Neutrinos raise questions faster than they answer them, but in science that’s a good thingby Philip Ball / October 6, 2017 / Leave a comment
Neutrinos were always trouble. These elusive little fundamental particles were first proposed in 1930 by the physicist Wolfgang Pauli to explain where some of the energy and momentum went during the process of radioactive beta decay of atomic nuclei. The Italian Enrico Fermi took up the idea, and helped to coin the name, in a 1934 paper that got rejected by Nature as too speculative and was published in an Italian journal to such scant interest that Fermi became an experimentalist instead. (Eight years later he created the first ever nuclear reactor in Chicago.)
At this stage neutrinos were just a hypothetical convenience to make the nuclear sums come out right. They seemed so bland as to verge on the pointless: as they have no electrical charge and seemed at first perhaps to have no mass, ordinary matter barely “feels” them at all. They weren’t detected until 1956 (in work that belatedly won the 1995 Nobel prize), because they are so damned hard to see.
Then in the 1960s, experiments using neutrino detectors—buried deep underground to shield them from false signals made by cosmic rays—showed that these particles weren’t being produced by the nuclear reactions in the Sun at anything like the predicted rate. More head-scratching ensued, until scientists figured that neutrinos must be able to switch in flight between three different varieties (“flavours”). Another Nobel prize (2015) followed for that discovery, not least because it resolved another long-standing issue: neutrinos must after all have some mass, because only then are these “oscillations” between flavours possible.
The mass is tiny, for sure—perhaps a fraction of a per cent of the mass of electrons, which are themselves lightweights in the particle world. But there are so many neutrinos in the universe that some researchers think they might explain at least some of the mysterious “dark matter” known to exist throughout the cosmos only because of its gravitational effects on stars and galaxies.
The point is that neutrinos are bordering on the perverse: their very existence seems to raise questions faster than it answers them. But in science that’s a good thing. Neutrinos are the grit in the oyster of fundamental physics: they create a productive discomfort that might ultimately lead to shiny new theories.
That’s the motivation behind a project called the Deep Underground Neutrino Experiment (DUNE), based at Fermilab near Chicago, the particle-physics establishment that arose around Fermi’s wartime nuclear pile. In DUNE, underground detectors will make measurements on a beam of neutrinos produced by the Fermilab particle accelerator in an effort to understand several deep questions about fundamental physics, astrophysics and cosmology.
“Neutrinos create a productive discomfort that might ultimately lead to shiny new theories in physics”
The UK has committed to investing £56 million ($88m) in the project. This is the first ever “umbrella” agreement on science between the UK and US, and was signed on 20th September in Washington DC, making the UK the largest investor in this international project outside the US. The temptation to suggest cynically that the UK is going to need science partners elsewhere as European collaborations unravel is not entirely to be resisted, but the fact is that having a stake in DUNE looks like a wise move.
It’s rather astonishing, really, how many of these deep questions neutrinos can illuminate. Physicists are particularly eager to escape from the safe confines of what is called the Standard Model, the theoretical framework that incorporates all the known particles and their forces of interaction. The discovery of the Higgs boson at the European particle-physics lab CERN in Geneva in 2013 completed the Standard Model, but it’s clear that this can’t be the whole picture. It doesn’t explain why the particles and forces have the properties and relationships that they do, for example, and the Standard Model has nothing obvious to say about outstanding issues such as the dark matter thought to pervade the universe. Yet until anyone sees a definite signature of a kind of physics the Standard Model can’t accommodate, it’s not clear how to get any further. String theory represents one attempt to do so, but it hasn’t yet come up with any predictions that can be easily tested against experiments.
The Higgs boson is responsible for giving mass to a certain class of particles that feature in the theory of the so-called electroweak interaction: a unified description of two of the four fundamental forces, electromagnetism and the nuclear weak force. Because the weak force underlies the beta decay process in which neutrinos are formed, neutrinos are implicated in this “Higgs mechanism.” In short, the non-zero mass of the three types of neutrino and the oscillations that cause switches between them can’t be fully squared with the Higgs mechanism, and seem to demand some kind of physics beyond the Standard Model. So investigating the flavour-changing property of neutrinos might supply vital clues to that issue of what comes next.
At the root of that problem is the fact that neutrinos seem to be left-handed. Here’s what that means. Neutrinos are members of one of the two general classes of fundamental particle called fermions (that man Fermi left quite a mark); bosons are the other class. All fermions come in mirror-image versions, said to be left- and right-handed. To acquire mass, fermions of both handedness have to interact with the Higgs boson. But no one has ever seen a right-handed neutrino. Does this mean they don’t exist, so neutrinos get their mass another (unknown) way? Or have we just not seen right-handed neutrinos yet (and if not, why not)? These are the kinds of questions that could open a crack in the carapace of the Standard Model, and DUNE might elucidate them.
“Fermilab will create a beam of high-energy protons; these smash into a solid target, and the fragments eventually decay into neutrinos”
There’s more. Neutrinos relate also to another apparent lack of symmetry: the fact that there seems to be much more matter than antimatter in the universe, even though the current theory of the Big Bang suggests that they should have been created initially in equal quantities. Precise measurements on neutrinos might help us see why this is. It’s also widely presumed that, just as the electromagnetic and weak forces are known to have been a single unified force early in the Big Bang, all four of the known forces (the other two are the strong nuclear force and gravity) were once a single Ur-force. So-called Grand Unified Theories (GUTs) attempt to describe this primal unity of forces. But they generally make a prediction that has never been confirmed experimentally: the nuclear particles called protons aren’t eternal, but decay into other particles—perhaps including neutrinos. Observation of proton decay—if it happens at all—would be a shoe-in for another Nobel.
Knowing more about neutrinos could also improve our understanding of the explosion of stars at the end of their life cycle in the cosmic cataclysms called supernovae, because these outbursts generate neutrinos in nuclear reactions.
The potential payoff of a dedicated neutrino experiment like DUNE is therefore huge. So is the experiment itself. Fermilab will generate neutrinos by using an accelerator to create a beam of high-energy protons; these smash into a solid target, and the fragments produced eventually decay into neutrinos that continue to move in the same direction as the original protons, heading down into the Earth at a shallow angle. About 200m from the start of the neutrino beam, a neutrino detector will sit 60m underground directly in the path of the beam. But there’s a second detector too, a full 1300km away at the Sanford Underground Research Facility in South Dakota, on the site of a former gold mine where the first neutrino detector saw a deficit of neutrinos from the Sun in the 1960s. Here a tank of 40,000 tons of liquid argon 1500m underground will detect the neutrino beam and see what has become of it on its journey from Fermilab—looking for example at what proportion of neutrinos has “oscillated” during transit through the rock. Because neutrinos interact so little with ordinary matter, rock is no impediment; solar neutrinos hitting the Earth mostly go right through it.
Construction of the Fermilab neutrino beam facility and the Sanford detector caverns began in July, but the beam isn’t likely to be ready to fire up until 2026. It should be worth the wait.