Introduction 187By now we can firmly state that no matter what physics should lies beyond
the SM, necessarily such new physics will necessarily have to reproduce the SM
with great accuracy at energies of the order of 100 GeV.
And, yet, in spite of all this glamorous success of the SM in reproducing an
impressive set of experimental electroweak results, we are deeply convinced of
the existence of new physics beyond this model. We see two main motivations
pushing us beyond the SM.
First, we have theoretical ‘particle physics’ reasons to believe that the SM is
not the whole story. The SM does not truly unify the elementary interactions (if
nothing else, gravity is left out of the game), it leaves the problem of fermion
masses and mixings completely unsolved and it exhibits the gauge hierarchy
problem in the scalar sector (namely, the scalar Higgs mass is not protected by
any symmetry and, hence, it would tend to acquire large values of the order of the
energy scale at which the new physics sets in). This first class of motivation for
new physics is well known to particle physicists. Less familiar is a second class of
reasons which finds its origin in some relevant issues of astroparticle physics. We
refer to the problems of the solar and atmospheric neutrino deficits, baryogenesis,
inflation and dark matter (DM). In a sense these aspects (or at least some of
them, in particular the solar and atmospheric neutrino problems and DM) may
be considered as the only ‘observational’ evidence that we have at the moment
for physics beyond the SM.
As for baryogenesis, if it is true that in the SM it is not possible to give
rise to a sufficient amount of baryon–antibaryon asymmetry, still one may debate
whether baryogenesis should have a dynamical origin and, indeed, whether
primordial antimatter is absent. Coming to inflation, again one has to admit that
in the SM there seems to be no room for an inflationary epoch in its scalar sector,
but, as nice as inflation is in coping with several crucial cosmological problems,
its presence in the history of the universe is still debatable. Finally, let me come
to the main topic of this chapter, namely the relation between the DM issue and
physics beyond the SM.
There exists little doubt that a conspicuous amount of the DM has to be in
non-baryonic nature. This is supported both by the upper bound on the amount
of baryonic matter from nucleosynthesis and by studies of galaxy formation. The
SM does not have any viable candidate for such non-baryonic DM. Hence the DM
issue constitutes a powerful probe in our search for new physics beyond the SM.
In this chapter we will briefly review the following aspects.
- The main features of the SM such as its spectrum, the Lagrangian and its
symmetries, the Higgs mechanism, the successes and shortcomings of the
SM. - The experimental evidence for the existence of DM.
- Two major particle physics candidates for DM: massive (light) neutrinos
and the lightest supersymmetric (SUSY) particle in SUSY extensions of the
SM withRparity (to be defined later on). Light neutrinos and the lightest