SO LET US exit the tiny airport we landed at and head out to explore the surrounding Neutrino Sector on foot.
The fact that neutrinos have different masses, and mix, indicates that there is indeed a mixing matrix for leptons, just as there is for quarks. So we should expect that the airports on the Isle of Leptons are somewhat displaced from the population centres, just as the Down-prime, Strange-prime and Bottom-prime airports are displaced. This is the case, but the picture is rather different.
On the Isle of Quarks, the two different versions of down quark, with and without the prime, are close enough that it is obvious that they belong together. However, for the neutrinos, the mixing matrix is very different. To explain the observations, the mixing must be much larger than on the Isle of Quarks.
The only way we have of fully exploring the Neutrino Sector is by air. The weak interaction connects three airports: Electron Neutrino, Muon Neutrino and Tau Neutrino. SNO and Super-Kamiokande – and other experiments at accelerators and reactors that have followed them – have identified the fact that these airports are not the whole story; there are cities which the airports serve. These are versions of the neutrinos with definite mass. But the weak interaction seems to be running a budget airline into the Neutrino Sector. The cities are miles away from the airports, and in fact we still aren’t even sure where they are exactly. This is not a very satisfactory state of affairs.
There is an interesting possibility here though. Remember, the mixing matrix for the quarks breaks the combined symmetry of charge-conjugation and parity, introducing a real distinction between the northern and southern hemispheres of our map, between matter and antimatter. The matrix we have just been forced to introduce for neutrinos also includes this possibility.
In the quark case, this effect has been measured. Neutrinos are harder to study, because they interact too rarely, and this aspect of the mixing matrix has not yet been measured. If it turns out there is a large effect, it could help explain how the universe evolved to be mostly made of matter, not antimatter.
Our exploration of the Neutrino Sector of the Isle of Leptons has been very fruitful, but there might be still more out there. There are still other explorers braving the wilderness, trying to establish whether the neutrino is even the same kind of particle as all the other leptons, whether the Dirac equation even applies to it at all, or whether it dances to a different drum. The neutrino has a startling possibility open to it, unavailable to the other matter particles. It might be its own antiparticle. We might have the Isle of Leptons wrong. The wilderness of the Neutrino Sector might extend right down to the equator – we still do not know, but we are trying to find out.
Matter is the same as antimatter except that all the ‘charges’ – the quantum numbers that determine the interactions with the forces – are the opposite. So the electron is negatively charged, and its antiparticle, the positron, is positively charged and therefore obviously different from the electron. Likewise, for a quark carrying a color charge (call it ‘blue’), the corresponding antiquark will carry the opposite charge (antiblue, or yellow if you want to stick with the analogy to real colours).
Before we knew they had mass, we knew that neutrinos don’t feel the electromagnetic or strong forces. And even the weak force acts only on the left-handed neutrinos. Back when neutrinos were thought to have zero mass, the Standard Model only contained those left-handed neutrinos. But now we know they have mass, that doesn’t work any more. The mass implies that right-handed neutrinos must exist too, mixed up with the left-handed ones. These right-handed neutrinos have zero charge with respect to all the forces – no electric charge, no color charge for the strong force, and no weak charge. So inverting the charge makes no difference – minus zero is the same as plus zero! Therefore there is a possibility – and many theorists would say a high probability – that the right-handed neutrino and the left-handed antineutrino are the same particle, with just the helicity flipped.
This kind of particle would appear in the equations of physics in a different way from the quarks and the other leptons; especially, the way their mass appears is different.fn1 The Neutrino Sector is currently being scoured for such a so-far-hypothetical particle. The search focuses on some very rare and special nuclear decays which, if observed, would have huge implications for physics and cosmology.
The rare decay in question is ‘neutrinoless double-beta decay’. We have come across beta decay already. It is the process in which either a neutron becomes a proton, or a proton becomes a neutron, inside an atomic nucleus, changing the atomic number by +1 or −1 respectively. The process was first observed around the end of the nineteenth century, and the ‘beta particle’ which is emitted is now known to be an electron in the first case, and a positron in the second.
Remember, beta decay provided the first indication of the existence of neutrinos. The emission of a neutrino, even if unobserved, allows the electron or positron that is emitted to have a spectrum of energies rather than a fixed value, as measured in experiments.
As the name suggests, in double-beta decay two neutrons or protons transform at once, two electrons or positrons are emitted, along with two neutrinos. This sounds unlikely, but for some nuclei the energy balance is such that this is the only way they can decay. The process is rare, but has been observed and measured in several different isotopes.
The existence of nuclei which undergo double-beta decay opens up a new and intriguing possibility. Emitting a particle is in many senses the same as absorbing an antiparticle. So if the neutrino can mix with its antiparticle, the same neutrino could be both absorbed and emitted in a double-beta decay, meaning that overall no neutrinos go in or out. This would be neutrinoless double-beta decay. In this case the pair of electrons would carry the fixed energy of the decay – just as the electron in single-beta decay would have had a fixed energy if no neutrino were emitted.
If neutrinoless double-beta decay were to be observed, it would show that the neutrino is, at least in part, its own antiparticle, because it would have to be emitted by one beta-decaying neutron as a particle, and absorbed by the other as an antiparticle. This would make it a fundamentally different kind of object from all the other particles of the Standard Model. That could lead to an understanding of why neutrino masses are so small. It could also provide still another source of violation of the combined charge-conjugation and parity symmetry, helping to explain why the universe contains so much more matter than antimatter. It would certainly give some important pointers as to what lies in the far east, off the edge of our current map.
The challenge for experiments searching for rare decays is to eliminate, as far as possible, the noise from natural background radiation. Every component has to be screened for traces of natural radioactivity (mostly uranium and thorium) using specialised instruments. The detector construction must be carried out in a carefully controlled, clean environment to avoid any contamination during assembly. Unsurprisingly building such an instrument takes a long time. The experiments then have to be installed far underground, and watched carefully for a long time. Several groups are building and running experiments right now.
We have learned much more than one might have expected from exploring the Neutrino Sector: it forced a significant change in the Standard Model, it holds out a possible new source of matter–antimatter asymmetry, and it may have given us some important clues as to what is happening far in the east, at energies beyond our current reach. The voyages of exploration and discovery continue, deep into the Neutrino Sector. It may not be wilderness for long. But what lies in store for our band of explorers right now is a long-haul flight back east to the airline hub. The land of Bosonia, home of the W, the Z, the photon and the gluon, requires our attention. And a mystery lies at its heart.