We review progress in the study of antinuclei, starting from Dirac's equation and the discovery of the positron in cosmic-ray events. The development of proton accelerators led to the discovery of antiprotons, followed by the first antideuterons, demonstrating that antinucleons bind into antinuclei. With the development of heavy-ion programs at the Brookhaven AGS and CERN SPS, it was demonstrated that central collisions of heavy nuclei offer a fertile ground for research and discoveries in the area of antinuclei. In this review, we emphasize recent observations at Brookhaven's Relativistic Heavy Ion Collider and at CERN's Large Hadron Collider, namely, the antihypertriton and the antihelium-4, as well as measurements of the mass difference between light nuclei and antinuclei, and the interaction between antiprotons. Physics implications of the new observations and different production mechanisms are discussed. We also consider implications for related fields, such as hypernuclear physics and space-based cosmic-ray experiments.

Antinuclei in Heavy-Ion Collisions

We consider the hyperfine structure, the atomic spectrum, and the decay channels of the bound ${\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$ system (dimuonium). The annihilation lifetimes of low-lying atomic states of the system lie in the ${10}^{\ensuremath{-}12}$ s range. The decay rates could be measured by detection of the decay products (high-energy photons or electron-positron pairs). The hyperfine-structure splitting of the dimuonic system and its decay rate are influenced by electronic vacuum polarization effects in the far timelike asymptotic region. This constitutes a previously unexplored kinematic regime. We evaluate next-to-leading-order radiative corrections to the decay rate of low-lying atomic states. We also obtain order ${\ensuremath{\alpha}}^{5}{m}_{\ensuremath{\mu}}$ corrections to the hyperfine splitting of the $1S$ and $2S$ levels.

Bound μ + μ − system

Bound µ + µ - system

The production of atoms formed by (..pi../sup +/..pi../sup -/), (..pi../sup -/K/sup +/), (..pi../sup +/K/sup -/), and (K/sup +/K/sup -/) mesons is considered. The conclusion is drawn that it is possible to observe these atoms and to measure their lifetime and the value of the square of the wave function for zero distance between the particles.

Elementary relativistic atoms

Hydrogenlike atoms consisting of a pion and a muon can be formed in ${{K}_{L}}^{0}\ensuremath{\rightarrow}\ensuremath{\pi}\ensuremath{\mu}\ensuremath{\nu}$ decays. In an intense, high-energy ${{K}_{L}}^{0}$ beam, 320 pi-mu atoms were detected and simultaneously the ${{K}_{L}}^{0}$ flux was monitored by recording ordinary ${{K}_{L}}^{0}\ensuremath{\rightarrow}\ensuremath{\pi}\ensuremath{\mu}\ensuremath{\nu}$ decays. The first measurement is reported of the branching ratio $R=\frac{\ensuremath{\Gamma}({{K}_{L}}^{0}\ensuremath{\rightarrow}\mathrm{p}\mathrm{i}\ensuremath{-}\mathrm{m}\mathrm{u}\phantom{\rule{0ex}{0ex}}\mathrm{a}\mathrm{t}\mathrm{o}\mathrm{m}+\ensuremath{\nu})}{\ensuremath{\Gamma}({{K}_{L}}^{0}\ensuremath{\rightarrow}\ensuremath{\pi}\ensuremath{\mu}\ensuremath{\nu})}=(3.88\ifmmode\pm\else\textpm\fi{}0.41)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}$, using a subset of 155 atoms. This ratio may be sensitive to anomalous interactions between the pion and the muon. In the absence of such interactions, theory predicts $R=(4.43\ifmmode\pm\else\textpm\fi{}0.12)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}$.

Measurement of the Rate of Formation of Pi-Mu Atoms in K L 0 Decay

We have studied the rare weak radiative hyperon decay Xi degrees -->Sigma degrees gamma in the KTeV experiment at Fermilab. We have identified 4045 signal events over a background of 804 events. The dominant Xi degrees -->Lambdapi degrees decay, which was used for normalization, is the only important background source. An analysis of the acceptance of both modes yields a branching ratio of B(Xi degrees -->Sigma degrees gamma)/B(Xi degrees -->Lambdapi degrees ) = (3.34+/-0.05+/-0.09)x10(-3). By analyzing the final state decay distributions, we have also determined that the Sigma degrees emission asymmetry parameter for this decay is alpha(XiSigma) = -0.63+/-0.09.

Measurement of the branching ratio and asymmetry of the decay Ξ° → σ°γ

Hydrogenlike atoms consisting of a pion and a muon can be formed in K/sub L//sup 0/..--> pi mu nu.. decays. In an experiment at Fermilab, 320 pi-mu atoms were detected and simultaneously the K/sub L//sup 0/ flux was monitored by recording ordinary K/sub L//sup 0/..--> pi mu nu.. decays. The first measurement of the branching ratio R = GAMMA(K/sub L//sup 0/..-->..pi-mu atom+..nu..)/GAMMA(K/sub L//sup 0/..--> pi mu nu..) = (3.90 +- 0.39) x 10/sup -7/ is reported, based on a subset of 155 atoms. This ratio may be sensitive to anomalous interactions between the pion and the muon. In the absence of such interactions, theory predicts R = (4.31 +- 0.08) x 10/sup -7/.

Measurement of the rate of formation of pi-mu atoms in KL0 decay.

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Measurement of the Rate of Formation of Pi-Mu Atoms in K L 0 Decay

Measurement of the branching ratio and asymmetry of the decay Ξ° → σ°γ

Measurement of the rate of formation of pi-mu atoms in KL0 decay.