S PECIAL F EATURE: E INSTEIN AND THE L AST O NE H UNDRED Y EARS www.iop.org/journals/physed Antihydrogen on tap Michael Charlton Department of Physics, University of Wales Swansea, Singleton Park, Swansea SA2 8PP, UK E-mail: [email protected] Abstract Plentiful quantities of antihydrogen, the bound state system of the antiparticles the positron and the antiproton, have recently been made under very controlled conditions in experiments at the European Laboratory of Particle Physics (CERN) near Geneva. In this article I describe how that was done, and why. Preamble For a few decades now it has become apparent Positrons and antiprotons are now to physicists that fundamental asymmetries are routinely made and stored for use in hidden deep within nature. For example, in the collider experiments. 1950s it was discovered that the weak nuclear interaction (which is responsible for β-decay) The above is quoted from the Advancing Physics violates parity conservation. (Parity conservation A2 textbook [1]1. We hope that this article will means that left and right and up and down are convince you that the next few sentences in a future indistinguishable in the sense that an atomic edition should describe how they are also routinely nucleus throws off decay products up as often as manipulated and used to create atomic antimatter. down, and left as often as right [2].) Specifically, β−-decay involves the emission of an antineutrino that always spins in a left-handed corkscrew It’s a challenging experiment—so why + bother? fashion. Conversely, β -decay only produces neutrinos that are right-handed. However, the Bringing together the two antiparticles, the defective parity ‘mirror’ can be mostly repaired positron and the antiproton, to form antihydrogen by adding so-called charge conjugation, which, is a formidable experimental challenge and one loosely speaking, means that interactions are that was not undertaken lightly. For a start, the unaffected when every particle is substituted by antiparticles annihilate readily on contact with its antiparticle. For a while it was believed that matter, so they must be stored for use in high the laws of nature would obey the combination vacuum conditions. Moreover, both species are + of parity reversal and charge conjugation. But produced (from β -decay for positrons and as a by the mid-1960s this was found to be untrue result of high energy proton–proton collisions for for a small class of reactions involving unusual, antiprotons) with kinetic energies much too high to fleeting particles called K-mesons. Since then it allow them to become bound. Thus, sophisticated has been assumed, and there is some experimental techniques and apparatus to cool and handle them evidence to support this, that the small blemish in must be developed. So, we must provide an answer the combined charge conjugation/parity reversal to the question: “why is it so important to pursue ‘mirror’ can be corrected by the application of this difficult goal”. time-reversal. 1 This is from a UK course with this book aimed at 17–18 However, this sort of three-way switch year-olds. involving charge, parity and time differs from 0031-9120/05/030229+09$30.00 © 2005 IOP Publishing Ltd P HYSICS E DUCATION 40 (3) 229 M Charlton the three discrete symmetries, or any two-way was not. Indeed it has turned out that gravity combination of them. The charge/parity/time still eludes a satisfactory unified description (with combination exists as a theorem that can be proved electromagnetism and the weak and strong nuclear using the basic postulates of quantum field theory. forces). Any new test of gravity (usually referred Such theories are the cornerstone of our current to as tests of the Weak Equivalence Principle) understanding of the universe, but are widely can offer new information and potential pointers recognized as being incomplete. So testing this towards a consistent theory of quantum gravity. unique three-way switch is going to the heart of Currently we do not have a theory that unites our understanding of nature. quantum mechanics with gravitation. Like an Just why this is so pertinent is as follows. Our unfinished story, at the moment we have a quantum current picture of the beginning of the universe theory that works on a microscopic level (e.g. involves the so-called Big Bang, which is thought particles, atoms and molecules) and a classical to have been an energetic event that created theory that works on the macroscopic scale of the equal amounts of matter and antimatter. Why universe. then did they not all annihilate one another and So where does antihydrogen fit in? Well, leave a universe devoid of matter—a universe to date, we have absolutely no information on with no stars or planets, and no prospects for the gravitational interaction of antimatter. If we life? Searches for large amounts of remnant invoke our earlier three-way switch of charge, antimatter (e.g. via signatures of galaxy–anti- parity and time reversal, all we can say is that an galaxy annihilation) in the universe, have failed antiatom will fall to an anti-Earth with the same to find any trace. Though investigations continue, acceleration as an atom falls to Earth! A moment’s with ever increasing sensitivity, we can currently thought will show the logic of this: our three-way say that our universe seems to be matter dominant, switch not only throws the falling particle, but also in other words asymmetric. The other fact to add the object it is falling towards. (It becomes an to this potent brew is that the amount of asymmetry anti-Earth!) Thus, our notions of symmetry can we can currently identify via numerous studies of say nothing about the antiparticle falling towards fleeting and rare particles isn’t enough to explain Earth. Gravitational experiments on individual (or the existence of the material universe. even small collections of) antihydrogen atoms will So the bottom line is this: we don’t really not be easy, but the potential pay-off is high. It is understand the evolution of the universe and why an experimental area crying out for investigation. there is anything much out there, or down here, at We will return to this in our concluding section all. This makes testing the symmetries of nature too. of paramount importance. We don’t have anything much to go on from theory (thought there is a The experiment resurgent interest in this area), so, experimentally, In 2002, two groups working at CERN, which we have to look where and when we can. The has unique facilities for producing low energy creation of cold antihydrogen in amounts suitable antiprotons, announced that they succeeded in for study has opened a new door on symmetry, producing cold antihydrogen [3,4]. The author is hopefully one that will be amenable to precision a member of one of these groups, the ATHENA laser spectroscopic comparisons with the spectral collaboration, and as such the remainder of this lines of hydrogen. We will discuss this further in article will be mainly based upon the methodology the final section of the article. and findings of that team. A detailed review of the The other major area where modern physics achievements of the ATRAP collaboration can be needs input from experiment is gravity. In 2005, found elsewhere [5]. the International Year of Physics, or Einstein Year as it is known in the UK, it is appropriate to recall that Einstein spent many of the latter years of his Antiprotons life searching for a unified theory of the forces of What does CERN have to offer that can’t be found nature based upon his own success in describing elsewhere? Accelerator physicists at CERN have gravitation. We now know that, although the always been proud of the diverse capabilities of the unification ambition was sound, the methodology machines they have built and the techniques they 230 P HYSICS E DUCATION May 2005 Antihydrogen on tap have pioneered in order to run these instruments kinetic energy below 5 keV) are maximized. The up to and beyond design specifications. One ATHENA trap is a so-called Penning trap and important difference between CERN and all other utilizes a strong magnetic field (3 T) along the particle physics laboratories is that, early on, they axis of the instrument to radially confine the realized the physics benefits that would accrue antiprotons, and an electric field formed by the from decelerating antiprotons so that experiments voltages applied to a set of cylindrical metal could be performed at low energies, with the electrodes to provide axial confinement. A antiprotons stopped in a small volume. schematic illustration is given in figure 1(a). Why is deceleration necessary? In order to The antiprotons in the burst with kinetic form antiprotons in the first place one must supply energies less than 5 keV (by now we have energy in the form of energetic bursts of protons unfortunately lost most of them, with just 104 (about 1013 per burst at kinetic energies over remaining) are reflected by the end electrode of 20 GeV), and collide these with a fixed target— the trap (figure 1(b)), and before they can return also a source of protons, but now stationary. It is to the foil the latter is rapidly raised to 5 kV and another application of the famous equation E the antiprotons are captured (figure 1(−c)). If this mc2; the more ‘E’ you put in the more ‘m’ you can= were all that was going on then the antiprotons create. The antiproton production reaction goes would pass back and forth along the length of the something like: catching trap for long periods.