VITAL CLUES TO A THEORY OF EVERYTHING
Ninety years after its prediction and 25 years since the Nobel Prize was awarded for its discovery, the neutrino particle is still surprising us. It may, in fact, be the key to understanding everything…
How mysterious ghost particles could help us understand the Universe.
Physicists are homing in on the mass of the neutrino, nature’s most elusive subatomic particle. The latest super-accurate measurement, made by an experiment in Germany, shows that the neutrino is around half a million times less massive than the electron, the lightest particle of normal atomic matter. According to the Standard Model, the high point of 300 years of physics which describes the fundamental building blocks of matter and three non-gravitational forces that glue them together, the neutrino should be massless. So why should we care about a mass measurement (no matter how tiny) of a neutrino? Well, it may provide vital clues to the fabled ‘theory of everything’ – a deeper, more fundamental theory of physics of which the Standard Model is believed to be but an approximation.
HUNTING THE ELUSIVE GHOST PARTICLE
The latest neutrino measurement was made in Karlsruhe, Germany, where physicists exploited the ‘beta decay’ of tritium. Tritium is a heavy type – or ‘isotope’ – of hydrogen. In beta decay, the unstable core – the ‘nucleus’ – of an atom sheds surplus energy by spitting out an electron and an antineutrino (the neutrino and its ‘antimatter’ twin have the same mass). Neutrinos are fantastically antisocial, interacting so rarely with normal matter that they could pass unhindered through several light-years of lead. Consequently, the physicists at the Karlsruhe Tritium Experiment, or KATRIN, must infer the neutrino mass from measurements made on their electrons. They can do this because the amount of energy emitted by the tritium nuclei is always the same. The energy is divided between the electron and the neutrino – if an electron has lots of energy, then it must mean that its associated neutrino only has a little bit. So if the physicists only allow the most energetic electrons to reach their detector, it ensures that their associated neutrinos will have very little energy – this allows them to make a more accurate reading of the neutrinos’ mass.
KATRIN is an extraordinary piece of engineering. After 18 years of planning and building, it weighs 200 tonnes and cost about €50m (£42m). It is operated by a team of 150 people from six international institutions, and yielded its first result after only one month of operation after observing two million electrons. The experiment found that the neutrino cannot weigh more than 1.1eV (because Einstein showed that mass is a form of energy, physicists measure the masses of subatomic particles in energy terms – an eV is an electron volt). By comparison, an electron has a mass of 500,000eV. “The result is an incredible achievement,” says Dr Melissa Uchida, a neutrino physicist at the University of Cambridge. “The uncertainty in the mass limit is 100 times better than the previous best estimate.”
There is a twist to this story – a major one. The electronneutrino is merely one of three types, or ‘flavours’, of neutrino. The electron-neutrino is associated with the electron, but there is also the muon-neutrino associated with the heavier ‘muon’ particle, and the tau-neutrino with the even heavier ‘tau’ particle.
There are three distinct mass states of the neutrino. But, crucially, each does not correspond to a flavour – in fact, each neutrino