What is this
particle like?
Constraints imposed
by various areas of
theoretical physics can
help scientists pin down
the axiflavonLightweight particle
The axiflavon’s mass must lie
between one 100th and one
10 billionth of an electronVolt.
For comparison, the mass of
an individual electron is about
500,000 electronVolts.
Coupling with light
The axiflavon’s photon coupling
- the strength of its effects on
radiation, including light – is very
weak,butincreasesifithasa
higher mass.
Gamma-ray signal?
The axiflavon’s weak effect
on electromagnetic radiation
might still be enough to detect a
pattern in the energies of cosmic
gamma rays.
Axiflavon sources
Depending on their mass,
axiflavons could be steadily
released from the cores of normal
stars, or only forged in the
explosive hearts of supernovae.
Plain particle
In order to behave in the ways
required the axiflavon should
have no electric charge, and also a
particle spin of zero.
Flavons,toputitassimplyaspossible,are
associated with the ‘flavon field’, a hypothetical
force field affecting quark particles that make up
the protons and neutrons in the cores of atoms.
Quarks come in six different f lavours, but can
sometimes change from one f lavour to another
using the so-called weak interaction, one of nature’s
fourknownfundamentalforces;thisishowatoms
changetheiridentityincertainformsofradioactive
decay. In order to explain how the different quark
flavours display wildly different masses, physicists
suggested another layer of complexity on top of the
Standard Model – a ‘f lavon field’ that linked quarks
with the mass-producing Higgs boson. Interactionsin this new field would be carried by a new
‘messenger particle’ known simply as the f lavon.
At around the same time, other physicists were
puzzled over a problem with the ‘strong interaction’,
an even more powerful nuclear force that binds
particles together in the atomic nucleus. Known
as the ‘strong CP’ problem, the issue was just why
certain strong-force interactions are symmetric
when the Standard Model suggests that they
shouldn’t be.
According to the Standard Model the strong
interaction is expected to break so-called ‘CP’ or
‘charge-parity’ symmetry – a rule that interactions
will look the same if the electric charges of the
particlesinvolvedarereversed,whiletheirspatial
coordinates are also f lipped. The fact that the strong
interaction does not break symmetry in this way
led physicists Roberto Peccei and Helen Quinn
at Stanford University to hypothesise another set
of unseen interactions taking place at the tiniest
scales involving a previously undetected force field
keeping things in line. Princeton physicist Frank
Wilczek extended the theory by suggesting that the
force field should have an associated particle, for
which he coined the name ‘axion’.
Atfirstglancetheaxionandflavonseemtohave
little in common, but the world of nuclear physics
isstrange,andparticlesandforcesthatseemvery
differentineverydaycircumstancescanreveal
unexpected similarities at high temperatures and
energies, such as those that occurred in the Big
Bang itself. By colliding particles at close to the
speedoflightinacceleratorssuchastheLarge
Hadron Collider (LHC), physicists have foundAbove: If the
axiflavon
exists
and has a
relatively
strong
coupling
to photons,
then normal
stars could
lose mass by
generating
them
through their
lifetime.Normal stars
could lose
mass by
generating
the
axiflavon.
Evidence
for mass
loss could
be seen in
studies of the
oldest stars
© CERN; The Hubble Heritage Team; Mark GarlickSolving the universe