  The action functional of the Standard Model of particle physics is intimately
related to a specific class of first order differential operators called Dirac
operators of Pauli type ("Pauli-Dirac operators"). The aim of this article is
to carefully analyze the geometrical structure of this class of Dirac operators
on the basis of real Dirac operators of simple type. On the basis of simple
type Dirac operators, it is shown how the Standard Model action (STM action)
may be viewed as generalizing the Einstein-Hilbert action in a similar way the
Einstein-Hilbert action is generalized by a cosmological constant. Furthermore,
we demonstrate how the geometrical scheme presented allows to naturally
incorporate also Majorana mass terms within the Standard Model. For reasons of
consistency these Majorana mass terms are shown to dynamically contribute to
the Einstein-Hilbert action by a "true" cosmological constant. Due to its
specific form, this cosmological constant can be very small. Nonetheless, this
cosmological constant may provide a significant contribution to dark
matter/energy. In the geometrical description presented this possibility arises
from a subtle interplay between Dirac and Majorana masses.
