Now is the winter (of our discontent)
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The absence of new physics at the electroweak scale scale appears as a paradox to most of us.
But possibly Nature has a way, hidden to us, to realize a deeper form of naturalness
at a more fundamental level. Indeed the picture suggested by the last 20 years of
data is simple and clear: just take the standard model (SM), extended to include Majorana neutrinos,
as the theory valid up to very high energy. It is impressive to me that, if one forgets
the fine tuning problem, the SM can stand up well beyond the LHC range with only
a few additional ingredients. The most compelling evidence for new physics is Dark
Matter. But a minimal explanation for Dark Matter could be provided by axions,
introduced originally to solve the strong CP problem {45}, which only need a modest
enlargement of the SM with some heavy new particles and a Peccei-Quinn additional
global symmetry [46, 47, 48]. The Majorana neutrino sector with violation of B-L and
new sources of CP violation offers an attractive explanation of baryogenesis through
leptogenesis [49]. Coupling unification and the explanation of the quantum numbers
of fermions in each generation in a non SUSY context can be maintained in SO(10)
with two (or more ) steps of symmetry breaking at MGUT and at an intermediate scale
MI. We have recently discussed an explicit example of a non-SUSY SO(10) model [43],
with a single intermediate breaking scale MI between MGUT and the electroweak scale, compatible with the following requirements: unification of couplings at a large enough
scale MGUT compatible with the existing bounds on the proton life-time; a Yukawa
sector in agreement with all data on flavour physics, fermion masses and mixings, also
including neutrinos, as well as with leptogenesis as the origin of the baryon asymmetry
of the Universe; an axion, which arises from the Higgs sector of the model, suitable to
solve the strong CP problem and to account for the observed amount of Dark Matter. It turns out that imposing all these requirements is very constraining, so that most of the
possible breaking chains of SO(10) must be discarded and the Pati Salam symmetry at
the intermediate scale emerges as the optimal solution. We show that all these different
phenomena can be satisfied in this fully specified, although schematic, GUT model, with
a single intermediate scale at MI∼1011 GeV. In fact, within this breaking chain, the seesaw
and leptogenesis mechanisms can both be made compatible with MI∼1011 GeV,
which is consistent with the theoretical lower limit on the lightest heavy right-handed
neutrino for sufficient leptogenesis [50] given by MI≥109 GeV. The same intermediate
scale MI is also suitable for the axion to reproduce the correct Dark Matter abundance.
If this scenario is realized in nature one should one day observe proton decay and
neutrinoless beta decay. In addition, none of the alleged indications for new physics
at colliders should survive (in particular even the claimed muon (g-2) [32] discrepancy
should be attributed, if not to an experimental problem, to an underestimate of the
theoretical errors or, otherwise, to some specific addition to the above model [51]). This
model is in line with the non observation of µ→eγ at MEG [52], of the electric dipole
moment of the neutron [53] etc. It is a very important challenge to experiment to falsify
this scenario by establishing a firm evidence of new physics at the LHC or at another
”low energy” experiment.
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