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The positron, the antiparticle of the electron, predicted by Dirac in 1931 and discovered by Anderson in 1933, plays a key role in many scientific and everyday endeavours. Notably, the positron is a constituent of antihydrogen, the only long-lived neutral antimatter bound state that can currently be synthesized at low energy, presenting a prominent system for testing fundamental symmetries with high precision. Here, we report on the use of laser cooled Be+ ions to sympathetically cool a large and dense plasma of positrons to directly measured temperatures below 7 K in a Penning trap for antihydrogen synthesis. This will likely herald a significant increase in the amount of antihydrogen available for experimentation, thus facilitating further improvements in studies of fundamental symmetries., Positrons are key to the production of cold antihydrogen. Here the authors report the sympathetic cooling of positrons by interacting them with laser-cooled Be+ ions resulting in a three-fold reduction of the temperature of positrons for antihydrogen synthesis.

We overview our recent calculations of scattering cross sections of positronium–antihydrogen collisions. We discuss the cross sections calculated in the vicinity of the four-body resonances of $$\bar{\mathrm {H}}\mathrm {Ps}$$ and above the energy threshold for the formation of the antihydrogen positive ions ( $$\bar{\mathrm {H}}^+$$ ). In the former energy region where a Rydberg series of resonances dominated by the $$\bar{\mathrm {H}}^+$$ - $$\hbox {e}^-$$ interaction appears, we show a good agreement between the resonant profiles in the cross sections and the resonance energies calculated by the complex coordinate rotation method. $$\bar{\mathrm {H}}^+$$ production cross sections near the threshold energy are reported for the positronium being initially in the (n, l) states with $$n\le 3$$ , together with all competing elastic/inelastic cross sections. Comparison of our results with the latest continuum distorted wave final state calculation is presented. A qualitative discrepancy concerning the dependence of cross sections on the angular momentum of Ps is indicated.

The ALPHA collaboration has successfully demonstrated the production and the confinement of cold antihydrogen, $\overline{\mathrm{H}}$. An analysis of trapping data allowed a stringent limit to be placed on the electric charge of the simplest antiatom. Charge neutrality of matter is known to a very high precision, hence a neutrality limit of $\overline{\mathrm{H}}$ provides a test of CPT invariance. The experimental technique is based on the measurement of the deflection of putatively charged $\overline{\mathrm{H}}$ in an electric field. The tendency for trapped $\overline{\mathrm{H}}$ atoms to be displaced by electrostatic fields is measured and compared to the results of a detailed simulation of $\overline{\mathrm{H}}$ dynamics in the trap. An extensive survey of the systematic errors is performed, with particular attention to those due to the silicon vertex detector, which is the device used to determine the $\overline{\mathrm{H}}$ annihilation position. The limit obtained on the charge of the $\overline{\mathrm{H}}$ atom is \mbox{$ Q = (-1.3\pm1.8\pm0.4)\times10^{-8}$}, representing the first precision measurement with $\overline{\mathrm{H}}$., Comment: 5 pages, 3 figures

At the historic Shelter Island Conference on the Foundations of Quantum Mechanics in 1947, Willis Lamb reported an unexpected feature in the fine structure of atomic hydrogen: a separation of the 2S

The photon—the quantum excitation of the electromagnetic field—is massless but carries momentum. A photon can therefore exert a force on an object upon collision1. Slowing the translational motion of atoms and ions by application of such a force2,3, known as laser cooling, was first demonstrated 40 years ago4,5. It revolutionized atomic physics over the following decades6–8, and it is now a workhorse in many fields, including studies on quantum degenerate gases, quantum information, atomic clocks and tests of fundamental physics. However, this technique has not yet been applied to antimatter. Here we demonstrate laser cooling of antihydrogen9, the antimatter atom consisting of an antiproton and a positron. By exciting the 1S–2P transition in antihydrogen with pulsed, narrow-linewidth, Lyman-α laser radiation10,11, we Doppler-cool a sample of magnetically trapped antihydrogen. Although we apply laser cooling in only one dimension, the trap couples the longitudinal and transverse motions of the anti-atoms, leading to cooling in all three dimensions. We observe a reduction in the median transverse energy by more than an order of magnitude—with a substantial fraction of the anti-atoms attaining submicroelectronvolt transverse kinetic energies. We also report the observation of the laser-driven 1S–2S transition in samples of laser-cooled antihydrogen atoms. The observed spectral line is approximately four times narrower than that obtained without laser cooling. The demonstration of laser cooling and its immediate application has far-reaching implications for antimatter studies. A more localized, denser and colder sample of antihydrogen will drastically improve spectroscopic11–13 and gravitational14 studies of antihydrogen in ongoing experiments. Furthermore, the demonstrated ability to manipulate the motion of antimatter atoms by laser light will potentially provide ground-breaking opportunities for future experiments, such as anti-atomic fountains, anti-atom interferometry and the creation of antimatter molecules., The successful laser cooling of trapped antihydrogen, the antimatter atom formed by an antiproton and a positron (anti-electron), is reported.

International audience; The GBAR experiment aims to measure the gravitational acceleration of antihydrogen H̅. It will use H̅+ ions formed by the interaction of antiprotons with a dense positronium cloud, which will require about 1010 positrons to produce one H̅+. We present the first results on the positron accumulation, reaching 3.8±0.4×108 e+ collected in 560 s.

International audience; The GBAR (Gravitational Behavior of Antihydrogen at Rest) experiment at CERN requires efficient deceleration of 100 keV antiprotons provided by the new ELENA synchrotron ring to synthesize antihydrogen. This is accomplished using electrostatic deceleration optics and a drift tube that is designed to switch from -99 kV to ground when the antiproton bunch is inside – essentially a charged particle “elevator” – producing a 1 keV pulse. We describe the simulation, design, construction and successful testing of the decelerator device at -92 kV on-line with antiprotons from ELENA.

In 1906, Theodore Lyman discovered his eponymous series of transitions in the extreme-ultraviolet region of the atomic hydrogen spectrum1,2. The patterns in the hydrogen spectrum helped to establish the emerging theory of quantum mechanics, which we now know governs the world at the atomic scale. Since then, studies involving the Lyman-α line—the 1S–2P transition at a wavelength of 121.6 nanometres—have played an important part in physics and astronomy, as one of the most fundamental atomic transitions in the Universe. For example, this transition has long been used by astronomers studying the intergalactic medium and testing cosmological models via the so-called ‘Lyman-α forest’3 of absorption lines at different redshifts. Here we report the observation of the Lyman-α transition in the antihydrogen atom, the antimatter counterpart of hydrogen. Using narrow-line-width, nanosecond-pulsed laser radiation, the 1S–2P transition was excited in magnetically trapped antihydrogen. The transition frequency at a field of 1.033 tesla was determined to be 2,466,051.7 ± 0.12 gigahertz (1σ uncertainty) and agrees with the prediction for hydrogen to a precision of 5 × 10−8. Comparisons of the properties of antihydrogen with those of its well-studied matter equivalent allow precision tests of fundamental symmetries between matter and antimatter. Alongside the ground-state hyperfine4,5 and 1S–2S transitions6,7 recently observed in antihydrogen, the Lyman-α transition will permit laser cooling of antihydrogen8,9, thus providing a cold and dense sample of anti-atoms for precision spectroscopy and gravity measurements10. In addition to the observation of this fundamental transition, this work represents both a decisive technological step towards laser cooling of antihydrogen, and the extension of antimatter spectroscopy to quantum states possessing orbital angular momentum., The observation of the 1S–2P Lyman-α transition in the antihydrogen atom, the antimatter counterpart of hydrogen, is reported.

Antihydrogen, a positron bound to an antiproton, is the simplest anti-atom. Its structure and properties are expected to mirror those of the hydrogen atom. Prospects for precision comparisons of the two, as tests of fundamental symmetries, are driving a vibrant programme of research. In this regard, a limiting factor in most experiments is the availability of large numbers of cold ground state antihydrogen atoms. Here, we describe how an improved synthesis process results in a maximum rate of 10.5 ± 0.6 atoms trapped and detected per cycle, corresponding to more than an order of magnitude improvement over previous work. Additionally, we demonstrate how detailed control of electron, positron and antiproton plasmas enables repeated formation and trapping of antihydrogen atoms, with the simultaneous retention of atoms produced in previous cycles. We report a record of 54 detected annihilation events from a single release of the trapped anti-atoms accumulated from five consecutive cycles., Antihydrogen studies are important in testing the fundamental principles of physics but producing antihydrogen in large amounts is challenging. Here the authors demonstrate an efficient and high-precision method for trapping and stacking antihydrogen by using controlled plasma.

International audience; We have developed a PbWO 4 (PWO) detector with a large dynamic range to measure the intensity of a positron beam and the absolute density of the ortho-positronium (o-Ps) cloud it creates. A simulation study shows that a setup based on such detectors may be used to determine the angular distribution of the emission and reflection of o-Ps to reduce part of the uncertainties of the measurement. These will allow to improve the precision in the measurement of the cross section for the (anti) hydrogen formation by (anti) proton-positronium charge exchange and to optimize the yield of antihydrogen ion which is an essential parameter in the GBAR experiment.

The feasibility of producing the molecular antihydrogen anion ${{\overline{\mathrm{H}}}_{2}}^{\ensuremath{-}}$ in the laboratory is investigated. Utilizing reaction rates calculated here involving the interaction of laser excited-state antihydrogen atoms held in magnetic minimum traps, key processes are identified that could lead to anion production, as well as competing effects leading to anti-atom loss. These are discussed in the context of present-day and near-future experimental capabilities.

We have developed a coupled-rearrangement-channel method allowing the rigorous non-adiabatic treatment of the multi-channel scattering problem for four particles. We present the study of the binding, resonant and collisional properties of the (H) over bar -Ps system with the total angular momentum J = 0(+) (singlet positronic configuration). The binding energy, the life-times of the resonant states and the collisional cross sections are calculated and discussed. We present the preliminary cross sections for the elastic and inelastic (H) over bar -Ps scattering, notably for the excitation of Ps and for the rearrangement reaction producing the (H) over bar (+) ions.

The binding energies of antihydrogen atoms formed when antiprotons are mixed with positron plasmas having densities ranging from 1013–1015 m−3, and at temperatures of 5–30 K, have been investigated using simulations. Major changes in the distribution of binding energies are observed, with more strongly bound states evident at the higher densities, and at lower temperatures. For deeper binding, the distribution of binding energies follows a power-law which is found to be strongly dependent upon plasma properties and the strength of the applied magnetic field. The underpinning role of collisions in determining the binding energies is explored.

Near-threshold production of antihydrogen ion ( H ̄ + ) in positronium–antihydrogen collisions is predicted by a rigorous four-body scattering calculation. The convergence of the cross sections for the rearrangement and for all competing reactions (elastic scattering, de/excitation and de/polarization of positronium) is carefully examined against partial waves. The multi-channel scattering solutions are composed of functions that diagonalize the four-body Hamiltonian, and scattering functions that satisfy the correct asymptotic boundary conditions. The production rates of H ̄ + show large discrepancies compared to more approximate calculations.

For the GBAR (Gravitational Behaviour of Antihydrogen at Rest) experiment at CERN's Antiproton Decelerator (AD) facility we have constructed a source of slow positrons, which uses a low-energy electron linear accelerator (linac). The driver linac produces electrons of 9 MeV kinetic energy that create positrons from bremsstrahlung-induced pair production. Staying below 10 MeV ensures no persistent radioactive activation in the target zone and that the radiation level outside the biological shield is safe for public access. An annealed tungsten-mesh assembly placed directly behind the target acts as a positron moderator. The system produces $5\times10^7$ slow positrons per second, a performance demonstrating that a low-energy electron linac is a superior choice over positron-emitting radioactive sources for high positron flux., published in NIM A. 33 pages 9 figures

Synopsis We present the 4-body calculation of the antihydrogen-positronium scattering aiming at the prediction of cross sections for the production of antihydrogen positive ions. The latter are expected to be a useful source of ultra-cold atoms for the test of matter-antimatter gravity. We convert the Schrödinger equation to a set of coupled integro-differential equations that involve intermediate states and are solved using the compact finite difference method. We will present the investigation of the rearrangement reaction between the ground-state antihydrogen atom and the excited positronium.

We present a 4-body calculation of scattering between an antihydrogen atom ( H ¯ ) and a positronium (Ps) aiming at the prediction of cross sections for the production of antihydrogen positive ions H ¯ + . The antihydrogen positive ions are expected to be a useful source of ultra-cold anti-atoms for the test of matter-antimatter gravity. We convert the Schrödinger equation to a set of coupled integro-differential equations that involve intermediate states which facilitate the internal region description of the scattering wavefunction. They are solved using a compact finite difference method. Our framework is extended to scattering between an excited Ps and H. Cross sections of the reactions, Ps (1s/2s/3s) H ¯ → e − + H ¯ + , in s-wave collisions, are calculated. It is found that the reactions originating from Ps (1s/2s) + H ¯ produce H ¯ + with a constant cross section within 0.05 eV above the threshold while the reaction cross section from Ps (3s) decreases as the collision energy increases in the same energy interval. Just above the threshold, the cross section of H ¯ + production from Ps (3s) + H ¯ in s-wave collision is 7.8 times larger than that from Ps (1s) + H ¯ in s-wave and 2.3 times larger than that from Ps (2s) + H ¯ in s-wave. The near-threshold de-excitation reaction from Ps (3s) + H ¯ occurs more rapidly than the H ¯ + production.

I give an overview of experimental and theoretical results for antiproton and antihydrogen scattering with atoms and molecules (in particular H, He). At low energies ($\lesssim1$~keV) there are practically no experimental data available. Instead I compare the results from different theoretical calculations, of various degrees of sophistication. At energies up to a few 10:s of eV, I focus on simple approximations that give reasonably accurate results, as these allow quick estimates of collision rates without embarking on a research project., 13 pages, 9 figures. This article is part of the Theo Murphy meeting issue "Antiproton physics in the ELENA era"

International audience; The simultaneous control of the density and particle number of non-neutral plasmas confined in Penning-Malmberg traps is demonstrated. Control is achieved by setting the plasma’s density by applying a rotating electric field while simultaneously fixing its axial potential via evaporative cooling. This novel method is particularly useful for stabilizing positron plasmas, as the procedures used to collect positrons from radioactive sources typically yield plasmas with variable densities and particle numbers; it also simplifies optimization studies that require plasma parameter scans. The reproducibility achieved by applying this technique to the positron and electron plasmas used by the ALPHA antihydrogen experiment at CERN, combined with other developments, contributed to a 10-fold increase in the antiatom trapping rate.

In 1928, Dirac published an equation1 that combined quantum mechanics and special relativity. Negative-energy solutions to this equation, rather than being unphysical as initially thought, represented a class of hitherto unobserved and unimagined particles—antimatter. The existence of particles of antimatter was confirmed with the discovery of the positron2 (or anti-electron) by Anderson in 1932, but it is still unknown why matter, rather than antimatter, survived after the Big Bang. As a result, experimental studies of antimatter3–7, including tests of fundamental symmetries such as charge–parity and charge–parity–time, and searches for evidence of primordial antimatter, such as antihelium nuclei, have high priority in contemporary physics research. The fundamental role of the hydrogen atom in the evolution of the Universe and in the historical development of our understanding of quantum physics makes its antimatter counterpart—the antihydrogen atom—of particular interest. Current standard-model physics requires that hydrogen and antihydrogen have the same energy levels and spectral lines. The laser-driven 1S–2S transition was recently observed8 in antihydrogen. Here we characterize one of the hyperfine components of this transition using magnetically trapped atoms of antihydrogen and compare it to model calculations for hydrogen in our apparatus. We find that the shape of the spectral line agrees very well with that expected for hydrogen and that the resonance frequency agrees with that in hydrogen to about 5 kilohertz out of 2.5 × 1015 hertz. This is consistent with charge–parity–time invariance at a relative precision of 2 × 10−12—two orders of magnitude more precise than the previous determination8—corresponding to an absolute energy sensitivity of 2 × 10−20 GeV., The shape of the spectral line and the resonance frequency of the 1S–2S transition in antihydrogen agree very well with those of hydrogen.

The observation of hyperfine structure in atomic hydrogen by Rabi and co-workers1,2,3 and the measurement4 of the zero-field ground-state splitting at the level of seven parts in 1013 are important achievements of mid-twentieth-century physics. The work that led to these achievements also provided the first evidence for the anomalous magnetic moment of the electron5,6,7,8, inspired Schwinger’s relativistic theory of quantum electrodynamics9,10 and gave rise to the hydrogen maser11, which is a critical component of modern navigation, geo-positioning and very-long-baseline interferometry systems. Research at the Antiproton Decelerator at CERN by the ALPHA collaboration extends these enquiries into the antimatter sector. Recently, tools have been developed that enable studies of the hyperfine structure of antihydrogen12—the antimatter counterpart of hydrogen. The goal of such studies is to search for any differences that might exist between this archetypal pair of atoms, and thereby to test the fundamental principles on which quantum field theory is constructed. Magnetic trapping of antihydrogen atoms13,14 provides a means of studying them by combining electromagnetic interaction with detection techniques that are unique to antimatter12,15. Here we report the results of a microwave spectroscopy experiment in which we probe the response of antihydrogen over a controlled range of frequencies. The data reveal clear and distinct signatures of two allowed transitions, from which we obtain a direct, magnetic-field-independent measurement of the hyperfine splitting. From a set of trials involving 194 detected atoms, we determine a splitting of 1,420.4 ± 0.5 megahertz, consistent with expectations for atomic hydrogen at the level of four parts in 104. This observation of the detailed behaviour of a quantum transition in an atom of antihydrogen exemplifies tests of fundamental symmetries such as charge–parity–time in antimatter, and the techniques developed here will enable more-precise such tests.

The method of complex scaling is used to calculate S- and P-wave resonance energies and widths below the Li(3s, 3p, 4s, 4p) excitation thresholds and positronium n = 2 formation threshold in the po ...

International audience; Simulations have been performed of the radial transport of antiprotons in positron plasmas under ambient conditions typical of those used in antihydrogen formation experiments in Penning traps. The simulations were performed using classical trajectories of both antiprotons and surrounding positrons with randomized initial conditions. In addition, friction and fluctuation forces on the antiproton due to interaction with the positron plasma were included. This gives rise to both axial and radial diffusion of the antiprotons at a rate given by the magnetic field, the positron temperature and the plasma density, and the parameter range explored includes at least two values of each. As the antiprotons travel through the positron plasma they undergo repeated cycles of antihydrogen formation and re-ionization. We find that when these effects are added to the simulations the properties of the radial diffusion alter dramatically, even changing from normal to anomalous diffusion. We attribute this to the azimuthal drift of the antiprotons in the crossed magnetic (from the trap) and electric fields (from the space charge of the plasma). When the antiprotons are neutralised through antihydrogen formation this drift is temporarily interrupted, and the antiproton (carrying a positron) continues in the tangential direction. On average this motion will give an increase of the radial position of the antiproton, relative the trap axis. At low positron plasma temperatures, repeated cycles of antihydrogen formation and destruction are the dominant source of radial (cross magnetic field) transport. On time scales of a few milliseconds (depending on plasma parameters) the antiprotons will reach the edge of the cylindrical plasma, where the radial transport will cease, and thus the antiprotons will accumulate there.

We present an investigation of resonances with natural and unnatural parities in the positron-potassium system using the complex scaling method. A model potential is used to represent the interaction between the core and the valence electron. Explicitly correlated Gaussian wave functions are used to represent the correlation effects between the valence electron, the positron and the K+ core. Resonance energies and widths for two partial waves (S- and P-wave) below the K(4p, 5s, 5p, 4d, 4f) excitation thresholds and positronium n = 2 formation threshold are calculated for natural parity. Resonance states for P-e below the K(4d) excitation threshold and positronium n = 2, 3 formation thresholds are calculated for unnatural parity which has not been previously reported. Below both positronium thresholds we have found a dipole series of resonances, with binding energies scaling in good agreement with exceptions from an analytical calculation. The present results are compared with those in the literature.

© 2016 Macmillan Publishers Limited. All rights reserved. Antimatter continues to intrigue physicists because of its apparent absence in the observable Universe. Current theory requires that matter and antimatter appeared in equal quantities after the Big Bang, but the Standard Model of particle physics offers no quantitative explanation for the apparent disappearance of half the Universe. It has recently become possible to study trapped atoms - of antihydrogen to search for possible, as yet unobserved, differences in the physical behaviour of matter and antimatter. Here we consider the charge neutrality of the antihydrogen atom. By applying stochastic acceleration to trapped antihydrogen atoms, we determine an experimental bound on the antihydrogen charge, Qe, of |Q| < 0.71 parts per billion (one standard deviation), in which e is the elementary charge. This bound is a factor of 20 less than that determined from the best previous measurement of the antihydrogen charge. The electrical charge of atoms and molecules of normal matter is known to be no greater than about 10-21e for a diverse range of species including H2, He and SF6. Charge-parity-time symmetry and quantum anomaly cancellation demand that the charge of antihydrogen be similarly small. Thus, our measurement constitutes an improved limit and a test of fundamental aspects of the Standard Model. If we assume charge superposition and use the best measured value of the antiproton charge, then we can place a new limit on the positron charge anomaly (the relative difference between the positron and elementary charge) of about one part per billion (one standard deviation), a 25-fold reduction compared to the current best measurement.

Magnetically trapped antihydrogen atoms at low temperatures were used to test the charge neutrality of this antiatomic system, by precisely placing a limit to its electrical charge. A new method for measuring the gravitational interaction of antimatter was also developed, and prospects for making a precise measurement of this interaction are discussed. Finally, progress towards a precise, direct test of CPT symmetry by laser spectroscopy of antihydrogen is presented.

The 1S–2S transition in magnetically trapped atoms of antihydrogen is observed, and its frequency is shown to be consistent with that expected for hydrogen. Testing to high precision whether matter behaves like antimatter could provide clues to one of the biggest puzzles in modern physics: why does the observable Universe consist almost entirely of matter? After all, the Standard Model predicts that after the Big Bang the Universe should have been made up of equal amounts of matter and antimatter. Antimatter is difficult to produce and characterize because it annihilates when it comes in contact with matter. But recent advances at CERN's Antiproton Decelerator have allowed researchers to trap and measure both antiprotons and antihydrogen. Now the ALPHA Collaboration at CERN reports the first spectroscopic characterization of antihydrogen, exciting the 1S to 2S transition with laser light. The transition frequency is consistent with that of hydrogen. The spectrum of ordinary hydrogen has been characterized to extremely high precision, so improvements in antihydrogen spectroscopy will yield highly sensitive tests of matter–antimatter symmetry. The spectrum of the hydrogen atom has played a central part in fundamental physics over the past 200 years. Historical examples of its importance include the wavelength measurements of absorption lines in the solar spectrum by Fraunhofer, the identification of transition lines by Balmer, Lyman and others, the empirical description of allowed wavelengths by Rydberg, the quantum model of Bohr, the capability of quantum electrodynamics to precisely predict transition frequencies, and modern measurements of the 1S–2S transition by Hansch1 to a precision of a few parts in 1015. Recent technological advances have allowed us to focus on antihydrogen—the antimatter equivalent of hydrogen2,3,4. The Standard Model predicts that there should have been equal amounts of matter and antimatter in the primordial Universe after the Big Bang, but today’s Universe is observed to consist almost entirely of ordinary matter. This motivates the study of antimatter, to see if there is a small asymmetry in the laws of physics that govern the two types of matter. In particular, the CPT (charge conjugation, parity reversal and time reversal) theorem, a cornerstone of the Standard Model, requires that hydrogen and antihydrogen have the same spectrum. Here we report the observation of the 1S–2S transition in magnetically trapped atoms of antihydrogen. We determine that the frequency of the transition, which is driven by two photons from a laser at 243 nanometres, is consistent with that expected for hydrogen in the same environment. This laser excitation of a quantum state of an atom of antimatter represents the most precise measurement performed on an anti-atom. Our result is consistent with CPT invariance at a relative precision of about 2 × 10−10.

We study, numerically and experimentally, the momentum distribution of atoms cooled in optical lattices. Using semiclassical simulations, we show that this distribution is bimodal, made up of a central feature corresponding to "cold," trapped atoms, with tails of "hot," untrapped atoms, and that this holds true also for very shallow potentials. Careful analysis of the distribution of high-momentum untrapped atoms, both from simulations and experiments, shows that the tails of the distribution do not follow a normal law, hinting at a power-law distribution and nonergodic behavior. We also revisit the phenomenon leading to the existence of an optimal cooling point, i.e., a potential depth below which the temperature of the atoms starts increasing.

The ALPHA project is an international collaboration, based at CERN, with the experimental goal of performing precision spectroscopic measurements on antihydrogen. As part of this endeavor, the ALPHA experiment includes a silicon tracking detector. This detector consists of a three-layer array of silicon modules surrounding the antihydrogen trapping region of the ALPHA apparatus. Using this device, the antihydrogen annihilation position can be determined with a spatial resolution of better than 5 mm. Knowledge of the annihilation distribution was a critical component in the recently successful antihydrogen trapping effort. This paper will describe the methods used to reconstruct annihilation events in the ALPHA detector. Particular attention will be given to the description of the background rejection criteria.

Precision comparisons of hyperfine intervals in atomic hydrogen and antihydrogen are expected to yield experimental tests of the CPT theorem. The CERN-based ALPHA collaboration has initiated a program of study focused on microwave spectroscopy of trapped ground-state antihydrogen atoms. This paper outlines some of the proposed experiments, and summarizes measurements that characterize microwave fields that have been injected into the ALPHA apparatus.

Precision spectroscopic comparison of hydrogen and antihydrogen holds the promise of a sensitive test of the Charge-Parity-Time theorem and matter-antimatter equivalence. The clearest path towards realising this goal is to hold a sample of antihydrogen in an atomic trap for interrogation by electromagnetic radiation. Achieving this poses a huge experimental challenge, as state-of-the-art magnetic-minimum atom traps have well depths of only ∼1 T (∼0.5 K for ground state antihydrogen atoms). The atoms annihilate on contact with matter and must be ‘born’ inside the magnetic trap with low kinetic energies. At the ALPHA experiment, antihydrogen atoms are produced from antiprotons and positrons stored in the form of non-neutral plasmas, where the typical electrostatic potential energy per particle is on the order of electronvolts, more than 104 times the maximum trappable kinetic energy. In November 2010, ALPHA published the observation of 38 antiproton annihilations due to antihydrogen atoms that had been trapped for at least 172 ms and then released—the first instance of a purely antimatter atomic system confined for any length of time (Andresen et al., Nature 468:673, 2010). We present a description of the main components of the ALPHA traps and detectors that were key to realising this result. We discuss how the antihydrogen atoms were identified and how they were discriminated from the background processes. Since the results published in Andresen et al. (Nature 468:673, 2010), refinements in the antihydrogen production technique have allowed many more antihydrogen atoms to be trapped, and held for much longer times. We have identified antihydrogen atoms that have been trapped for at least 1,000 s in the apparatus (Andresen et al., Nature Physics 7:558, 2011). This is more than sufficient time to interrogate the atoms spectroscopically, as well as to ensure that they have relaxed to their ground state.

In efforts to trap antihydrogen, a key problem is the vast disparity between the neutral trap energy scale (\(\sim\!50\,\upmu\mathrm{eV}\)), and the energy scales associated with plasma confinement and space charge (~1 eV). In order to merge charged particle species for direct recombination, the larger energy scale must be overcome in a manner that minimizes the initial antihydrogen kinetic energy. This issue motivated the development of a novel injection technique utilizing the inherent nonlinear nature of particle oscillations in our traps. We demonstrated controllable excitation of the center-of-mass longitudinal motion of a thermal antiproton plasma using a swept-frequency autoresonant drive. When the plasma is cold, dense and highly collective in nature, we observe that the entire system behaves as a single-particle nonlinear oscillator, as predicted by a recent theory. In contrast, only a fraction of the antiprotons in a warm or tenuous plasma can be similarly excited. Antihydrogen was produced and trapped by using this technique to drive antiprotons into a positron plasma, thereby initiating atomic recombination. The nature of this injection overcomes some of the difficulties associated with matching the energies of the charged species used to produce antihydrogen.

Spectroscopy of antihydrogen has the potential to yield high-precision tests of the CPT theorem and shed light on the matter-antimatter imbalance in the Universe. The ALPHA antihydrogen trap at CERN's Antiproton Decelerator aims to prepare a sample of antihydrogen atoms confined in an octupole-based Ioffe trap and to measure the frequency of several atomic transitions. We describe our techniques to directly measure the antiproton temperature and a new technique to cool them to below 10 K. We also show how our unique position-sensitive annihilation detector provides us with a highly sensitive method of identifying antiproton annihilations and effectively rejecting the cosmic-ray background., 10 pages, 5 figures

Antihydrogen spectroscopy promises precise tests of the symmetry of matter and antimatter, and can possibly offer new insights into the baryon asymmetry of the universe. Antihydrogen is, however, difficult to synthesize and is produced only in small quantities. The ALPHA collaboration is therefore pursuing a path towards trapping cold antihydrogen to permit the use of precision atomic physics tools to carry out comparisons of antihydrogen and hydrogen. ALPHA has addressed these challenges. Control of the plasma sizes has helped to lower the influence of the multipole field used in the neutral atom trap, and thus lowered the temperature of the created atoms. Finally, the first systematic attempt to identify trapped antihydrogen in our system is discussed. This discussion includes special techniques for fast release of the trapped anti-atoms, as well as a silicon vertex detector to identify antiproton annihilations. The silicon detector reduces the background of annihilations, including background from antiprotons that can be mirror trapped in the fields of the neutral atom trap. A description of how to differentiate between these events and those resulting from trapped antihydrogen atoms is also included.

I discuss rearrangement in low-energy collisions between atoms (ions) and antihydrogen. For many atoms there exists a critical internuclear distance below which the leptons are no longer bound to the heavy particles. At this critical distance the Born–Oppenheimer approximation breaks down. I briefly discuss the case of general atoms. Numerical examples are given for the simple proton–antihydrogen system, which has the advantage that the Born–Oppenheimer approximation can be solved exactly. I study the convergence of the optical potential approach for this system.

In this paper, we describe calculations that we have carried out of cross sections for rearrangement processes in very low-energy helium + antihydrogen ( H ¯ ) scattering that result in He + p ¯ + Ps or He p ¯ + e + or α p ¯ + Ps - . A significantly more accurate method from that used previously [E.A.G. Armour, S. Jonsell, Y. Liu, A.C. Todd, Nucl. Instr. and Meth. B 247 (2006) 127] is used to calculate the He + H ¯ entrance channel wave function. Results are presented for the first two processes. Mention is made of the use of the method in calculations of low-energy e + H 2 scattering.

The creation of atoms of antihydrogen under controlled conditions has opened up a new era in physics with antimatter. We describe the experimental realisation of low energy antihydrogen, via the mixing of carefully prepared clouds of positrons and antiprotons, and some of the progress that has been made in the last few years in characterising properties of the nascent anti-atoms. Ongoing efforts aimed at trapping the anti-atoms in magnetic field minima are discussed. Some of the motivations for undertaking experiments with antihydrogen are presented.

We examine a method used by Zygelman B et al (2004 Phys. Rev. A 69 042715) to evaluate the transition matrix for rearrangement in atom–antihydrogen scattering. We make a small change to the expression used previously and generalize it to all kinds of rearrangement processes.

We present an investigation of resonances in positron-sodium scattering using the complex scaling method. For the target sodium atoms, the interaction between the core and outer electron is treated ...

The GBAR project (Gravitational Behaviour of Anti hydrogen at Rest) at CERN, aims to measure the free fall acceleration of ultracold neutral anti hydrogen atoms in the terrestrial gravitational field. The experiment consists preparing anti hydrogen ions (one antiproton and two positrons) and sympathetically cooling them with Be (+) ions to less than 10 mu K. The ultracold ions will then be photo-ionized just above threshold, and the free fall time over a known distance measured. We will describe the project, the accuracy that can be reached by standard techniques, and discuss a possible improvement to reduce the vertical velocity spread.

We have observed a new mechanism for compression of a non-neutral plasma, where antiprotons embedded in an electron plasma are compressed by a rotating wall drive at a frequency close to the sum of the axial bounce and rotation frequencies. The radius of the antiproton cloud is reduced by up to a factor of 20 and the smallest radius measured is similar to 0.2 mm. When the rotating wall drive is applied to either a pure electron or pure antiproton plasma, no compression is observed in the frequency range of interest. The frequency range over which compression is evident is compared to the sum of the antiproton bounce frequency and the system's rotation frequency. It is suggested that bounce resonant transport is a likely explanation for the compression of antiproton clouds in this regime.

The scattering length for hydrogenic atoms with variable lepton mass, ranging from Ht o pµ, is calculated. The non-trivial sensitivity of the scattering length to non-adiabatic effects is investigated using the coupled-channel approach. The physical conditions for the Bose–Einstein condensation of exotic atoms are expressed in terms of this scattering length, which for the pµ atoms interacting via 3 � + potentials, is calculated to be 1.13 × 10 −2 au. The critical temperature for condensing the pµ atoms is estimated to be 0.5 K at a density of 2.2 × 10 20 cm −3 . Under these conditions the phonon velocity in the condensate is 2.2 m s −1 and the coherence length is nearly 5 µm.

A simple method to include the strong force in atom-antiatom scattering is presented. It is based on the strong-force scatteringn length between the nucleon and antinucleon. Using this method elastic and annihilation cross sections are calculated for hydrogen-antihydrogen and helium-antihydrogen scattering. The results are compared to first-order perturbation theory using a pseudo potential. The pseudo-potential approach works fairly well for hydrogen-antihydrogen scattering, but fails for helium-antihydrogen scattering where strong-force effects are more prominent., Comment: 9 pages, 2 figures, to be published in Nuclear Instruments and Methods B

We review theoretical results for low-energy scattering of antihydrogen and atomic hydrogen and helium. Various inelastic channels, such as rearrangement and annihilation, as well as elastic scattering, are considered. In particular we give a progress report on our He – H ¯ calculations and give preliminary results for the low-energy cross section for rearrangement into positronium and antiprotonic helium. As far as we are aware, this is the first time that these cross sections have been calculated.

We study the dynamics of the cooling of a gas of caesium atoms in an optical lattice, both experimentally and with 1D full-quantum Monte Carlo simulations. We find that, contrary to the standard interpretation of the Sisyphus model, the cooling process does not work by a continuous decrease of the average kinetic energy of the atoms in the lattice. Instead, we show that the momentum of the atoms follows a bimodal distribution, the atoms being gradually transferred from a hot to a cold mode. We suggest that the cooling mechanism should be depicted in terms of a rate model, describing the transfer between the two modes along with the processes occurring within each mode., 7 pages, 5 figures