Although the word “antimatter” is considered to be coined in 1898, the history of antimatter research began in 1928 with a paper by Paul Dirac, where he predicted existence of anti-electrons as a solutions of the Schrödinger equation with negative energies [1]. In a few years, in 1932, these anti-electrons, called positrons, were experimentally observed by Carl Anderson. Two decades after, in 1955 antiprotons were discovered, and in the next year, 1956, antineutrons were seen in experiments with antiproton – proton collisions. In 1965 formation of antimatter nuclei – antideuteron was observed for the first time, and then, in the 1970s, a few more complex antimatter nuclei were discovered. Finally, antimatter research in the XX century culminated in formation of antihydrogen atoms in 1995 [2].
The main motivation to study antimatter comes from some of the most fundamental questions about the Standard Model. Why does our universe consist mainly of matter, neither of equal parts of matter and antimatter, nor an ocean of photons produced by matter/antimatter annihilation, as it might be expected? The existence of our “matter universe” requires much stronger violation of CP symmetry, than was observed before. This search for sufficient CP violation involves comparison of matter and antimatter objects in terms of their fundamental properties, such as transition energies in antimatter atoms or mass, charge and electromagnetic moments of antimatter particles [3].
One of the most prominent objects of such studies is the antihydrogen atom. Antihydrogen research, started in 1995, was significantly accelerated in 2010 when first cold antihydrogen atoms were fabricated at the Antiproton Factory in CERN. Cold antihydrogen can be trapped for long time periods, which is required for high precision atomic spectroscopy. Currently there are several experiment running at CERN dedicated to antimatter research: ALPHA, ASACUSA, BASE, AEGIS and GBAR. The results have yielded characterization of the hyperfine structure of atomic transitions [4], charge [5], and gravitational properties [6] of antihydrogen atoms, and even probed the possibility of antimatter-dark matter interaction [7]. So far, no difference between matter objects and their antimatter counterparts was founded with fractional precision down to 10-15, so the main question about antimatter in our universe remains to be answered.
The Antimatter On A Chip (AMOC) project is devoted to development of the technology for fabrication and manipulation of antimatter at a laboratory away from CERN, currently the only source of sufficiently cold antiprotons. The main objective of the project is to make such tabletop experiments with antimatter possible, which could scale up antimatter research around the globe.
[1] G. Williams, “Antimatter and 20th century science,” Phys Educ, vol. 40, no. 3, pp. 238–244, Mar. 2005, doi: 10.1088/0031-9120/40/3/004.
[2] W. A. Bertsche, E. Butler, M. Charlton, and N. Madsen, “Physics with antihydrogen,” Journal of Physics B: Atomic, Molecular and Optical Physics, vol. 48, no. 23. Institute of Physics Publishing, Oct. 06, 2015. doi: 10.1088/0953-4075/48/23/232001.
[3] K. Khabarova, A. Golovizin, and N. Kolachevsky, “Antihydrogen and Hydrogen: Search for the Difference,” Symmetry, vol. 15, no. 8. Multidisciplinary Digital Publishing Institute (MDPI), Aug. 01, 2023. doi: 10.3390/sym15081603.
[4] E. Widmann, “Hyperfine Spectroscopy of Antihydrogen, Hydrogen, and Deuterium,” Physics of Particles and Nuclei, vol. 53, no. 4, pp. 790–794, Aug. 2022, doi: 10.1134/S1063779622040141.
[5] M. Ahmadi et al., “An improved limit on the charge of antihydrogen from stochastic acceleration,” Nature, vol. 529, no. 7586, pp. 373–376, Jan. 2016, doi: 10.1038/nature16491.
[6] E. K. Anderson et al., “Observation of the effect of gravity on the motion of antimatter,” Nature, vol. 621, no. 7980, pp. 716–722, Sep. 2023, doi: 10.1038/s41586-023-06527-1.
[7] C. Smorra et al., “Direct limits on the interaction of antiprotons with axion-like dark matter,” Nature, vol. 575, no. 7782, pp. 310–314, Nov. 2019, doi: 10.1038/s41586-019-1727-9.