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.     

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.     

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.     

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.     

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.     

Antihydrogen production and precision experiments. The ATHENA collaboration

The study of CPT invariance with the highest achievable precision in all particle sectors is of fundamental importance for physics. Equally important is the question of the gravitational acceleration of antimatter. In recent years, impressive progress has been achieved at the Low Energy Antiproton Ring (LEAR) at CERN in capturing antiprotons in specially designed Penning traps, in cooling them to energies of a few milli-electron volts, and in storing them for hours in a small volume of space. Positrons have been accumulated in large numbers in similar traps, and low energy positron or positronium beams have been generated. Finally, steady progress has been made in trapping and cooling neutral atoms. Thus the ingredients to form antihydrogen at rest are at hand. We propose to investigate the different methods to form antihydrogen at low energy, and to utilize the best of these methods to capture a number of antihydrogen atoms sufficient for spectroscopic studies in a magnetostatic trap. Once antihydrogen atoms have been captured at low energy, spectroscopic methods can be applied to interrogate their atomic structure with extremely high precision and compare it to its normal matter counterpart, the hydrogen atom. Especially the 1S-2S transition, with a lifetime of the excited state of 122 ms and thereby a natural linewidth of 5 parts in 10¹⁶, offers in principle the possibility to directly compare matter and antimatter properties at a level of 1 part in 10¹⁸. Additionally, comparison of the gravitational masses of hydrogen and antihydrogen, using either ballistic or spectroscopic methods, can provide direct experimental tests of the Weak Equivalence Principle for antimatter at a high precision. This revised version was published online in August 2006 with corrections to the Cover Date.     

The AEgIS antihydrogen gravity experiment

The experimental program of the AEgIS experiment at CERN’s AD complex aims to perform the first measurement of the gravitational interaction of antimatter, initially to a precision of about 1%, to ascertain the veracity of Einstein’s Weak Equivalence Principle for antimatter. As gravity is very much weaker than electromagnetic forces, such an experiment can only be done using neutral antimatter. The antihydrogen atoms also need to be very cold for the effects of gravity to be visible above the noise of thermal motion. This makes the experiment very challenging and has necessitated the introduction of several new techniques into the experimental field of antihydrogen studies, such as pulsed formation of antihydrogen via 3-body recombination with excited state positronium and the subsequent acceleration of the formed antihydrogen using electric gradients (Stark acceleration). The gravity measurement itself will be performed using a classical Moire deflectometer. Here we report on the present state of the experiment and the prospects for the near future.     

Adiabatic cooling of antiprotons in a Penning trap

An antiproton cloud cooled at 4.2 K in a Penning trap can be further cooled by adiabatic reduction of the trap magnetic and electric fields. It will be shown that the temperature can be reduced by two orders of magnitude. This cooling method may be useful to obtain ultra-low energy antiprotons for the measurement of their gravitational properties and the production of ultra-low energy antihydrogen atoms.     

Antimatter gravity studies with interferometry

There has never been a direct measurement of the gravitational force on antimatter. This paper describes a possible measurement of this force by measuring the phase shift of neutral antimatter in a transmission-grating interferometer caused by the Earth’s gravitational field. This experiment avoids the severe problem of shielding stray electromagnetic fields necessary for making a gravity measurement with charged particles, and also avoids the need to trap neutral particles. The neutral antimatter for this experiment could be either antihydrogen, positronium, or antineutrons. This revised version was published online in August 2006 with corrections to the Cover Date.     

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.     

Quantum phenomena in gravitational field

The subjects presented here are very different. Their common feature is that they all involve quantum phenomena in a gravitational field: gravitational quantum states of ultracold antihydrogen above a material surface and measuring a gravitational interaction of antihydrogen in AEGIS, a quantum trampoline for ultracold atoms, and a hypothesis on naturally occurring gravitational quantum states, an Eötvös-type experiment with cold neutrons and others. Considering them together, however, we could learn that they have many common points both in physics and in methodology.     

A Proposal to Measure Antimatter Gravity Using Ultracold Antihydrogen Atoms

The gravitational acceleration of antimatter has never been measured directly. Antihydrogen atoms, being both stable and neutral, are an ideal system for investigating antimatter gravity. Ultralow temperatures in the 10–100 K range are desirable for practical experiments. It is proposed to cool positive antihydrogen ions using laser-cooled ordinary ions. Ultracold neutral antihydrogen atoms might then be obtained by photodetachment. The gravitational acceleration can readily be determined from the time-of-flight between the photodetachment laser pulse and an annihilation detector.     

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     

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.     

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.     

ALPHA: antihydrogen and fundamental physics

Detailed comparisons of antihydrogen with hydrogen promise to be a fruitful test bed of fundamental symmetries such as the CPT theorem for quantum field theory or studies of gravitational influence on antimatter. With a string of recent successes, starting with the first trapped antihydrogen and recently resulting in the first measurement of a quantum transition in anti-hydrogen, the ALPHA collaboration is well on its way to perform such precision comparisons. We will discuss the key innovative steps that have made these results possible and in particular focus on the detailed work on positron and antiproton preparation to achieve antihydrogen cold enough to trap as well as the unique features of the ALPHA apparatus that has allowed the first quantum transitions in anti-hydrogen to be measured with only a single trapped antihydrogen atom per experiment. We will also look at how ALPHA plans to step from here towards more precise comparisons of matter and antimatter.     

GBAR

The GBAR project aims to perform the first test of the Equivalence Principle with antimatter by measuring the free fall of ultra-cold antihydrogen atoms. The objective is to measure the gravitational acceleration to better than a percent in a first stage, with a long term perspective to reach a much higher precision using gravitational quantum states of antihydrogen. The production of ~20 μK atoms proceeds via sympathetic cooling of $\mathrm{\overline{H}^+}$ ions by Be^?+? ions. $\mathrm{\overline{H}^+}$ ions are produced via a two-step process, involving the interaction of bursts of 10⁷ slow antiprotons from the AD (or ELENA upgrade) at CERN with a dense positronium cloud. In order to produce enough positronium, it is necessary to realize an intense source of slow positrons, a few 10⁸ per second. This is done with a small electron linear accelerator. A few 10¹⁰ positrons are accumulated every cycle in a Penning–Malmberg trap before they are ejected onto a positron-to-positronium converter. The overall scheme of the experiment is described and the status of the installation of the prototype positron source at Saclay is shown. The accumulation scheme of positrons is given, and positronium formation results are presented. The estimated performance and efficiency of the various steps of the experiment are given.     

Storage of an electron plasma in a sextupole radial antihydrogen trap

This work reports for the first time experimental data obtained with electrons stored in a Penning–Malmberg trap surrounded by a sextupole radial magnetic field. This trap geometry is one of the candidates for trapping antihydrogen atoms in the place where they are produced starting from cold antiprotons and positrons or positronium. The measurements show that electron plasmas with parameters matching the range used for positrons and electrons in the antihydrogen experiments (number of particles ranging from few 10⁶ up to several 10⁷ and densities of the order of 10⁸–10⁹ cm⁻³, radius of the order of 1–2 mm) can be transported with 100% efficiency in a trap region that simultaneously confines completely the charged particles and the neutral antihydrogen in the radial plane. Inside this trap plasma storage times of the order of several tens of seconds up to some hundreds of seconds are measured. The plasma storage times are consistent with those needed for antihydrogen production; however the increase of the plasma temperature due to the expansion is not negligible; the consequences of this effect on the antihydrogen trapping are outlined.     

Measurement of the antiproton-nucleus annihilation cross-section at very low energies

The ASACUSA collaboration at the Antiproton Decelerator at CERN is planning to measure the hyperfine splitting of the ground state of antihydrogen using an atomic beam line. This will be a measurement of the antiproton magnetic moment, and also a test of the CPT invariance. The planned experimental method and setup, including the radiofrequency resonance cavity, are described, and results of Monte Carlo simulations are shown. These simulations predict that the antihydrogen ground-state hyperfine splitting can be determined with a relative precision of ～10^???7.