In the Ardennes hills near Givet, in eastern France, stands Chooz nuclear power station, with its twin concrete cooling towers spewing steam. Here scientists may soon catch some neutrinos and break one of the most enigmatic secrets of the universe.
Chooz is best known for its two 1,450 MW nuclear reactors, commissioned in the late 90s. They are precursors of the second-most recent generation of French reactors. But there is also an international laboratory, which pools 35 research teams from eight countries. They are hosted by the French Centre for Scientific Research (CNRS) and the Atomic and Alternative Energies Commission (CEA).
The scientists have installed an ultra-sophisticated neutrino detector in a cavern under a layer of rock 150 metres thick, which acts as a screen against cosmic rays. The detector rests at the bottom of a pit seven metres deep and as many wide.
It is very difficult to catch neutrinos, despite the fact that they are the commonest elementary particles in the universe. They may even have served as building blocks for all the matter that surrounds us. But they are nevertheless almost undetectable: in just one second several tens of billions of neutrinos pass through every square centimetre of our bodies without us ever noticing.
As their name – "little neutral one" in Italian– suggests, they are electrically neutral. No magnetic field diverts them from their course, shooting straight ahead at almost the speed of light. Above all, having nearly zero mass, they very rarely encounter any obstacles. Almost nothing stops them. Only if they happen to collide with an atom does a flash of energy reveal their passing. The physicists in the cave at Chooz are on the lookout for collisions of this sort, or more exactly a possible lack of collisions.
"There are many sources of neutrinos in nature," says Thierry Lasserre, the head of the experiment at CEA. "The cosmos, the sun, exploding stars and active galaxy cores all produce vast quantities of them. Nuclear reactors also release enormous amounts of electron neutrinos … or more exactly antineutrinos, with properties and characteristics considered to be identical."
The two reactors at Chooz each produce 1026 antineutrinos a day, which hare off in all directions. Given the detector's volume (10 cubic metres) and its distance from the source (1km), about 1021 of these particles pass through it every day. According to probability calculations, only 70 of them should actually hit the detector. Odd as it may seem, the physicists are hoping there will be fewer hits than predicted, as this would mean that on their way some antineutrinos have morphed, enabling them to slip through the net.
Neutrinos are remarkably tricky customers. There are three types or flavours: electron, muon and tau neutrinos, named after three other particles to which they give rise when they collide with an atom. The flavours correspond to the three families of elementary particle. "To produce all ordinary matter, just one of these families would suffice. So why did nature create three families? It is one of the great mysteries of physics," says Hervé de Kerret, the CNRS project leader.
To complicate matters further, the various types of neutrino do not occur in isolation. As they propagate through space they may oscillate between the three available flavours, shifting from one particle family to another. This gift for transformation has already been measured for two forms of mixture: the third has yet to be assessed.
This is the goal set for the Double Chooz experiment. It will seek to determine how many electron antineutrinos, out of the 70 predicted, fail to show, indicating that they have morphed in transit. To be absolutely certain, the instrument, currently being calibrated, will be duplicated. Between now and 2012 a second detector will be installed only 400 metres from the two reactors, its role being to gauge the initial stream of particles very precisely.
If this can be achieved, it should lead to a leap in our understanding of these intriguing particles. Costing $21m and tipped to beat similar projects in China and in South Korea, Double Chooz will operate for five years, in order to collate sufficient data.
Designing and installing the dual system has been a huge undertaking, requiring amazing attention to detail, says Patrick Perrin, the CEA operations manager. "The detectors," he explains, "are like Russian nesting dolls, with a series of superimposed containments, filled with a glittering liquid: mineral oil enriched with the heavy metal gadolinium. When a neutrino collides with a hydrogen nucleus it produces two flashes of purple light, which in turn are converted into electrical signals by 400 photo-sensitive detectors".
The scientists can barely contain their excitement. "The standard model of [particle] physics describes the structure of the universe with 12 elementary particles and four force carriers.
But this involves too many parameters," De Kerret and Lasserre explain. "We need to invent a different model, simpler yet more comprehensive." In particular, the standard model does not explain the mass of neutrinos and their oscillation, so it needs to be amended.
But that is not all. Neutrinos could clarify another mystery. In theory the Big Bang created equal quantities of matter and antimatter, but (fortunately) matter has become predominant in the universe. Just after the original bang, neutrinos may have disintegrated into particles of matter and antimatter, but with very slight asymmetry in favour of matter.
If Double Chooz discovers such asymmetry in the behaviour of its neutrinos and antineutrinos, it would support this hypothesis. Le Monde The Guardian
