Limits on particle physics beyond its standard model are mostly
sensitive to the bounds imposed on the 4He abundance. As described
earlier, the 4He abundance is predominantly determined by the
neutron-to-proton ratio just prior to nucleosynthesis. This ratio
is determined by the competition between the weak interaction rates
and the universal expansion rate. The latter can be modified from
its standard model prediction by the presence of ``new'' particles
beyond those known or expected on the basis of the standard model
of particle physics. For example, additional neutrino ``flavors''
(N
> 0), or other new particles, would
increase the
total energy density of the Universe, thus increasing the expansion
rate (see equations 4 & 5), leaving more neutrons to form more
4He. For
N
sufficiently small, the predicted
primordial
helium abundance scales nearly linearly with
N
:
Y
0.013
N
. As a result, an upper
bound to YP coupled with a lower bound to
(since YP
increases with increasing baryon abundance) will lead to an upper
bound to
N
and a constraint on particle
physics
[1].
The constraints on N
3 +
N
as a function
of the baryon-to-photon ratio
is shown in
Figure 5. For
``low-D/high-
'' there
is little room for any ``extra'' particles:
N
0.3.
This would eliminate a new neutrino flavor
(
N
= 1)
or a new scalar particle
(
N
= 4/7)
provided they were massless or light (m << 1 MeV), and
interacted at least as strongly as the ``ordinary'' neutrinos. In
contrast, much weaker constraints are found for
``high-D/low-
''
where N
as large as
5 (
N
2)
may be allowed (see Fig. 5).