Flemish physicists enter race to find new Higgs particle

Summary

A group of European scientists at the Study Centre for Nuclear Energy in Mol are racing against the clock to solve one of the last remaining puzzles in physics

 
©courtesy SCK-CEN

Uncovering a dark mystery

Neutrinos are some of the most intangible particles in the universe. A group of European physicists are now hoping to catch a new, mysterious variety of these ghostly particles in a nuclear reactor in Mol, Antwerp province. If they succeed, they could shed light on one of the biggest enigmas in physics – dark matter.

Almost three years have passed since the European Centre for Nuclear Research (CERN) in Geneva announced its discovery of the Higgs boson. Now, physicists from all over the world are entering the next particle race, one that is taking them into uncharted territory.

They don’t have much choice, really, since the detection of the Higgs particle effectively filled in the last blind spot in the Standard Model – the theory that brings together all of nature’s particles and forces.

But one of the biggest puzzles in modern physics remains: dark matter. The only thing we known about it is that it exists. And that it outnumbers normal, “atomic” matter. As its name suggests, it’s impossible to “see” dark matter since it doesn’t emit any detectable radiation.

Indirect measurements are the only reason that we know that dark matter even exists. Astronomers have noticed that many stars rotate around the centre of their galaxy at a higher than normal speed. Given the total visible mass of the galaxy and its corresponding gravitational pull, it’s impossible to explain how these stars continue to be in stable orbits. That is, unless an invisible form of matter exists that keeps them there.

A good hypothesis

To solve the dark matter enigma, scientists need to know what it consists of. Since all physicists love a good hypothesis, a bevy of candidate hypotheses exists.

Some proposals err on the side of caution. For instance, the Massive Compact Halo Objects (yes, that’s MACHOs) are fairly dull, non-light-emitting chunks of ordinary matter. Others are more exotic such as the Weakly Interacting Massive Particles (indeed, WIMPs), huge particles that don’t participate in the electromagnetic interaction. 

I’m convinced that we lie in pole position to catch the sterile neutrino first

- Physicist Nick Van Remortel

Still others are completely bonkers, like the spooky SUperSYmmetrical particles (SUSYs), supposedly the exact mirror image of every normal particle.

But a new dark matter candidate recently entered the scene – the sterile neutrino. This so far theoretical particle would represent an entirely new type of neutrino next to the three existing and already well-known types – electron, muon and tau.

Neutrinos are some of the most intangible particles in the universe. It was just recently that physicists were able to prove that they indeed have a non-zero mass, but it’s so small that it’s hard to express in numbers.

Their almost ghostly-light weight means that they rarely interact with ordinary matter. For instance, if a lead wall was one light year thick, it would still only stop half the neutrinos that pass through it. In fact, we’re being struck by millions of neutrinos every second here on Earth – most of them coming from the sun.

So how do physicists dare speak of another type of neutrino, one that doesn’t even belong to the Standard Model?

“We call it ‘sterile’ because this neutrino doesn’t participate in the weak interaction, the force that mediates radioactive decay and that governs the behaviour of the three known neutrino types,” explains Dirk Ryckbosch, a physicist and neutrino expert at the University of Ghent. “The sterile neutrino is also immune to the strong nuclear force, which keeps the atomic nucleus together, and the electromagnetic force. Only gravity has a grip on it.”

The fact that the sterile neutrino is not affected by three of the four fundamental forces is critical. “Otherwise we would have seen it pop up in experiments a long time ago,” says Ryckbosch.

So how does this explain the connection with dark matter? “Because they’re only sensitive to gravity, sterile neutrinos make a good candidate,” Ryckbosch says. “In theory, they could be responsible for the mass of the other three neutrinos. Just like the Higgs boson is responsible for the generation of mass in ordinary matter.”

Mol enters the race

To prove that sterile neutrinos are indeed behind the dark matter puzzle, scientists all over the world are feverishly looking for it. Because reactors spew out massive amounts of neutrinos during nuclear fission (when a nucleus splits into multiple, smaller fragments), their experiments often take place inside nuclear reactors.

By meticulously recording the extremely rare occasions when a neutrino is stopped by a detector, physicists can count how many neutrinos of each type the reactor produces. By comparing their results with the theory, they can find out what’s missing. And if there’s not too much noise on the recording, they can identify the missing part.

A group of Flemish, British and French physicists at the Study Centre for Nuclear Energy (SCK-CEN) in Mol, Antwerp province, recently joined the neutrino race. Inside the reactor compartment of Belgian Reactor 2 (BR2), they are currently constructing the SoLid detector, which should be ready by the end of this year.

Built in 1962, BR2 is one of the oldest, but also most powerful, research reactors in the world. These days, it is mainly used for the production of medical isotopes. But the reactor will take a three-year sabbatical in the beginning of 2016, since it requires maintenance.

“During this period, the reactor will keep on producing neutrinos,” says Eric Van Walle, director-general of SCK-CEN. “It would be a pity to just let them fly away and not use them.”

Though it has only just entered  the race, the SoLid team is confident about its chances. “I’m convinced that we lie in pole position to catch the sterile neutrino first – if it exists,” says Nick Van Remortel, a particle physicist at Antwerp University who learned the tricks of the trade at CERN during the Higgs hunt. “Our experiment is designed in such a way that by the end of 2018 we’ll know for sure if the sterile neutrino exists or not. So we’ll definitely get a ‘yes’ or ‘no’, and nothing in-between.”

A perfect reactor

It’s exactly such inconclusive answers that have haunted many of the other neutrino experiments currently underway or already completed. But the SoLid project has something that the others don’t, says Antonin Vacheret, a particle physicist at Oxford University, who is leading the neutrino experiment in Mol. 

It’s not at all clear who came up with the idea of a sterile neutrino first

- Physicist Dirk Ryckbosch

“To catch enough neutrinos, we need to place our detector very close to the reactor – within 10 metres of the reactor core,” he says. “Because the core of BR2 is so compact, this is actually possible. That’s why this reactor is perfect.”

Another advantage, he continues, “is the low background radiation, because we need to make a clear distinction between what’s coming straight out of the reactor and what’s not. Last but not least, BR2 is powerful enough to deliver almost 1,000 events per day So by the end of 2018, we’ll have enough data to give a clear answer.” (An “event” is physics jargon for when a neutrino is caught by the detector).

The SoLid detector consists of more than 20,000 small, hard plastic cubes, put together in a huge cube of building blocks. Every cube is a small scintillator; this means that when struck by an incoming particle, it produces a flash of light.

Every cube is also wired to the computer, so that the physicists know exactly where a neutrino has crashed into the plastic. “This extreme segmentation gives us a very good localisation of the interactions,” says Vacheret.

Higgs mania

So let’s suppose for a second that the SoLid team realises its ambitions and that it finds the sterile neutrino in 2018. What impact would this have on the physics community? Would it be comparable to the Higgs mania we witnessed in the summer of 2012?

“Finding the sterile neutrino would definitely be a big thing,” admits Ryckbosch, who’s also involved in the project. “For the first time, we would have found something beyond the Standard Model, something that sheds light on a new kind of physics. And, of course, the fact that the sterile neutrino could explain dark matter only makes it more appealing.”

It’s not clear who would receive the credit should the sterile neutrino be proven to exist. The Higgs scenario could repeat itself, for instance, with the theoretical fathers of the particle celebrated. Or the honour could go to those who conducted the experiment and actually caught the particle.

“Actually, it’s not at all clear who came up with the idea of a sterile neutrino first, so this could well be to our advantage,” smiles Ryckbosch.

Still, instead of fantasising about the 2018 limelight, the SoLid team is currently firmly fixing its attention on assembling 20,000 plastic building blocks. “We want to start running our experiment in the beginning of 2016, when the BR2 goes into maintenance,” says Van Remortel. “This is a unique opportunity that we simply can’t miss.”

Photo: A SCK-CEN scientist in front of the SoLid detector