摘要：The standard model of particle physics explains many things, but the strange behaviour of neutrinos isn't one of them, writes Chanda Prescod-Weinstein
THE standard model of particle physics is so named because it is a model of our subatomic universe that is now the standard description of reality for physicists. It is not only widely accepted, but also extensively tested. Although it is unable to account for gravity, it describes every other force and every particle we have ever seen in a laboratory or particle collider – from the familiar electron, which carries a charge and makes up the outer layers of atoms, to the Higgs, which is a complex part of the picture, one that gives mass to most particles in the model.
Even so, mysteries remain. There is the strong CP problem, which I’ve described in earlier columns and which gives rise to the hypothetical axion particle, my favourite dark matter candidate. And of course the aforementioned inability to bring the standard model into line with gravity.
But at a more basic level, there is another big issue. This concerns a class of particles in the model whose basic properties continue to confuse us: the neutrinos.
These particles are elusive. We know they are there, but they are very hard to capture because they only interact through gravity, which isn’t very powerful, and the weak nuclear force, which is only effective at very short distances. Trillions of them pass through our bodies every second without pause. Neutrinos are also very low in mass. We have an upper limit on this property – they are maybe a million times less massive than the electron, itself a featherweight. However, we still don’t have an exact number for their mass or a solid theory for how they gain it. The Higgs mechanism that works so well for other particles can’t explain neutrino mass.
These mysteries are tied up in a strange feature of neutrinos. Before we get to that, you need to know that these particles come in three forms, or flavours as physicists say: electron neutrino, muon neutrino and tau neutrino.
What is unusual about neutrinos, compared with other fundamental particles in the standard model, is that they can change flavour. An electron neutrino flying through space will randomly become a muon neutrino or a tau neutrino, and the same is true for all three forms.
Neutrinos, rather than having a static identity, can shift to any one of the three identities at any time. As a queer person, I’ve enjoyed recognising that this means neutrinos are non-trinary, rather than being permanently fixed in one of a trio of identities. In technical language, we call these changes neutrino oscillations because the neutrinos are oscillating between flavour types.
The fact that neutrinos don’t have a static identity may seem bizarre because in everyday life we are so used to things staying the same. But even in human relations, we are coming to see that beliefs about fixed identities don’t necessarily make much sense and that distinctions we once clung to are outdated.
On a subatomic scale, neutrinos are certainly set apart because their leptonic family members that I mentioned earlier don’t display this behaviour. For example, an electron won’t randomly turn into a muon or a tau particle as it moves through space. The evidence for neutrino oscillations is now extensive. We know they happen in the sun and we know they also happen in our atmosphere. They have also been observed in nuclear reactors and particle accelerators. Experiments have been incredibly successful in capturing evidence for this.
Yet challenges lie ahead. There remain open questions about why neutrino oscillations occur, and there still isn’t a satisfactory theoretical explanation for them. This is related to our inability to calculate a mass for neutrinos: as their identities tend to mix, their masses must also mix. Plus, the traditional Higgs mechanism that gives mass to other particles relies on those particles having a form of handedness, a sense of left or right at the quantum property level.
The neutrinos that we have seen seem to only be lefties, and that suggests that maybe there are right-handed neutrinos that we just haven’t observed yet. The most popular model for neutrino mass, the see-saw mechanism, adds a right-handed neutrino to the mix and allows the Higgs mechanism to provide all neutrinos with a mass. These right-handed neutrinos are very massive, which tips the scales away from the left-handed ones – making them very light.
The standard model is a stunning success in our quest to understand the basic building blocks of visible matter in the universe, but in some ways it is still a work in progress. The fact that it is incomplete is a wonderful thing from my point of view. It not only means that there is much for me to do, but that there probably will still be questions for the next generation to pursue, too.