BEACH 04 J. Piedra 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 2008University of Wisconsin

University of Massachusetts Amherst Physics Colloquium

g g s

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The AtomIn 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

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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

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Elements in same column have similar chemical properties

Different types of quantum orbits

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We Observed New Physics

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How could the nucleus exist?

Positive protons all bound together in the

the atomic nucleus

One type of atom could convert

itself into another type of atom

Nuclear beta decay

Charge of atom changed and

electron emitted

Needed a new theory

Already there were some clear problems

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The ForcesBest 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!

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EM Weak Strong Gravity

Couples to:Particles with electric charge

Protons, Neutrons and electrons

Protons and Neutrons

All particles with mass

ExampleAttraction between

protons and electrons

Nuclear beta decay and nuclear fission

Holds protons and neutrons together the

nucleasOnly attractive

Strength in an Atom

F = 2.3x10-8NDecays can take

thousands of yearsF = 2.3x102N F = 2.3x10-47N

How do we understand the Forces? Fundamental differences in strengths!

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

electron

photon

How Do the Forces Work

For a very quick interaction we can see individual photon exchanges

Exchange of many photons allows for a smooth force(EM field)

The new QED EM Theory has one very interesting additional feature

Can rotate diagrams in any direction

Time goes from left to right. What is an electron going backward in time?

electron

electron

photon electro

n

electron

photon

???

???

Particle Annihilation or Creation

Antiparticles! Anti-electron 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

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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

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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

Electroweak Symmetry BreakingConsider 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 bosonA primary goal of the Tevatron and LHC

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Weak and EM Force: Strength

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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

P 2/(q2+M2)2

For EM force

For weak force

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

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EM Weak Strong Gravity

Couples to:Particles with electric charge

Weak charge: quarks and electrons

Color charge: quarksAll particles with

mass

ExampleAttraction between

protons and electrons

Nuclear beta decay and nuclear fission

Holds nucleons, quarks together in the

nucleusOnly 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 ModelWhat 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

bosonsOnly observing the Higgs Boson is left to

complete the experimental program

associated with the SM

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Searching for the HiggsHow do we search for the Higgs Boson

Use the idea of particle anti-particle annihilation

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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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Searching for the Higgs: ProductionThe 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!

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t

t_

10 orders of magnitude smaller cross section than total inelastic cs

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Searching for the Higgs: DecayWe 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

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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

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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

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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

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

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The CDF Detector

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Higgs analysis uses most of the capabilities of the CDF

detector

Detector designed to

measure all the SM

particles

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The Real CDF Detector

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Searching for the Higgs: Low Mass

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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

Example: CDF WHlbb - 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

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

Results at mH = 115GeV: 95%CL Limits/SM

Higgs Search: WHlbb

Worlds most sensitive low mass Higgs

search - Still a long way to go!

We gain our full sensitivity by searching for the

Higgs in every viable production and decay mode

Analysis: Limits Exp. Limit

obs. Limit

CDF WHWWW 33 31

DØ WHWWW 20 26

CDF VHqqbb 37 37

CDF H 25 31

DØ WHbb 42 35

DØ H 23 31

DØ ttH 45 64

Analysis Lum (fb-1) Higgs Events

Exp. Limit

Obs. Limit

CDF NN: ZHllbb 2.7 2.2 9.9 7.1

DØ NN,BDT 2.3 2.0 12.3 11.0

CDF NN: VHMETbb 2.1 7.6 5.5 6.6

DØ BDT 2.1 3.7 8.4 7.5

CDF Comb: WHlbb 2.7 8.4 4.8 5.8

DØ NN 1.7 7.5 8.5 9.3

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.

Low Mass Higgs Searches

-

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Searching for the Higgs: High Mass

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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

HWWll - 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

HHμ+

νW-

W+

e-

ν

W-

W+

Spin correlation: Charged leptons

go in the same direction

Higgs Search: HWW

Most sensitive Higgs search channel at the Tevatron

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

Results at mH = 165GeV : 95%CL Limits/SMBoth experiments

Approaching

SM sensitivity!

Let’s Combine the Results.

SM Higgs Search: HWW

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)

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

95%CL Limits/SM

SM Higgs Excluded: mH = 170 GeV

We exclude at 95% C.L. the production of a

SM Higgs boson of 170 GeV

SM Higgs Combination

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

Projections

Discovery projections: chance of 3 or 5 discoveryTwo 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.

Discovery

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Conclusions

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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

Backup

SM Higgs Combined LimitsLimits 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

HWW 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

ATLAS preliminary

Exclusion at 95% CL

LHC Prospects: SM HiggsLHC experiments have the potential to observe a SM Higgs at 5 over a

large region of mass

Observation: ggH, VBF H, HWWll, and HZZ4l

Possibility of measurement in multiple channels

Measurement of Higgs properties

Yukawa coupling to top in ttH

Quantum numbers in diffractive production

CMS

All key channels

explored

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Example HEP Detector