By the end of the twentieth century, after hundreds of high energy experiments,
mankind had deduced that all
matter detected up to then in the laboratory is composed of three families
(or three generations) of quarks and leptons. The first family containing
the electron and three "colors" of "up" and "down" quarks, u and d, is
sufficient to describe all stable atoms. The other two families existed
in equilibrium with the first in the very early universe and are now recreated
only briefly in the highest energy accelerators. They are, however, thought
to have played an essential role in the development of the asymmetry between
matter and anti-matter which allowed for the evolution of stars, planets,
and life as the universe cooled after the big bang.
2 Charged W Bosons:
Higgs Boson: H0
Forces are mediated by the gauge bosons of the standard model. They consist of
the "gluons" six of which cause horizontal transitions within each family
among the three colors of quarks and the
W's which cause vertical transitions. In addition two of the gluons as well as
the photon and Z boson are "diagonal" in their couplings; they do not affect
transitions between the quarks and leptons.
The gauge forces of the standard model do not distinguish between
the families and hence the primary question left unanswered by
twentieth century physics is why there is such a range in
masses among the quarks and leptons. The ratio of the top quark
mass to the electron mass is about 300,000. The masses of particles in
the standard model are proportional to the couplings of an as-yet undiscovered
Higgs boson. The puzzle then becomes why there is such a great hierarchy
in the Higgs couplings.
There are now significant clues from the relative values of the strong and
electroweak coupling constants and from the
that a new symmetry known as Supersymmetry (SUSY) exists in nature.
This theory, which has many aesthetic and theoretical advantages, predicts
that for every particle in the standard model there exists a partner particle
with the same quantum numbers except that its angular momentum differs from
that of the standard model partner by 1/2 unit of Planck's constant.
In exact SUSY the partners would also have the same mass but we know
experimentally that this is not possible. Hence we are expecting to find
in nature a broken supersymmetry where the SUSY partner particles are higher
in mass. The standard model particles shown above
are complemented then by SUSY particles as shown below. For each quark there
should be a "squark" and for each lepton a "slepton". Also the force
carriers have partner particles, gluinos, charginos, and neutralinos.
One prediction of the minimal SUSY model is that there will be two Higgs
particles accompanied by partner "Higgsinos". There will then be charged
Higgs whereas, in the standard model, the Higgs was neutral. In addition
the Higgsinos will mix with the partners of the electroweak gauge bosons
so that there will be four charginos and four neutralinos.
Extra Higgs Bosons:
Pseudoscalar Higgs: A
In exact Supersymmetry the partners should have the same masses which is
experimentally excluded. We must look therefore for a "broken" supersymmetry
in which the SUSY partners are higher in mass. There are reasons to believe
however that these partners should not be higher than 1 TeV in mass and they
should therefore be discovered early in the 21st century. This will then
constitute a doubling of the known constituents of matter analogous to what
happened in the twentieth century when anti-particles were discovered.
There is still a possibility that the gluinos and photino of supersymmetry
may be at relatively low mass (below that of the proton). Many experiments
have been done that could have seen direct evidence for a light gluino.
So far these
have all turned up negative. To see the current map of gluino windows, i.e.
allowed regions of gluino and squark masses not ruled out by experiment,