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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.
