Now is the winter (of our discontent)

The threat of the long night over Tera eV scale?

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 M_I. We have recently discussed an explicit example of a non-SUSY SO(10) model [43], with a single intermediate breaking scale M_I between M_GUT and the electroweak scale, compatible with the following requirements: unification of couplings at a large enough scale M_GUT 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 M_I∼10¹¹ GeV. In fact, within this breaking chain, the seesaw and leptogenesis mechanisms can both be made compatible with M_I∼10¹¹ GeV, which is consistent with the theoretical lower limit on the lightest heavy right-handed neutrino for sufficient leptogenesis [50] given by M_I≥10⁹ GeV. The same intermediate scale M_I 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.

The Higgs: so simple yet so unnatural

Guido Altarelli

(Submitted on 2 Aug 2013 (v1), last revised 4 Dec 2013 (this version, v2))

A long time ago, there came a night that lasted a generation. Experimenters froze to death in their laboratories like the phenomenologists in their nontenured positions while theorists smothered their speculations rather than see them starve, and wept, and felt the tears freeze on their cheeks...

(inspired from Fashionable) Folklore