Aungshuman Zaman Department of Physics and Astronomy Stony Brook University October 11 2010 What Is This Talk All About Why is the search for the Higgs Boson important Gauge theory and standard model ID: 315271
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Slide1
THE SEARCH FOR THE HIGGS BOSON
Aungshuman
Zaman
Department of Physics and Astronomy
Stony Brook University
October 11, 2010Slide2
What Is This Talk All About
Why is the search for the Higgs Boson important?
Gauge theory and standard model.
How can we detect Higgs boson?Direct and indirect search.
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2Slide3
How do we explain nature at its smallest scale?
Quantum Mechanics + Special Relativity
(No Gravity)
QFT
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3Slide4
Gauge Symmetry
We demand
Lagrangian
density is invariant under certain continuous local transformations--- Gauge Transformations.
These symmetry transformations form groups.This imposition of condition on the field theories gives us the force carrying particles.
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4Slide5
Gauge Theories
U(1)
Elecromagnetism
photon
SU(2) Weak Interaction
,
, Z
SU(3) Strong Interaction
8 gluons
Standard Model
SU(3)
×
SU(2)
×
U(1) ?
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BUT… …
There are problems with this picture
1. Weak force carriers are massive unlike the photon and gluon.
2. Leptons and quarks should not be massive.
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Here comes the Higgs
Englert
-
Brout-Higgs-Guralnik-Hagen-Kibble (1963-64)SU(2) not an exact symmetry.Introduce one extra scalar field--- HIGGS field
with non-zero vacuum expectation value
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The Mexican hat potential: The ground state lacks the symmetry of the whole system.
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Higgs Boson completes the SM picture
The electroweak symmetry is spontaneously broken.
Electroweak gauge bosons acquire mass through the “Higgs Mechanism.”
According to the simplest model, Higgs boson is a scalar particle with couplings to other particle. This coupling is responsible for the mass of leptons and quarks.
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So search for the Higgs boson is very important for our understanding of the universe.
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Experimental Search for Higgs
Indirect:
Precision
Electroweak ConstraintsPrecision measurement of the W,Z and t masses has been used to establish indirect limits on SM Higgs mass.(Fermilab, LEP and SLD)
exclusion of a
SM Higgs
boson having a mass greater than 2
85
GeV/c2 at 95%
CL. (2006)
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Experimental Search for Higgs
Direct
Search
LEP (1989-2000; electron-positron at 45-200 GeV)Tevatron (proton-antiproton at 2 TeV)LHC (proton-proton at 7-14
TeV; The discovery of the Higgs particle was a primary motivation for the LHC
.)
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Large Electron-Positron collider (LEP)
LEP data sets the
experimental
lower bound for the mass of the
SM Higgs boson at 114.4 GeV/
c
2
(95% CL) In 2000, data from LEP suggested inconclusively that the Higgs Particle of a mass around 115 GeV might have been observed.
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Important parameters
Higgs cross section
Higgs Branching Ratio
BackgroundAZAMAN, 10.11.10
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Higgs Cross section (in pb)
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Higgs Decay Ratio
While searching for the Higgs particle in a given mass range, the decay modes are selected on the basis of branching ratio as well as the relative background for the process in that mass range.
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Low mass region: MH
<135
Gev
/c2
pp H b
(
B
0
, B±
, Λb, π0, π
± )
Higgs branching ratio (BR) is roughly 85%
Background: p
(q
) b
Signal to Background ratio (S/B) is
very poor
!!
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So at Tevatron……
Signal:
(
)
WH
l ν
ZH
l+ l -
ZH
ν
Background:
W+
; e.g. W +
Wb
Wb
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At LHC… …
pp
H
γ γBranching Ratio ~ 10-4
So we are throwing away 99.99% of the data.
Larger energy makes S/B even
worse
WHY??
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LHC
Two
high
energy
photons
set the Higgs process apart from the regular processes (
q¯q
→
γ , gg
→ γ
and quark
bremsstralung
).
A
bump in the di-photoninvariant mass spectrum.
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High mass region; MH>135
GeV
/c
2Both Tevatron and LHCEasier, S/B comparatively good
Dominant channel: H WW(*)Background:
pp
WW
(*)
l ν l ν WZ l ν l‘ ν’
ZZ l ν l‘ ν’
Angular correlation between final state leptons.
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Higgs mass range narrows down at Tevatron
In 2010
,
data
from CDF and D0 experiments
at the Tevatron
exclude
the Higgs boson in the range between 158 GeV/
c2 and 175 GeV/c2
(95% CL)
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So, Where do we stand?
Status as of August 2010, to 95% confidence
interval.
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Bibliography
Professor John Hobbs
, Stony Brook
University. Professor Patrick Meade, Stony brook University.
Introduction to elementary particles, D. GriffithsTests of the Standard Electroweak Model at the Energy
Frontier,
John D.
Hobbs, Mark
S. Neubauer and Scott Willenbrock Precise
predictions for Higgs cross sections at the Large Hadron Collider, Robert Harlandera
Indirect limit on the standard model Higgs boson mass from the precision Fermilab
, LEP, and SLD
data,
J
. H.
Field
SEARCHES
FOR THE HIGGS BOSON AT LHC, M. DELMASTRO, on behalf of the ATLAS and CMS collaborations, European Laboratory for Particle Physics (CERN)Wikipedia
,ScholarpediaAZAMAN, 10.11.1025