Proceedings of the 12th International Conference on Low Energy Antiproton Physics (LEAP2016)

Abstract

I review briefly string-inspired models of the Early Universe in which the Robertson-Walker geometry is extended to involve appropriate torsion fields that are constant in the frame of a cosmological observer. Fermion fields in such geometries are described by Lagrangians that fall in the simplest Lorentz-and CPT-violating extensions of the Standard Model (SME), in which the fermions couple to a constant axial vector background b µ γ 5 , with the b µ "axion-like" field associated to torsion. In the talk I also review mechanisms to suppress the torsion in the current era, compatible with the existing stringent upper bounds, which are of relevance to searches for CPT violation of interest to this conference. KEYWORDS:Early Universe, Torsion, Strings, Standard Model Extension, CPT Violation Theoretical motivations for Lorentz and CPT ViolationBaryogenesis/Leptogenesis, that is processes in the early Universe that lead to the observed matter-antimatter asymmetry in the Cosmos, represent a long-standing unresolved issue in cosmology [1]. Many approaches to the problem proposed in the literature [2] but no consensus has emerged on the potential mechanisms. Many of them predict lepton number violation, and in particular some of them entail [3] a µ → e − γ interaction with predictions on the corresponding branching ratio which may be falsified at the next generation experiments, but others are more difficult to falsify.A standard measure of the abundance of baryons over that of antibaryons is defined by the ratiowhere n B is the number density of baryons, nB is the number density of antibaryons and n γ is the density of photons (proportional to the entropy density s). This number was determined with accurate measurements of the CMB radiation by the experiments WMAP [5] and Planck [6]. However, there is no experimental evidence for primordial antimatter in the visible universe. Similarly, the generation of an asymmetry between leptons and antileptons is known as leptogenesis. This is expected to be of the same order of magnitude as Y ∆B . If B, the net baryon number, is conserved in Nature, the matter asymmetry can only originate from an asymmetric initial condition B 0. However, such an asymmetry would rapidly diminish during inflation, and extreme fine tuning of the initial condition would become necessary. This is highly unsatisfactory from a theoretical point of view. Consequently a mechanism for the dynamical generation of a baryon asymmetry is required. In the seminal paper [7], Sakharov identified three sufficient conditions that must be satisfied in order to produce a net baryon number.(1) The theory must allow for interactions that violate B conservation. These interactions must become effective at high energy scales in order to guarantee the stability of the proton. (2) Both discrete symmetries C (charge conjugation) and CP (where P denotes parity) are violated.In fact C violation is not enough, as correlations between the spins of particles and antiparticles 1