Background-free observation of cold antihydrogen with field-ionization analysis of its states

A background-free observation of cold antihydrogen atoms is made using field ionization followed by antiproton storage, a detection method that provides the first experimental information about antihydrogen atomic states. More antihydrogen atoms can be field ionized in an hour than all the antimatter atoms that have been previously reported, and the production rate per incident high energy antiproton is higher than ever observed. The high rate and the high Rydberg states suggest that the antihydrogen is formed via three-body recombination.     

Antihydrogen atom formation in a CUSP trap towards spin polarized beams

The ASACUSA collaboration has been making a path to realize high precision microwave spectroscopy of ground-state hyperfine transitions of antihydrogen atom in flight for stringent test of the CPT symmetry. For this purpose, an efficient extraction of a spin polarized antihydrogen beam is essential. In 2010, we have succeeded in synthesizing our first cold antihydrogen atoms employing a CUSP trap. The CUSP trap confines antiprotons and positrons simultaneously with its axially symmetric magnetic field to form antihydrogen atoms. It is expected that antihydrogen atoms in the low-field-seeking states are preferentially focused along the cusp magnetic field axis whereas those in the high-field-seeking states are defocused, resulting in the formation of a spin-polarized antihydrogen beam.     

Cold antihydrogen atoms

Cold antihydrogen atoms have been produced recently by mixing trapped antiprotons with cold positrons. The efficiency is remarkable: more than 10% of the antiprotons form antihydrogen. Future spectroscopy of antihydrogen has the potential to provide new extremely precise tests of the fundamental symmetry between matter and antimatter. In addition, cold antihydrogen atoms might permit the first direct experiments investigating antimatter gravity. A novel method to measure the gravitational acceleration of antimatter using ultra-cold antihydrogen atoms is proposed. PACS 04.80.Cc; 32.80.Pj; 36.10.-k     

Antihydrogen synthesis by the reaction of antiprotons with excited state positronium atoms

Aspects of the possible reactions of trapped antiprotons with excited state positronium atoms to form antihydrogen are discussed. Conditions are identified whereby the antihydrogen produced may be suitable for capture in a neutral trap. A discussion is given of possible use of antihydrogen to test the quantization of electric charge involving precision comparisons of hydrogen and antihydrogen (Rydberg constants), and proton and antiproton cyclotron frequencies.     

Gravitational states of antihydrogen near material surface

We present a theoretical study of the motion of antihydrogen atoms in the Earth??s gravitational field near a material surface. We predict the existence of long-living quasistationary states of antihydrogen in a superposition of the gravitational and Casimir-van der Waals potentials of the surface. We suggest an interferometric method of measuring the energy difference between such gravitational states, hence the gravitational mass of antihydrogen.     

Towards the production of an ultra cold antihydrogen beam with the AEGIS apparatus

The AEGIS (Antimatter Experiment: Gravity, Interferometry, Spectroscopy) experiment is an international collaboration, based at CERN, with the experimental goal of performing the first direct measurement of the Earth’s gravitational acceleration on antihydrogen. In the first phase of the experiment, a gravity measurement with 1% precision will be performed by passing a beam of ultra cold antihydrogen atoms through a classical Moiré deflectometer coupled to a position sensitive detector. The key requirements for this measurement are the production of ultra cold (T～100?mK) Rydberg state antihydrogen and the subsequent Stark acceleration of these atoms. The aim is to produce Rydberg state antihydrogen by means of the charge exchange reaction between ultra cold antiprotons (T～100?mK) and Rydberg state positronium. This paper will present details of the developments necessary for the successful production of the ultra cold antihydrogen beam, with emphasis on the detector that is required for the development of these techniques. Issues covered will include the detection of antihydrogen production and temperature, as well as detection of the effects of Stark acceleration.     

Alternative method for reconstruction of antihydrogen annihilation vertices

The ALPHA experiment, located at CERN, aims to compare the properties of antihydrogen atoms with those of hydrogen atoms. The neutral antihydrogen atoms are trapped using an octupole magnetic trap. The trap region is surrounded by a three layered silicon detector used to reconstruct the antiproton annihilation vertices. This paper describes a method we have devised that can be used for reconstructing annihilation vertices with a good resolution and is more efficient than the standard method currently used for the same purpose.     

Measurement of the gravitational acceleration of antihydrogen

The gravitational force acting on antiparticles has never been directly measured to date. A method for measuring the gravitational effects on antihydrogen by equilibrating the gravitational force with a magnetic gradient is discussed. The systematic and statistical errors inherent to the measurement will be presented. It will be shown that a measurement of gravity at 1% can be realised using ∼ 5 × 10⁵ antihydrogen atoms. The production of antihydrogen atoms in conditions suitable for the measurement is also discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Trapped antihydrogen for spectroscopy and gravitation studies: Is it possible?

Possibilities for trapping and cooling antihydrogen atoms for spectroscopy and gravitational measurements are discussed. A measurement of the gravitational force on antihydrogen seems feasible if antihydrogen can be cooled to of order 1 milli-Kelvin. Difficulties in obtaining this low energy are discussed in the hope of stimulating required experimental and theoretical studies.     

Measurement of the hyperfine structure of antihydrogen in a beam

A measurement of the hyperfine structure of antihydrogen promises one of the best tests of CPT symmetry. We describe an experiment planned at the Antiproton Decelerator of CERN to measure this quantity in a beam of slow antihydrogen atoms.     

Spatial distribution of cold antihydrogen formation

Antihydrogen is formed when antiprotons are mixed with cold positrons in a nested Penning trap. We present experimental evidence, obtained using our antihydrogen annihilation detector, that the spatial distribution of the emerging antihydrogen atoms is independent of the positron temperature and axially enhanced. This indicates that antihydrogen is formed before the antiprotons are in thermal equilibrium with the positron plasma. This result has important implications for the trapping and spectroscopy of antihydrogen.     

Positron plasma diagnostics and temperature control for antihydrogen production

Production of antihydrogen atoms by mixing antiprotons with a cold, confined, positron plasma depends critically on parameters such as the plasma density and temperature. We discuss nondestructive measurements, based on a novel, real-time analysis of excited, low-order plasma modes, that provide comprehensive characterization of the positron plasma in the ATHENA antihydrogen apparatus. The plasma length, radius, density, and total particle number are obtained. Measurement and control of plasma temperature variations, and the application to antihydrogen production experiments are discussed.     

Molecular-dynamics simulations of cold antihydrogen formation in strongly magnetized plasmas

Employing a high-order symplectic integrator and an adaptive time-step algorithm, we perform molecular-dynamics simulations of antihydrogen formation, in a cold plasma confined by a strong magnetic field, over time scales of microseconds. Sufficient positron-antiproton recombination events occur to allow a statistical analysis for various properties of the formed antihydrogen atoms. Giant-dipole states are formed in the initial stage of recombination. In addition to neutral atoms, we also observe antihydrogen positive ions (H(+)), in which two positrons simultaneously bind to an antiproton.     

Proposed measurement of the ground-state hyperfine structure of antihydrogen

The ASACUSA collaboration at CERN-AD has recently submitted a proposal to measure the hyperfine splitting of the ground state of antihydrogen in an atomic beam line. The spectrometer will consist of two sextupoles for spin selection and analysis, and a microwave cavity to flip the spin of the antihydrogen atoms. Numerical simulations show that such an experiment is feasible if ~200 antihydrogen atoms per second can be produced in the ground state, and that an accuracy of better than 10^–7 can be reached. This measurement will be a precise test of the CPT invariance. B. Juhász serves as one of the authors of this article on behalf of the ASACUSA collaboration.     

Quantum beats in hydrogen and antihydrogen atoms in an external electric field

An effect of quantum beats that arises due to the coherent excitation of 2s and 2p states of hydrogen and antihydrogen atoms in an external electric field is described. It is shown that the quantum beat signal contains terms linear in electric field, i.e., is of opposite sign for the hydrogen and antihydrogen atoms. The conditions for the observation of this effect are discussed.     

Measuring the gravitational acceleration of antimatter with an antihydrogen interferometer

The gravitational force on antimatter has never been directly measured. A method is suggested for making this measurement by directing a low-energy beam of neutral antihydrogen atoms through a transmission-grating interferometer and measuring the gravitationally-induced phase shift in the interference pattern. A 1% measurement of the acceleration due to the Earth's gravitational field (¯ g) should be possible from a beam of about 10⁵ or 10⁶ atoms. If more antihydrogen can be made, a much more precise measurement of¯ g would be possible. A method is suggested for producing an antihydrogen beam appropriate for this experiment.     

The ASACUSA CUSP: an antihydrogen experiment

In order to test CPT symmetry between antihydrogen and its counterpart hydrogen, the ASACUSA collaboration plans to perform high precision microwave spectroscopy of ground-state hyperfine splitting of antihydrogen atom in-flight. We have developed an apparatus (“cusp trap”) which consists of a superconducting anti-Helmholtz coil and multiple ring electrodes. For the preparation of slow antiprotons and positrons, Penning-Malmberg type traps were utilized. The spectrometer line was positioned downstream of the cusp trap. At the end of the beamline, an antihydrogen beam detector was located, which comprises an inorganic Bismuth Germanium Oxide (BGO) single-crystal scintillator housed in a vacuum duct and surrounding plastic scintillators. A significant fraction of antihydrogen atoms flowing out the cusp trap were detected.     

Fast antihydrogen beam spectroscopy

The motivation for production and precision spectroscopy of antihydrogen atoms is outlined. An experimental configuration is considered, concerning laser-microwave spectroscopy of a fast hydroten beam with characteristics similar to those of an antihydrogen beam emanating from an antiproton-positron overlap region in an antiproton storage ring. In particular, a possible experiment for the measurement of the ground state hyperfine structure splitting is described.     

Towards antihydrogen trapping and spectroscopy at ALPHA

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.