PAOLO
LOMBARDO/INFN-MI,
US
DEPARTMENT
OF
ENERGY
Thesearetheelementaryparticles,
whichtogethermakeuptheStandard
Modelofparticlephysics.Allofthe
atomsintheUniversearebuiltusing
onlytheelectronsandthe‘up’and
‘down’quarks.Theseinteractwith
eachotherandsticktogetherwith
thehelpofgluonsandphotons.
Gluonstransmitwhatis knownas
the‘strongforce’thatbindstogether
quarkstomakeprotonsandneutrons,
thebuildingblocksofatomicnuclei.
Photonstransmittheelectromagnetic
forcethatactsbetweenelectrically
chargedparticles,likeelectrons.
Theotherparticlesinthetableare
alsoimportant,butforlessevidentreasons.Forexample,around 60
billionelectronneutrinosstream
througheverysquarecentimetre
ofyourbodyeverysecond.These
neutrinosaremadeinsidetheSun,
asa by-productoftheprocessthat
fuseshydrogenintohelium.The‘weak
force’is responsibleforthisprocessof
nuclearfusionandis transmittedby
theWandZ particles.
Theparticlesinthesecondand
thirdcolumnsoftheStandardModel
arelikeheaviercopiesofthosein
thefirstcolumn.Theexistenceof
theseheavierparticleswascrucial
ingoverningthebehaviourofthe
UniverseshortlyaftertheBigBang.STANDARDMODELOF
ELEMENTARYPARTICLESu
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
Wboson
QUARKS LEPTONS GAUGEBOSONS SCALARBOSONS
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.