PAOLO LOMBARDO/INFN-MI, US DEPARTMENT OF ENERGY
These are the elementary particles,
which together make up the Standard
Model of particle physics. All of the
atoms in the Universe are built using
only the electrons and the ‘up’ and
‘down’ quarks. These interact with
each other and stick together with
the help of gluons and photons.
Gluons transmit what is known as
the ‘strong force’ that binds together
quarks to make protons and neutrons,
the building blocks of atomic nuclei.
Photons transmit the electromagnetic
force that acts between electrically
charged particles, like electrons.
The other particles in the table are
also important, but for less evidentreasons. For example, around 60
billion electron neutrinos stream
through every square centimetre
of your body every second. These
neutrinos are made inside the Sun,
as a by-product of the process that
fuses hydrogen into helium. The ‘weak
force’ is responsible for this process of
nuclear fusion and is transmitted by
the W and Z particles.
The particles in the second and
third columns of the Standard Model
are like heavier copies of those in
the first column. The existence of
these heavier particles was crucial
in governing the behaviour of the
Universe shortly after the Big Bang.STANDARD MODEL OF
ELEMENTARY PARTICLESu
up
c
charm
t
top
g
gluon
d
down
s
strange
b
bottom
Y
photon
e
electron
μ
muon
τ
tau
Z
Z boson
Ve
electron
neutrinoVμ
muon
neutrinoVτ
tau
neutrinoW
W boson
QUARKS LEPTONS GAUGE BOSONS SCALAR BOSONS
H
Higgs
2 is a different mix of all three masses. Imagine an animal that
is 25 per cent cat, 25 per cent dog and 50 per cent giraffe. This
conveys some idea of the weirdness of neutrinos. As each type
flies through space, its individual mass components travel at
different speeds, and consequentially the relative proportions of
each mass state changes. This results in a neutrino ‘oscillating’
between an electron-, muon- and tau-neutrino.
Measurements of neutrino oscillations will provide estimates of
the differences between masses of the three neutrinos. Importantly,
KATRIN pins down an upper limit on one mass. Crucially,
however, we still don’t know the neutrino-mass hierarchy –
whether electron-, muon- and tau-neutrinos get progressively
more massive as do electrons, muons and taus.
Underdstanding neutrino oscillations and neutrino masses is
vitally important. If the ‘mixing’ between neutrino mass states is
big enough, it could indicate that nature permits a process that,
in the jargon ‘violates charge-parity symmetry’. This would make
antineutrinos behave differently from neutrinos. By favouring the
production of matter over antimatter, this could solve one of the
outstanding puzzles of cosmology: why we live in a Universe of
matter. “According to the Standard Model, all fundamental particle
processes create equal quantities of matter and antimatter,” saysNEUTRINOS FE ATURE
Uchida. “We therefore should not exist [because when matter
and antimatter particles meet, they annihilate]!”.RETHINKING THE EARLY UNIVERSE
Neutrino oscillations may reveal the existence of a fourth, ‘sterile’
neutrino, interacting with matter so rarely it makes the other three
flavours appear positively sociable. The total mass of all three
(or more) types of neutrino has consequences for the Universe
because neutrinos are the second most common subatomic
particle, after photons. In the early Universe, their considerable
gravity would have helped matter clump together to make the
first galaxies. The more massive neutrinos are, the earlier they
would have slowed down after the Big Bang and the clumpier
our Universe should be. Consequently, knowing the masses of
the neutrinos helps pin down
the cosmological model that
best describes our Universe.
If astronomers’ observations
of clumpiness contradict that
model, then it will be strong
evidence of physics beyond
the Standard Model.by MARCUS CHOWN
Marcus is a science writer and journalist.
His new book, The Magicians (£12.99,
Faber & Faber), tells the story of the
prediction and discovery of the neutrino
alongside many other stories.