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.     

The antiprotonic helium atom

The Born-Oppenheimer approximation is used to discuss the high rotational and vibrational state of the (He p⁻)e⁻ system in the electronic 1sσ and 2pσ states. Very high angular momentum states, in which the antiproton is well outside the electron orbit, have a Rydberg-like character. states in which the antiproton is within the 1sσ electron orbit have enhanced radiative lifetimes due to the polarization of the 1sσ state by the antiproton. This effect may account for the long-lived component observed in antiproton destruction in He. Preliminary results on the effect of coupling to the 2pσ well, in which the polarization effects enhance the decay rate, are also presented. Some consequences for the suggestion that metastable antiprotonic He atoms may be used to promote antihydrogen formation are discussed.     

Driven production of cold antihydrogen and the first measured distribution of antihydrogen states

Cold antihydrogen is produced when antiprotons are repeatedly driven into collisions with cold positrons within a nested Penning trap. Efficient antihydrogen production takes place during many cycles of positron cooling of antiprotons. A first measurement of a distribution of antihydrogen states is made using a preionizing electric field between separated production and detection regions. Surviving antihydrogen is stripped in an ionization well that captures and stores the freed antiproton for background-free detection.     

Laser-induced radiative recombination of antihydrogen in a magnetic field through a quasi-stationary state

To increase the efficiency of laser-induced recombination of antihydrogen from cold antihydrogen—positron plasma in a trap, it is proposed to use a new resonance mechanism with the participation of positron quasi-stationary states, arising under the joint action of an antiproton Coulomb field and a strong magnetic field of the trap. The recombination rate is expressed through the atomic laser ionization cross section whose frequency dependence is nonmonotonic due to the presence of quasi-stationary states against the background of the continuum. The estimates with the use of the ionization cross sections calculated earlier demonstrate the possibility of improving the efficiency of the laser-induced recombination at an optimally selected laser frequency.     

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.     

Production of relativistic antihydrogen atoms by pair production with positron capture and measurement of the Lamb shift

A beam of relativistic antihydrogen atoms — the bound state ( e⁺) — can be created by circulating the beam of an antiproton storage ring through an internal gas target. An antiproton which passes through the Coulomb field of a nucleus will create e⁺e⁻ pairs, and antihydrogen will form when a positron is created in a bound instead of continuum state about the antiproton. The cross section for this process is roughly 3Z ² pb for antiproton momenta about 6 GeV/c. A sample of 600 antihydrogen atoms in a low-emittance, neutral beam will be made in 1995 as an accidental byproduct of Fermilab experiment E760. We describe a simple experiment, Fermilab Proposal P862, which can detect this beam, and outline how a sample of a few-10⁴ atoms can be used to measure the antihydrogen Lamb shift to 1 %. Work supported in part by Department of Energy contract DE-AC03-76SF00515 (SLAC). Work supported by Fondo Nacional de Investigación Científica y Tecnológica, Chile.     

Relativistic antihydrogen production

Antihydrogen has recently been produced in collisions of antiprotons with ions. While passing through the Coulomb field of a nucleus an antiproton will create an electron-positron pair. In rare cases the positron is bound by the antiproton and an antihydrogen atom produced. We calculate the production of relativistic antihydrogen atoms by bound-free pair production. The cross section is calculated in the semiclassical approximation (SCA), or equivalently in the plane wave Born approximation (PWBA) using exact Dirac-Coulomb wave functions. We compare our calculations to the equivalent photon approximation (EPA). Received: 19 December 1997 / Published online: 10 March 1998     

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.     

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.     

Quadrupole-induced resonant-particle transport in a pure electron plasma

Small transverse magnetic quadrupole fields sharply degrade the confinement of non-neutral plasmas held in Malmberg-Penning traps. For example, a quadrupole magnetic field of only 0.02 G/cm doubles the diffusion rate in a trap with a 100 G axial magnetic field. Larger quadrupole fields noticeably change the shape of the plasma. The transport is greatest at an orbital resonance. These results cast doubt on plans to use magnetic quadrupole neutral atom traps to confine antihydrogen atoms created in double-well positron/antiproton Malmberg-Penning traps.     

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.     

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.     

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.     

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.     

Laser-stimulated formation and stabilization of antihydrogen atoms

Laser-stimulated radiative transitions from states close to the ionization threshold to low-lying atomic levels are considered for protons (antiprotons) in a cold electron (positron) plasma and estimates for the resulting formation rate of hydrogen (antihydrogen) atoms in the ground state are given. The estimates apply to both laser-stimulated recombination and induced radiative stabilization of high Rydberg levels. First experiments concerning laser-stimulated recombination in merged beams of electrons and protons are discussed, which have confirmed the rate predictions for this process. In view of antihydrogen formation in a cold trapped positron plasma, the use of two successive stimulated transitions is considered for obtaining a high formation rate of ground-state atoms at relatively low radiation intensity.     

Antimatter plasmas in a multipole trap for antihydrogen

We have demonstrated storage of plasmas of the charged constituents of the antihydrogen atom, antiprotons and positrons, in a Penning trap surrounded by a minimum-B magnetic trap designed for holding neutral antiatoms. The neutral trap comprises a superconducting octupole and two superconducting, solenoidal mirror coils. We have measured the storage lifetimes of antiproton and positron plasmas in the combined Penning-neutral trap, and compared these to lifetimes without the neutral trap fields. The magnetic well depth was 0.6 T, deep enough to trap ground state antihydrogen atoms of up to about 0.4 K in temperature. We have demonstrated that both particle species can be stored for times long enough to permit antihydrogen production and trapping studies.     

Antihydrogen synthesis in a double-CUSP trap towards test of the CPT-symmetry

The aim of the ASACUSA-CUSP experiment at CERN is to produce a cold, polarised antihydrogen beam and perform a high precision measurement of the ground-state hyperfine transition frequency of the antihydrogen atom and compare it with that of the hydrogen atom using the same spectroscopic beam line. Towards this goal a significant step was successfully accomplished: synthesised antihydrogen atoms have been produced in a CUSP magnetic configuration and detected at the end of our spectrometer beam line in 2012 [1]. During a long shut down at CERN the ASACUSA-CUSP experiment had been renewed by introducing a new double-CUSP magnetic configuration and a new semi-cylindrical tracking detector (AMT) [2], and by improving the transport feature of low energy antiproton beams. The new tracking detector monitors the antihydrogen synthesis during the mixing cycle of antiprotons and positrons. In this work the latest results and improvements of the antihydrogen synthesis will be presented including highlights from the last beam time.     

Theory of X-ray grazing incidence reflection in the presence of nuclear resonance excitation

ATHENA, one of the three approved experiments at the new facility for low energy antiprotons (AD) at CERN, has the primary goal to test CPT invariance by comparing the atomic energy levels of antihydrogen to those of hydrogen. The extended experimental program also contains studies on differences in gravitational acceleration of antimatter and matter. The production of antihydrogen atoms and their spectral response to laser light will be monitored by a sophisticated detector for the end products of antiproton and positron annihilations. This revised version was published online in August 2006 with corrections to the Cover Date.     

Resonance spectroscopy of gravitational states of antihydrogen

We study a method to induce resonant transitions between antihydrogen ( \(\bar {H}\) ) quantum states above a material surface in the gravitational field of the Earth. The method consists in applying a gradient of magnetic field which is temporally oscillating with the frequency equal to a frequency of a transition between gravitational states of antihydrogen. Corresponding resonant change in a spatial density of antihydrogen atoms can be measured as a function of the frequency of applied field. We estimate an accuracy of measuring antihydrogen gravitational states spacing and show how a value of the gravitational mass of the \(\bar {H}\) atom can be deduced from such a measurement.