FE ATURE NEUTRINOS
KATRIN COLLABORATION, GET T Y IMAGES X2
hysicists 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 electron-
neutrino 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 2A RADICAL IDEAÉ
Zurich, Switzerland, December 1930. Wolfgang
Pauli was having the worst year of his life. His
mother had committed suicide two years
earlier, causing him to turn his back on the
Catholic Church. He had recently married a
Berlin cabaret dancer but she had already
moved in with her chemist boyfriend down the
road. He had even appealed for help from the
great psychoanalyst Carl Jung. But Pauli’s main
distraction from his personal distress was
physics – and one physics problem in particular
had captured his attention.
In the late 1920s, physicists were pulling their
hair out over the puzzle of beta decay. In beta
decay, an unstable, or ‘radioactive’ nucleus
sheds its surplus energy by spitting out an
electron. The peculiar thing is that the ejected
electrons do not always have the same energy.
Think how bonkers this is. In dropping from an