All of the news on low-energy antiprotons.
Low-energy antiproton physics is an interdisciplinary field that spans particle, nuclear, atomic and applied physics, as well as astrophysics. It confronts directly the relationship between matter and antimatter, in particular CPT symmetry, one of the foundations of the theory of particle physics. CPT is so fundamental that its violation would require a complete rewriting of particle-physics textbooks. Precision studies with antiprotons may also shed light on the question of why the universe is made almost exclusively of matter but not antimatter. Recent months have witnessed dramatic breakthroughs in the field at CERN’s Antiproton Decelerator (AD), including the trapping of antihydrogen atoms and developments towards an antihydrogen beam. Satellite and balloon experiments are searching for cosmic antimatter, the results of which could have profound implications on cosmology. Antiprotons are also being used to study the properties and structures of atoms, nuclei and hadrons, for which the start of the Facility for Antiproton and Ion Research (FAIR) in Darmstadt will usher in a new era.
Dialogue across disciplines
It was against this stimulating backdrop that LEAP 2011 – the 10th International Conference on Low Energy Antiproton Physics – took place at TRIUMF in Vancouver on 27 April – 1 May. The conference was organized and supported by the Canadian institutions involved in the ALPHA experiment at the AD (the universities of British Columbia, Calgary, Simon Fraser, York and TRIUMF), with additional support from the Canadian Institute of Nuclear Physics, and was chaired by Makoto Fujiwara of TRIUMF/
Calgary, with Mary Alberg of Seattle as co-chair. LEAP 2011 was the first of the series in North America; the conferences have traditionally been held in Europe, with the exception of Yokohama in 2003. It attracted nearly 100 participants and featured more than 60 invited plenary speakers, with an emphasis on promoting young researchers. Several review talks by senior physicists facilitated dialogue across the disciplines. In addition, a dozen posters were presented and presenters were allowed a two-minute talk to advertise their work at a plenary, a format that worked quite effectively. This report presents some of the highlights of a packed programme.
The conference began with a session on antihydrogen physics, with reports on the recent trapping of antihydrogen by the ALPHA experiment and the ASACUSA collaboration’s developments towards an antihydrogen beam, both at the AD. The two results were together voted the number one physics breakthrough for 2010 by Physics World. Key techniques that enabled ALPHA’s trapping of antihydrogen are evaporative cooling and autoresonant excitation of antiproton plasmas. The conference heard how the collaboration’s work has led to the successful confinement of antihydrogen for 1000 s. The next major goal for ALPHA is to perform microwave spectroscopy on trapped antihydrogen. ASACUSA also has plans to use microwave spectroscopy to measure ground-state hyperfine splitting with an antihydrogen beam.
The ATRAP collaboration, again at the AD, presented new results on adiabatic cooling of antiprotons, with up to 3 × 106 antiprotons cooled to 3.5 K, and described the first demonstration of centrifugal separation of antiprotons and electrons, suggesting a new method for isolating low-energy antiprotons. The team also has a scheme for improved antihydrogen production via interactions with positronium atoms, created in the interactions of excited caesium atoms with positrons. Other talks described new possibilities for antimatter gravity experiments with antihydrogen at the AD: AEGIS, already under preparation, and the proposed Gbar.
Ion traps with single-particle sensitivity are another powerful tool. A team from Heidelberg and Mainz has recently observed a single proton spin-flip, a result that paves the path for the comparison of the magnetic moments of protons and antiprotons. At TRIUMF, an ion trap system, TITAN, is being used at the ISAC facility for precision studies of radioactive nuclei.
Talks on applications and new techniques with antiprotons included the ACE experiment at the AD, which is studying the possible use of antiprotons for cancer therapy, and developments towards spin-polarized antiprotons. The session on atomic physics also covered some novel techniques that have possible applications to antihydrogen. One proposal concerns a new pulsed Sisyphus scheme for (anti)hydrogen laser cooling. Another involves using an atomic coil-gun, which can stop beams of paramagnetic species, to trap hydrogen isotopes, followed by single-photon cooling techniques. A Lyman-α laser for antihydrogen cooling is being developed at Mainz.
The positron, or anti-electron, is the other ingredient in antihydrogen atoms. A review on positron accumulation techniques was given by Clifford Surko of the University of California, San Diego – the inventor of the Surko trap now used by many of the antihydrogen experiments. Studies were reported using variations of the Surko trap by ATRAP and the University of Swansea groups. Measurement of hyperfine splitting in positronium could provide precision tests of QED. One experiment on positronium atoms at the University of Tokyo has made the first direct measurement of this splitting, employing a novel sub-THz source, while another aims at precise measurements via the Zeeman effect.
This year marks the 20th anniversary of the discovery of long-lived antiprotonic helium at KEK. Studies of such exotic atoms and fundamental symmetries are an important part of antiproton physics. ASACUSA has made recent progress on precision studies on antiprotonic helium and on microwave measurements of antiprotonic 3He atoms. Recent but still controversial results on muonic hydrogen spectroscopy at the Paul Scherrer Institute indicate a much smaller size for the proton radius than is generally accepted. Hadronic and radioactive atoms were featured in review talks at the conference, focusing on pionic and kaonic atoms, as well as on the fundamental symmetries programme at TRIUMF. The final results of the TWIST experiment at TRIUMF, a precision measurement of muon decay parameters, have greatly reduced systematic uncertainties, providing improved limits for constraining extensions to the Standard Model.
An important pillar of antiproton physics is hadron and QCD physics at “low energy”, ranging from stopped antiprotons to a beam of 15 GeV. At the lower energy end, ASACUSA is studying antiproton in-flight annihilation on nuclei. Following hints from an experiment at KEK, an experiment in a low-momentum antiproton beam at the Japan Proton Accelerator Research Complex (J-PARC) will search for a φ-meson–nucleus bound state using antiproton annihilation on nuclei. Also at J-PARC, a study of double anti-kaonic nuclear clusters in antiproton–3He annihilation has been proposed. Further into the future, the research programme for the major PANDA detector at FAIR, which is expected to start running in 2018, encompasses a breadth of physics that includes searches for exotic states and studies of double Λ hypernuclei. Back to the present, hot news from the Brookhaven National Laboratory concerned the discovery of the anti-alpha nucleus, the heaviest anti-nucleus observed.
The theory talks at the conference covered topics ranging from atomic collisions to cosmology. There were reviews on atomic collision physics with antiprotons and on interactions of antihydrogen with ordinary matter atoms. Calculations of gravitational effects on the interaction between antihydrogen and a solid surface suggest that the antiatoms would settle in long-lived quantum states, the study of which could provide a new way to measure the gravitational force on antihydrogen. Theoretical ideas based on the so-called Standard Model Extension, an effective theory that incorporates CPT and Lorentz violation, could offer the opportunity for probing Planck-scale physics as well as antimatter gravity in antihydrogen experiments. On the hadron physics side, antiproton–proton and antiproton–nucleus collisions provide ways to test theories of strangeness production, the latter offering a window onto the behaviour of strange particles in the nuclear medium that complements heavy-ion studies. In cosmology, baryon asymmetry – or the dominance of matter over antimatter – is a long-standing puzzle, as is the nature of dark matter. Could hidden antibaryons be the dark matter? Such a possibility could explain the two mysteries in one go.
LEAP 2011 featured two dedicated sessions on the universe. In the first, CERN’s John Ellis discussed the nature of dark matter and its connection to low-energy hadron physics and William Unruh, from the University of British Columbia, reported on fascinating experimental work that confirms aspects of Hawking radiation in an analogue system, confirming his own theoretical prediction from some 30 years ago. The second of the sessions focused on experimental searches for antimatter in the universe – a hot topic as the conference was held not long before the launch into space of the Alpha Magnetic Spectrometer. The latest results from the PAMELA detector, which has been in space since 2006, continue to show an anomaly in the positron flux at high energies (PAMELA’s quest for answers to cosmic questions). BESS-Polar II, the second flight of the Balloon-borne Experiment with a Superconducting Spectrometer (BESS) over Antarctica, has a new measurement of the antiproton spectrum based on 24.5 days in which 4.7 × 109 cosmic-ray events were collected, yielding a sensitivity complementary to satellite experiments. The proposed General Antiparticle Spectrometer (GAPS) would be a balloon experiment to search for anti-deuterons from dark-matter annihilations using exotic atom techniques.
Looking to the future, the construction of FAIR at Darmstadt will allow for a dedicated Facility for Low-energy Antiproton and Ion Research (FLAIR), while Fermilab has a proposal to use its Antiproton Source – the world’s most intense – for low-energy experiments once the Tevatron programme comes to an end later this year. Finally the conference returned to the AD, when the proposal for the Extra Low ENergy Antiproton ring (ELENA) was described by Walter Oelert, from the Jülich Research Centre, whose experiment at CERN observed the first antihydrogen atoms in 1996. The conference ended with his remarks on the prospects for antiproton physics. Just a few weeks after the conference, CERN Council approved the construction of ELENA, which will provide significantly enhanced opportunities for antiproton physics at CERN in the coming decade (ELENA prepares a bright future for antimatter research).
This successful conference was capped off by a social programme that included a dinner cruise in Vancouver’s spectacular English bay, and a well-attended public lecture by John Ellis at the University of British Columbia. The future of low-energy antiproton physics appears bright. The next LEAP meeting is planned for Uppsala in 2013, chaired by Tord Johansson.
• For full details of the speakers and many of the presentations, see http://leap2011.triumf.ca. The proceedings will be published in Hyperfine Interactions.