In 1964, in the context of the research of a classification scheme of the numerous
family of the new elementary particles, mesons and baryons, that had been discovered since
1947, he proposed an unitary classification scheme ,based on particle octets, that are
considered no more elementary, but consisting of the union of two or three truly
elementary particles, with a fractional electric charge ( 2/3 or 1/3 of that of the
electron ) and spin 1/2, to which he gave the mysterious name of "quark".
Quarks, individuated with the u (up), d (down) and s (strange) symbols, interact among
them, three by three, to form baryons (proton, neutron and particles having a mass greater
than the one of proton ), while to form the mesons quark-antiquark pairs are considered,
being the /u, /d and /s the antiparticles of the quarks.
By means of his unitary scheme, Gell-Mann succeeded besides to foresee the existence and
the mass of a new baryon, W-, that was discovered shortly
afterwards, furnishing a bright confirmation of validity of the three quark model,
that constituted the base for the development of the present "standard model ".
He was conferred the Nobel prize in 1969.
Since the first sixties, independently each from the others, in the context of a
unification work, whose first example goes up again to Maxwell, that treated in an unitary
scheme electric and magnetic phenomena, they elaborated a unified theory of
electromagnetic and weak forces (the so-called electro-weak theory ), using and extending
the calculation methods introduced by Feynmann and Schwinger to improve quantum
electrodynamics, and, as made the Japanese physicist Yukawa in 1935 for the theory of
strong interactions, hypotisizing the exchange of three heavy virtual particles, that are
the vectors of the electro-weak force.
The aforesaid particles, individuated by the W+ , W- and Z° symbols, are some
intermediate vector bosons with an unitary spin and masses much greater of the one of
proton, about hundred times the proton mass.
A so high value of the mass-energy of the three virtual vectors bosons , mediators of the
interaction, implicates, for analogy with the calculations made by Yukawa for the mass of p meson (the pion), a short range of the weak force ( 10-15
cm, a millionth of billionth of centimeter ), imposed by uncertainty principle and
conservation of the energy .
The exchange of the charged bosons W+ and W- explains the b
decay , whose first theory was developed by Enrico Fermi in 1934.
For example, in an atomic nucleus, the transformation of a neutron in a proton with the
emission of an electron and of an antineutrino, can be explained by the exchange between
neutron and proton, of a virtual boson W- , that decaies into an electron and an
antineutrino.
Likewise it can be explained, with the exchange of a virtual boson W+, the b decay with positron emission ( interactions with weak charged
currents) .
The neutral boson Z° instead mediates weak forces in the cases when there isn't a
transfer of electric charge ( interaction with weak neutral currents ).
For example, the interaction between an electron and a positron that disappear producing a
Z° boson, which decays into a muon-antimuon pair, showed that the electromagnetic forces
and the weak ones derive from only one force, the electro-weak force , that, for symmetry
breakup at the low energies, differentiates in both them.
It can be verified, in fact, experimentally,that weak forces increase with increasing
energy, until assume, at about 100 Gev, the same intensity of the electromagnetic ones.
They were conferred the Nobel prize in 1979.
Carlo Rubbia , that already in the early seventies had contributed at the CERN
(Ginevra) to important detection experiments of the neutral weak currents produced by
intense beams of neutrinos, programmed a series of important experiments finalized to
detection of the vectors bosons foreseen by electro-weak theory.
The exceptional skill of Carlo Rubbia as a researcher and as a manager, together with the
precious experience of Van Der Meer in using again, strengthening them, the detectors and
the existing particle accelerators, allowed to realize in a short time the whole research
project and to get in 1983 abundant and evident tests of the existence of the W and Z°
bosons and of the validity of electro-weak theory.
They were conferred the Nobel prize in 1984.
