Abstract
The Standard Model (SM) of particle physics is a big success. However, it lacks explanations for cosmic inflation, the matter-anti-matter asymmetry of the Universe, dark matter, neutrino oscillations, and the feebleness of CP violation in the strong interactions. The latter may be explained by a complex scalar field charged under a spontaneously broken global U(1) Peccei-Quinn (PQ) symmetry. Moreover, the pseudo Nambu-Goldstone boson of this breaking -- the axion -- may play the role of the dark matter. Furthermore, the modulus of the PQ field is a candidate for driving inflation. If additionally three extra SM singlet neutrinos (whose mass is induced by the PQ field) are included, the five aforementioned problems can be addressed at once. We review the SM extension dubbed SMASH -for SM-Axion-Seesaw-Higgs portal inflation-, discuss its predictions and tests in astrophysics, cosmology, and laboratory experiments. Variants of SMASH are also considered and commented on.
Highlights
The Standard Model (SM) is arguably the most successful theory in physics
We have provided an overview of SMASHy extensions of the Standard Model which feature a new mass scale vσ —of the order of 1011 GeV in the simplest models, but which could be tied to a Grand Unification scale around 1016 GeV—and provide a falsifiable framework that addresses the following problems in particle physics and cosmology: inflation, baryogenesis, neutrino masses, dark matter and the strong CP problem
Whenever the dynamics of the most economical model (Ballesteros et al, 2017a,b), called SMASH in this review, is realized in other extensions, the models reviewed here predict a tensor-to-scalar-ratio r 0.004, a running of the spectral index α −8 × 10−4, and a deviation in the effective number of relativistic neutrino species Nνeff ∼ 0.03, values which can be probed in future cosmic microwave background (CMB) experiments, such as CMB-S4, LiteBIRD, and the Simons Observatory
Summary
The SM is arguably the most successful theory in physics. It describes the known particles and their interactions remarkably well. The baryogenesis and dark matter problems can be solved simultaneously if M1 ∼ keV and M2 ∼ M3 ∼ GeV In this case N2,3 create flavored lepton asymmetries from CP-violating oscillations in the early Universe, which generate which promotes the Higgs field to an inflaton candidate. Given the current experimental uncertainties, a definite conclusion cannot yet be drawn (see e.g., Buttazzo et al, 2013; Bednyakov et al, 2015) These obstacles of the νMSM can be neatly circumvented in SMASH-type (Ballesteros et al, 2017a,b; Ernst et al, 2018) extensions of the SM which are built around the axion for the solution of the strong CP problem (Peccei and Quinn, 1977; Weinberg, 1978; Wilczek, 1978), as well as for dark matter, and allow inflation to be driven by (a mixture of the modulus of the Higgs field with) the modulus of the Peccei-Quinn field – sometimes called saxion field (Pi, 1984; Fairbairn et al, 2015).
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