Antihydrogen Research Articles

Quenching of para-H2with an ultracold antihydrogen atomH¯1s

In this work we report the results of calculation for quantum-mechanical rotational transitions in molecular hydrogen, ${\mathrm{H}}_{2}$, induced by an ultracold ground-state antihydrogen atom ${\overline{\mathrm{H}}}_{1s}$. The calculations are accomplished using a nonreactive close-coupling quantum-mechanical approach. The ${\mathrm{H}}_{2}$ molecule is treated as a rigid rotor. The total elastic-scattering cross section ${\ensuremath{\sigma}}_{\mathrm{el}}(\ensuremath{\epsilon})$ at energy $\ensuremath{\epsilon}$, state-resolved rotational transition cross sections ${\ensuremath{\sigma}}_{{\mathit{jj}}^{'}}(\ensuremath{\epsilon})$ between states $j$ and ${j}^{'}$, and corresponding thermal rate coefficients ${k}_{{\mathit{jj}}^{'}}(T)$ are computed in the temperature range $0.004 \mathrm{K}\ensuremath{\lesssim}T\ensuremath{\lesssim}$ 4 K. Satisfactory agreement with other calculations (variational) has been obtained for ${\ensuremath{\sigma}}_{\mathrm{el}}(\ensuremath{\epsilon})$.

Antihydrogen formation dynamics in a multipolar neutral anti-atom trap

Antihydrogen production in a neutral atom trap formed by an octupole-based magnetic field minimum is demonstrated using field-ionization of weakly bound anti-atoms. Using our unique annihilation imaging detector, we correlate antihydrogen detection by imaging and by field-ionization for the first time. We further establish how field-ionization causes radial redistribution of the antiprotons during antihydrogen formation and use this effect for the first simultaneous measurements of strongly and weakly bound antihydrogen atoms. Distinguishing between these provides critical information needed in the process of optimizing for trappable antihydrogen. These observations are of crucial importance to the ultimate goal of performing CPT tests involving antihydrogen, which likely depends upon trapping the anti-atom.

Proposal for making a beam of antihydrogen by two charge exchange events

We have performed calculations of two successive charge transfers in a geometry that could generate a beam of antihydrogen atoms () or reliably produce cold without having to reach extremely cold plasma temperatures. The basic idea is similar to that proposed by Hessels et al (1998 Phys. Rev. A 57 1668) except that the order of the charge transfers is reversed. A beam of highly-excited (Rydberg) Cs atoms passes through an antiproton () plasma where a charge transfer can take place; the result is an exotic Rydberg atom (Cs+ ion and a bound ) which has approximately the original velocity of the Cs atom since the Cs+ mass is much greater than that of the . This exotic Rydberg atom travels into a positron plasma where a second charge exchange gives antihydrogen (). The velocity distribution of the resulting is directly related to the original velocity of the Rydberg Cs atom. The binding energy of the is roughly that of the original Cs Rydberg atom; thus, the starting state of the can be controlled by choosing the initial state of the Cs atom. Because the Cs+ binding energy can be controlled in the charge transfer, this first step, by itself, could be of interest to the exotic atom community.

Developments of the cusp trap to synthesize antihydrogen atoms for high precision spectroscopy of ground state hyperfine splitting

We have been developing a so-called cusp trap to synthesize antihydrogen atoms, and to make high precision microwave spectroscopy. The cusp trap consists of an anti-Helmholtz coil and a multi-ring trap, which yield axially symmetric magnetic and electric fields. This axial symmetry is quite important to realize stable confinements of antiproton and positron plasmas, which guarantees stable operation during the antihydrogen synthesis.

Possible mechanism for enhancing the trapping and cooling of antihydrogen

We propose a usage of microwave radiation in a magnetic trap for improving the cooling and trapping of cold antihydrogen atoms which are initially produced in high magnetic moment states. Inducing transitions toward lower magnetic moments near the turning points of the atom in the trap, followed by spontaneous emission, should enhance the number of trappable atoms. We present results of simulations based on a typical experimental condition of the antihydrogen experiments at CERN. This technique should also be applicable to other trapped high magnetic moment Rydberg atoms.

Precision spectroscopy of metastable hydrogen and antihydrogen with a Lamb-shift polarimeter

A spinfilter, the most important component of a Lamb-shift polarimeter, can be used to produce a beam of metastable hydrogen (deuterium) atoms in one hyperfine state (α1, α2 and together with the Sona transition β3). As function of a magnetic field separated transitions between the 2S1/2 metastable hyperfine states seem to be observable as well as single transitions into the short-lived 2P1/2 and 2P3/2 states. The Breit-Rabi diagrams for these states can be measured with good precision. Furthermore, the hyperfine splittings and the Lamb shift can be observed as well. Application of this method to anti-hydrogen atoms are suggested.

Influence of an external electric field on the probabilities of two-photon transitions between 2s, 2p and 1s levels for hydrogen and antihydrogen atoms

The probabilities of two-photon decay for hydrogen (H) and antihydrogen (\( \bar H \)) atoms in the presence and absence of an external electric field are analytically calculated. In particular, the probabilities of the E1E2 and E1M1 transitions between the 2p and 1s levels are calculated for the case when emitted photons are characterized by polarization vectors and wavevectors. It is shown that, in an external electric field, the decay probabilities for 2s and 2p levels differ for H and \( \bar H \) atoms because of interference terms linear in field. Coulomb Green’s function method is used for summing over intermediate states.

Antihydrogen Physics at ALPHA/CERNThis paper was presented at the International Conference on Precision Physics of Simple Atomic Systems, held at University of Windsor, Windsor, Ontario, Canada on 21–26 July 2008.

Cold antihydrogen has been produced at CERN (Amoretti et al. (Nature, 419, 456 (2002)), Gabrielse et al. (Phys. Rev. Lett. 89, 213401 (2002))), with the aim of performing a high-precision spectroscopic comparison with hydrogen as a test of the CPT symmetry. Hydrogen, a unique system used for the development of quantum mechanics and quantum electrodynamics, has been continuously used to produce high-precision tests of theories and measurements of fundamental constants and can lead to a very sensitive search for CPT violation. After the initial production of cold antihydrogen atoms by the ATHENA group, the ALPHA Collaboration ( http://alpha.web.cern.ch/ ) has set forth on an experiment to trap and perform high-resolution laser spectroscopy on the 1S-2S transition of both atoms. In this contribution, we will review the motivations, goals, techniques, and recent developments towards this fundamental physics test. We present new discussion on predicted lineshapes for the 1S-2S spectroscopy of trapped atoms in a regime not discussed before.

Monte Carlo simulation of an antiproton annihilation detector system

A renewed antiproton annihilation detector system has been developed for the ATRAP (Antihydrogen Trap Collaboration) experiment at CERN (European Organization for Nuclear Research). It counts the antihydrogen atoms and determines the annihilation vertex of the atoms. This diagnostic element will allow to optimize the production of cold antihydrogen sufficiently to permit the optical observations and measurements. Extensive Monte Carlo simulations concerning the detector system have been developed. Different event generators as well as different geometry representations were established and evaluated. Real-time measurement with the detector system was performed and the results are calibrated via the Monte Carlo simulations.

Spin exchange in hydrogen–antihydrogen collisions

We consider spin exchange in ultracold collisions of hydrogen (H) and antihydrogen atoms. The cross sections for transitions between various hyperfine states are calculated. We show that the process of spin exchange in –H collisions is basically driven by the strong force between a proton and an antiproton.

Design of a Superconducting Magnet for Trapping Antihydrogen Atoms at CERN

The ATRAP collaboration (Antihydrogen Trapping) operates an experiment at the Antiproton Decelerator (AD) at CERN with the objective to create antihydrogen atoms and confine them at a very low temperature. The goal is to perform precise spectroscopy on antihydrogen in comparison to hydrogen as a high precision test of charge, parity and time invariance (CPT).

The antiproton annihilation detector system of the ATRAP experiment

The ATRAP (antihydrogen trap collaboration) at CERN (European organization for nuclear research) has developed a completely new, larger and more robust apparatus in the second experimental zone. The antiproton annihilation detector system consists of 10 layers of scintillating fibers, counts the antihydrogen atoms and determines the annihilation vertex of the atoms. This diagnostic element will allow to optimize the production of cold antihydrogen sufficiently to permit the optical observations and measurements. Using this new apparatus thousands of antihydrogen atoms have been produced within a combined Penning-Ioffe trap. These observed antihydrogen atoms resolve a debate about whether positrons and antiprotons can be brought together to form antihydrogen atoms within the divergent magnetic fields of a quadrupole Ioffe trap.

Antihydrogen formation from antiprotons in a pure positron plasma

This paper investigates the evolution in binding energy of antihydrogen atoms formed from stationary antiprotons located within a strongly magnetized positron plasma. Three-body recombination and a collisional cascade to deeper binding, limited by a kinetic bottleneck at a binding energy of 4T, dominate the initial antihydrogen formation process. A classical Monte-Carlo simulation is used to determine the collisional transition rate between atomic binding energies, using the drift approximation for initial conditions that allow it, and full dynamics for initial conditions resulting in chaotic motion. These transition rates are employed in determining mean energy-loss rates for an ensemble of atoms, as well as in a numerical solution of the master equation to find the rate at which atoms are formed over a range of binding energies. The highly excited atoms formed by this process separate into guiding-center drift atoms and chaotic atoms. The phase-space distributions of the atoms are investigated, along with their implications for magnetic confinement and radiative energy loss. Estimates of radiative energy loss indicate that radiation is unimportant for guiding-center atoms, but increases rapidly near the chaotic regime, taking over as the dominant energy-loss process for parameters typical of recent experiments. Furthermore, the fraction of low-magnetic field seekers is considerably larger than suggested by estimates of the magnetic moment based on guiding-center dynamics, due to effects associated with chaos.

Hydrogen-Bonded Networks Constructed with 5-Nitrobarbiturate

A series of nine compounds including tetra-alkylammonium 5-nitrobarbiturates and their urea and thiourea adducts have been obtained and characterized by single crystal X-ray analysis. The hydrogen-bonded networks in these compounds feature three different kinds of dimeric 5-nitrobarbiturate linkages (para-to-para, ortho-to-ortho and para-to-ortho), which are stabilized by formation of the cyclic urea dimer supramolecular synthon. In four thiourea complexes, the 5-nitrobarbiturate dimer is the dominant building unit, while in the urea complexes no such dimer is present due to the comparable hydrogen bonding capability of urea and 5-nitrobarbiturate. The main connection modes in the urea complexes are the traditional head-to-tail and shoulder-to-shoulder types, as well as a specific nitro-urea hydrogen motif. In [2(CH3)4N+]·[2(C4H2N3O5)−]·(NH2)2CS (3), the thiourea molecule uses its syn and anti hydrogen atoms as a clamp and strut, respectively, to connect two nearly parallel hydrogen-bonded 5-nitrobarbitur...

Two-species mixing in a nested Penning trap for antihydrogen trapping

There exists an international quest to trap neutral antimatter in the form of antihydrogen for scientific study. One method that is being developed for trapping antihydrogen employs a nested Penning trap. Such a trap serves to mix positrons and antiprotons so as to produce low energy antihydrogen atoms. Mixing is achieved when the confinement volumes of the two species overlap one another. In the work presented here, a theoretical understanding of the mixing process is developed by analyzing a mixing scheme that was recently reported [G. Gabrielse et al., Phys. Rev. Lett. 100, 113001 (2008)]. The results indicate that positron space charge or collisions among antiprotons may substantially reduce the fraction of antiprotons that have an energy suitable for antihydrogen trapping.

Radial Compression of an Antiproton Cloud for Production of Intense Antiproton Beams

We report here the radial compression of a large number of antiprotons ( approximately 5 x 10(5)) in a strong magnetic field under ultrahigh vacuum conditions by applying a rotating electric field. Compression without any resonant structures was demonstrated for a range of frequencies from the sideband frequency of 200 kHz to more than 1000 kHz. The radial compression achieved is a key technique for synthesizing and manipulating antihydrogen atoms and antiprotonic atoms.

Probabilities of single-photon 2s–1s transition in hydrogen and antihydrogen atoms in an external electric field

The probabilities of single-photon 2s–1s transition in hydrogen (H) and antihydrogen (\( \bar H \)) atoms in an external electric field are calculated. These quantities are different for H and \( \bar H \) due to the presence of a T odd term linearly dependent on the electric field strength. The relative difference between the probabilities of the considered process for hydrogen and antihydrogen is given, and the conditions under which this difference can be measured are discussed.

A novel antiproton radial diagnostic based on octupole induced ballistic loss

We report results from a novel diagnostic that probes the outer radial profile of trapped antiproton clouds. The diagnostic allows us to determine the profile by monitoring the time history of antiproton losses that occur as an octupole field in the antiproton confinement region is increased. We show several examples of how this diagnostic helps us to understand the radial dynamics of antiprotons in normal and nested Penning–Malmberg traps. Better understanding of these dynamics may aid current attempts to trap antihydrogen atoms.

Towards trapped antihydrogen

Substantial progress has been made in the last few years in the nascent field of antihydrogen physics. The next big step forward is expected to be the trapping of the formed antihydrogen atoms using a magnetic multipole trap. ALPHA is a new international project that started to take data in 2006 at CERN’s Antiproton Decelerator facility. The primary goal of ALPHA is stable trapping of cold antihydrogen atoms to facilitate measurements of its properties. We discuss the status of the ALPHA project and the prospects for antihydrogen trapping.

Rearrangement in antihydrogen–atom scattering

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.