The ghosts and the machine

Studying the diaphanous neutrino will be America’s contribution to a new generation of physics
neutrino1DEEP beneath the plains of Illinois, in a man-made cavern filled with racks of scientific equipment, someone has spray-painted a white circle onto the bare rock wall. Stand in front of it and you are standing in the path of the most powerful beam of neutrinos in the world, which is emerging from a nearby particle accelerator at Fermilab, America’s main particle-physics laboratory. With any other kind of accelerator, standing in the beam would have spectacular and fatal consequences. But your correspondent was not vapourised—nor, several weeks later, has he developed either cancer or superpowers.

And that is the point: neutrinos are ghostly things. Billions a second stream through every cubic centimetre of space. But because they feel only the two weakest of the four fundamental physical forces—gravity and the aptly named weak nuclear force, rather than electromagnetism and the strong nuclear force—they hardly interact with the rest of creation.

They are, nonetheless, a hot topic, for studying them could reveal the physics that must exist beyond the Standard Model. This is the theoretical framework of particle physics, which was completed in 2012 when its missing element, the Higgs boson, was run to ground by the Large Hadron Collider (LHC) at CERN, Europe’s chief particle-physics laboratory. Looking beyond the Standard Model is physics’s next big thing. And a report on the future of American physics, published last month, outlined a plan to make America the world’s leader in neutrino research.

Neutrinos are interesting because they do not obey the rules. The Standard Model said they should be massless. According to Einstein, this means they necessarily travel at the speed of light—and, as a consequence, do not experience the flow of time. But in 1998 an experiment in Japan showed that neutrinos spontaneously transform between the three varieties of them that exist (known as electron, muon and tau neutrinos) as they zip through space—a process known as oscillation.

Variations on a theme
Oscillation means neutrinos experience time. This, in turn, means they must travel slower than light and so have mass after all. That is a lever to break the Standard Model open. Steve Weinberg, a doyen of particle physics who was one of the Model’s architects, has described neutrino mass as the most important discovery in particle physics for a quarter of a century.

But the details of oscillation remain incomplete, which is where Fermilab’s neutrino beam comes in. By the end of July work should have finished on building NOVA, an experiment designed to pin those details down. The beam that passes through the white circle will carry on for 810km (500 miles) through the Earth to a detector in northern Minnesota. When it arrives, some of the muon neutrinos in it will have transformed themselves into electron neutrinos. NOVA will measure precisely how often this occurs.

Besides their three varieties, neutrinos have three different masses. Scientists know that two of these are closely matched, while the third is either much bigger or much smaller. NOVA should be able to discover which. That, in turn, should shed light on how neutrinos acquire mass in the first place.

All this may sound rather technical. But neutrino physics could have some very big implications indeed. Specifically, it could explain why there is any matter in the universe at all. Like other kinds of matter, neutrinos have antimatter opposites. Matter and antimatter, as is well known, annihilate each other on contact. That is a problem because existing theory says the Big Bang should have produced equal quantities of both, and this means the modern universe ought to be little more than a thin soup of post-annihilation radiation—which, evidently, it is not.

The best explanation for the preponderance of matter is that it and antimatter are not quite equal and opposite. A few inequalities are known, involving particles called B-mesons and K-mesons, but they are insufficient to explain the amount of matter around. Many physicists suspect that neutrinos oscillate in a different way from their antineutrino counterparts, and that this could be a big enough effect to fill in the gap and explain why the universe is as it appears. By switching its beam between neutrinos and antineutrinos, NOVA may be able to glimpse such differences. And an even more sensitive successor experiment, the Long-Baseline Neutrino Facility (LBNF), planned to be finished in 2022, would be able to explore them more comprehensively.

The LBNF will detect neutrinos by watching for flashes of light caused when one of them deigns to interact with an atom in a giant tank of liquid argon. But neutrinos are not the only things that cause such flashes. The Standard Model says that protons, which help make up the nuclei of atoms, are stable, but many post-Standard Model theories disagree. They predict protons should decay into lighter particles. The fact that no proton has been seen to do so suggests such events are exceedingly rare. But the LBNF’s 34,000-tonne tank of argon will contain an awful lot of carefully watched protons. If these particles really can combust spontaneously, LBNF stands a fair chance of spotting it.

It could even act as a telescope. Neutrinos are generated in prodigious quantities in stellar explosions known as supernovas, and their ghostliness lets them escape the superdense centre of such an explosion unhindered. By registering the neutrino pulse escaping a supernova, the LBNF, alongside other, existing neutrino detectors, could let astronomers probe what takes place in such extreme environments.

Assuming, that is, the LBNF gets built. As physicists probe deeper and deeper into the subatomic world they must make use of bigger and bigger machines. Particle physics is, increasingly, an international endeavour, as governments try to spread the cost of the kit. America will be the biggest contributor to the LBNF’s $1.5 billion budget, but crucial cash must be raised from other countries, including Brazil, Britain and India. Still, next to the $9 billion the LHC cost, that might seem a bargain.



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