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NEWS FEATURE MAXIMILIEN BRICE/CERN

CERN’s new decelerator, ELENA, is to start slowing the down for study this year. THE RACE Competing are hunting for new in the shadow of the Large Hadron .

n a high-ceilinged hangar at CERN, six rival BY ELIZABETH GIBNEY Hangst, who leads the known as experiments are racing to understand the , says that in principle at least, his team I of one of the ’s most elusive 182 around, that feeds from the same can now do with anything others mat­erials. They sit just metres apart. In places, accelerators as the lab’s bigger and more famous do with . “For me, this period is what they are literally on top of one another: the sibling, the (LHC). I’ve worked towards for 25 years,” he says. metallic beam of one criss-crosses another enter the machine travelling close The experiments have a lot riding on them: like a shopping-centre escalator, its multi- to the speed of . 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 and antimatter could explain overhead. ing a stream of antiprotons from which experi- why anything exists at all. As far as “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 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 . and so blasted each other into oblivion. But first to discover how antimatter — matter’s rare For decades, have worked to pin that didn’t happen, and the origin of this fun- mirror image — responds to . down antiprotons, and the antihydrogen damental imbalance remains one of the biggest Doser and his competitors have little choice they can be used to build, for long mysteries in physics. but to get cosy. CERN, Europe’s -physics enough to study. The past few years have seen The CERN efforts are unlikely to crack near Geneva, Switzerland, boasts rapid advances: experimentalists can now the case any soon. Antimatter has so the ’s only source of antiprotons — par- control enough to start probing far proved maddeningly identical to matter, ticles that seem identical to in every antimatter in earnest and to perform increas- and many physicists think it remain that way except for their opposite and . ingly precise measurements of its fundamen- way, because any difference would the The lab’s is a ring, tal properties and internal structure. Jeffrey foundations of . But the six

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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 , 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 with up to using and then by slowing them using and magnetic fields (which make them 100 more particles. strong electric fields. Beams of also in a circle). The team can store antiprotons for The dozens of physicists working on the exchange with the antiprotons, cooling but more than a year, and has used the 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 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- ratio with record precision3. ter will yield a major discovery. But “if you can other. The overall process slows the anti­protons The also uses a complex method to reveal get your hands on some”, he says, “it would be to one-tenth of the . 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 . 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 wrote Each ‘shot’ of 30 million antiprotons fed to an lating microwave . Mastering the technique an that described an 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 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 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 ’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 , you have to work with this stuff.” vary enormously over a tiny , changing its trajectory bent in the opposite direction. by 1 — the strength of a car-lifting scrap­ Physicists soon realized that positrons were RACE FOR THE PRIZE yard — over just 1 millimetre. Even so, routinely produced in : 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 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 , 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 . being built). Each has its own way of working ade after that, when the teams learnt to trap Creating positrons is fairly straightfor- with antiprotons, and although some do unique the antiatoms for minutes at a stretch6. They ward. The particles are produced in certain experiments, they often compete to measure the have since measured properties such as charge types of , and can be readily same properties and independently corroborate and mass and used light to probe energy caught with electric and magnetic fields. But each other’s values (see ‘Wrangling antimatter’). levels7. On page 66, ALPHA reports its latest the higher-mass antiproton is another story. The experiments share one beam, which advance: the most precise measurement yet Antiprotons can be made by slamming pro- means that in any two-week period, just three of of anti­hydrogen’s , the tiny tons into a dense metal, but they emerge from the five experiments get beam time, each taking internal energy shifts caused by such collisions moving too fast to be held by an their turn in an 8-hour shift. A weekly coordi- between its antiproton and positron8. electromagnetic trap. nation meeting ensures that each experiment Together, the CERN experiments explore a Antimatter hunters needed a way to mas- knows when its neighbours’ magnet will be run- range of antimatter properties, any of which sively slow down, or cool, the particles. CERN’s ning, so as not to ruin sensitive measurements. could display a difference from matter. The goal

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NEWS FEATURE

Six experiments at the particle-physics lab CERN are on the hunt for subtle di erences between the properties of antimatter and matter. All sit inside the Antiproton Decelerator, which slows antiprotons down to manipulable speeds. From there, the experiments WANGLING ANTIMATTE further slow the particles and some combine them with positrons to make antihydrogen. THE ANTIMATTE THE DECELEATOS

ANTIPROTON POSITRON ANTIPROTON DECELERATOR The ’s antimatter This lower-mass particle, The 182--circumference ring uses electromagnetic elds and beams of electrons counterpart is produced the electron’s antimatter to slow incoming particles to around 10% of their initial speed over 100 seconds. in high-energy collisions counterpart, is produced and needs heavy in radioactive decays. deceleration.

ANTIHYDROGEN ANTIPROTON PRODUCTION An antiproton and a positron can be combined to Protons from the CERN make antihydrogen, opening up opportunities to accelerator complex are red study new properties. into an target to create 5 antiprotons. Antiproton Positron 3

4

2 1 Antiproton decelerator Researchers also build this exotic hybrid, in which ELENA an antiproton takes the place of an electron in a 6 Beginning later this year, helium atom. this ring will further slow antiprotons before they are delivered to experiments. Helium nucleus Antiproton Electron Existing connection Large Hadron Planned connection Collider

THE EXPEIMENTS

1 ALPHA 2 ASACUSA 3 ATRAP Started: 2005 Started: 2000 Started: 2000 Studies: Charge, and acceleration under Studies: Mass of antiproton (relative to the electron), Studies: and charge-to-mass gravity of antihydrogen. spectroscopy of antihydrogen and antiprotonic helium. ratio of antiprotons; plans to study the spectroscopy How it works: Mixes antiprotons and positrons in a How it works: Traps antiatoms then turns them into a of antihydrogen. complex electromagnetic trap to create antihydrogen, polarized beam, which can be probed with microwave How it works: Studies trapped antiprotons and which physicists probe with . or lasers. antihydrogen atoms, which it makes by mixing cold positrons with antiprotons.

Magnetic trap

4 BASE 5 AEGIS 6 GBAR Started: 2014 Started: 2015 Starting: 2017 Studies: Charge-to-mass ratio and magnetic moment Studies: Gravitational acceleration of antihydrogen Studies: Gravitational acceleration of antihydrogen of antiprotons. atoms. atoms. How it works: Stores antiprotons in a reservoir, before How it works: Observes the produced by How it works: Laser-cooled beryllium chill observing their trajectory in a trap or shuttling particles parallel beams of excited low-energy antihydrogen antihydrogen ions containing two positrons. A laser to various traps to measure di erent properties. atoms as they pass through a grating. knocks o one positron, and the antihydrogen atom free-falls under gravity.

Detector Detector

Antihydrogen Antihydrogen ion in Laser

Moiré Detector deectometer SOURCE: CERN

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FEATURE NEWS

Six experiments at the particle-physics lab CERN are on the hunt for subtle di erences between the properties of antimatter and matter. All sit inside the Antiproton Decelerator, for all of them is to keep shrinking the uncer- which slows antiprotons down to manipulable speeds. From there, the experiments tainty, says antimatter veteran Masaki Hori. He WANGLING ANTIMATTE further slow the particles and some combine them with positrons to make antihydrogen. leads the ASACUSA experiment, which uses lasers to study antiatoms in flight, free from the disruptive of traps. Last year, the team THE ANTIMATTE THE DECELEATOS made a precise measurement of the ratio of MAXIMILIEN BRICE/CERN antiproton mass to electron mass, using exotic ANTIPROTON POSITRON ANTIPROTON DECELERATOR The 182-metre-circumference ring uses electromagnetic elds and beams of electrons helium atoms in which an antiproton takes the The proton’s antimatter This lower-mass particle, 9 counterpart is produced the electron’s antimatter to slow incoming particles to around 10% of their initial speed over 100 seconds. place of an electron . Like other measurements in high-energy collisions counterpart, is produced so far, it showed no difference between matter and needs heavy in radioactive decays. deceleration. and antimatter. But each result is a more strin- gent of whether matter and antimatter really ANTIHYDROGEN are exact mirror images. ANTIPROTON PRODUCTION An antiproton and a positron can be combined to Protons from the CERN make antihydrogen, opening up opportunities to accelerator complex are red WHAT DIFFERENCE DOES IT MAKE? study new properties. into an iridium target to create 5 If the experiments were to detect any differ- antiprotons. ence between matter and antimatter, it would Antiproton 3 Positron be a radical discovery. It would mean the vio- 4 lation of a principle called charge, parity and time reversal (CPT) . According to 2 1 ANTIPROTONIC HELIUM Antiproton this principle, a mirror-image Universe that is decelerator Researchers also build this exotic hybrid, in which filled with antimatter and in which time runs ELENA an antiproton takes the place of an electron in a 6 Beginning later this year, backwards will have the same laws of physics The ELENA antiproton decelerator began tests in late 2016. helium atom. this ring will further slow as our own. CPT symmetry is the backbone of antiprotons before they are delivered to experiments. such as relativity and field acceleration under gravity. Physicists generally that sits inside the Antiproton Decelerator and Helium . Breaking it would, in a way, break phys- expect antimatter to fall just like matter. But is designed to further slow the antiprotons com- nucleus Antiproton Electron ics. In fact, only exotic theories predict that the some fringe theories predict that it has ‘nega- ing from the machine. Eventually, ELENA will Existing connection antimatter experiments will find anything at all. tive mass’ — it would be repelled by, rather than supply particles to all of the experiments, nearly Large Hadron Planned connection For this reason, the physicists at the LHC attracted to, matter. Antimatter with this prop- simultaneously. The antiprotons will be slower Collider tend to view the antimatter researchers next erty might account for the effects of by a factor of seven and arrive in sharper beams. door “with bemused attention”, says Doser, who and , the identities of which are still Because they’ll be more efficiently cooled at has been working on antimatter for 30 years. unknown. But most mainstream theorists say early stages, experiments should be able to trap “They think this stuff is fun and interesting, such a Universe would be inherently unstable. more of the particles. THE EXPEIMENTS but unlikely to lead to something new,” he says. Now that the teams can manipulate and test CERN theorist Urs Wiedemann seems to con- UP IS DOWN antimatter, Hangst says, more and more physi- 1 ALPHA 2 ASACUSA 3 ATRAP firm that. He says that the experiments’ ability Measuring antihydrogen in free fall will, as cists are becoming interested in the work. They Started: 2005 Started: 2000 Started: 2000 to manipulate antimatter is “-boggling” ever, be a question of making it cool enough. even pitch ideas for experiments and values to Studies: Charge, spectroscopy and acceleration under Studies: Mass of antiproton (relative to the electron), Studies: Magnetic moment and charge-to-mass and that such tests of theory are essential, but Even the tiniest thermal fluctuations will mask check. And the groups are looking outwards, gravity of antihydrogen. spectroscopy of antihydrogen and antiprotonic helium. ratio of antiprotons; plans to study the spectroscopy “if you ask is there a firm physics motivation the signal of a falling atom. And only neutral for ways in which their could How it works: Mixes antiprotons and positrons in a How it works: Traps antiatoms then turns them into a of antihydrogen. complex electromagnetic trap to create antihydrogen, polarized beam, which can be probed with microwave How it works: Studies trapped antiprotons and that at some accuracy something new will be particles such as antihydrogen can be used, aid other areas of research. The GBAR team, which physicists probe with lasers. radiation or lasers. antihydrogen atoms, which it makes by mixing cold discovered, I think a fair statement is, ‘No’”. because even distant sources of electromag- for example, is working on a portable trap to positrons with antiprotons. Still, the LHC has fared little better in solv- netic fields can expose charged particles to carry antiprotons to a CERN experiment called ing the antimatter mystery. Experiments dat- forces bigger than gravity. ISOLDE, where they can be used to map the ing back to the 1960s have shown that some Next year, Hangst’s group aims to use proven neutrons in unstable radioactive atoms. physical processes, such as the decay of exotic — a vertical version of its ALPHA Assuming a technical impasse doesn’t grind kaon particles into more familiar ones, have experiment — to get a definitive determination to a halt, Doser reckons that by the end tiny biases in favour of producing matter. LHC of whether antimatter falls up or down. “Obvi- of the 2020s, physicists will be adept enough experiments have been hunting more such ously I think we’ll succeed first, or I wouldn’t at handling antimatter to be able to replicate a biases, and even a raft of as-yet-undiscovered get into it,” he says. But two other experiments range of atomic-physics feats, including con- Magnetic trap particles whose behaviour in the early Universe — Doser’s AEGIS and the antimatter facil- structing antimatter atomic clocks. “I see lots could have accounted for the huge matter– ity’s newest member, GBAR — are hot on the of ideas popping up now, and that’s a sign the 4 BASE 5 AEGIS 6 GBAR antimatter imbalance that remains. There has team’s heels. Both use laser-cooling techniques field is moving forward quickly,” he says. “I Started: 2014 Started: 2015 Starting: 2017 been good reason to suspect such particles exist: to boost precision, which will enable them to hope CERN never kicks me out, because I’ve Studies: Charge-to-mass ratio and magnetic moment Studies: Gravitational acceleration of antihydrogen Studies: Gravitational acceleration of antihydrogen they were predicted by , a theory pick up subtler differences between the accel- got plans for the next 30 years.” ■ of antiprotons. atoms. atoms. that was proposed to tie up some troubling loose eration of antimatter and matter than ALPHA How it works: Stores antiprotons in a reservoir, before How it works: Observes the pattern produced by How it works: Laser-cooled beryllium ions chill ends in . But no such particles currently can. AEGIS will measure the bend Elizabeth Gibney is a senior reporter for observing their trajectory in a trap or shuttling particles parallel beams of excited low-energy antihydrogen antihydrogen ions containing two positrons. A laser to various traps to measure di erent properties. atoms as they pass through a grating. knocks o one positron, and the antihydrogen atom have turned up in eight years of searching. Now, of a horizontal beam of antihydrogen, whereas Nature in London. free-falls under gravity. the simplest, most elegant versions of supersym- GBAR will let its antiatoms free-fall for 20 cen- 1. Dirac, P. A. M. Proc. R. Soc. A 117, 610–624 (1928). metry — the ones that made the idea appealing timetres. Both aim to bring the antiatoms’ tem- 2. Baur, G. et al. Phys. Lett. B 368, 251–258 (1996). Detector Detector in the first place — have been largely ruled out. perature down to a few thousandths of a degree 3. Ulmer, S. et al. Nature 524, 196–199 (2015). “Today, the LHC is looking for hypothetical par- above absolute zero, allowing measurements of 4. Nagahma, H. et al. Nature Commun. 8, 14084 Antihydrogen (2017). Antihydrogen ion in free fall ticles, which may or may not be there, with very gravitational acceleration as sensitive as 1 part 5. Amoretti, M. et al. Nature 419, 456–459 (2002). 6. ALPHA Collaboration Nature Phys. 7, 558–564 Laser little guidance from theory. In a way, this is the in 100, and have plans to go even further. same situation we’re in,” says Doser. Later this year, GBAR will be the first to ben- (2011). 7. Ahmadi, M. et al. Nature 541, 506–510 (2017). A few teams are now jumping into the next efit from ELENA, a new 25-million-Swiss-franc 8. Ahmadi, M. et al. Nature 548, 66–69 (2017). Moiré Detector deectometer big challenge: the race to measure antimatter’s ($26-million), 30-metre-circumference ring 9. Hori, M. et al. 354, 610–614 (2016).

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