NEWS FEATURE MAXIMILIEN BRICE/CERN CERN’s new antiproton decelerator, ELENA, is set to start slowing the particles down for study this year. THE ANTIMATTER RACE Competing experiments are hunting for new physics in the shadow of the Large Hadron Collider. n a high-ceilinged hangar at CERN, six rival BY ELIZABETH GIBNEY Hangst, who leads the experiment known as experiments are racing to understand the ALPHA, says that in principle at least, his team I nature of one of the Universe’s most elusive 182 metres around, that feeds from the same can now do with antihydrogen anything others mat erials. They sit just metres apart. In places, accelerators as the lab’s bigger and more famous do with hydrogen. “For me, this period is what they are literally on top of one another: the sibling, the Large Hadron Collider (LHC). I’ve worked towards for 25 years,” he says. metallic beam of one criss-crosses another Antiprotons enter the machine travelling close The experiments have a lot riding on them: like a shopping-centre escalator, its multi- to the speed of light. As the name implies, the even a slight difference between the proper- tonne concrete support hanging ominously decelerator slows the particles down, provid- ties of matter and antimatter could explain overhead. ing a stream of antiprotons from which experi- why anything exists at all. As far as physicists “We’re constantly reminded of each other,” ments must take turns to sip. All this must be know, matter and antimatter should have been says physicist Michael Doser, who leads done carefully; upon meeting matter, the anti- created in equal amounts in the early Universe AEGIS, an experiment that is vying to be the particles vanish in a puff of energy. and so blasted each other into oblivion. But first to discover how antimatter — matter’s rare For decades, scientists have worked to pin that didn’t happen, and the origin of this fun- mirror image — responds to gravity. down antiprotons, and the antihydrogen damental imbalance remains one of the biggest Doser and his competitors have little choice atoms they can be used to build, for long mysteries in physics. but to get cosy. CERN, Europe’s particle-physics enough to study. The past few years have seen The CERN efforts are unlikely to crack laboratory near Geneva, Switzerland, boasts rapid advances: experimentalists can now the case any time soon. Antimatter has so the world’s only source of antiprotons — par- control enough antiparticles to start probing far proved maddeningly identical to matter, ticles that seem identical to protons in every antimatter in earnest and to perform increas- and many physicists think it will remain that way except for their opposite charge and spin. ingly precise measurements of its fundamen- way, because any difference would shake the The lab’s Antiproton Decelerator is a ring, tal properties and internal structure. Jeffrey foundations of modern physics. But the six 20 | NATURE | VOL 548 | 3 AUGUST© 22017017 Mac millan Publishers Li mited, part of Spri nger Nature. All ri ghts reserved. ©2017 Mac millan Publishers Li mited, part of Spri nger Nature. All ri ghts reserved. FEATURE NEWS experiments, the latest in a line of investiga- first dedicated attempt to decelerate and store But despite the close proximity, teams usually tions that began at CERN more than 30 years antimatter began in 1982, with the Low Energy find out about breakthroughs made by their ago, are attracting attention as the LHC contin- Antiproton Ring (LEAR). In 1995, the year neighbours by reading about them in a paper. ues to draw a blank in its hunt for particles that before LEAR was slated to be shut down, a “This is built on competition, and that’s good. could explain the antimatter paradox. More- team used antiprotons from the facility to pro- That motivates you,” says Hangst. over, the teams’ rapid advances in manipu- duce the first antihydrogen atoms2. Today, only one of the six experiments — lating antimatter have earned them a major LEAR’s replacement, the Antiproton BASE — directly studies the antiprotons from upgrade to the facility’s antiproton factory Decelerator, came online in 2000 with three the Antiproton Decelerator. BASE holds the — a cutting-edge decelerator that will start experiments. Similar to its predecessor, it particles in a Penning trap, a complex array of operation by the end of this year and eventu- tames antiparticles, first by focusing them electric fields (which pin particles vertically) ally enable experiments to work with up to using magnets and then by slowing them using and magnetic fields (which make them orbit 100 times more particles. strong electric fields. Beams of electrons also in a circle). The team can store antiprotons for The dozens of physicists working on the exchange heat with the antiprotons, cooling but more than a year, and has used the orbits of anti- CERN experiments know they face a tough not touching them because the particle types protons in the trap to determine the particle’s challenge. Antimatter is exasperating to work with, the competition between teams is intense and the odds of finding anything new seem low. But CERN’s antimatter wranglers are “THIS PERIOD IS WHAT I’VE WORKED motivated by the thrill of opening a new win- dow on the Universe. “These are such tour de force experiments that, no matter what answer TOWARDS FOR 25 YEARS.” you get, you can be proud that you do this,” says Hangst. There’s no guarantee that antimat- are both negatively charged and so repel each charge-to-mass ratio with record precision3. ter will yield a major discovery. But “if you can other. The overall process slows the anti protons The group also uses a complex method to reveal get your hands on some”, he says, “it would be to one-tenth of the speed of light. That is still the antiproton’s magnetic moment4 — akin to completely reprehensible not to look.” too fast to work with, so each of the six experi- its intrinsic magnetism. The measurement ments uses techniques to further slow and trap involves switching individual particles rap- THE FACT OF THE MATTER the antiprotons. idly between two separate traps and detecting The roots of antimatter physics can be traced to There is plenty of attrition along the way. changes caused by minuscule shifts in an oscil- 1928, when British physicist Paul Dirac wrote Each ‘shot’ of 30 million antiprotons fed to an lating microwave field. Mastering the technique an equation that described an electron moving experiment starts by smashing 12 trillion pro- has become a passion for collaboration leader close to the speed of light1. Dirac realized that tons into a target. By the time Hangst’s ALPHA Stefan Ulmer, a physicist at RIKEN in Wako, there had to be both a positive and a negative experiment, for example, has slowed its anti- Japan. “My entire heart is in this,” he says. solution to his equation. He later interpreted protons enough to pair them with positrons Antihydrogen, which is studied by the other this mathematical quirk as suggestive of the and create antihydrogen, just 30 of the particles experiments at CERN, comes with its own existence of an anti-electron, now called a posi- remain, the rest having escaped, been annihi- challenges. Because it has a neutral charge, it is tron, and theorized that antimatter equivalents lated or been discarded because they are too immune to electric fields, and so nearly impos- should exist for every particle. fast or in the wrong condition to study. Experi- sible to control. Experiments must exploit the Experimentalist Carl Anderson confirmed menting with such tiny numbers of antiatoms antiatoms’ weak magnetic properties, restrain- the positron’s existence in 1932, when he found is a real pain, says Hangst: “You get a whole ing the particles with a ‘magnetic bottle’. For the a particle that seemed like an electron except new attitude about all the rest of physics when bottle to work, the magnetic fields inside must that when it travelled through a magnetic field, you have to work with this stuff.” vary enormously over a tiny distance, changing its trajectory bent in the opposite direction. by 1 tesla — the strength of a car-lifting scrap- Physicists soon realized that positrons were RACE FOR THE PRIZE yard magnet — over just 1 millimetre. Even so, routinely produced in collisions: smash particles Antimatter research at CERN will eventually the antihydrogen atoms must have a tempera- together with enough energy and some of that have some competition from the Facility for ture of less than 0.5 kelvin, or they will escape. energy can turn into matter–antimatter pairs. Antiproton and Ion Research, a €1-billion The first antihydrogen atoms, created using By the 1950s, researchers had begun to (US$1.16-billion) international accelerator antiprotons on the move, lasted about 40 bil- exploit this energy-to-particle conversion to complex in Darmstadt, Germany, that will be lionths of a second. In 2002, two experiments, produce antiprotons. But it took decades to find completed around 2025. But for the moment, ATRAP and ALPHA’s predecessor ATHENA, a way to make enough of them to capture and CERN has the monopoly on producing anti- became the first to slow antiprotons enough study. One motivation was the tantalizing idea protons slow enough to study. to make significant amounts of antihydrogen, that antiprotons and positrons could be paired Today, there are five experiments running amassing many thousands of the atoms each5. to make antihydrogen, which could then be at the antimatter facility (one, GBAR, is still The major breakthrough came almost a dec- compared with the well-studied hydrogen atom.