Primordial Black Holes and Cosmological Phase Transitions Report ...

PBHs and Cosmological Phase Transitions 35
Figure 10: The particle content of the SMPP (http://wwwsldnt.slac.stanford.edu)
as regards fundamental fermions and bosons. Since
there are eight different types of gluons (g), two W bosons and the (still
hypothetical) Higgs boson (H) – not shown – there are a total of 25 particles.
families of leptons, and 13 gauge bosons (i.e., particles that act as carriers of
the fundamental interactions): eight massless gluons, Z 0 , W ± ; the massless
photon; the (yet to be discovered) scalar Higgs (e.g. Boyanovsky et al., 2006).
These particles interact in only three ways: the electromagnetic interaction, the
weak interaction and the strong interaction. Note that gravity is left outside
the SMPP because we do not yet have a theory of quantum gravity.
The classification of fundamental particles is performed taking into account
certain properties such as the rest mass, the electric charge and the spin. The
spin must be an integer or an half–integer and is normally expressed in units
of . Quantum particles with integer spin are called bosons and quantum particles
with half–integer spin are called fermions. Fermions obey the Exclusion
Principle (identical fermions cannot be at the same state at the same time) but
bosons do not obey the Exclusion Principle.
A particle which does not react to the strong interaction is called a lepton
(see Table 4, Figure 10). In the SMPP six of the 12 fermions are leptons:
three electric charged particles (electron – e − , muon – µ − , tau – τ − ) and their
associated neutrinos (νe, νµ, ντ). The remaining six fermions are quarks –
particles which react to the strong interaction (see Table 5, Figure 10). Quarks
invariant under certain symmetry transformation groups. When they are invariant under a
transformation identically performed at every space–time point they are said to have a global
symmetry. Gauge theory extends this idea by requiring that the Lagrangians must possess
local symmetries which enable symmetry transformations in a particular region of space–time
without affecting what happens in another region. This requirement is a generalized version
of the Equivalence Principle of general relativity.