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.     

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     

AEGIS at CERN: Measuring Antihydrogen Fall

The main goal of the AEGIS experiment at the CERN Antiproton Decelerator is testing fundamental laws such as the weak equivalence principle (WEP) and the CPT symmetry. In the first phase of AEGIS, a beam of antihydrogen will be formed whose fall in the gravitational field is measured in a Moirè deflectometer; this will constitute the first test of the WEP with antimatter.     

Extremely cold antiprotons for antihydrogen production

The possibility to produce, trap and study antihydrogen atoms rests upon the recent availability of extremely cold antiprotons in a Penning trap. Over the last five years, our TRAP Collaboration has slowed, cooled and stored antiprotons at energies 10¹⁰ lower than was previously possible. The storage time exceeds 3.4 months despite the extremely low energy, which corresponds to 4.2 K in temperature units. The first example of measurements which become possible with extremely cold antiprotons is a comparison of the antiproton inertial masses which shows they are the same to a fractional accuracy of 4×10⁻⁸. (This is 1000 times more accurate than previous comparisons and large additional increases in accuracy are anticipated.) To increase the number of trapped antiprotons available for antihydrogen production, we have demonstrated that we can accumulate or “stack” antiprotons cooled from successive pulsed injections into our trap.     

反氢和反原子

自从狄立预言反粒子的存在后，虽然人们已经找到了几乎每个粒子的反粒子，但几代物理学家苦苦寻找着由反粒子组成的以原子，１９９６年１月ＣＥＲＮ宣布制成反氢原子，打开了通向反物质世界的大门子，引起轰动，文章叙述这一重大发现的物理背景，报道了上述发现，并展望由此开辟的崭新领域。     

Possible antihydrogen formation in antiproton-positronium reactions: Where we've been and where we're going

We present a discussion of the development of a programme whose aim is to synthesise antihydrogen from antiproton-positronium reactions for its eventual use in precise spectroscopic comparisons with hydrogen. We describe how cold antiprotons and large bursts of positronium atoms must be used and present an estimate of the reaction rate and show, in principle, how the antihydrogen can be detected. The implications of using the proposed Antiproton Decelerator (AD) machine, rather than LEAR, for this work are explored. This revised version was published online in August 2006 with corrections to the Cover Date.     

Capture and cooling of antiprotons

Approximately one million antiprotons have been captured at a few keV in a half meter long cylindrical Penning trap from a single, fast extracted pulse of antiprotons at the Low Energy Antiproton Ring (LEAR) at CERN. By electron cooling more than 65% were collected in a harmonic Penning trap. Here they can be confined for more than three hours. This is one of the first important steps towards the synthesis of antihydrogen at rest for tests of CPT. This revised version was published online in August 2006 with corrections to the Cover Date.     

AD performance and its extension towards ELENA

The CERN’s Antiproton Decelerator (AD) is devoted to special experiments with low energy antiprotons. A main topic is the antihydrogen production with the present aim to produce these antimatter atoms with such low energy that they can be trapped in a magnetic gradient field. First very convincing results have been published recently by ALPHA. Still, it appears to be cumbersome, time consuming and ineffective when collecting the needed large numbers and high densities of antiproton clouds with the present AD. Both the effectiveness and the availability for additional experiments at this unique facility would drastically increase, if the antiproton beam of presently 5 MeV kinetic energy would be reduced by an additional decelerator to something like 100 keV. Such a facility ”ELENA”, as an abbreviation for Extra Low ENergy Antiproton Ring and first discussed in 1982 for LEAR, was freshly proposed with a substantial new design and revised layout and is presently under consideration. ELENA will increase the number of useful antiprotons by up to two orders of magnitude and will allow to serve up to four experiments in parallel.     

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.     

Control of Non-Neutral Plasmas and Generation of Ultra-Slow Antiproton Beam

The Antiproton Decelerator (AD) devoted primarily to atomic physics experiments has been stably operated since 2000. Until now, three proposals have been approved, two of which are on the production and spectroscopy of antihydrogen, and the third one is on atomic collisions and precision spectroscopy of antiprotonic atoms, ASACUSA collaboration. One of the unique features of the ASACUSA collaboration is to develop intense slow and ultra slow antiproton beams of high quality, which will open a new multidisciplinary field involving atomic physics, nuclear physics and elementary particle physics. The ultra slow antiprotons will be prepared by combining the AD (down to 5.3 MeV), the RFQD (Radio Frequency Quadrupole Decelerator) (down to several tens keV), and an electron cooling device which will be called “MUSASHI” (Monoenergetic Ultra Slow Antiproton Source for High-precision Investigations) (down to several eV). MUSASHI produces the eV antiproton beam through an electron cooling of trapped antiprotons and a radial compression followed by an extraction through a transport beam line. The transport beam line is specially designed so that the pressure at the trap region can be maintained more than six orders of magnitude better than the collision region and at the same time the transport efficiency is kept at almost 100%. The ultra slow antiproton beam allows for the first time to study collision dynamics such as antiprotonic atom formation and ionization processes under single collision conditions, and also to study spectroscopic nature of various metastable antiprotonic atoms such as p, He⁺, He⁺⁺, etc. Metastable p are particularly interesting because they allow to make high precision spectroscopy of two body exotic atoms. Production and spectroscopy of antiprotonic atoms consisting of unstable exotic nuclei will also be discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Extremely cold positrons for antihydrogen production

The storage of extremely cold (4 K) antiprotons in a Penning trap is an important step toward the creation and study of cold antihydrogen. The other required ingredient, the largest possible number of comparably cold positrons, is still lacking. These would be recombined in a high vacuum with the trapped antiprotons, already stored at a pressure below 5×10⁻¹⁷ Torr, thereby avoiding annihilation of the antihydrogen atoms before they can be used in high accuracy measurements or in controlled collision experiments. In an exploratory experiment, positrons from a 18 mCi²²Na source follow fringing field lines of a 6 T superconducting solenoid through tiny apertures in the electrodes of a Penning trap to strike a tungsten (reflection) moderator. The positron beam is chopped mechanically and a lock-in directly detects a positron current of 2.5×10⁶e⁺/s on the moderator. The use of a moderator, unlike an earlier experiment in which < 100 positrons were confined in vacuum, should greatly increase the number of positrons trapped in high vacuum.     

AD: low-energy antiproton production at CERN

Well into its 10th year of running for physics, the Antiproton Decelerator (AD) supplies antiprotons to 4 different physics collaborations: ATRAP, ALPHA, ASACUSA and ACE. Antiprotons are injected at 3.5 GeV/c, then cooled and decelerated before being extracted at 100, 300 or 500 MeV/c in single or multi-batch mode. Here we will discuss the challenges of keeping reliability and performance at adequate levels, prospects of future physics scheduling and also possible additional experiments and machine improvements.     

Metastable antiprotonic helium atoms: Past,present and future

This review briefly summarises the experimental studies carried out on metastable antiprotonic helium over the last few years, and points out a possible way ahead, assuming that very low-energy antiprotons continue to be available. Very recently, an abundance of new results has been found by the Japanese-European collaboration working in these exotic atoms at LEAR, the Low Energy Antiproton Ring at CERN. Together with tremendous progress in several of the techniques needed for antihydrogen synthesis, this has provided new hope that some way will be found to continue this fruitful line of research after the closure of this excellent machine.     

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.     

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.     

Antihydrogen and fundamental symmetries

The prospects for testing CPT invariance and the weak equivalence principle (WEP) for antimatter with spectroscopic measurements on antihydrogen are discussed. The potential precisions of these tests are compared with those from other measurements. “If there is negative electricity, why not negative gold, as yellow...as our own, with the same boiling point and identical spectrallines...” A. Schuster [1], 1898     

ATRAP antihydrogen experiments and update

Antihydrogen (Hbar) was first produced at CERN in 1995. Over the past decade our ATRAP collaboration has made massive progress toward our goal of producing large numbers of cold Hbar atoms that will be captured in a magnetic gradient trap for precise comparison between the atomic spectra of matter and antimatter. The AD at CERN provides bunches of 3 × 10⁷ low energy antiprotons approximately every 90 s. We capture and cool to 4 K, 0.1% of these in a cryogenic Penning trap. By stacking many bunches we are able to do experiments with 3 × 10⁵ Antiprotons. Approximately 100 positrons (e+)/s from a 22 Na radioactive source are captured and cooled in the trap, with 5 × 10⁶ available experiments. We have developed two ways to make Hbar from these cold ingredients, namely three-body collisions, and two-stage Rydberg charge exchange. We have also developed techniques to measure the excited-state distribution of the Hbar and measure their velocity. A new apparatus is being used this year that includes a e+ accumulator built at York University providing many more e+. The new antiproton annihilation detector provides spatial information of annihilations. Windows allow lasers to enter the trap for spectroscopic measurements and for laser cooling of the Hbar. Possibly the most exciting inclusion in this new apparatus is the inclusion of a neutral particle trap which may, for the first time, capture the Hbar and lead to the first atomic spectrum from antimatter.     

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.     

Extraction of ultra-low energy antiprotons from Penning traps

Approximately one million antiprotons have been captured in a large Penning trap at the Low Energy Antiproton Ring at CERN. These antiprotons have subsequently been cooled by electron cooling. This has opened new discussions of the possible use of ultra-low energy antiprotons for nuclear, atomic, and gravitational physics. For most of these experiments, it will be necessary to extract the antiprotons from the trap in a continuous or bunched beam, allowing the timing structure to be used for post-acceleration schemes or as a time tag for the subsequent measurements. We have designed an extraction scheme to accomplish this and have tested portions of it using a smaller Penning trap loaded with protons. First results in generating a time-correlated beam of particles from a Penning trap are presented.     

Designing an RFQ ring trap for antiproton confinement and trapping and laser cooling Mg+ ions

Summary In the true sense of Bernie's approach to physics, an old idea (the Radiofrequency Quadrupole Trap) was taken and upgraded to be applied to a new area of physics (formation of an antihydrogen beam). During the course of the development work, new applications were identified and immediately put to use. While the collaboration is still pursuing its original goal, formation of antihydrogen by Bernie's reaction, collisions between positronium atoms and antiprotons, many new experiments have been found possible and are actively pursued. These include atomic and nuclear physics studies with ultra-low energy antiprotons ejected from the initial catching trap of the antihydrogen project, and the formation and study of exotic atoms and molecules in ultra-thin targets using trapped antiprotons (an extension of the work by the PS205 collaboration at CERN described elsewhere in this volume [10]). A large physics community has grown around these ideas and may even succeed in obtaining its very own antiproton source, which is a true sign of the recognition of the importance of this field. The work in this area will hopefully continue for many years to come, but we will truly miss the motivation and drive of our friend and colleague, Bernie Deutch.