The Origin of Mass in

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Lecture VI Symmetry Breaking
The Origin of Mass in Particle Physics
60th Compton Lectures Ambreesh Gupta
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Quantum Spin
Particle spin s(h/2?) s 0,1,2
Bosons s1/2,3/2 Fermions
Massless Spin 1 particle
Massive Spin 1 particle
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Vacuum
Greek Philosophy starts with a double negation
It denies the concept of nothing.
Some Vacuums Earth atmosphere 250 km above
surface 10-7 mbar, 109 molecules/cm3 Vacuum tube
of LEP ring 10-9 mbar,
107molecules/cm3 Best Vacuum reached
10-13 mbar, 103 molecules/cm3
Casimir Effect
Vacuum is seething fluctuation of
electromagnetic field
The Quantum Oscillator
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Peculiar Higgs Field In its lowest energy state
of a Higgs field is not ZERO
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Spontaneous Symmetry Breaking
Ferromagnet Above Curie Temperature A random
direction of magnetization is chosen Below
Curie Temperature
Glodstone Theorem Every symmetry of nature, that
is not also a symmetry of the ground State
implies existence of an elementary particle.
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Spontaneous Symmetry Breaking Higgs Mechanism
Introduce a field that does not vanish in
vacuum. Higgs field provides a frame in which
the orientation of isotopic spin arrow can
be determinedthe Higgs field conceals the the
symmetry.
Yang-Mills fields are mass less vector i.e., have
two state. Yang-Mills quantum eats Higgs
particle and acquires mass and extra spin state.
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Cocktail partyroom full of peoplelike Higgs
field
A celebrity walks in
people crowd around celebrityresisting his
movementlike acquiring Mass.
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Why bother with Gauge Symmetry?
Theories exhibiting gauge symmetries are
renormalizable. (Repeat this sentence until it
rolls trippingly on your tongue, then try it
out at lunch.)
- Leon Lederman, in The God Particle
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ElectroWeak Theory Late 60s Early 70s
In 1967 Weinberg-Salam-Glashow combined the
electromagnetic and the weak force in to a
single electroweak theory.
In 1971 Gerardus t hooft Martinus Veltman
showed the theory was Renormalizable.
Ph.D thesis!
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Discovery of Neutral Current 1973
Beta decay n ? p e ? d ? u e
?
A looking simple reaction ? u ? ? u
Gargamelle Bubble Chamber Detector at CERN For
1.5 million pictures, 5 to 30 event events
expected 3 gold plated events observed 1972
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Discovery of W/Z Bosons 1983
Super Proton Synchrotron (SPS) proton anti-proton
collider at CERN
Carlo Rubbia and Simon Van der Meer 1985 Nobel
Prize
UA1 Detector
First Z event
First W event
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An unexpected surprise observation of quark Jet
(1981)
UA 2 detector
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Standard Model Relations

• electron g-2 0.004 ppm
• GF muon life-time 9 ppm
• MZ LEP 1 lineshape 23 ppm

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Large Electron Positron Collider (LEP)
LEP I (1989-1993) Z physics. 18 million Z
bosons produced LEP II (1996-2000) W physics.
80,000 Ws produced. (Energies
from 161 GeV 209 GeV) Ws
produced in
pairs.
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The LEP Experiments
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Collider Detectors 101
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Standard Model Parameters
Z boson at LEP I
W pairs at LEP II
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Top Quark Discovery at Fermilab Tevatron 1995
Top Mass 174.3 ? 5.1 GeV
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What about the Higgs?
Since Top mass is measured, it can be used along
with other precision measurements as an input to
Standard Model relations. ? Indirect evidence of
the Higgs Boson.
Direct Search for Higgs at LEP Higgs-strahlung
process, ee-?ZH,
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Strong Forces
Existence of quarks Gellman, Neeman, Zweig
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Standard Model