Abstract
It is well known that a fundamental theorem of Quantum Field Theory (QFT) set in flat spacetime ensures the CPT invariance of the theory. This symmetry is strictly connected to the Lorentz covariance, and consequently to the fundamental structure of spacetime. Therefore it may be interesting to investigate the possibility of departure from this fundamental symmetry, since it can furnish a window to observe possible effects of a more fundamental quantum gravity theory in a “lower energy limit”. Moreover, in the past, the inquiry of symmetry violations provided a starting point for new physics discoveries. A useful physical framework for this kind of search is provided by astroparticle physics, thanks to the high energy involved and to the long path travelled by particles accelerated by an astrophysical object and then revealed on Earth. Astrophysical messengers are therefore very important probes for investigating this sector, involving high energy photons, charged particles, and neutrinos of cosmic origin. In addition, one can also study artificial neutrino beams, investigated at accelerator experiments. Here we discuss the state of art for all these topics and some interesting new proposals, both from a theoretical and phenomenological point of view.
Highlights
Symmetries are fundamental in every physical theory formulation, since they express the visible variations of detected results after changes in the experimental framework
A theoretical model including the possibility of LIV or CPT violation usually aims to find an appropriate geometry to formulate an extended version of the “usual” theory, making possible some kind of unification between quantum field theory and General Relativity (GR), i.e., Quantum Field Theory (QFT) and gravity [52]
Even Very Special Relativity (VSR) [59] and HMSR [49] are settled in the Finsler geometry, which naturally emerges in this case as the appropriate one from the kinematic perturbation introduced via the Modified Dispersion Relation (MDR) [60]
Summary
Symmetries are fundamental in every physical theory formulation, since they express the visible variations of detected results after changes in the experimental framework. Invariance under CPT symmetry, the combined transformation given by the product of C, P, and T in any order, is guaranteed in flat spacetime by a fundamental result: The so-called CPT theorem that was implicit in ‘51 Schwinger’s work [1] and was more formally proved by Lúders [2] and by Pauli [3] a few years later. In the specific case of the Lorentz and CPT invariance the search for modifications of these fundamental principles is motivated by the idea that small violations of space-time symmetries could provide an experimental window on the Planck scale effects caused by a more fundamental quantum gravity theory. We discuss the phenomenology that can be induced by CPT violation and by LIV in the astrophysical sector, in particular in gamma-ray bursts, Cosmic Rays (CRs), and the neutrino sector
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