GUT Physics in the Era of the LHC

Abstract

Grand Unified Theories (GUTs) are one of the most interesting high-energy completions of the Standard Model, because they provide a rich, powerful and elegant group-theoretical framework able to resolve a variety of problems remaining in our current understanding of particle physics. They usually act as motivators for many low energy BSM theories, such as left-right symmetric or supersymmetric models, and they serve to fill the gap between the experimentally reachable low energies and the physics in the ultraviolet. In recent years, however, they have fallen slightly from the spotlight, in favour of "simplified" models with more specific phenomenological predictions. The aim of this review is to summarize the state of the art on GUTs and argue for their importance in modern physics. Recent advances in experiments permit to test the predictions of GUTs at different energy scales. First, as GUTs can play a role in the inflationary dynamics of the early Universe, their imprints could be found in the CMB observations by the Planck satellite. Remarkably enough, GUTs could manifest themselves also in terrestrial tests; several planned experiments aim to probe the proton stability and to establish order of magnitude higher bounds on its lifetime. Moreover, the predictions of specific GUT models could be tested even at the LHC thanks to its high energy reach, via searches for exotic states or additional contributions to flavour anomalies.

Dynamical axions and gravitational waves

“…The analysis in this paper leaves open the question of phase transitions in models with N F > 4, such as the recently proposed high-scale color confinement model [71]. The determinant operator is irrelevant for N F > 4, and the strength of the phase transition in the linear 10 This m η mϕ upper value estimate comes from the fact that (assuming large separations of scales µSSI and Λ) the η gets its mass from confinement and therefore m 2 η ∼ Λ 4 f 2 , where f is the chiral symmetry breaking scale and is estimated to be Λ ≤ 4πf . Then m 2 η ≤ (4π) 2 Λ 2 and since mϕ ∼ Λ, then m η mϕ ≤ 10. sigma model may instead correlate with a further explicit breaking, or the generation of the η mass in such scenarios.…”

“…It is seen that in this case, the ratio is not bounded from above. For N F = 4, we will study a larger range, m η /m ϕ = [1,10], noting that a large mass ratio is a natural expectation of models such as [30]. We repeat the calculation described in the previous subsection, for 50 parameter points as before.…”

Predictive power of grand unification from quantum gravity

“…The recent renewed interest in GUTs, see, e.g., [94], and [95] for a recent review, includes the exploration of asymptotic safety in GUTs without gravity [96][97][98][99] as well as the coupling of asymototically free GUTs [100] to quadratic gravity [101][102][103][104][105]. Throughout this paper, we will focus on the non-supersymmetric case whenever we discuss specific examples.…”

SO(10) unification with horizontal symmetry

“…The quest for the unification of fundamental forces remains to be a major motivation over the last few decades for investigating theories beyond the Standard Model. Although the smallest possible group, SU(5) [1], has been ruled out, the idea of unification is very much alive, resting on supersymmetric versions, and/or bigger unification groups like SO (10) [2][3][4][5][6][7][8][9][10][11][12][13][14][15]. In fact, SO(10) is the smallest unification group that contains all fermion fields of a single generation in one irreducible multiplet (irrep).…”