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.     

A scheme to produce a dense positronium plasma for an antihydrogen experiment

A 6 MeV industrial electron linac with 0.2 mA average current will be installed in December 2007 in CEA-Saclay. Equipped with a tungsten target and moderator, it is aimed at producing rates of order 10⁸ s⁻¹ slow positrons. This setup is part of a project to demonstrate the feasibility of an experiment to produce the ion for a free fall measurement of . The energy is below the neutron activation threshold. Its small size and cost could be of interest for a university laboratory or industry, and could be envisaged as a replacement source for the antihydrogen experiments at CERN.     

Off-axial plasma displacement suitable for antihydrogen production in AEgIS experiment

Antihydrogen experiments are currently based on non neutral electron, positron or antiproton plasma manipulation techniques in cylindrical Malmberg-Penning traps. An experimental study of a plasma manipulation technique based on off-axis diocotron displacement is presented. The use of the autoresonant excitation of (1, 0) diocotron mode of pure electron plasma allows a precise positioning of the plasma by moving it across the magnetic field and allows dumping such plasma in a desired angular position. The experimental procedure described here will pave the way to positron loading into an off-axial Penning trap terminated with a positronium converter target as it is proposed for the AEgIS experimental apparatus. The technique was studied over a range of confining magnetic field values and reproduces experimental conditions similar to most of the currently running antihydrogen experiments. The efficiency of the autoresonant excitation – in terms of plasma expansion rate and particle loss – is analyzed, studying the behaviour of electron plasma subjected to large off-axial displacements, showing that this method fulfills the requirements imposed by the AEgIS experiment.     

Beyond antihydrogen: testing CPT with the molecular antihydrogen ion

Measurements of Zeeman, Zeeman-hyperfine and ro-vibrational transitions in \(\bar {H}_{2}^{-}(\bar {p}e^{+}\bar {p})\) compared to \(H_{2}^{+}\) have the potential for more precise tests of CPT than can be obtained from antiprotons and antihydrogen. In particular, measurements of ro-vibrational transitions have a potential sensitivity to a difference between antiproton and proton mass three orders of magnitude higher than antihydrogen/hydrogen. Methods are outlined for precision measurements on a single \(\bar {H}_{2}^{-}\) or \({H}_{2}^{+}\) ion in a cryogenic Penning trap, with non-destructive state identification using the continuous Stern-Gerlach effect or changes in mass. \(\bar {H}_{2}^{-}\) can be produced using the \(\bar {H}^{+}+\bar {p} \rightarrow \bar {H}_{2}^{-} + e^{+}\) reaction.     

Particle detectors for the future antiproton Paul trap of the ASACUSA experiment at CERN

Two detectors which will be used to commission a superconducting radiofrequency Paul trap for antiprotons, now being constructed at CERN and MPQ, are described. One is a microwire secondary electron emission monitor which will nondestructively measure the spatial profile of a low energy (E= 10?100 keV) antiproton beam. The other is a system of electromagnetic shower counters which will detect the secondary particles emerging from the antiproton annihilations occurring in the trap.     

ASACUSA: atomic spectroscopy and collisions using slow antiprotons

The ASACUSA experiment [1] was approved last year for the Antiproton Decelerator at CERN. Its aim is to study basic atomic processes of slow antiprotons: stopping power and ionization in low-pressure gases, Coulomb capture of antiprotons and to make laser spectroscopy of antiprotonic transitions. This revised version was published online in August 2006 with corrections to the Cover Date.     

Towards antihydrogen confinement with the ALPHA antihydrogen trap

ALPHA is an international project that has recently begun experimentation at CERN’s Antiproton Decelerator (AD) facility. The primary goal of ALPHA is stable trapping of cold antihydrogen atoms with the ultimate goal of precise spectroscopic comparisons with hydrogen. We discuss the status of the ALPHA project and the prospects for antihydrogen trapping.     

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.     

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     

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     

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.     

ASACUSA MUSASHI: New progress with intense ultra slow antiproton beam

Our group “ASACUSA MUSASHI” has established an efficient way for accumulating antiprotons and extracting them as intense ultra-slow mono-energetic beams at the CERN-AD facility. This novel beam opens new frontiers for investigating a variety of physics. For realizing H? spectroscopy and the test for charge-parity-time symmetry, we have also developed the cusp trap, a combination of an anti-Helmholz superconducting coil and a multi-ring electrode trap, for trapping both antiprotons and positrons and then synthesizing antihydrogens. Recently, the cusp trap was practically used to accumulate antiprotons. The last piece for synthesizing antihydrogens in the cusp trap is the positron accumulator. We have developed a compact system to effectively accumulate positrons based on N₂ gas-buffer scheme with a specially designed high precision cylindrical multi-ring electrode trap. The recent progress of the developing work is an important milestone for upcoming antihydrogen science of ASACUSA MUSASHI.     

Trapped positrons for antihydrogen

Positrons from a 12 mCi²²Na source are slowed by a W(110) reflection moderator and then captured in a Penning trap, by damping their motion with a tuned circuit. Because of the stability of the Penning trap and the cryogenic ultra-high vacuum environment, we anticipate that positrons can be accumulated and stored indefinitely. A continuous loading rate of 0.14 e⁺/s is observed for 32 h in this initial demonstration. More than 1.6×10⁴ positrons have thus been trapped and stored at 4 K, with improvements expected. The extremely high vacuum is required for compatibility with an existing antiproton trap, which has already held more than 10⁵ antiprotons at 4 K, for producing antihydrogen at low temperatures. The extremely cold positrons in high vacuum may also prove to be useful for cooling highly stripped ions.     

Spectroscopy with relativistic antihydrogen

A technique is described for measuring the Lamb shift in antihydrogen in Fermilab experiment E862. The sensitivity of the measurement is expected to be 4.5%. This revised version was published online in August 2006 with corrections to the Cover Date.     

CPT-symmetry studies with antihydrogen

Various approaches to physics beyond the Standard Model can lead to small violations of CPT invariance. Since CPT symmetry can be measured with ultra-high precision, CPT tests offer an interesting phenomenological avenue to search for underlying physics. We discuss this reasoning in more detail, comment on the connection between CPT and Lorentz invariance, and review how CPT breaking would affect the (anti)hydrogen spectrum.     

Laser cooling of antihydrogen

We present a short review of the essential techniques of cooling free atoms by resonant laser radiation. The different contributions to the light forces are explained and their application to the problem of damping the thermal motion of free atoms is described. Due to quantum mechanical fluctuations of the light force there exists a limit temperature for a given atomic transition. Deceleration of atomic beams by the radiation pressure demands techniques to maintain the resonance condition while the Doppler shift of the decelerated atom is rapidly changing. Radiation forces may serve to compress and deflect slow atomic beams as well as to trap cold atoms. The possible use of pulsed laser radiation is discussed.     

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.     

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.