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<strong>Axion</strong>s as Pseudo Nambu-Goldstone Bosons<br />
Georg Raffelt, Max-Planck-Institut für Physik, München, Germany<br />
PONT d‘Avignon, 21-25 April 2008, Avignon, France<br />
• The realization of the Peccei-Quinn mechanism involves a new chiral<br />
U(1) symmetry, spontaneously broken at a scale f a<br />
• <strong>Axion</strong>s are the corresponding Nambu-Goldstone mode<br />
E ≈ f a<br />
V(a)<br />
• U PQ (1) spontaneously broken<br />
• Higgs field settles in<br />
“Mexican hat”<br />
a<br />
E ≈ Λ QCD ≪ f a<br />
V(a)<br />
• U PQ (1) explicitly broken<br />
by instanton effects<br />
• Mexican hat tilts<br />
• <strong>Axion</strong>s acquire a mass<br />
_<br />
Θ=0<br />
a
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