 1 (7+8 TeV) combination

– Excess in ttH,multilepton final state (WW)

– Beware: large uncertainties

Christopher Neu 37

JHEP08(2016)045

JHEP08(2016)045

ttH, H bb at 13 TeV

Christopher Neu 38

Overview: Hbb

• Hbb is a prime target of ttH analyses:

– Largest Higgs BR for MH=125

• CMS and ATLAS consider two topologies:

– Single-lepton channel: one high pT

isolated e/μ, ≥4 jets, ≥2 b tags

– Dilepton channel: two opposite-sign leptons, ≥2 jets, ≥2 b tags

• Split selected events into

categories based on jet, b-tags

• Exploit discriminant in each category for signal extraction

• Perform simultaneous fit across all categories

• Low-signal categories serve to help constrain backgrounds.

Christopher Neu 39

Big Issue: Understanding the tt+HF Background

• tt+bb production poses irreducible background:

– Poorly known theoretically

– Measurements of ttbb CRUCIAL

• Modeling of this process:

– Powheg+Pythia8, normalized to NNLO prediction

– Separate templates for tt + b, tt + bb, tt + 2b, tt + cc, tt + LF

– 50% rate uncertainty per tt + jets process, uncorrelated in final fit

• Among the leading uncertainties

– Add. sources include parton shower, hadronisation, PDF, ISR/FSR

Christopher Neu 40

Measurement of ttbb production at CMS

CMS-TOP-16-010

Measurements like this one performed in Prof. Kim’s group here at Hanyang University are essential for the success

of the tth,Hbb campaign at the LHC

https://cds.cern.ch/record/2202803

ttH,Hbb at CMS

• Single variable not very sensitive

– Jet energy resolution

– Combinatorics in jet assignment

• Different choices and combinations of more advanced classifiers

– Boosted-Decision-Trees (BDTs) and Neural Networks (NNs)

• Combination of various input variables, trained per category

• Separation against all tt +X processes

• Used in signal vs. background separation and in event reconstruction

– Matrix-Element-Method (MEM) classifiers

• Likelihood of event kinematics under signal or background hypothesis

• Particularly powerful against difficult tt + bb background

Christopher Neu 41

ttH,Hbb at CMS

Christopher Neu 42

ttH,Hbb at CMS

• Great deal of power added to the final analysis by combining these different multivariate approaches.

• For instance, the MEM discriminant can be used as an input to a BDT along with other kinematic variables

Christopher Neu 43

ttH,Hbb at CMS

Christopher Neu 44

Or one can cut on one of the variables and

use the other in signal discrimination.

ttH,Hbb at CMS

• Exploit boosted category in ttH,Hbb search

– ttH has already three heavy objects –hard to kick these objects into the boosted regime

– tt+jets background in boosted regime still difficult to discriminate

Christopher Neu 45

ttH,Hbb at CMS

Christopher Neu 46

Combined simultaneous fit across all categories, profiling nuisances.

μ < 2.6 (3.6) observed (expected) at 95% CLBest-fit: μ = -2.0 ± 1.8

ttH,Hbb at ATLAS

• Qualitatively very similar approach used at ATLAS in ttH,Hbb

Christopher Neu 47


μ < 4.0 (1.9) observed (expected) at 95% CLBest-fit: μ = +2.1 +1.0

-0.9

Going Forward

• Several important ingredients needed to move from low-precision search to high-precision measurement era

• tt+bb in ttH,Hbb search:

– ad-hoc 50% systematic uncertainty on tt+HF contribution

– can be reduced by using higher-precision predictions for xsec and kinematics

– need data-driven validation and feedback to theory community to improve overall treatment

• tt+charm completely uncovered in theory community in high-precision calculations

• LHC Higgs Cross Section Working Group organizing work. See this link

Christopher Neu 48

https://twiki.cern.ch/twiki/bin/view/LHCPhysics/LHCHXSWG

ttH, H multileptons (WW, ZZ, ττ) at 13 TeV

Christopher Neu 49

ttH, H multileptons

• ttH, H➞leptons:– Targeted Higgs decays and BR H➞WW* (~20%) ,𝛕𝛕 (6%), ZZ (3%)

– leptons originate from Higgs and top system

• Targeted experimental signatures include multiple leptons– 2 same-sign leptons (2lss)

– ≥3 leptons

– 4 leptons (ATLAS only for now)

• – lepton = e,μ

Christopher Neu 50

ttH decay modes signatures

ttH, H multileptons

Christopher Neu 51

ttH, H multileptons

Christopher Neu 52

• CMS excess from Run 1 (in μμ channel) not evident in CMS 13 TeV data

• All observations consistent within uncertainties

• Except for ATLAS eμ channel!

• Excesses bouncing around from period to period

• Takeaway: – Not yet achieving sensitivity to

expected SM-level of ttH, but getting there

ttH, H multileptons

Christopher Neu 53


CMS: μ < 3.4 (1.3) observed (expected) at 95% CL Best-fit: μ = +2.0 +0.8-0.7

ATLAS: μ < 4.9 (2.3) observed (expected) at 95% CL Best-fit: μ = +2.5 ±0.7 +1.1-0.9

ttH, H γγ at 13 TeV

Christopher Neu 54

ttH, H γγ at CMS

• Very rare process – yet very pure signature

• Important: – Only ttH search channel in which one can reconstruct a clear mass peak

• Event selection:

– 2 photons (requirements on BDTγID and EM deposits

– (sub)leading γ pT/mγγ > 0.5 (0.25)

– |η| < 2.5

– 100 < mγγ < 180 GeV

– Categorize events according to ttbar system decay:

• Leptonic:

– ≥1 pT>20 e or μ, ≥2 pT>25 jets, ≥1 b-tag

• Hadronic:

– ==0 e or μ, ≥5 pT>25 jets, ≥1 b-tag

Christopher Neu 55

ttH, H γγ at CMS

• Backgrounds so low – allows for very simple signal extraction:

– Determine signal shape in mγγ

exploiting superior resolution of CMS crystal ECAL

– Assume a falling exponential in mγγ for the uncorrelated diphoton background

– See what amount of signal is favored in the data for a specific MH

hypothesis

Christopher Neu 56

ttH, H γγ at CMS

• Results from two ttH categories combined:

– μttH = 1.91 + 1.5– 1.2 , assuming MH = 126 GeV

• Uncertainty driven by statistics

Christopher Neu 57

ttH, H γγ at ATLAS

• Qualitatively similar approach employed at ATLAS as well

– Similar categorization

– Tighter selection, mγγ window

• Minor deficit in observation but sensitivity is lacking:– Result: μ = -0.25 + 1.26

– 0.99

Christopher Neu 58

Single top + Higgs Searches

Christopher Neu 59

tHq Analyses: Different Approach to Yt

• Hence this process is dependent on the signof the top-Higgs coupling

• Interference effects suppress tHq production in the SM, but if Yt is negative there is considerable enhancement:

– For 8 TeV,

• σSM(tHq) = 18 fb

• σ(tHq, Yt= -1) = 230 fb

• ttH is far less sensitive to this negative coupling

Christopher Neu 60

EPJC 75 (2015) 212

κV = gHVV / gHVV(SM)

Yt = κt Yt(SM)

σ(tHq) ≈ a κt2 + b κV

2 + c κt κV

κV

κt

+

tHq Analyses at CMS

Christopher Neu 61

tHq, H bbLeptonic W decay3-,4-tag categories

MVA for tHq v. ttbar

tHq, H WW, ττSame-sign 2lep and 3lep

MVA for tHq v. bkgd

tHq, H γγEnhancement on

production and decay side.No events survive in data.

σ/σ(Yt = -1) < 5.4 (7.6) σ/σ(Yt = -1) < 5.0 (6.7) σ/σ(Yt = -1) <4.1 (4.1)

Expected (observed) upper limits:

arXiv:1509.08159, submitted to JHEP

Combined upper limit σ/σ(Yt = -1) < 2.0 (2.8)

Summary

• Characteristics of this Higgs boson need to be measured with high precision. The measurement campaign has so far revealed no significant deviations from the predictions of the SM

• A few crucial ones remain to be measured – the most important being the coupling between the top quark and the Higgs boson

• The most essential process to pursue is ttH production, in all of the accessible decay modes

• Signal strength summary:

• First direct measurement of the top-Higgs coupling is among the primary goals of the LHC physics program.

Christopher Neu 62

μ = σ/σSM √s ATLAS CMS

Hbb7,8 1.1± 1.0

13 +2.1 +1.0-0.9 -2.0 ± 1.8

multilepton

s

7,8 5.0 +1.8-1.7

13 +2.5 ±0.7 +1.1-0.9 2.0 +0.8

-0.7

Hγγ7,8 2.2 +1.6

-1.3

13 -0.25 +1.26-0.99 1.9+1.5

-1.2

Backup

Christopher Neu 63

The Standard Model

Christopher Neu 64

• Mathematical description of the building blocks of the universe and its interactions – matter and forces

• Matter: Quarks, leptons

• Forces: Gauge bosons

•Very successful model:

– Accommodates nearly all of the observed phenomena we see in our every day world

– Explains nearly all of the observations we have made in the exotic conditions created in particle physics experiments

•Particularly elegant feature:

– The unification of two of the four known forces into a single interaction

The Higgs boson is the last of the fundamental particles predicted by the Standard

Model to be observed.

The History of Discoveries

Christopher Neu 65

1860 202019401900 1980

1898

e1923

γ1937

μ1962

νμ

1968

u,d,s1974

c1976

τ

1977

b1979

g1982

W,Z

1995

t1956

νe

2000

ντ

2012H

The Electromagnetic and Weak Forces

Christopher Neu 66

Higgs Mechanism:

– Higgs field breaks EWK symmetry

– Explains masses of W, Z and photon

– Other particles interact with the Higgs field and acquire mass

– Additional consequence:

new particle predicted to exist,

the so-called Higgs boson

Symmetry is broken however:

– Photon massless

– W, Z very massive

– How?

Symmetry in description of Electromagnetic and Weak forces allows unification:

electroweak interaction

c W± t

Higgs boson:Mathematical curiosity or

something more?

1 GeV/c2 = 1.783×10−27 kg

Mass – What’s the Big Deal?

Christopher Neu 67

• Higgs boson credited with the “origin of mass”

• This is not the complete story:– Most of the visible universe is protons and neutrons

– Protons (p) and neutrons (n) are a bound state

of u, d quarks (~a few MeV apiece)

– The p, n masses (938 and 940 MeV) are much larger

than the constituent quarks alone

– Measured mass comes mostly from the

strong force holding the quarks together

Mnucleon ~ mquarks + 1/c2 (Energy in strong intxn)

– Strong force proceeds with or without the Higgs

u

u

d

proton

d

d

u

neutron

However…

Christopher Neu 68

• What if the fundamental particles were massless?

• Quark masses:– The up and down quark masses have been measured:

– mu = 2.3 +0.7-0.5 MeV and md = 4.8 +0.5

-0.3 MeV

– Important point: md > mu

– This small difference forces the neutron mass to be slightly larger than that of the proton

– If mu = md = 0, then Mproton > Mneutron , allowing…

This would be bad.

• Lepton masses:– If me = 0, the Bohr radius of atoms would be large

• Chemistry as we know it would not exist!

This too would be bad.

evnep

a º4pe0

2

mee2

u

u

d

proton

d

d

u

neutron

However…

Christopher Neu 69

• What if the fundamental particles were massless?

• Quark masses:– The up and down quark masses have been measured:

– mu = 2.3 +0.7-0.5 MeV and md = 4.8 +0.5

-0.3 MeV

– Important point: md > mu

– This small difference forces the neutron mass to be slightly larger than that of the proton

– If mu = md = 0, then Mproton > Mneutron , allowing…

This would be bad.

• Lepton masses:– If me = 0, the Bohr radius of atoms would be large

• Chemistry as we know it would not exist!

This too would be bad.

evnep

a º4pe0

2

mee2

u

u

d

proton

d

d

u

neutron

Understanding electroweak symmetry breaking and mass is

among the most significant puzzles in modern physics.

Hence, the search for the Higgs boson was one of the primary

goals of modern particle physics experiments!

Characterizing the Higgs Boson: Mass

• Exploit highest mass resolution channels in combination for precise determination of mH

– H γγ

– H ZZ 4lep

• First example of combination of precision Higgs measurement results across LHC exps

• Individual channel

results are each compatible within 10%

Christopher Neu 70

ChannelmH

[GeV]

±δStat

[GeV]

±δSyst

[GeV]

Hγγ 125.07 0.25 0.14

HZZ 125.15 0.37 0.15

Comb 125.09 0.21 0.11

mH = 125.09 ± 0.24 GeV (δm/mH = 1.9E-3)

Phys. Rev. Lett. 114, 191803

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Characterizing the Higgs Boson: Spin and Parity

Christopher Neu 71

• The Higgs boson of the SM is a CP-even scalar, JP = 0+

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Christopher Neu 72


– The spin and parity influence the Higgs decay kinematics in HVV decays

Generalization of pp X ZZ, WW, Zγ,γγ

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Christopher Neu 73



– Alternative spin-parity models can be tested

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Christopher Neu 74




• Spin 1 hypothesis disfavored by observation of Hγγ, but tested anyways

JP = 1+

Phys. Rev. D 92 (2015) 012004

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• Spin 1 hypothesis disfavored by observation of Hγγ, but tested anyways

• No known fundamental particle has spin 2. Many models predict them, however

Christopher Neu 75

JP = 1+

JP = 2+

Universal couplings

Just one example

Phys. Rev. D 92 (2015) 012004 arXiv:1506.05669, submitted to EPJC

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• All observations consistent with JP = 0+

• JP = pseudoscalar 0-, vector 1-, pseudovector 1+

excluded at >99% CL

• Mixed-parity spin-1 state excluded at 99.999%CL

• All tested JP = 2±x models excluded at > 98% CL, many at > 99.99% CL

Christopher Neu 76

Phys. Rev. D 92 (2015) 012004

arXiv:1506.05669, submitted to EPJC

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Spin-0 Higgs: Parity Structure and Higher-Order Interactions

• Scattering amplitude term to O(q2) for spin-0 H and VV (VV= ZZ,Zγ*,γ*γ*, WW):

Christopher Neu 77

All observations consistent with SM expectation

In SM: large for WW,ZZ

scale of new physics in HVV intxn

CP-even scalar term, tiny in SM

CP-odd pseudoscalar term, really tiny in SM

Eff

ecti

ve

xsec

fra

ctio

ns

f xco

s(φ

x)

= 0

SM

-lik

e

Phys. Rev. D 92 (2015) 012004

ATLAS has similar results: arXiv:1506.05669, submitted to EPJC.

Characterizing the Higgs Boson: Width

• The narrow width approximation is not really adequate for HZZ

Christopher Neu 78

• Off-shell contribution is sizable

– For mZZ > 2 mZ, the contribution is ~8%

• Can derive information on the Higgs width from on-shell / off-shell ratio

• Assumptions:– off-shell signal couplings are not different from

on-shell behavior

– no new light-mass statesPhys. Lett.B 736 (2014)64

See arXiv:1307.4935 [hep-ph]

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Characterizing the Higgs Boson: Width

These results are ~100 times more precise than direct/on-shell Higgs width limits.Christopher Neu 79

Phys. Lett.B 736 (2014)64 Eur. Phys. J. C (2015) 75:335

upper limit CMS ATLAS

ΓH [MeV] < 22 < 22.7

ΓH / ΓSM < 5.4 < 5.5

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Characterizing the Higgs Boson: Self-Coupling

Christopher Neu 80

• Ultimate characterization will come through examination of the Higgs self-coupling

• Accessible through Higgs pair production searches

– Very low production cross section

• Need significant increase in integrated luminosity:

– High-luminosity LHC era

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http://arxiv.org/pdf/1206.5001v2.pdf

Search for ttH Production, HMultileptons

Low-rate but low-background signatures.Christopher Neu 81

Same-sign dilepton

2 e/μ with pT > 20 ≥4 jets with pT > 25

≥1 b-tagged jet

Three lepton

1 e/μ with pT > 20 1 e/μ with pT > 10

1 e(μ) with pT > 7(5) ≥2 jets with pT > 25

≥1 b-tagged jet Veto Mll ~ MZ

Four lepton

1 e/μ with pT > 20 1 e/μ with pT > 10

2 e(μ) with pT > 7(5) ≥2 jets with pT > 25

≥1 b-tagged jet Veto Mll ~ MZ


Christopher Neu 82

Expected and observed yield in 19.6/fb at 8 TeV

• Signal dominated in each category by HWW mode

• Significant excess over expectation in SS 2lep μμ channel

JHEP 09 (2014) 087


Signal extraction:

•BDT used in SS 2-lep and 3-lep categories

•Njets alone used for 4-lep category

•Excess in μμ channel

Christopher Neu 83

3-lep 4-lep

μ±μ± e±e± e±μ±

JHEP 09 (2014) 087

Search for ttH Production

Christopher Neu 84

• Suite of ttH analyses not yet achieving sensitivity to level of ttH production predicted in the SM

• Place instead upper limits on the amount of ttH in the observed data

– CMS: μ = σ/σSM < 2.7 (4.5) expected (observed) at 95% CL [combination]

– ATLAS: μ < 2.2 (3.4) [Hbb]

μ < 2.4 (4.7) [Hmultilep]

JHEP 09 (2014) 087

Eur. Phys. J. C (2015) 75:349

arXiv:1506.05988, accepted by PLB

ATLAS ttH,Hγγ (omitted here): Physics Letters B 740 (2015)

Search for ttH Production: All Channels

Christopher Neu 85

arXiv:1408.1682, Accepted by JHEP • Can perform same kind of coupling-modifier study as described earlier

• Value in performing this solely within ttH analyses

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