BASE—Testing Fundamental Symmetries by High Precision Comparisons of the Properties of Antiprotons and Protons

This chapter reviews the experimental activities of the BASE collaboration at CERN [1], which operates advanced Penning trap systems to compare the fundamental properties of single protons and antiprotons with high precision [2,3,4]. Such measurements provide stringent tests of charge, parity, time (CPT) reversal invariance, which is the most fundamental symmetry in the standard model of particle physics [5]. CPT invariance has been tested with high sensitivity in very different sectors [6], and so far no indications for a violation have been found. In contrast, the non-observation of primordial antimatter and the matter excess in our Universe are a tremendous challenge for the Standard Model, since the tiny amount of CP-violation contained in the Standard Model is insufficient to reproduce the matter content by more than eight orders of magnitude [7]. Other activities of the BASE collaboration include contributions to searches for axions and axion like particles, light spinless bosons ($m_a≪1$ eV/$c^2$) originally proposed to resolve the strong CP problem of quantum chromodynamics. In recent work BASE investigated asymmetric couplings of dark-matter particles to fermions and antifermions [8], and used superconducting resonant single particle detection circuits in strong magnetic fields, to set, in a narrow mass range, competitive limits on the coupling of ALPs to photons [9].