1. Searches for the SM Higgs Boson
Forces of Nature
Unification of the Forces and the Higgs Particle
Searching for the Higgs/Higgs Searches Results
Matthew Herndon, Dec 2008
University of Wisconsin
University of Massachusetts Amherst Physics Colloquium
BEACH 04
J. Piedra 1

2. 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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3. The Periodic Table
Elements in same column have similar chemical properties
Different types of quantum orbits
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4. 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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5. 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! EM
Weak
Strong
Gravity
Couples to:
Particles with electric charge
Protons, Neutrons and electrons
Protons and Neutrons
All particles with mass
Example
Attraction between protons and electrons
Nuclear beta decay and nuclear fission
Holds protons and neutrons together the nucleas
Only attractive
Strength in an Atom
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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6. 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
electron
photon
Exchange of many photons allows for a smooth force(EM field)
For a very quick interaction we can see individual photon exchanges

7. Particle Annihilation or Creation
The new QED EM Theory has one very interesting additional feature
Can rotate diagrams in any direction
electron
photon
???
electron
photon
Antiparticles! Anti-electron or positron. This is going to be a useful way to make new particles.
Time goes from left to right. What is an electron going backward in time?
Also learned from studying EM force that the proton and neutron were made of smaller particles. up and down quarks. p=uud, n=udd

8. 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
So far just a theory - though a successful one
1eV = 1.6x10-19 J
Still working to fully understand EW=EM+Weak Unification
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9. 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

10. Weak and EM Force: Strength
P  2/(q2+M2)2
For EM force
For weak force
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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11. The Forces Revisited EM Weak Strong Gravity Couples to:
Particles with electric charge
Weak charge: quarks and electrons
Color charge: quarks
All particles with mass
Example
Attraction between protons and electrons
Nuclear beta decay and nuclear fission
Holds nucleons, quarks together in the nucleus
Only attractive
Quanta: Force Carrier
Photon
W and Z Boson
Gluon
Graviton
Mass
80 and 91 GeV
Strength in an Atom
Decay time:
10-18 sec
F = 2.3x10-8N
10-12 sec to
thousands of years
F = 2.3x102N
F = 2.3x10-47N
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12. The Standard Model What is the Standard Model? Basic building blocks
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 12

13. Searching for the Higgs
How do we search for the Higgs Boson
Use the idea of particle anti-particle annihilation
electron
Higgs Boson
positron
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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14. Searching for the Higgs: Production
The Higgs will couple best to the most massive particles and the W and Z
W and Z bosons: 80 and 91 GeV
The top quark: 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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15. 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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16. 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 > GeV
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17. 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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18. 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 and ~160GeV
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19. Higgs analysis uses most of the capabilities of the CDF detector
CDF Tracker
Silicon detector: 1 million channel solid state device!
96 layer drift chamber
Dedicated systems for finding different types of particles
Electrons and muons
Measurement of the energy of quarks(jets)
And if any energy is missing
Detector designed to
measure all the SM
particles
Higgs analysis uses most of the capabilities of the CDF detector
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20. The Real CDF Detector
Wisconsin Colloquium
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21. 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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22. Worlds most sensitive low mass Higgs search - Still a long way to go!
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!

23. 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
3.7
8.4
7.5
CDF Comb: WHlbb
4.8
5.8
DØ NN
1.7
8.5
9.3
Analysis: Limits
Exp. Limit
obs. Limit
CDF WHWWW 33 31
DØ WHWWW 20 26
CDF VHqqbb 37
CDF H 25
DØ WHbb 42 35
DØ H 23
DØ ttH 45 64
With all analysis combined we have a sensitivity of about ~2.4xSM at low mass.
A new round of DØ analysis, 2x data and 1.5x improvements will bring us to SM sensitivity.

24. 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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25. 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 H μ+ ν W- W+ e-
Spin correlation: Charged leptons
go in the same direction

26. Let’s Combine the Results.
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
DØ NN
15.6
1.9
2.0

27. SM Higgs Combination Exp. 1.2 @ 165, 1.4 @ 170 GeV Obs. 1.0 @ 170 GeV
High mass only
Exp. 165, 170 GeV
Obs. 170 GeV

28. SM Higgs Combination
Result verified using two independent methods(Bayesian/CLs)
SM Higgs Excluded: mH = 170 GeV
We exclude at 95% C.L. the production of a SM Higgs boson of 170 GeV
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
1.0
Method 2: Exp
1.1
Method 2: Obs
0.95

29. 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

30. 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.

31. 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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32. Backup

33. 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/

34. 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

35. 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

36. Example HEP Detector Transverse slice Path of muon
Punch-through can make a high-pt muon look low
Muons are “gold-plated” since they suffer less radiative loss in the tracker, are better measured (Detector Paper, 128). 36 36