Antimatter was first predicted in 1931, by Dirac. Work with high-energy antiparticles is now commonplace, and anti-electrons are used regularly in the medical technique of positron emission tomography scanning. Antihydrogen, the bound state of an antiproton and a positron, has been produced at low energies at CERN (the European Organization for Nuclear Research) since 2002. Antihydrogen is of interest for use in a precision test of nature's fundamental symmetries. The charge conjugation/parity/time reversal (CPT) theorem, a crucial part of the foundation of the standard model of elementary particles and interactions, demands that hydrogen and antihydrogen have the same spectrum. Given the current experimental precision of measurements on the hydrogen atom (about two parts in 10(14) for the frequency of the 1s-to-2s transition), subjecting antihydrogen to rigorous spectroscopic examination would constitute a compelling, model-independent test of CPT. Antihydrogen could also be used to study the gravitational behaviour of antimatter. However, so far experiments have produced antihydrogen that is not confined, precluding detailed study of its structure. Here we demonstrate trapping of antihydrogen atoms. From the interaction of about 10(7) antiprotons and 7 × 10(8) positrons, we observed 38 annihilation events consistent with the controlled release of trapped antihydrogen from our magnetic trap; the measured background is 1.4 ± 1.4 events. This result opens the door to precision measurements on anti-atoms, which can soon be subjected to the same techniques as developed for hydrogen.

[1] At an industrial site on a sand aquifer overlying a clayey silt aquitard in Connecticut, a zone of trichloroethylene dense nonaqueous phase liquid (DNAPL) at the aquifer bottom was isolated in late 1994 by installation of a steel sheet piling enclosure. In response to this DNAPL isolation, three aquifer monitoring wells located approximately 330 m downgradient exhibited strong TCE declines over the next 2-3 years, from trichloroethylene (TCE) concentrations between 5000 and 30,000 mg/L to values leveling off between 200 and 2000 mg/L. TCE concentrations from analysis of vertical cores from the aquitard below the plume and also from depth-discrete multilevel systems in the aquifer sampled in 2000 were represented in a numerical model. This shows that vertical back diffusion from the aquitard combined with horizontal advection and vertical transverse dispersion account for the TCE distribution in the aquifer and that the aquifer TCE will remain much above the MCL for centuries.

A baseball fielder will arrive at the right place at the right time to catch a fly ball if he runs at the only constant velocity for which the rate of change of tangent of the elevation angle of the ball and the bearing angle of the ball both remain constant. Remarks are made concerning curve balls. Baseball is not a scientific game—although, unknown to the players, many principles of physics can be applied to it.

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We report the application of evaporative cooling to clouds of trapped antiprotons, resulting in plasmas with measured temperature down to 9 K. We have modeled the evaporation process for charged particles using appropriate rate equations. Good agreement between experiment and theory is observed, permitting prediction of cooling efficiency in future experiments. The technique opens up new possibilities for cooling of trapped ions and is of particular interest in antiproton physics, where a precise CPT test on trapped antihydrogen is a long-standing goal.Historically, forced evaporative cooling has been successfully applied to trapped samples of neutral particles [1], and remains the only route to achieve Bose-Einstein condensation in such systems [2]. However, the technique has only found limited applications for trapped ions (at temperatures ∼ 100 eV [3]) and has never been realized in cold plasmas. Here we report the application of forced evaporative cooling to a dense (∼ 10 6 cm −3 ) cloud of trapped antiprotons, resulting in temperatures as low as 9 K; two orders of magnitude lower than any previously reported [4].The process of evaporation is driven by elastic collisions that scatter high energy particles out of the confining potential, thus decreasing the temperature of the remaining particles. For charged particles the process benefits from the long range nature of the Coulomb interaction, and compared to neutrals of similar density and temperature, the elastic collision rate is much higher, making cooling of much lower numbers and densities of particles feasible. In addition, intraspecies loss channels from inelastic collisions are non-existent. Strong coupling to the trapping fields makes precise control of the confining potential more critical for charged particles. Also, for plasmas, the self-fields can both reduce the collision rate through screening and change the effective depth of the confining potential.The ALPHA apparatus, which is designed with the intention of creating and trapping antihydrogen [5], is located at the Antiproton Decelerator (AD) at CERN [6]. It consists of a Penning-Malmberg trap for charged particles with an octupole-based magnetostatic trap for neutral atoms superimposed on the central region. For the work presented here, the magnetostatic trap was not energized and the evaporative cooling was performed in a homogeneous 1 T solenoidal field. Figure 1a shows a schematic diagram of the apparatus, with only a subset of the 20.05 mm long and 22.275 mm radius, hollow cylindrical electrodes shown. The vacuum wall is cooled using liquid helium, and the measured electrode temperature is about 7 K. The magnetic field, indicated by the arrow, is directed along the axis of cylindrical symmetry and confines the antiprotons radially: due to conservation of angular momentum, antiprotons do not readily escape in directions transverse to the magnetic field lines [7]. Parallel to the magnetic field, antiprotons are confined by electric fields generated by the electrodes.Also shown are the two ...

Measurements on electrons confined in a Penning trap show that extreme quadrupole fields destroy particle confinement. Much of the particle loss comes from the hitherto unrecognized ballistic transport of particles directly into the wall. The measurements scale to the parameter regime used by ATHENA and ATRAP to create antihydrogen, and suggest that quadrupoles cannot be used to trap antihydrogen.

We present the results of an experiment to search for trapped antihydrogen atoms with the ALPHA antihydrogen trap at the CERN Antiproton Decelerator. Sensitive diagnostics of the temperatures, sizes, and densities of the trapped antiproton and positron plasmas have been developed, which in turn permitted development of techniques to precisely and reproducibly control the initial experimental parameters. The use of a position-sensitive annihilation vertex detector, together with the capability of controllably quenching the superconducting magnetic minimum trap, enabled us to carry out a high-sensitivity and low-background search for trapped synthesised antihydrogen atoms. We aim to identify the annihilations of antihydrogen atoms held for at least 130 ms in the trap before being released over ∼ 30 ms. After a three-week experimental run in 2009 involving mixing of 10 7 antiprotons with 1.3 × 10 9 positrons to produce 6 × 10 5 antihydrogen atoms, we have identified six antiproton annihilation events that are consistent with the release of trapped antihydrogen. The cosmic ray background, estimated to contribute 0.14 counts, is incompatible with this observation at a significance of 5.6 sigma. Extensive simulations predict that an alternative source of annihilations, the escape of mirror-trapped antiprotons, is highly unlikely, though this possibility has not yet been ruled out experimentally.