Let us now consider how the predictions for the precision observables are
modified in the presence of supersymmetry. There are basically three
implications for the precision results. The first, and most important, is
in the Higgs sector. In the standard model the Higgs mass is arbitrary. It
is controlled by an arbitrary quartic Higgs coupling, so that 
could
be as small as 60 GeV (the experimental limit) or as heavy as a TeV. The
upper bound is not rigorous: larger values of 
would correspond to
such large quartic couplings that perturbation theory would break down.
This cannot be excluded, but would lead to a theory that is qualitatively
different from the (perturbative) standard model. In particular, there are
fairly convincing triviality arguments, related to the running of the
quartic coupling, which exclude a Higgs which acts like a distinct elementary
particle for 
above 
GeV) [25].
However, in supersymmetric extensions of the standard model the quartic
coupling is no longer a free parameter. It is given by the squares of
gauge couplings, with the result that all supersymmetric models have at
least one Higgs scalar that is relatively light, typically with a mass
similar to the Z mass. In the minimal supersymmetric standard model
(MSSM) one has 
GeV
, which generally acts just like the standard
model Higgs except that it is necessarily light.
In the standard model there is a large 
correlation, and
the global fit yields
We have seen that for 
GeV this corresponds to
However, in the MSSM one has the smaller range 
GeV,
leading to
which is on the lower side of the CDF/D0 range, 
GeV).
Because of the lower 
, one obtains 
and 
, which
differ slightly from the values in Table 3.
There can be additional effects on the radiative corrections due to
sparticles and the second Higgs doublet that must be present in the MSSM.
However, for most of the allowed parameter space one has 
, and
the effects are negligible by the decoupling theorem. For example, a large
splitting would contribute to the 
(
-breaking) parameter (to be discussed below), leading to a smaller
prediction for 
, but these effects are negligible for 
. Similarly, there would be new contributions to the 
vertex for 
, 
, or 
.
The MSSM yields a better fit to the precision data than the standard
model [26], but that is mainly due to the
anomalous experimental value of 
.
There are only small windows of allowed parameter space for which the new
particles contribute significantly to the radiative corrections. Except
for these, the only implications of supersymmetry from the precision
observables are: (a) there is a light standard model-like Higgs, which in
turn favors a smaller value of 
. Of course, if a light Higgs were
observed it would be consistent with supersymmetry but would not by itself
establish it. That would require the direct discovery of the
superpartners, probably at the LHC. (b) Another important implication of
supersymmetry, at least in the minimal model, is the absence of other
deviations from the standard model predictions. (c) In supersymmetric
grand unification one expects the gauge coupling constants to unify when
extrapolated from their low energy values [21]. This is consistent
with the data in the MSSM but not in the ordinary standard model (unless
other new particles or thresholds are added). This is not actually a modification of the
precision experiments, but a prediction for the observed gauge couplings.
Of course, one could have supersymmetry without grand unification.