Transcript THE SEARCH FOR THE HIGGS BOSON

THE SEARCH FOR THE
HIGGS BOSON
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.
• How can we detect Higgs boson?
– Direct and indirect search.
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How do we explain nature at its
smallest scale?
Quantum Mechanics + Special Relativity
(No Gravity)
QFT
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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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Gauge Theories
• U(1)
• SU(2)
• SU(3)
Elecromagnetism
photon
Weak Interaction
𝑊 +, 𝑊 −, Z
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 Constraints
– Precision 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 285 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/c2 (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
• Background
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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
b𝑏
(B0, B±, Λb, π0, π± )
• Higgs branching ratio (BR) is roughly 85%
• Background: p𝑝 (q𝑞 )
b𝑏
• Signal to Background ratio (S/B) is very poor!!
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So at Tevatron……
• Signal:
𝑝𝑝 (𝑞 𝑞)
WH
ZH
ZH
l ν 𝑏𝑏
l+ l - 𝑏𝑏
ν ν 𝑏𝑏
• Background:
𝑝𝑝
𝑝𝑝
W+ 𝑞 𝑞 ; e.g. W + 𝑏𝑏
𝑡𝑡
Wb Wb
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At LHC… …
• pp
H
γγ
• Branching Ratio ~ 10-4
• So we are throwing away 99.99% of the data.
WHY??
Larger energy makes S/B even worse
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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-photon
invariant mass spectrum.
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High mass region; MH>135 GeV/c2
Both Tevatron and LHC
• Easier, S/B comparatively good
• Dominant channel: H
WW(*)
• Background: pp
WW(*)
lν 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
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Professor John Hobbs, Stony Brook University.
Professor Patrick Meade, Stony brook University.
Introduction to elementary particles, D. Griffiths
Tests 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,
Scholarpedia
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