\stackrel{{\rm \bf Many\,\,\, people...}}{\Longrightarrow}
      {\rm \bf Grand\,\,\, Unified\,\,\, Theory}$

    This is an unsuccessful generalization of the electroweak unification scheme based on a semisimple gauge group ($SO(10), SU(5),\cdots$) spontaneously broken by a generalized Higgs mechanism to $SU(3)\times U(1)$. There are two main problems:

    -   New massive and massless particles predicted may mediate proton disintegration (not observed).

    -   Unification of coupling constants should occur at the energy at which the symmetry is restored, but this does not seems to work.

8.  ${\rm \bf Bosons}\bigoplus{\rm \bf Fermions}
      \stackrel{{\rm \bf Golfand, Likhtman, Volkov, Akulov, Soroka,
          Wess\, and\, Zumino}}{\Longrightarrow}   {\rm \bf Superfields}$

    This is a new kind of unification based in an increase of (global spacetime) symmetry to *supersymmetry*, which should also be spontaneously broken by a yet unknown super-Higgs mechanism. It has many interesting properties:

    -   It is the most general extension of the Poincaré and [Yang-Mills]{} symmetries of the S-matrix (Haag-Lopuszanski-Sohnius theorem).

    -   This new symmetry can be combined with Yang-Mills-type symmetries (super-Yang-Mills theories) and with GUT models in which, in some cases, unification of coupling constants can be achieved.

    -   It can also be combined with g.c.t.’s, making it local (*supergravity* theories). We can have supergravity theories with Yang-Mills fields etc., but 