2008 Nobel Prize in Physics

The 2008 Nobel Prize in Physics was awarded, in half, to Yoichiro Nambu for his groundbreaking work on the mechanism responsible for the spontaneous breaking of symmetries in elementary particle physics. Nambu’s work, published in 1961 in two seminal articles co-authored by Giovanni Jona-Lasinio:

showed that a strong interaction among (nearly) massless fermions will lead the formation of a condensate of fermion-antifermion pairs in the vacuum, which breaks the symmetry associated with massless fermions, i.e. chiral symmetry, endowes the fermions with a dynamical mass, and leads to the emergence of massless bosons called Goldstone bosons. When applied to the isospin doublet of up- and down-quarks, the NJL mechanism explains the relative lightness and other properties of the pions, π+, π0, and π, as a consequence of their Goldstone boson character.

Nambu’s mechanism today is understood to be realized in the fundamental interaction of the strong nuclear interaction, quantum chromodynamics (QCD). Because of the analytical intractability of QCD, the NJL model is still used extensively an an effective model of chiral symmetry breaking in QCD. The central goal of relativistic heavy ion collisions is to heat the QCD vacuum to such a high temperature that the interaction among quarks is weakened and the broken symmetry is restored.

It has been recognized in recent years that chiral symmetry breaking is intimately linked to the other fundamental property of the QCD vacuum, quark confinement. Naïvely, the restoration of chiral symmetry and quark-deconfinement could occur at different temperatures. Numerical simulations of lattice QCD have shown that they occur together. An extension of the NJL model to include symmetry aspects of the color force, the PNJL model

provides an explanation of the mechanism responsible for the linkage of the two phenomena and thus for the existence of a single critical temperature threshold for the formation of a quark-gluon plasma exhibiting the full symmetries of QCD.

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