Download Higgs colloquium - High Energy Physics

Searches for the SM Higgs Boson
ggs
Forces of Nature
Unification of the Forces and the Higgs Particle
Searching for the Higgs/Higgs Searches Results
Matthew Herndon, Dec 2008
BEACH 04
University of Wisconsin
University of Massachusetts
Amherst Physics Colloquium
J. Piedra
1
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 detailed
understanding of the
properties of matter
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The Periodic Table
Different types of
quantum orbits
Elements in same
column have
similar chemical
properties
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We Observed New Physics
Already there were some clear problems
One type of atom could convert
itself into another type of atom
Nuclear beta decay
Charge of atom changed and
electron emitted
How could the nucleus exist?
Positive protons all bound together in the
the atomic nucleus
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
between protons
and electrons
Nuclear beta decay
and nuclear fission
Holds protons and
neutrons together
the nucleas
Only attractive
F = 2.3x10-8N
Decays can take
thousands of years
F = 2.3x102N
F = 2.3x10-47N
How do we understand the Forces?
Fundamental differences in strengths!
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How Do the Forces Work
Relativistic quantum field theory (QFT): Quantum electrodynamics(QED)
Unification of relativity(the theory space time and gravity) and quantum mechanics(the
theory of atoms as described by the EM Force)
Description of the particles and the forces at one time
Allowed for a possible unification of the forces - description by one theory
Electromagnetic force comes about from exchange of photons.
Electromagnetic
repulsion via emission
of a photon
Exchange of many
photons allows for a
smooth force(EM field)
electron
photon
electron
For a very quick interaction we can see individual photon exchanges
Particle Annihilation or Creation
The new QED EM Theory has one very interesting additional feature
Can rotate diagrams in any direction
???
electron
photon
???
photon
electron
electron
electron
Time goes from left to right. What is
an electron going backward in time?
Antiparticles! Antielectron or positron.
This is going to be a
useful way to make
new particles.
Also learned from studying EM force that the proton and neutron were
made of smaller particles. up and down quarks. p=uud, n=udd
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
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
1eV = 1.6x10-19 J
So far just a theory - though a
successful one
Still working to fully understand EW=EM+Weak Unification
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Electroweak Symmetry Breaking
Consider the Electromagnetic and the Weak Forces
SM says that they are two aspects of one force and governed by the same rules
They should be the same strength, but EM always active, weak decays can take
thousands of years!
Coupling probabilities at low energy: EM: ~2, Weak: ~2/(MW,Z)4
Fundamental difference in the coupling strengths at low energy, but apparently governed
by the same constant
Difference due to the massive nature and short lifetime of the W and Z bosons.
At high energy the strengths become the same. We say the forces are symmetric
SM postulates a mechanism of electroweak symmetry breaking via the
Higgs mechanism
Predicts a field, the Higgs field, and an associated particle, the Higgs boson.
Introduces terms where particles interact with themselves: self energy or mass
Directly testable by searching for the Higgs boson
A primary goal of the Tevatron and LHC
Weak and EM Force: Strength
For EM force
For weak force
P  2/(q2+M2)2
P  2/(q2+MW2)2
Coupling strength:
Same as EM force
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!
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
However it’s only a theory. Have to find the Higgs boson!
30 years of searching and no luck yet!
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The Forces Revisited
EM
Couples to:
Example
Quanta: Force
Carrier
Mass
Strength in an
Atom
Particles with
electric charge
Weak
Weak charge:
quarks and
electrons
Strong
Gravity
Color charge:
All particles with
quarks
mass
Attraction
between protons
and electrons
Nuclear beta decay
and nuclear fission
Holds nucleons,
quarks together in
the nucleus
Only attractive
Photon
W and Z Boson
Gluon
Graviton
0
80 and 91 GeV
0
0
Decay time:
Decay time:
10-18 sec
10-12 sec to
F = 2.3x102N
F = 2.3x10-47N
F = 2.3x10-8N
thousands of years
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The Standard Model
What is the Standard Model?
Explains the hundreds of common particles:
atoms - protons, neutrons and electrons
Explains the interactions between them
Basic building blocks
6 quarks: up, down…
6 leptons: electrons…
Bosons: force carrier particles
All common matter particles are
composites of the quarks and leptons
and interact by exchange of the bosons
Only observing the Higgs Boson is left to
complete the experimental program
associated with the SM
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Searching for the Higgs
How do we search for the Higgs Boson
Use the idea of particle anti-particle annihilation
positron
Higgs Boson
electron
Annihilate high energy electrons and positrons or high energy quarks and
anti-quarks inside of protons and anti-protons
Problem: The probability or strength of Higgs interactions is proportional
to the mass of the particle. Electrons and u and d quarks are very light!
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Searching for the Higgs: Production
The Higgs will couple best to the most massive
particles and the W and Z
t
W and Z bosons: 80 and 91 GeV
The top quark: 172.6 GeV: Gold atom
We need to produce Higgs using interactions
with those particles!
_
t
10 orders of magnitude smaller cross section than total inelastic cs
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Searching for the Higgs: Decay
We need decays of the Higgs involving massive particles
Higgs particle is probably not massive enough to decay to top quarks
So we look for the interactions involving the W and Z and the next most
massive particle, the b quark, 4.5GeV
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Higgs Search at LEP
Searched for the Higgs using an electron positron collider
Achieved an energy of 209GeV which allowed it to search for Higgs
particle up to a mass of ~115GeV
Final Result mH > 114.4 GeV
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Indirect Higgs Search
Measuring the mass of the most massive quarks and boson should
allow you to calculate the Higgs mass.
Current Result mH < 160 GeV
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Tevatron Higgs Search
The search for Higgs continues of the Tevatron Accelerator
1.96TeV proton anti-proton accelerator
Enough energy to produce the Higgs.
However, the rate is expected to be very small - 3fb-1 of data per experiment
Two experiments designed to find the Higgs: CDF and DØ
Wisconsin participates in the Higgs search at the CDF experiment
The stage is set.
We can produce the Higgs
We know where to look
The Higgs boson mass is
between 114.4 and ~160GeV
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The CDF Detector
CDF Tracker
Silicon detector: 1 million channel
solid state device!
96 layer drift chamber
Detector designed to
measure all the SM
particles
Dedicated systems for finding
different types of particles
Electrons and muons
Measurement of the energy
of quarks(jets)
And if any energy is missing
Higgs analysis uses most of
the capabilities of the CDF
detector
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The Real CDF Detector
Wisconsin Colloquium
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Searching for the Higgs: Low Mass
At Higgs masses well below
160GeV we search for Higgs
decays to b quarks.
b hadrons are long lived.
Low efficiency to tag long
lifetime.
Many different searches.
Associated production with a
vector boson, VH: Leptonic
decays W and Z are distinctive
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Higgs Search: WHlbb
Example: CDF WHlbb - signature: high pT lepton, MET and b jets
Key issues: Maximizing lepton acceptance and b tagging efficiency
Backgrounds: W+bb, W+qq(mistagged), single top, Non W(QCD)
Single top: yesterdays new physics signal is today’s background
Innovations: acceptance from isolated/forward tracks. Multiple or NN b tagging methods.
Multivariate discriminants: example - Matrix Element Method (probability of any decay
configuration based on the SM calculation compared between signal and background)
Factor of 1.5 improvement in the expected limits in the last year from innovations
Results at mH = 115GeV: 95%CL Limits/SM
Analysis
Lum
(fb-1)
Higgs
Events
Exp.
Limit
Obs.
Limit
CDF NN+ME+BDT
2.7
8.4
4.8
5.8
DØ NN
1.7
7.5
8.5
9.3
Worlds most sensitive low mass Higgs
search - Still a long way to go!
Low Mass Higgs Searches
We gain our full sensitivity by searching for the
Higgs in every viable production and decay mode
Analysis
Lum (fb-1)
Higgs
Events
Exp.
Limit
Obs.
Limit
CDF NN: ZHllbb
2.7
2.2
9.9
7.1
DØ NN,BDT
2.3
2.0
12.3
11.0
CDF NN: VHMETbb
2.1
7.6
5.5
6.6
DØ BDT
2.1
3.7
8.4
7.5
CDF Comb: WHlbb
2.7
8.4
4.8
5.8
DØ NN
1.7
7.5
8.5
9.3
Analysis: Limits
Exp.
Limit
obs.
Limit
CDF WHWWW
33
31
DØ WHWWW
20
26
CDF VHqqbb
37
37
CDF H
25
31
With all analysis combined we have a
sensitivity of about ~2.4xSM at low mass.
DØ WHbb
42
35
DØ H
23
31
A new round of DØ analysis, 2x data and 1.5x
improvements will bring us to SM sensitivity.
DØ ttH
45
64
Searching for the Higgs: High Mass
At Higgs masses around
160GeV we search for Higgs
decays to W bosons.
Leptonic W decay
-
Uses the excellent charged
lepton fining ability of our
detectors
Also a primary channel for the
LHC
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Higgs Search: HWW
HWWll - signature: Two high pT leptons and MET
Key issue: Maximizing lepton acceptance
Primary backgrounds: WW and top in di-lepton decay channel
Innovations: CDF/DØ : Inclusion of acceptance from VH and VBF
CDF : Combination of ME and NN approaches
W+
H
W+
W-
W-
e-
μ+
ν
ν
Spin correlation: Charged leptons
go in the same direction
SM Higgs Search: HWW
Most sensitive Higgs search channel at the Tevatron
Both experiments
Approaching
SM sensitivity!
Let’s Combine the
Results.
Results at mH = 165GeV : 95%CL Limits/SM
Analysis
Lum
(fb-1)
Higgs
Events
Exp.
Limit
Obs.
Limit
CDF ME+NN
3.0
17.2
1.6
1.6
DØ NN
3.0
15.6
1.9
2.0
SM Higgs Combination
High mass only
Exp. 1.2 @ 165, 1.4 @ 170 GeV
Obs. 1.0 @ 170 GeV
SM Higgs Combination
Result verified using two independent methods(Bayesian/CLs)
95%CL Limits/SM
M Higgs(GeV)
160
165
170
175
Method 1: Exp
1.3
1.2
1.4
1.7
Method 1: Obs
1.4
1.2
1.0
1.3
Method 2: Exp
1.2
1.1
1.3
1.7
Method 2: Obs
1.3
1.1
0.95
1.2
SM Higgs Excluded: mH = 170 GeV
We exclude at 95% C.L. the production of a
SM Higgs boson of 170 GeV
Projections
Goals for increased sensitivity
achieved
Goals set after 2007 Lepton Photon
conference
First stage target was sensitivity for
possible exclusion
Second stage goals still in progress
Expect large exclusion, or evidence,
with full Tevatron dataset and further
improvements.
Run II Preliminary
Discovery
Discovery projections: chance of 3 or 5 discovery
Two factors of 1.5 improvements examined relative to summer Lepton
Photon 2007 analyses.
First 1.5 factor achieved for summer ICHEP 2008 analysis
Resulted in exclusion at mH = 170 GeV.
Conclusions
Finding the Higgs Boson would add fundamental information
to our understanding of the forces of nature
Without the Higgs boson we don’t understand the nature of
the weak force: Why it is so much weaker than the
electromagnetic force?
The Higgs boson search is in its most exciting era ever
The Tevatron experiments have achieved sensitivity to the SM Higgs
boson production cross section at high mass
We exclude at 95%C.L. the production
of a SM Higgs boson of 170 GeV
Expect large exclusion, or evidence, with
full Tevatron data set and improvements
SM Higgs Excluded: mH = 170 GeV
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Backup
SM Higgs Combined Limits
Limits calculating and combination
Using Bayesian and CLs methodologies.
Incorporate systematic uncertainties using pseudo-experiments (shape and rate
included) (correlations taken into account between experiments)
Backgrounds can be constrained in the fit
Winter conferences
combination
April: hep-ex/0804.3423
HWW Systematic Uncertainties
Shape systematic evaluated for
Scale variations, ISR, gluon pdf, Pythia vs. NL0 kinematics, jet energy scale: for signal and
backgrounds. Included in limit setting if significant.
Systematic treatment developed in collaboratively between CDF and DØ
LHC Prospects: SM Higgs
LHC experiments have the potential to observe a SM Higgs at 5 over a
large region of mass
Observation: ggH, VBF H, HWWll, and HZZ4l
Possibility of measurement in multiple channels
Measurement of Higgs properties
Yukawa coupling to top in ttH
Quantum numbers in diffractive production
All key channels
explored
Exclusion at 95% CL
CMS
ATLAS preliminary
Example HEP Detector
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