discovery of the higgs boson - indico · 2019-06-04 · higgs boson in the 80’s experimental...

Discovery of the Higgs Boson

Jianming QianUniversity of Michigan

May 6-8, 2019, Duke University

A Journey of half a century

Professor Goshaw’s career spans the entire history of the Higgs boson!

Al served on the IAC of the Symposium on the First Year of LHC Physics at Michigan

4

The BeginningIn 1964, three teams published proposals on how mass could arise in local gauge theories. They are now credited for the Brout-Englert-Higgs (BEH) mechanism.

L to R: Kibble, Guralnik, Hagen, Englert, and BroutHiggs

5

The Standard Model

2

1L=

4

L R

F F

D

D V

G

Gauge-sectorKinetic energies and self-interactions

Flavor-sectorKinetic energies and interactions with gauge bosons

Higgs-sector (EWSB)Gauge boson masses and couplings

Ad hoc Fermion mass termsCouplings (Yukawa) to Higgs fields

?

Three years later, Weinberg and Salam incorporated the BEH mechanism into a theoretical model, the Standard Model as it is known today.

6

The Higgs Sector

2

† †

2

Introduce scalar fields with the potential:

(

Spontaneous symmetry breaking if 0.

V

The theory of particle interactions is based on “gauge” symmetry,but the symmetry prevents particles from having masses.

Particles “acquire” masses through their interactions with the Higgs field via the Brout-Englert-Higgs (BEH) mechanism.

The BEH mechanism can explain the masses of all fundamental particles except those of neutrinos and predicts the existence of a massive neutral scalar particle, the Higgs boson (H).

7

The Predictions

We know how it can be produced and how it decays (instantaneously) Know exactly how and where to look for it !

H

f

f

H

,V W Z

,V W Z

22 V

HVV

mg

2 f

Hff

mg

The Higgs boson mass, mH, is the only unknown parameter in theHiggs sector. Once mH is known, other Higgs boson properties canbe calculated within the SM.

Decay branching ratio

22Hm

8

WW Scattering Argument

2

Problem:

The WW WW scattering amplitude diverges as for . WM ss

Historic precedent:

W boson is needed to

make finitee e

Solution:Introducing a scalar particleto cancel the divergence

9

Nucl. Phys. B106 (1976) 292

Higgs Boson in the 70’s

Phenomenological studies began in the 70’s after t’Hooft and Veltmanshowed that the theory is renormalizable..

10

Higgs Boson in the 80’s

Experimental limits on a light Higgs boson from nuclear decays, nucleon scatterings, Kaon and B meson decays …

Simulation studies of searches at SLC/LEP and LHC/SSC

Rept. Prog. Phys. 52 (1989) 389

Most of the Higgs phenomenology had been worked out by the end of 80’s

11

The Higgs Hunter's Guide is a definitive and comprehensive guide to the physics of Higgs bosons. In particular, it discusses the extended Higgs sectors required by those recent theoretical approaches that go beyond the Standard Model, including supersymmetry and superstring-inspired models.

Higgs Boson in the 90’s

12

Searches pre-LEP (before 1989)

G. Marel, PhD thesis, Orsay (1968)

Recherche d'un boson masse 960 MeV par l'étude du spectre de masse manquante au deuton produit dans l'interaction p-p à 3.8 GeV/c(Search for a Higgs mass of 960 MeV by studying the missing mass spectrum deuteron produced in pp interactions at 3.8 GeV/c)

NA31 Collaboration, Phys. Lett. B235 (1990) 356-362

Usually an after thought, not a major physics goal as Ellis et al suggested…

p p d H

13

Summary of the pre-LEP SearchesNicely summarized by the Particle Data GroupPhys. Lett. B204 (1988) 1

14

In summary, the only cast-iron constraint on the Higgs mass is

MeV. A combination of theoretical arguements and

bounds from B, , and K decays probably excludes the range

below 4 GeV.

HM

14

Large Electron-Positron (LEP) Collider

An collider operated at 209 GeV

LEP I: (1989-1995) collected 17 million Z bosons

LEP II: (1996-2000) WW and searches for Higgs boson

Z

Z

Z

e e s M

s M

s M

L3

DELPHI

ALEPH

OPAL

CERN

15

LEP: A Decade Long Precision Physics

16

Higgs Boson Production at LEP

- Dominance of Z resonance production- Sharp kinematic threshold from narrow Higgs boson width

*

*

Z Z H

Z ZH

LEP I:

LEP II:

17

Searches at LEP I

A. Sopczak, Nucl. Phys. B 37C (1995) 168

No evidence, set a lower Higgs

boson mass limit of 64 GeV

* , ,Z Z He e qq H

18

Searches at LEP II: Final Results

114.4 GeV @ 95% CLHm

Phys. Lett. B565 (2003) 61

*

Dominated by

limited by

Z ZH

s

206

115

H Z

H

m s M

s

m

Reach in the mass

For GeV,

GeV

19

Indirect Constraint

29

2494

By the time, top quark mass had been measured with a high precision. Thus the Higgsboson mass can be inferred from the global fit to precisionelectroweak data:

and a 95%

CL bo

Ge

und was

VHm

152

set

V

:

GeHm

2 % correctiontm 2log % sub- correctionHm

20

Tevatron Collider

collider with s 1.96 TeVpp Running between 1990-2011 with two experiments: CDF and DØ

DØ

CDF

Fermilab

21

Spokesperson Quartet

22

Higgs Boson Production at Tevatron

1 pb @ 125 GeVpp H X

dominated by

sizeable production

enhanced from collisions

gg H

WH ZH

pp

GeVHm

VH

ggF

VBF

23

Higgs Boson Decay

*

,

Low mass

High mass

WH bb

ZH bb bb

gg H WW

(*)

135

135

Dominant decays:

Low mass: GeV

High mass: GeV

H bb

H WW

Tevatron is mostly sensitive to Higgs boson mass below 200 GeV

(T. Han, hep-ph/9807424)

24

arXiv:0804.3423arXiv:0911.3930arXiv:1007.4587

Searches at Tevatron at a Glance

arXiv:1107.5518

Exclusion 156-177 GeV

25

Large Hadron Collider (LHC)

CERN

26

Higgs Boson Production at the LHCDominant processes:

gluon-gluon fusion gg→Hvector-boson fusion qq→qqH

@125 GeV: 19.5 pb, 1.6 pb,

0.70 pb, 0.39 pb, 0.13 pb

ggH VBF

WH ZH ttH

27

Search Status: EPS 2011

First ATLAS and CMS combination was exercised, but the results have never made public, a right decision in retrospect.

*

At EPS 2011 in Grenoble, both ATLAS and CMS had broad exccesses

at 145 GeV, driven by the exccesses in the searchH WW

EPS 2011: http://hep2011.insight-outside.fr/

28

Search Status: December 2011The rumor about a potential 125 GeV Higgs boson was made “official” at the CERN council meeting in December, 2011:https://indico.cern.ch/event/166949/

CMS results

H

* 4H ZZ

29

Search Status: December 2011

ATLAS results

A 3.6 excess around 126 GeVbased on the 7 TeV data

30

Dawn of Discovery

*

*

H WW

H ZZ

Most of the mass range has been excluded by the and

searches, leaving open a narrow 10 GeV mass window

arXiv:1203.3774Status of early 2012

31

Discovery: The Seminar

Both ATLAS and CMS collaborations claimed excesses of ~5 significance

Seminars on July 4, 2012 at CERN:https://indico.cern.ch/event/197461/

32

Discovery: The Papers

The discovery was made based on the analysis of bosonic decays of7 and 8 TeV data.

There were no results of fermionic decays at the time of the discovery.

Paper submission on July 31, 2012ATLAS: Phys. Lett. B716 (2012) 1

H→, H→ZZ*→4l, H→WW*→ll

CMS: Phys. Lett. B716 (2012) 30H→, H→ZZ*→4l

The seminar was hastily scheduled at 9:00am at CERN to coincide with the start of the 36th ICHEP conferencein Melbournehttps://indico.cern.ch/event/181298/

33

Discovery: ATLAS Results

ATLAS: Phys. Lett. B716 (2012) 1

*

*

4

5.9

Based on

observed a local significance of

standard deviations from a

combined 7 and 8 TeV dataset

H

H ZZ

H WW

34

Discovery: CMS Results

CMS: Phys. Lett. B716 (2012) 30

* 4

5.0

H H ZZ Analyzed the and decays of the 7 and 8 TeV

data, observed an excess with a significance of standard deviations

35

ATLAS Significance Evolution

36

Results from Tevatron

Submitted after the 4th

of July seminar, but 4 days before the submissions by ATLAS and CMS collaborations

,

2.8

A broad excess in

with a significance of

at 125 GeV, combining CDF

and D0 collaborations.

H bb

H bb

37

Higgs Boson MassPhys. Lett. B784 (2018) 345

38

Higgs Boson Decays

* *, ,

,

H H ZZ H WW

H H bb

The discovery was based on the three bosonic decay modes:

Since then, two fermionic decay modes have been observed:

ATLAS-CONF-2019-005

39

Production Modes

Four major production modeshave been observed as well

ATLAS-CONF-2019-005

40

Coupling Measurements

2

2

2,

2

2 ,

2

f

Hff

VHVV

f

Hff F

VV

HVV

mg

mg

mg

mg

ATLAS-CONF-2019-005

Measured couplings are very Standard Model like

41

Spin/CP PropertiesEur. Phys. J C75 (2015) 476Higgs boson spin/CP properties

can be studied from the angularand transverse momentum distributions of its decay products

*pp ZZ X e e X

The results are consistent with theproperties of a CP-even scalarparticle, predicted by the SM.

42

Higgs + Top ProductionDirect probe of the interaction between the two heaviest particles in the Standard Model

Ph

ys. L

ett.

B7

84

, 17

3 (

20

18

)

Observed (expected): 6.3 (5.1)(Combining H→, H→bb and H→l’s)

ATL

AS-

CO

NF-

20

19

-00

4

with ttH H

43

What’s Next? The question remains:

Is the new boson solely responsible for the electroweak symmetry breaking?

Two parallel approaches:• Precision measurements of production and decays properties• Direct searches of exotic decays and additional particles

LHC is the place to be to study the Higgs boson and search for additional Higgs bosons in the foreseeable future. It is a Higgs factory !

Precision measurements of Higgs boson properties to sub-percentlevel or better will require complementary precision programs. Proposed Higgs factories will be able to achieve these precisions.

44

LHC Schedule

Discovery Now

Collected 15x more data since the discoveryWill collect 20x more only 5% of the data has been collected

45

Coupling to the 2nd GenerationATL-P

HYS-P

UB

-20

18

-00

6

,

,

Hff fg m

H

nd

Because only Higgs boson decays to the 3rd generation of

fermions have been observed, observation of decays to the 2 generation

of fermions, such as will be a major focus of studies.

1

About 1000 fb of data is required for

the observation of the H decay

H

46

Higgs Boson Potential

Direct probe of the Higgs boson self-coupling (and therefore the Higgspotential), but the rates are low and backgrounds are high

10 pb @ 8 TeV

40 pb @ 13 TeV

gg hh

gg hh

30%Extremely challenging: expected for HL-LHC

2

2 † † 3 4

4V h h

2

2

1

82SM:

Small shallow potential well

hm

47

HL-LHC Coupling ProjectionsMany studies done for US Snowmass process, Europe ECFA studies,and recently updated for the European Strategy Studies.

170 millions of Higgs bosons will be produced per experiment

arX

iv:1

90

2.0

01

34

At HL-LHC, precisions for major couplings are projected to 1-4%.

48

e+e- Collider

Electroweak production cross sections are predicted with(sub)percent level precisions in most cases

Relative low ratecan trigger on every event

Well defined collision energyallow for the “missing” massreconstruction (eg recoiling mass)

Clean events, smaller backgroundsmall number of processes

Ideal for precisions: measurements or searches

49

Electron-Positron Collider Proposals

JapanILC 250: 2032

CERNCLIC 350: 2035

CERNFCC-ee: 2039

ChinaCEPC: 2030

50

Higgs Boson Production in e+e- CollisionsAt 240 250 GeV, production is maximum and

dominates with a smaller contribution from .

s ee ZH

ee H

Beyond that, the cross section decreases asymptotically as 1 for and increases logarithmically for .s ee ZH ee H

250 GeV: 200 fb, 10 fbZH Hs

51

Higgs Boson Tagging

2 22

Z Zm s

ee

p

ZH

E

recoil

Identifying the Higgs boson without

looking at it. Measuring

Re

coil mass reconstructi

independent of it

on:

s dec

ay !

Unique to lepton colliders, the energy and momentum of the Higgsboson in can be measured by looking at the Z kinematics

only: , H Z H Z

ee ZH

E s E p p

H

Z

LHC always measures , no model-independent way to

distangle decay from production!

BR

52

The Glory of Standard Model

53

Beyond the Standard ModelThe SM has been remarkably successful, but it cannot be a complete theory. It does not explain the origin of neutrino masses, the nature of dark matter, matter-antimatter asymmetry, ….

54

Journey Through and Beyond…

It took about 25 years before the search for the Higgs boson became a serious experimental pursuit, another 20 years before its discovery. It has been a remarkable Journey through the Standard Model.

The discovery of the Higgs boson represents the end of beginning. The Higgs boson offers a new tool for exploration. We hope it will lead to a Journey Beyond the Standard Model !

Additional Slides

56

Discoveries of the W and Z Bosons

Phys. Lett. B122, 103 (1983)

UA1 Collaboration

Phys. Lett. B126, 398 (1983)

UA1 Collaboration

,

pp W X e X

pp Z X ee X

CERN Press Conference: June 1, 1983

57

LEP: A Success StoryLEP program was a great success even though no new particle was discovered.

• Determined the three light-neutrino species;

• Codified the Standard Model;

• Set in motion the subsequent searchesfor the Higgs boson

58

Searches at LEP II: 115 GeV CandidatesJust before the LEP was scheduled to be shutdown, multiple candidates consistent with a 115 GeV boson were reported…

59

115 GeV Higgs OdysseyJ. Ellis, arXiv:hep-ex/0011086

LEP was scheduled to be shutdown on Oct. 1, 2000.

At the LEPC meeting on Sept. 5, 2000, LEP experiments showed indication of a 115 GeV Higgs boson

LEP running was extended forone month.

On Nov. 3, 2000, the experimentsshowed an excess of ~3 sigma significance with a measured Higgs boson mass

1.3

0.9115 GeVHm

LEP was shutdown forever at 8:00am on Nov. 2, 2000 to pave the way for the LHC construction

60

Superconducting Super Collider (SSC)Search for the Higgs boson was supposed to be a race between the LHC at CERN and SSC in Texas. But Congress cancelled the SSC after ~$2B investment.

Like LHC, SSC was a proton-proton collider but with 3x of the LHC energy.

With the Tevatron shutdown in 2011,the energy frontier research shifted from US to Europe.

61

Search Status: 2011 Easter Episode

Simultaneous discoveries of the Higgs boson and physics beyond the Standard Model !

62

Fermion Decay Modes

Observed 6.4 , Expected 5.4 Observed 5.4 , Expected 5.9

Run 1 and Run 2 combinationPhys. Lett. B 786 (2018) 59

Run 1 and Run 2 combinationPhys. Rev. D99 (2019) 072001

H H bb

63

Latest Winter’19 Results

CMS-PAS-HIG-18-029 ATLAS-CONF-2018-018

64

Hadron Colliders

11

QCD production dominates

tiny S/B ratio: 10

ˆunknow event level

messy collision environment

h tot

s

On the other hand…

ˆbroad band in

much large Higgs cross section

s

Huge background

Trigger is the key!

At HL-LHC170 millions of Higgs events

65

Accessible Decay Modes6 8Numbers of Higgs events: 10 at Higgs factories, 10 at HL-LHC

Limitations: statistics at Higgs factories, trigger and systematics at (HL-)LHC

Higgs factories are sensitive to unknown decays whileHL-LHC can only be sensitive to “known” decays.