The existence of an elusive atomic particle, crucial to the theory of the origin of the universe, may soon be established
SCIENTISTS in the us and Japan are now
on the verge of establishing the existence
of an elusive elementary atomic particle
called the axion. Till date, the axion has
only been theorised and has helped
explain a discrepancy between the
theory of the origin of the universe and
the facts available to scientists. If its
presence is established, particle physics
is in for an exciting time. But if it
proves to be a theoretical necessity not
supported by fact, theory will have to
change accordingly.
The search for new elementary
particles has always been a continuing endeavour for particle physicists.
Unfortunately, most of the fundamental
building-blocks of nature required by
theory are already well known. Among
the important exceptions to this are the
Higgs boson and the axion.
The Higgs boson is required by
,theory to make it consistent and will
most likely be seen in the next generation of accelerators. The axion, because of its weird properties, will never be seen
in accelerators and hence ingenious
experiments have to be designed to
discover it. Two such experiments are
currently underway and researchers
are waiting for the results to solve the
mystery of this elusive particle.
Proposed about two decades ago,
the axion is required by the currently
accepted theory of strong interactions,
quantum chromodynamics (QCD). The
theory describes how quarks, the elementary constituents of nucleons, and
gluons interact. This should be identical
when we run the interaction backwards
in time. That is, the interaction should
be time-reversal symmetric. This and
other requirements of the theory could
be met by the introduction of a particle
in the model. This hypothetical particle
was called the axion by Frank Wilczek.
Restoring the consistency Of QCD is
only one of the important things the
axion does. It also provides a solution to
the missing mass problem. That the
amount of visible matter in the universe
is much less than the amount of matter
inferred by its gravitational force is
termed the missing mass problem or the
dark matter problem. Many particles
and exotic objects have been proposed
to solve the mystery of the missing mass.
These have ranged from known particles
like neutrinos, to innovative objects like
wimps (Weakly Interacting Massive
Particles) and MACHOS (Massive Compact
Halo Objects). If the axion exists, it
would be a natural candidate for solving
the missing mass problem. Axions are
thought to be extremely light, each one
of them weighing only a thousand-billionth of an electron. Nevertheless, there
could be so many axions in the universe
that they could potentially provide a
large fraction of the missing mass.
Ever since they were hypothesised,
people have attempted to detect axions
without success. But two experiments
now operational could change that.
One of them is based at the Lawrence
Livermore Laboratory in the us and is a
collaboration between one Russian and
six us institutions. The setup is a copper
cavity that is cooled to extremely
low temperatures (-270'c),and is put in
the field of a giant superconducting
magnet. When the axion interacts with
the magnetic field, it should convert
into a photon (electromagnetic waves).
If the cavity is tuned to the correct
frequency, it can detect the photon and
hence the axion. Data has been collected
for more than a year and are being
analysed.
The other experiment has been set
up at the University of Kyoto, Japan,
and has just started recording data.
The basic technique is similar, though
photon detection from axions is
done differently in this experiment.
Here the photons are detected by being
absorbed by Rydberg atoms, in which
the electrons are so far away from the
nucleus that they are very loosely
attached it.
If the mysterious axion is detected
in these experiments, it will arouse
great interest in the fields of particle
physics and cosmology. If not, there
will always be newer, improved models
of nature to take care of the eventuality.
Either way, there promises to be a lot
of excitement.
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