A survey of antimatter reactions is presented, including the formation of the antihydrogen atom and anionic, cationic, and molecular species by collisional and radiative processes. Our approach is rooted in the detailed knowledge available for many matter counterpart (hydrogenic) reactions, due to their importance in controlling early Universe chemistry. We point out that the availability of trapped antihydrogen at densities similar to those pertaining to the epoch of hydrogen chemistry will soon be available. In addition, using modern atomic physics techniques, it should be feasible to control antimatter in the laboratory to facilitate antihydrogen chemistry. Our purpose is to summarize what is known from hydrogen chemistry that is of relevance for antimatter and to indicate, based on possible reaction rates, which processes may be fruitful to pursue to create new antimatter entities as probes of fundamental symmetries. We include antihydrogen, positrons, and antiprotons in our discussion and additionally the electron due to its propensity to form positronium and perhaps to participate in certain reactions. We attempt to indicate whether further theoretical/computational work is necessary to add to the assessment of reaction rates, and we discount processes where the projected rates are too low to be of interest, given foreseeable experimental capabilities.

The antimatter equivalent of atomic hydrogen—antihydrogen—is an outstanding testbed for precision studies of matter–antimatter symmetry. Here we report on the simultaneous observation of both accessible hyperfine components of the 1S–2S transition in trapped antihydrogen. We determine the 2S hyperfine splitting in antihydrogen and—by comparing our results with those obtained in hydrogen—constrain the charge–parity–time-reversal symmetry-violating coefficients in the standard model extension framework. Our experimental protocol allows the characterization of the relevant spectral lines in 1 day, representing a 70-fold improvement in the data-taking rate. We show that the spectroscopy is applicable to laser-cooled antihydrogen with important implications for future tests of fundamental symmetries.

This paper describes the ALPHA-2 apparatus, used at the CERN Antiproton Decelerator facility for the study of trapped antihydrogen atoms. Details of both the construction and performance are included. Prominence is given to both the new and the improved features, with respect to the original ALPHA assembly, of the apparatus including a stand-alone antiproton catching trap, the traps used to mix antiparticles to produce the anti-atoms, the magnetic atom trap used to hold some of them, and access for laser light to facilitate excitation of the antihydrogen 1S-2P and 1S-2S transitions.

Magnetically trapped antihydrogen atoms can be cooled by expanding the volume of the trap in which they are confined. We report a proof-of-principle experiment in which antiatoms are deliberately released from expanded and static traps. Antiatoms escape at an average trap depth of 0.08±0.01K (statistical errors only) from the expanded trap while they escape at average depths of 0.22±0.01 and 0.17±0.01K from two different static traps. (We employ temperature-equivalent energy units.) Detailed simulations qualitatively agree with the escape times measured in the experiment and show a decrease of 38% (statistical error<0.2%) in the mean energy of the population after the trap expansion without significantly increasing antiatom loss compared to typical static confinement protocols. This change is bracketed by the predictions of one-dimensional and three-dimensional semianalytic adiabatic expansion models. These experimental, simulational, and model results are consistent with obtaining an adiabatically cooled population of antihydrogen atoms that partially exchanged energy between axial and transverse degrees of freedom during the trap expansion. This result is important for future antihydrogen gravitational experiments which rely on adiabatic cooling, and it will enable antihydrogen cooling beyond the fundamental limits of laser cooling.

In this paper we present the first pilot calculation of the elastic and inelastic cross sections for the 5-body scattering of antihydrogen ions with positronium atoms. These cross sections have not been calculated before and are not known experimentally. In particular, we focus on the collisional rearrangement reactions which deplete the ions and result in stable atomcules . To better understand the mechanism of this rearrangement, we study the 3-dimensional, angle resolved positron densities of the 3- and 4-body fragments in the initial and final states of the rearrangement collision.

Antiprotons created by laser ionization of antihydrogen are observed to rapidly escape the ALPHA trap. Further, positron plasmas heat more quickly after the trap is illuminated by laser light for several hours. These phenomena can be caused by patch potentials-variations in the electrical potential along metal surfaces. A simple model of the effects of patch potentials explains the particle loss, and an experimental technique using trapped electrons is developed for measuring the electric field produced by the patch potentials. The model is validated by controlled experiments and simulations.

Here we present a four-channel model that incorporates a magnetically tunable Feshbach resonance in a system of three atoms that interact via pairwise van der Waals interactions. Our method is designed to model recent experiments where the tunability of the scattering length has been used to study three-body Efimov states, which appear in the limit of a diverging two-body scattering length. Using this model, we calculate three-body adiabatic and effective potential curves and study how the strength (or width) of the Feshbach resonance affects the three-body effective hyperradial potential that is connected to the Efimov effect. We find that the position of the repulsive barrier, which has been used to explain the so-called van der Waals universality in broad resonances, is slightly shifted as the narrow-resonance limit is approached and that this shift is correlated to the appearance of two avoided crossings in the adiabatic energy landscape. More importantly, the attractive well is markedly shifted upward in energy and is extremely shallow for the narrowest resonance. We argue that this behavior is connected to the breakdown of van der Waals universality for weak (narrow) resonances.

Einstein’s general theory of relativity from 1915 remains the most successful description of gravitation. From the 1919 solar eclipse to the observation of gravitational waves, the theory has passed many crucial experimental tests. However, the evolving concepts of dark matter and dark energy illustrate that there is much to be learned about the gravitating content of the universe. Singularities in the general theory of relativity and the lack of a quantum theory of gravity suggest that our picture is incomplete. It is thus prudent to explore gravity in exotic physical systems. Antimatter was unknown to Einstein in 1915. Dirac’s theory appeared in 1928; the positron was observed in 1932. There has since been much speculation about gravity and antimatter. The theoretical consensus is that any laboratory mass must be attracted by the Earth, although some authors have considered the cosmological consequences if antimatter should be repelled by matter. In the general theory of relativity, the weak equivalence principle (WEP) requires that all masses react identically to gravity, independent of their internal structure. Here we show that antihydrogen atoms, released from magnetic confinement in the ALPHA-g apparatus, behave in a way consistent with gravitational attraction to the Earth. Repulsive ‘antigravity’ is ruled out in this case. This experiment paves the way for precision studies of the magnitude of the gravitational acceleration between anti-atoms and the Earth to test the WEP.

We report on the first production of an antihydrogen beam by charge exchange of 6.1 keV antiprotons with a cloud of positronium in the GBAR experiment at CERN. The 100 keV antiproton beam delivered by the AD/ELENA facility was further decelerated with a pulsed drift tube. A 9 MeV electron beam from a linear accelerator produced a low energy positron beam. The positrons were accumulated in a set of two Penning-Malmberg traps. The positronium target cloud resulted from the conversion of the positrons extracted from the traps. The antiproton beam was steered onto this positronium cloud to produce the antiatoms. We observe an excess over background indicating antihydrogen production with a significance of 3-4 standard deviations.

We compare effective hyperspherical three-body potentials calculated using the S-wave part of the Faddeev equations to calculations using the full Schrödinger equation. As two-body model potential we test both a short-range potential and a Lennard-Jones potential with van der Waals tail. In the former case we find excellent agreement between the two methods for the lowest adiabatic state, indicating that the Faddeev method can be a useful tool also for numerical computations. For excited states the two methods show important differences, but agree for hyperradii larger than about five times the range of the potential (independent of the value of the scattering length and of the number of bound states). For the van der Waals potential, we focus on how well the Faddeev method reproduces the so-called van der Waals universality. We find that indeed the universality is manifest also using this method, but at a slightly different value of the universal parameter κ*. We further derive an efficient method to solve the integro-differential equation arising in the Faddeev method.