WE ARE getting used to Big Physics. Giant telescopes stare up at the heavens, huge doughnut-shaped machines coax atomic nuclei into fusing, and mammoth accelerators smash subatomic particles together. But for a group of particle physicists who want to study neutrinos, these projects are small fry. They want a laboratory the size of the Earth.
If they get their way, they’ll soon be building an entirely new type of particle accelerator, known as a neutrino factory, that will produce vast quantities of neutrinos and fire them through the Earth at detectors on the other side of the world. It’s a machine that might help answer some of nature’s most perplexing mysteries.
Until now, physicists have mostly relied on neutrinos from natural sources. But there’s a problem with this. Neutrinos produced in the Sun and in nuclear reactors have too little energy. If we want to shed light on the puzzling differences between matter and antimatter, for instance, we need neutrinos with high energies. And although the neutrinos in cosmic rays can carry huge amounts of energy, there simply aren’t enough to satisfy physicists’ demands. Another problem is that neutrinos come in three different types: the electron neutrino and its relatives the muon and tau neutrino. Nature’s neutrinos are an ill-defined mixture of these three.
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If physicists are to find out what’s really going on, they need to make more neutrinos with higher energies and know exactly how many of each type they’ve produced. That’s where the neutrino factory comes in. “A neutrino factory changes observations that you get for free into real science,” explains Ken Peach, a particle physicist at the Rutherford Appleton Laboratory in Oxfordshire and a member of the 300-strong team designing the machine. Siting the detectors on the other side of the world is also crucial because neutrinos change their identity as they travel over long distances (see “Reluctant heroes”).
Creating a neutrino beam isn’t going to be easy, though. While protons and electrons can be produced by breaking apart everyday matter, neutrinos are harder to create on demand. The best way is to first create particles called muons, which are similar to electrons, only heavier. Unfortunately, muons aren’t part of everyday matter either – they have to be produced in a chain reaction that starts with high-energy protons.
So a neutrino factory begins by slamming an extremely high-power beam of protons into a target roughly the size of a brick. The target has to be strong enough to sustain the impact of the 4-megawatt collision, enough to make most known materials explode (see Graphic). The neutrino factory team is weighing up the options: at the moment, a jet of liquid metal such as mercury looks promising. Another idea is to have a string of solid targets that are moved into the firing line one after another as they are successively blown to pieces.
When the protons hit the target, they produce unstable particles called pions. Within an instant the pions decay into muons, which fly in all directions. This is bad news because the ultimate goal is a narrow, high-energy beam of neutrinos. So the muons have to be tamed. Although it’s never been done before, the researchers think they know how.
On their neutrino production line, the muons will first hit a target made of liquid hydrogen, where collisions with electrons will slightly reduce their energy and velocity. Once they’re moving more slowly, they’re easier to steer in the direction you want. Because muons have an electric charge, they can then be accelerated by a high voltage that increases their velocity in one direction only. Once the velocity of the muons has been boosted, they collide with some more liquid hydrogen, and so on. After repeating this step a few times, the muons lose all their sideways velocity and are travelling in a tidy, well-directed beam. But this beam still isn’t quite intense enough, so it needs to be squeezed even more using large magnets that focus the charged muons just as a glass lens focuses light.
Once the muons have been tamed and squeezed they’ll be fired into an accelerator, a large oval machine that whips them up to velocities close to the speed of light. Finally they’ll be injected into the “muon decay ring”, where magnets will guide them in their last moments of life before they decay into muon neutrinos and electron antineutrinos.
To produce an intense beam of neutrinos, the muons must decay while they’re going in a straight line. The best way to achieve this seems to be a “decay ring” shaped like a bow tie. Neutrinos would be allowed to fly off from the straight sections and head towards detectors on the other side of the Earth.
All this has to happen in less than a millionth of a second – the lifetime of a muon. Fortunately, there’s a trick for making muons live longer. “Relativity is our friend here,” says team member Giles Barr of Oxford University. In Einstein’s special theory of relativity, he showed that time flows slower close to the speed of light. This idea is often illustrated by the “twins paradox” in which one twin brother is sent off into space in a rocket accelerating closer and closer to the speed of light. As far as people on Earth can tell, time seems to pass more slowly for the high-speed twin, who ages less than his brother back home. So if you can get the muons to move fast enough in the accelerator, you’ll fool them into living hundreds of times longer than if they were at rest. And that’s long enough, says Barr.
The project to build a neutrino factory is still at a very early stage of planning, but will it ever work? While your local hardware store may look at you blankly if you asked for a proton source and a target to smash the protons into, to particle physicists this first stage of the neutrino factory would hardly be a show-stopper. Building an accelerator to get the muons up to speed also involves tried-and-tested technology, although it won’t come cheap. And we know how to dig tunnels and build detectors. “None of this stuff is trivial,” says Ed McKigney of Imperial College in London, but he reckons the biggest challenge will be getting all the muons to travel in the same direction inside the liquid hydrogen. “We simply haven’t done it before,” he explains.
McKigney and his collaborators are hoping to test their idea for taming muons soon in a separate experiment at the Rutherford Appleton lab. He’s confident they’ll succeed: “On paper everything’s OK, but until you’ve actually done the experiment no one’s going to believe that this thing works.”
However, if it all goes according to plan, within a decade we could be creating neutrinos to order, passing them through the Earth and catching them on the other side. Forget looking to the heavens to understand the Universe’s secrets. Neutrinos produced in our own back yard – and flung through the centre of the planet – might hold the key.