A possible particle of dark matter

Fundamental physics
Axion stations

Dec 19th 2006
The Economist
A possible particle of dark matter

WHEN Frank Wilczek, a Nobel laureate, proposed the existence of a new
type of elementary particle in 1977 he named it an "axion", after a
type of detergent, because it cleaned up a profound physical problem.

This problem is that the amount of visible stuff in the universe is far
smaller than is needed to account for the apparent effects of gravity.
In particular, galaxies behave as though they are much heavier than
they actually look.

One way of solving this conundrum is to invoke a type of matter that
has a gravitational field, but cannot interact with light or other
forms of electromagnetic radiation, and is therefore invisible. In
other words, dark matter. Axions are the most popular proposal for what
this dark matter might actually be.

Unfortunately, they have since created a mess of their own. Many
experiments have looked for axions. Most have not found them. Indeed,
they have proved so hard to detect that many physicists question
whether they exist.

Earlier this year, though, an Italian experiment did see something that
suggested their existence. Now a paper by Piyare Jain and Gurmukh Singh
of the State University of New York, Buffalo, also offers some evidence
that axions really do exist. It is published in the January edition of
the Journal of Physics G: Nuclear and Particle Physics.

The pair had another look at some photographic plates from an
experiment conducted a decade ago at CERN, the European particle
physics laboratory near Geneva. Though that experiment had not been
intended to make axions, the two researchers wondered if it had created
such particles as a by-product. After re-analysing the plates, Dr Jain
and Dr Singh concluded that they may have found axions that exist so
fleetingly that they are not noticed by the modern electronic methods
of particle detection that have replaced the use of photographic
plates.

If their interpretation is correct, that would be exciting, as it would
establish the existence of a new class of matter. Unfortunately, it
would not solve the dark-matter problem, since what the two researchers
think they have found would be too short-lived to form the
"missing" matter. But other particles in the class might plug the
gap. An experiment under way at DESY, a laboratory in Hamburg, Germany,
is seeking to make and detect axions that would fall into this
longer-lived category. Should it succeed, scientists will have come
closer to cleaning up the mystery.

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