In the late 1980s, experiments that looked at the effect of gravity
as a function of distance from earth claimed to have found evidence for
a new type of force.
All their results could be explained by ordinary gravitational forces
-- because the expected behavior of gravity as you move away from the
surface of the earth depends in detail on the profile of the rocks or
soil nearby and their densities. Unexpected fissures or extra dense
rocks could cause all the effects seen.
there is at least one additional type of
beyond the four known forces (weak,
This force is needed to explain how all the
masses are generated. This part of the theory is the least tested
experimentally, so there are a number of different competing ideas on
how it may work.
The simplest version introduced one more force--the
force--and one more particle type--the Higgs particle--related to this
force. Searches for this particle and efforts to learn more about how
particle masses occur are one active area of particle studies. Other
models introduce more complicated explanations for particle masses.
In addition, there are many speculations about physics beyond the
Standard Model that introduce additional types of extremely-weak
interactions. These interactions can only be observed if they mediate
a process, such as proton
that is otherwise totally forbidden. So far, no experimental evidence
for such processes has been found. However physicists like this idea,
since such additional processes are predicted when we try to unify the
strong, weak and
into a single "Grand Unified Theory." Such unification is suggested by
the similarities of the underlying mathematical theories for these
very different interactions.
Unification of all four force types, including gravity, is also a
goal for particle physics. Gravity has a different mathematical
structure, and so far no complete quantum theory of gravity has been
suggests possible answers, but much work remains to be done.
One part of the Standard
Model is not yet well established. We do not know what causes the
Fundamental particle to have masses. The simplest idea
is called the Higgs mechanism. This mechanism involves one
additional particle, called the Higgs boson, and one
additional force type, mediated by
exchanges of this
The Higgs particle has not yet been observed. Today we
can only say that if it exists, it must have a mass greater than about
80GeV/c2. Searches for a more massive the Higgs boson are
beyond the scope of the present facilities at SLAC or elsewhere.
Future facilities, such as the
Hadron Collider at
or upgrades of present facilities to higher energies are intended to
search for the Higgs particle and distinguish between competing
Thus, this one aspect of the Standard Model does not yet
have the status of "theory"
but still remains in the realm of hypothesis or model.