Transcript The Higgs Particle - University of Houston

The Higgs Particle
Sarah D. Johnson
University of La Verne
August 22, 2002
August 22, 2002
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Outline
I.
II.
Mass in the Standard Model
Electro-Weak Force Unification and the
Higgs Mechanism
III. Searches for the Higgs Particle
IV. Future Prospects
V. What We Will Learn When We Find It
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I.
Mass in the Standard Model
What is the origin of the particle masses?
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Particle Masses (GeV/c2)
Up
0.003
Down
0.006
νe
<1 x 10-8
Electron
0.000511
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Charm
1.3
Strange
0.1
νμ
<0.0002
Muon
0.106
Top
175
Bottom
4.3
ντ
<0.02
Tau
1.7771
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Photon
0
Gluon
0
Z
91.187
W±
80.4
Questions:
Why is there such a large range of quark masses?
Why is there such a large range of lepton masses?
Why are the neutrino masses so small?
Why do the W and Z have mass, but the photon and the gluon
do not?
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II. Electroweak Force Unification
and the Higgs Mechanism
1961 – 1968 Glashow, Weinberg and Salam (GWS)
developed a theory that unifies the electromagnetic and
weak forces into one electroweak force.
Electromagnetic Force – mediator: photon (mass = 0)
felt by electrically charged particles
Weak Force – mediators: W+,W-, Z0(mass ~ 80-90 GeV/c2)
felt by quarks and leptons
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For two protons in a nucleus the electromagnetic force
is 107 times stronger than the weak force, but, at much shorter
distances (~10-18 m), the strengths of the weak and the
electromagnetic forces become comparable..
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GWS Electroweak Theory
The theory begins with four massless mediators
for the electroweak force: Wμ1,2,3 and Bμ.
Wμ1,2,3, Bμ
W+, W-, Z0, γ
This transformation is the result of a phenomenon known
as Spontaneous Symmetry Breaking. In the case of the
electroweak force, it is known as the Higgs Mechanism.
August 22, 2002
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Spontaneous Symmetry Breaking
This is a phenomenon that can occur when the
symmetries of the equations of motion of a system do
not hold for the ground state of the system.
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Higgs Mechanism
Goldstone’s Theorem - The spontaneous breaking of a
continuous global symmetry is always accompanied by the
appearance of massless scalar particles called Goldstone
bosons.
In the Higgs Mechanism, as the result of choosing the
correct gauge, the massless gauge field “eats” the Goldstone
bosons and so acquires mass. In addition, a “mass-giving”
Higgs field and its accompanying Higgs boson particle
emerge.
W’s
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W+
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W-
Z0
The Higgs Field and Higgs Boson
The neutral Higgs field permeates space and all particles
acquire mass via their interactions with this field.
The Higgs Boson
• neutral
• scalar boson (spin = 0)
Ho
• mass = ?
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III. Searches for the Higgs Particle
What properties are important?
• The strength of the Higgs coupling is proportional to
the mass of the particles involved so its coupling is
greatest to the heaviest decay products which have mass
< mH/2. For example, if mH > 2Mz then the couplings for
decay to the following particle pairs:
Z0Z0 : W+W- : τ+τ- : pp : μ+μ- : e+eare in the ratio
1.00 : 0.88 : 0.02 : 0.01 : 0.001 : 5.5 x 10-6
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• Mass constraints from self-consistency* of the Standard Model :
130 GeV/c2 < MH < 190 GeV/c2
*The discovery of a Higgs boson with a mass less than 130 GeV/c2 would imply
“new physics” below a grand unification (GUT) scale energy of 1016 GeV/c2
• Dominant Production Mechanisms :
LEP:
e+e-  H0 Z0
Tevatron:
gg  H0
qq  H0W or H0Z
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Searches at the Large Electron-Positron
Collider (LEP) at CERN
Final States with Good Sensitivity to Higgs Boson:
1. e+e-  (H0bb) (Z0qq)
BR 60%
2. e+e-  (H0bb) (Z0νν)
BR 17%
3. e+e-  (H0bb) (Z0e+e- , μ+μ-)
BR 6%
4. e+e-  (H0τ+τ-) (Z0qq)
e+e-  (H0qq) (Z0 τ+τ-)
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BR 10%
Aerial view of LEP at CERN
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LEP Search Results
LEP1: 17 million Z0 decays
mH > 65 GeV/c2
LEP2: 40,000 e+e-  W+W- events
e+e-  H0Z0 has background from W+W- and Z0Z0 events, but b-tagging and
kinematic constraints can reduce these backgrounds.
In 2000 at LEP2 with a center of mass energy of > 205 GeV:
ALEPH: signal three standard deviations above background with
mH  115 GeV/c2
All four experiments: signal reduced to two standard deviations
above background
mH  115.6 GeV/c2
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mH > 114.1 GeV/c2
Searches at the Tevatron
Search Methods:
qq  (H0  bb)(W  l ν)
qq  (H0  bb)( Z0  l+l-) (l = e, μ)
CDF: also hadronic decays of W,Z Dzero: also Z  ν ν
Run I: CDF and DZero took 100 pb-1 of data each and no
signal seen though cross section limits were set
Run II: CDF and DZero expect 10 fb-1 of data each
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IV. Future Prospects
The Large Hadron Collider (LHC):
2007
• pp collider with a center of mass energy of 14 TeV
• ATLAS and CMS detectors optimized for Higgs searches
• Higgs mass range between 100 GeV/c2 and 1TeV/c2
Next Linear Collider:
after 2010
• e+e- collisions at 500+ GeV
• precision measurements of Higgs couplings to a few percent
• measurements of self-interaction via two Higgs final states
August 22, 2002
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V. What We Will Learn When We Find It
• If H0 found at the expected Standard Model mass, it will
validate the GWS Electroweak Theory and complete the model.
• Measurements of the Higgs couplings and comparison with
particle masses will verify mass-generating mechanism.
• A lighter than 130 GeV/c2 mass Higgs boson could support a
theory beyond the Standard Model, known as Supersymmetry.
• If a Higgs boson with a mass < 1 TeV is not found, it would
indicate that the Electroweak symmetry must be broken by a
means other than the Higgs mechanism.
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Supersymmetry
• Supersymmetry is a theory beyond the Standard Model that
predicts that every particle will have a super-partner.
• The Minimal Supersymmetric Standard Model (MSSM)
contains five Higgs particles: h0, H0, A0, H+, H• In the MSSM the lightest Higgs, h0, is expected to have a
mass less than 130 GeV/c2
• The current mass limits on MSSM Higgs are:
mH0 > 89.8 GeV/c2
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mA0 > 90.1 GeV
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mH > 71.5 GeV