by Harry Cliff
While interesting in its own right, the real significance of Super-K’s discovery was that neutrinos could only pull off this quantum mechanical Jekyll and Hyde act if they had mass. Until that point, no experiment had seen any direct evidence that neutrinos had anything but fantastically tiny masses, so it had not unreasonably been assumed that they had no mass at all. In fact, neutrinos do have masses, they’re just so stupendously minuscule that we haven’t been able to measure them. All we can say is that they must have masses smaller than 0.5 electron volts, more than a million times lighter than an electron. The question is, why are their masses millions of times smaller than the other matter particles?
The most popular answer to this question is known as the “seesaw mechanism,” which, as the name kind of implies, counterbalances the lightness of the ordinary neutrinos by proposing the existence of three additional extremely heavy neutrinos. You can picture these heavyweight neutrinos as being like rugby prop forwards sitting at one end of a theoretical seesaw, while the ordinary light neutrinos are like ballerinas stranded high in the sky at the other end.
Now just in case you’re scratching your head at this point, you are correct in thinking that I haven’t actually explained why neutrinos have such tiny masses apart from some vague visual analogies to do with seesaws. Unfortunately, the full explanation involves too much hard math to go into here. But the important point is that if these heavyweight neutrinos exist—and to be absolutely clear, so far we have zero evidence that they do—then they could have been responsible for making matter during the big bang.
To explain the incredible lightness of ordinary neutrinos, these heavy neutrinos would have to be absolute whoppers, with masses of between a billion and a thousand trillion protons (that’s between 109 GeV and 1015 GeV), far, far heavier than any particle we have ever seen to date and at least one hundred thousand times higher in energy than the LHC can reach. However, although it’s impossible to create them in colliders today due to their enormous masses, they could have been made in the furious conditions of the very, very early universe when, as we’ve seen, temperatures were unimaginably high.
It is these heavy neutrinos that may have been responsible for the imbalance of matter over antimatter in the universe. As the universe expanded and cooled down, there wouldn’t have been enough energy left to make more of them and they would have started to decay into Higgs bosons and ordinary leptons (that’s the three light neutrinos and the electron, the muon, and the tau).
If these heavy neutrinos break charge-parity symmetry, then it is possible for them to decay into antileptons more often than leptons, leading to a universe with more antiparticles than particles. Now this may not sound particularly helpful, as surely we want a universe with more particles than antiparticles, and not the reverse? This is where our old friend the sphaleron rides in to save the day.
Remember we said that sphalerons can convert antiparticles into particles? Well, a bit later in the universe’s history (although we’re still talking within the first trillionth of a second) sphalerons would have converted all these excess antileptons into ordinary matter particles, including quarks and electrons, giving rise to the basic ingredients that would go on to form everything we see around us today.
You’d be forgiven for thinking that this recipe for matter is speculation built on speculation, and you’d be right. If these heavy neutrinos exist, then they’re way out of reach of any particle accelerator we can imagine today. So how can we possibly test this idea?
This is where Super-K comes in. One of the key ingredients of this recipe is that these heavy neutrinos broke matter-antimatter symmetry when they decayed just after the big bang. Given that we can’t get our hands on these heavy neutrinos, there’s no way to test that directly, but if we can catch ordinary neutrinos breaking matter-antimatter symmetry then that would be a vital clue that their heavy cousins can do it too.
The Tokai to Kamioka (T2K) experiment begins 295 kilometers to the east of the vast Super-K neutrino observatory, at Tokai on the Pacific coast. Here, a powerful particle accelerator slams protons into a graphite target, creating a shower of particles. Some of these particles decay into neutrinos, which tunnel directly through the Earth toward the Super-K observatory on the other side of the country. Thanks to their ghostly properties, the neutrinos are completely unbothered by 295 kilometers of rock, with only a small fraction being absorbed as they travel.
Critically, T2K has the ability to produce beams of either muon neutrinos or their antimatter versions, muon antineutrinos. As they travel through the Earth toward Super-K, the neutrinos start to morph into other flavors, and by the time they arrive at the observatory a certain fraction of them have converted into electron neutrinos. A tiny fraction of these electron neutrinos smack into a water molecule in Super-K’s huge tank, producing electrons that emit a flash of light as they whizz through the liquid. By counting the number of electrons created, T2K can measure the probability that a muon neutrino morphs into an electron neutrino. By switching to a beam of muon antineutrinos, it can also measure how often a muon antineutrino turns into an electron antineutrino.
If neutrinos respect matter-antimatter symmetry, then T2K should measure equal probabilities for muon neutrinos to turn into electron neutrinos as for muon antineutrinos to turn into electron antineutrinos. But in April 2020 the team announced that they had seen compelling evidence that neutrinos were more likely to switch flavor than their antimatter versions. What’s more, the numbers they saw suggest that neutrinos don’t just break matter-antimatter symmetry a little bit, they break it by the maximum amount possible.
This result is really quite exciting. If neutrinos actually do break matter-antimatter symmetry it suggests that their heavy cousins could have done the same thing at the start of the universe, sowing the seeds for creation of ordinary matter. That said, the results aren’t yet precise enough to be absolutely certain of the effect. Future upgrades to T2K and giant new neutrino experiments in Japan and the United States should be able to clear up the picture in the coming years.
However, even if T2K’s results are confirmed, we’ll still only have suggestive evidence that heavy neutrinos were responsible for making matter during the big bang. The heavy neutrinos themselves will most likely be forever beyond our reach. Here we run into a problem that will become increasingly frustrating as we push closer and closer to the big bang. The energies that were present at the dawn of time were far, far higher than we could achieve even in particle physicists’ most feverish dreams, meaning that these theories are ultimately only loosely tethered to the firm ground of experimental observations. This is one of the main attractions of the previous recipe for making matter when the Higgs field switched on—it can actually be tested in experiments either today or in the near future. On the other hand, if heavy neutrinos were responsible for making matter during the big bang, we may never know for certain.
But we shouldn’t be too gloomy. If the history of science teaches us anything it is that many of the biggest breakthroughs begin with an unexpected experimental result that radically challenges accepted principles or assumptions. Hardly anyone expected nature to break mirror symmetry before Wu’s experiment showed that it did. The fact that quarks can break matter-antimatter symmetry too was a bolt from the blue. An experiment with the potential to throw up just such a result is currently going on at CERN. It is the stuff of science fiction, a place where a small team of scientists make, store, and study the most volatile substance in the universe.
THE ANTIMATTER FACTORY
On a hot late-summer morning I found myself waiting outside a large but otherwise nondescript metal warehouse deep in the sprawling CERN site. I have always assumed that CERN’s buildings were numbered by someone with a sense of humor, given that they seem to be scattered more or less at random across the 500-acre laboratory, which can make finding an unfamiliar building an interesting ch
allenge. Luckily, locating Building 393 turned out to be a fair bit easier than I’d feared thanks to a giant blue sign bolted to one of its corrugated walls, proclaiming “ANTIMATTER FACTORY.”
As a result, I was fifteen minutes early to meet Jeffrey Hangst, spokesperson of the ALPHA experiment, and so did my best to look innocent as I loitered by the security door. After all, if Hollywood has taught us anything about particle physics it’s that unscrupulous individuals will do almost anything to get their hands on some antimatter.
Bang on time, Jeffrey came striding down the road toward me. Tall, lithe, and sporting a black T-shirt, shades, and short gray stubble, he looked more rocker than physicist; the only giveaway was the CERN lanyard hanging around his neck. As I was about to find out, ALPHA is a pretty rock ’n’ roll experiment. Inside the Antimatter Factory we were hit by a wall of noise: humming machinery, the rhythmic chirrup of compressors, and the occasional blast of a siren as a bridge crane slid across the roof high above our heads. Growing up in a small town in Pennsylvania’s steel country, Jeffrey had been told that if he didn’t study hard and get into university he’d end up at the local steelworks, so it’s a strange quirk of fate that he goes to work in a factory each day, albeit one of a rather different kind.
Here at the Antimatter Factory, Jeffrey and his fifty collaborators on ALPHA manufacture and study atoms of antihydrogen, the simplest atom of antimatter. This is no mean feat. Since there aren’t any handy reserves of antimatter in our cosmic neighborhood, ALPHA has to create its anti-atoms from scratch by carefully mixing positively charged antielectrons with negatively charged antiprotons. And even once you’ve got some, holding on to antihydrogen long enough to study it is a seriously difficult task. After all, how do you contain a substance that annihilates immediately on contact with ordinary matter? When I carelessly referred to ALPHA as a “detector” Jeffrey flashed me a scornful look. “ALPHA is not a detector. Detection is just a tool for us. The real art is learning how to trap neutral anti-atoms. That’s really hard. We make antihydrogen so that when it’s born, it’s trapped and we’re the only experiment in the world that knows how to do this. So, when you call it a detector, it really gets my hackles up.”
Trapping antielectrons or antiprotons is relatively straightforward. Since they’re electrically charged, a judicious arrangement of electric and magnetic fields can keep them floating safely in the center of a vacuum vessel, but once they combine to make neutral antihydrogen, you’re in a whole different ball game. With no net electric charge, antihydrogen atoms are far, far harder to manipulate. ALPHA was the first experiment in the world to crack the problem. In late 2010, they managed to hang on to thirty-eight antihydrogen atoms for around a sixth of a second; today they can store a thousand more or less indefinitely.
As Jeffrey himself put it, when he started out at CERN a couple of decades ago this would have seemed like science fiction. In fact, it literally was. In 2008 he showed Ron Howard and Tom Hanks around the experiment while they were shooting the movie adaptation of Dan Brown’s thriller Angels and Demons, whose plot centers around a nefarious organization stealing a canister of antimatter from CERN in a dastardly plot to blow up the Vatican. In reality, if you could gather together all the antihydrogen that ALPHA has ever trapped, it wouldn’t be enough to blow up a housefly, let alone a city.*5
ALPHA isn’t in the antimatter bomb business. Its real goal is to make exquisitely precise measurements of the properties of the spectrum of antihydrogen atoms. Just like in ordinary hydrogen, the antielectron orbits the antiproton in fixed quantum energy levels, jumping from one orbit to another by absorbing or emitting photons. By measuring the frequencies of these photons and comparing against the spectrum of ordinary hydrogen, Jeffrey and his fifty collaborators test for breaks in the symmetry between matter and antimatter, which maybe, just maybe, could provide a clue to how matter came to exist in the universe.
The symmetry that relates an ordinary hydrogen atom to its antimatter version is known as “charge-parity-time (CPT) symmetry.” Charge-parity symmetry we’ve already met; it involves flipping particles into antiparticles (and vice versa) and then reflecting the universe in a mirror, so that left becomes right and right becomes left. We know that nature breaks CP symmetry in the way quarks decay and possibly also in neutrino oscillations. However, if you add in time-reversal (T) symmetry, where in effect you reverse the directions that particles are traveling in, then it is believed that the laws of nature will ultimately remain unchanged. Or to put it another way, an antimatter universe, reflected in a mirror, where time runs backward, would look completely identical to the one we live in.
CPT symmetry is so fundamental to quantum field theory that most theorists think that it must be unbreakable and that Jeffrey and his colleagues are on a hiding to nothing. However, very few people expected either parity or charge-parity symmetry to be violated before experiments showed that they are. If CPT symmetry turned out to be broken too, it would be totally revolutionary and shake quantum field theory to its foundations. Or as Jeffrey put it, “The theory guys are like, ‘Okay it’s CPT, fuck it, CPT’s good,’ but these symmetries are always good until they’re not. The argument for CPT as an immutable thing assumes that quantum field theory is the final word, and that’s incredibly arrogant. There are so many things we don’t know. I just refuse to accept when people say we know CPT is a law because again and again they’ve been wrong. I think as an experimentalist you have to filter that out and just do the best experiment you can do.”
But even putting the potential theoretical implications aside, Jeffrey is clearly in this for the love of the challenge itself. “For me it’s how could you not do this?” he said. “It’s become possible in my lifetime to make and hold on to antimatter atoms. That’s incredible! If you could have seen how this started, it was a kind of ragtag bunch of people and nobody believed we would ever make antihydrogen, nobody believed we would ever capture it if we could make it, and nobody believed if we could do all that we would ever have enough. Now I can measure a single spectroscopic line in antihydrogen in one day. It’s routine now for us.”
The unquestionable coolness of what the team at ALPHA are doing has attracted a fair amount of celebrity attention. On our way down to the factory floor to see the experiment, Jeffrey pointed proudly to a whiteboard covered with signatures of the big names who have come to see ALPHA. Alongside a signed photo of Ron Howard were the signatures of Roger Waters, David Crosby and Graham Nash, Jack White, and the members of Muse, Slayer, Metallica, the Pixies, and the Red Hot Chili Peppers. There was clearly a strong rock bias. Jeffrey himself plays guitar in a band that performs annually at CERN’s Hardronic music festival (yes that really is a thing and it’s quite the event) and he is pretty selective about whose name goes up. “Anyone you’d like to get?” I asked. He replied without hesitation: “David Gilmour from Pink Floyd.”
Down another staircase and we were standing next to the experiment itself, an anarchic-looking arrangement of cables, pipes, electronic readouts, flashing lights, metal framework, and glistening insulating foil. At its center was a gleaming stainless-steel vessel—the antihydrogen trap itself.
To make antihydrogen, you first need antiprotons, which are created when one of CERN’s large particle accelerators fires protons into a target, creating a shower of particles and antiparticles. The antiprotons that come flying out are moving way too fast and chaotically to be used to make antihydrogen and so are first corralled and slowed by a unique machine called the Antiproton Decelerator, which then feeds them into ALPHA. However, even then the antiprotons still have far too much energy and have to be “cooled” further by shooting them through sheets of aluminum foil and then mixing them with electrons. After all this the antiproton temperature has dropped from billions of degrees down to just 100 Kelvin (-173 degrees Celsius). Meanwhile, antielectrons produced by a radioactive source on the other side of the setup are spiraled
through a magnetic field to cool them down before being brought into the antihydrogen trap.
At first, the antiprotons and antielectrons are kept apart by an electric field, which is slowly adjusted to bring the two clouds of oppositely charged particles together. As they mix, neutral antihydrogen atoms are formed, which thanks to their slight magnetism, can be kept away from the walls of the trap by an extremely powerful magnetic field. Repeating this process over an eight-hour shift, the ALPHA team can now store up to one thousand antihydrogen atoms at a time, an incredible achievement that has taken two decades of painstaking design, innovation, and hard work.
Once the team has trapped a cloud of antihydrogen atoms, the final step is to measure their spectrum. Laser light is fired into the trap, and if the frequency is right, it will kick some of the antielectrons from the lowest energy levels into a higher one. If an antielectron gets promoted, another photon can then knock it out of the atom altogether, which causes the resulting antiproton to drift into the trap walls and annihilate. The particles released by the annihilation can then be detected, and by counting the number of annihilations the team can tell whether their laser is set at the right frequency to cause an antielectron to make a quantum jump.
After they first successfully trapped antihydrogen in 2010, ALPHA was completely rebuilt, and in 2016 they finally managed to see a quantum jump in antihydrogen for the first time. Now they can do the same measurement in a single day and have determined the energy of the jump to a few parts in a trillion. “I still don’t believe that it’s possible today,” Jeffrey told me with undisguised pride. “We’ve surprised ourselves a lot.” So far, the spectrum of antihydrogen is in perfect agreement with ordinary hydrogen, and amazingly the precision of ALPHA’s measurements are rapidly closing in on the measurements of ordinary hydrogen. “We’ll get close to hydrogen soon. Hydrogen has been measured to parts in ten to the fifteenth [a thousand trillion]. We’re at parts in ten to the twelfth [a trillion] now, in only two years. They’ve had two hundred years!”
