Searches for the SM Higgs Boson

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Searches for the SM Higgs Boson

• Forces of Nature
• Unification of the Forces and the Higgs Particle
• Searching for the Higgs/Higgs Searches Results

BEACH 04
J. Piedra
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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 detailed understanding of the properties
  of matter

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The Periodic Table
Different types of quantum orbits
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We Observed New Physics

• 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

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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!

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
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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)

• For a very quick interaction we can see
  individual photon exchanges

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Particle Annihilation or Creation

• The new QED EM Theory has one very interesting
  additional feature
• Can rotate diagrams in any direction

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?
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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
• 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
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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
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Weak and EM Force Strength
P ? ?2/(q2M?2)2

• For EM force
• For weak force

P ? ?2/(q2MW2)2

• 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 its only a theory. Have to find the
Higgs boson!
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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 0 80 and 91 GeV 0 0
Strength in an Atom Decay time 10-18 sec F 2.3x10-8N Decay time 10-12 sec to thousands of years F 2.3x102N F 2.3x10-47N
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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

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Searching for the Higgs

• How do we search for the Higgs Boson
• Use the idea of particle anti-particle
  annihilation

• 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
• 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!

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

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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.

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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
• 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
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The Real CDF Detector
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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 Wbb, Wqq(mistagged), single top,
  Non W(QCD)
• Single top yesterdays new physics signal is
  todays 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 95CL Limits/SM
Analysis Lum (fb-1) Higgs Events Exp. Limit Obs. Limit
CDF NNMEBDT 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!
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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
DØ WH???bb 42 35
DØ H??? 23 31
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.

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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

Spin correlation Charged leptons go in the same
direction
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SM Higgs Search H?WW

• Most sensitive Higgs search channel at the
  Tevatron

Results at mH 165GeV 95CL Limits/SM
Both experiments Approaching SM
sensitivity! Lets Combine the Results.
Analysis Lum (fb-1) Higgs Events Exp. Limit Obs. Limit
CDF MENN 3.0 17.2 1.6 1.6
DØ NN 3.0 15.6 1.9 2.0
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SM Higgs Combination
High mass only
Exp. 1.2 _at_ 165, 1.4 _at_ 170 GeV
Obs. 1.0 _at_ 170 GeV
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SM Higgs Combination

• Result verified using two independent
  methods(Bayesian/CLs)

95CL 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
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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
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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.

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Conclusions

• Finding the Higgs Boson would add fundamental
  information to our understanding of the forces of
  nature
• Without the Higgs boson we dont 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 95C.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
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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
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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

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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
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Example HEP Detector
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