The Origin of Mass in

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Lecture V Electroweak Physics
The Origin of Mass in Particle Physics
60th Compton Lectures Ambreesh Gupta
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Radioactivity
Radioactivity discovered in 1896 by Becquerel
The rate in beta decay was much slower ( 10-8 sec
) than would be expected from electromagnetic
force ( 10-16 sec ) or strong force ( 10-23 sec ).
A different force at work for beta decays Weak
Force
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Decay of a Neutron Weak Interaction
A desperate remedy to conserve energy, Pauli
hypothesized neutrinos
Detection of neutrino, Cowan and Reins (1956
Nobel)
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Fermi Theory of Weak Interaction
In 1934 Fermi developed a theory of beta decay
taking account of Paulis neutrino suggestion.
Problem . The theory would unrealistic
probability at high energies ? Fermi Theory
Low energy approximation of some theory at
high energies.
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Decay of a Muon Weak Interaction
Muons were discovered In cosmic ray data in
1947 ?-decay, ?-decay and ?-absorption all
have same coupling
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Parity Violation Weak Interaction
Only left handed neutrino exist!
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A theory of weak interactions formulated in
1930s by Enrico Fermi accounted for some basic
features of the Weak interactions, but the
theory lacked local symmetry. It is now clear why
it took so long to make sense of these forces
the necessary gauge theory were not understood.
G. thooft
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Yang-Mills Theory
Global Change
Yang and Mill considered the consequence of
making global isotopic-spin symmetry into a
local symmetry
Local Change

• In order to preserve invariance of
• all observable quantities necessary
• to introduce six new fields.
• Novel idea but unrealistic
• three vector boson. One photon like filed and
  two mass less
• charged particles(!)
• - makes protons and neutrons indistinguishable

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Non/Abelian Gauge Theory
abelian
If emmision of a photon from an electron is A
and absorption is B AB BA (abelian) AB
? BA (non-abelian)
non-abelian
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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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Electro-Weak Theory
Weinberg, Salam and Glashow created the
electroweak theory based on Yang-Mills modelit
embraces both electromagnetic and weak Forces.
1979 Nobel Prize Predicted the existence of
bosons Discovered at
CERN in 1983 Carlo Rubbia and Simon van der
Meer. One of the big triumph of the Theory was
the prediction of Z0 boson. The only remaining
particle unseen in experiments directly is the
Higgs boson.
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Relation between W and Z

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

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The LEP Experiments
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