Download Discovery of Higgs Boson - High Energy Physics

The Discovery of the Higgs boson
Matthew Herndon, University of Wisconsin Madison
Phys 301: Physics Today
M. Herndon, Phys 301 2017
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The Atom
In the early twentieth century atomic
physics was well understood
The atom had a nucleus with protons
and neutrons.
An equal number of electrons to the
protons orbited the nucleus
The keys to understanding this were
the electromagnetic(EM) force and
the new ideas of quantum mechanics
The EM force held the
electrons in their orbits
Quantum mechanics told us
that only certain quantized
orbits were allowed
Allowed a detailed
understanding of the
properties of matter
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The Periodic Table
Different types
of quantum
orbits
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Elements in same
column have
similar chemical
properties
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We Observed New Physics
Already there were some
unexplained phenomena
One type of atom could
convert itself into another
type of atom
Nuclear beta decay
Charge of atom changed and an
electron was emitted
How could the nucleus exist?
Positive protons all bound
together in the the atomic nucleus
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Needed a new theory
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The Forces
Best way to think about the problem was from the viewpoints of the forces
Needed two new forces and at first glance they were not very similar to
the familiar electromagnetic and gravitational forces!
Couples to:
Example
Strength in an
Atom
EM
Weak
Strong
Gravity
Particles with
Protons, Neutrons
Protons and
All particles with
electric charge
and electrons
Neutrons
mass
Attraction
Nuclear beta decay
Attraction between
between protons
Not an attractive
protons and
and electrons
force
neutrons
F = 2.3x10-8N
Decays can take
thousands of years
F = 2.3x102N
Only attractive
F = 2.3x10-47N
How do we understand the Forces? Why so different in in properties?
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A New Theory
Relativistic quantum field theory (QFT):
Unification of special relativity(the theory space time) and
quantum mechanics(used to understand the atom)
Forces described by exchanging particles
Electromagnetic force comes
about from exchange of photons.
electron
photons
Example :Electromagnetic
repulsion via emission of a photon
Exchange of many photons allows
for a smooth force(EM field)
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electron
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Unification!
Maxwell had unified electricity and magnetism
Both governed by the same equations with the strengths of the forces
quantified using a set of constants related by the speed of light
The Standard Model of Particle
Physics (proposed 1960)
QFTs for EM, Weak and Strong
Unified EM and Weak forces - obey
a unified set of rules with strengths
quantified by single set of constants
All three forces appear to have
approximately the same strength at
very high energies. May also unify.
1eV = 1.6x10-19 J
A very successful theory
A Key component was missing to fully understand EM-Weak Unification
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Weak and EM Force: Strength
For EM force
For weak force
Coupling strength:
Same as EM force
P µ a2/(q2+Mg2)2
P µ a2/(q2+MW2)2
q momentum of
the W or Z bosons
Mass of the photon is 0, mass of the W and Z bosons is large
When the mass of the W boson is large compared to the momentum
transfer, q, the probability of a weak interaction is low compared to the EM
interaction! Too low to form a field and bound states.
At high energy when q was much larger than the mass of the weak
bosons the the weak and EM interaction have the same strength
The key missing element is to explain the mass of the W and Z bosons
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The Forces Revisited
EM
Couples to:
Particles with
electric charge
Attraction
Weak
Weak charge:
quarks and
electrons
Strong
Gravity
Color charge:
All particles with
quarks
mass
Nuclear beta decay
Attraction between
No attractive force
quarks/nucleons
Photon
W and Z Boson
Gluon
Graviton
Mass
0
80 and 91 GeV
0
0
Decay time/
Decay time:
Decay time:
Strength in an
10-18 sec
10-12 sec to
F = 2.3x102N
F = 2.3x10-47N
Atom
F = 2.3x10-8N
thousands of years
Example
between protons
and electrons
Quanta: Force
Carrier
M. Herndon, Phys 301 2017
Only attractive
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The SM Higgs Boson
SM postulates a mechanism of electroweak symmetry
breaking via the Higgs mechanism (proposed 1964)
Interaction with the Higgs field results in masses for the W and Z
vector bosons
A primary reason for the difference EM and Weak interactions
Fills in the key missing element of the SM
Can explain the mass of the fermions (quarks and leptons) as well
Also expect an observable quanta of the field: Higgs boson
à directly testable by searching for the Higgs boson!
The primary goal of the LHC Run 1
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Searching for the Higgs
How do we search for the Higgs Boson?
In the SM particles that carry the charge of a given force can
interact by absorbing or emitting the force carrier
The diagrams
(Feynman diagrams)
can be converted into
Also they can annihilate or pair produce
equations to
calculate the
probability of the
process occurring.
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Searching for the Higgs
The “charge” that the Higgs boson interacts with is mass
Particles with high mass will interact with higher probability with a Higgs
boson
W and Z bosons: 80 and 91 GeV - mass of a krypton atom
The top quark: 172.6 GeV - mass of a gold atom
However, the LHC collides protons made of quarks and gluon
Some thought needed to understand the best way to make Higgs bosons
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Searching for the Higgs
Also look for decay to massive particles
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Searching for the Higgs
Those decays should be to particles
that are easy to detect: i.e. uniquely
identify and measure the momentum of
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Need Every Advantage
LHC collision rates
LHC collides protons every 50 ps
20 proton-proton interactions each time
Probability of a Higgs interaction
11 orders of magnitude less.
1/100000000000 collisions produces a
Higgs boson
~1 Higgs every 10 minutes
100-1000 less, easily detected Higgs
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Plan of action
Calculate the probability of Higgs production and decay
expected in proton-proton collisions
A decade of work by dozens of theorists
Build a collider to collide the protons at high energy and
high enough rate
A decade of work by hundreds of collider physicists
Build experiments that can detect the Higgs boson
A decade of work by thousands of experimental physicists
Apply our best ideas to achieve the above
All built on decades of experience from previous
experiments.
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SM Higgs Production and Decay
Take advantage of large gg ® H production cross section
Had to calculate as a function of mass as Higgs mass was not predicted
Alternative production mechanisms
Primarily VBF: qq ® Hqq
Decay modes: H ® gg, H ® ZZ, Sensitive since they are well reconstructed
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The Large Hadron Collider
14 TeV proton-proton collider (now 8 TeV)
27 Km tunnel 100m underground
109 collisions/second
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The CMS Detector
Detector designed to
measure all the SM
particles
Tracker
Electromagnetic Calorimeter
Hadronic Calorimeter
Solenoid
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Muon System
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Particle Detection in CMS
Particle Detection
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LHC Collision
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LHC Collision
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Higgs Sensitivity
q
As of 2012 the Higgs
boson not yet been found
but the mass was
constrained to the range
mH: 115-130 GeV
q
UW
UW
Plot shows convolution of
production, decay and
detector capabilities
q
Strong sensitivity to 5
decay modes led by γγ
and ZZ
(below 1 means sensitivity to
SM Higgs production)
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Multiboson Physics
UW
UW
UW
UW
UW
Some preparation: Other SM processes with decays to bosons.
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Final Higgs Search
Once you have your best calculations, collider, detector
and have applied all your ideas the final analysis in
essence consists of looking for events with two photons or
two Z bosons where the combined mass of the two bosons
adds up to a consistent mass.
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LHC Higgs Event: gg
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LHC Higgs Event: ZZ ® µµee
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Higgs Searches
Events / 3 GeV
CMS Preliminary
s = 7 TeV, L = 5.05 fb-1 ; s = 8 TeV, L = 5.26 fb-1
12
Data
7 TeV 4e, 4µ , 2e2µ
8 TeV 4e, 4µ , 2e2µ
Z+X
10
UW
8
Zγ *,ZZ
mH=126 GeV
6
4
2
0
m4l [GeV]
80
100
120
140
160
180
m4l [GeV]
γγ: 4.0σ
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ZZ 3.2σ
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1
1σ
2σ
-1
10
10-2
10-3
10-4
10-5
10-6
10-7
10-8
10-9
10-10
10-11
10-12
3σ
Combined obs.
Exp. for SM Higgs
H → γγ
H → ZZ
UW
CMS Preliminary
H → ZZ + γ γ
s = 7 TeV, L = 5.1 fb-1
s = 8 TeV, L = 5.3 fb-1
6
5
4σ
4
5σ
3
6σ
Combined
H → γ γ (inc.)
H → γ γ (VBF)
CMS Preliminary
H → ZZ + γ γ
s = 7 TeV, L = 5.0 fb-1
s = 8 TeV, L = 5.2 fb-1
H → ZZ
2
1
7σ
116 118 120 122 124 126 128 130
Higgs boson mass (GeV)
σ/σSM
Local p-value
Higgs Observation!
0
123
124
125
126
127
128
129
Higgs Boson Mass (GeV)
mH=125.3±0.6(stat+sys
) experiment as well
Simultaneously observed by ATLAS
Combined 5.0σ!
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Higgs Properties
Couples proportional
to mass
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Conclusions
LHC has observed a Higgs boson with mass ~125 GeV!
Observed at the gold standard of statistical significance.
Simultaneous observation by independent experiments providing both
discovery and proof of reproducibility.
Now observed with the event rates expected for both W and Z
interactions with the Higgs.
All properties very consistent with SM expectation.
The last piece of the SM confirmed!
A beginning, not an end, for the LHC story
More mysteries to solve. Dark matter, unification of the forces
….
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