If caught, it could spell trouble for the Standard Model of particle physics; although success has eluded most scientists, a group of relentless researchers refuse to give up the search for a free quark
A SMALL drop is threatening the whole
ocean of thought that physicists have
nourished for quite some time now.
Most particle physicists believe that the
theory which governs the interaction of
matter at the smallest scale, the so-called
Standard Model of particle physics, has
Deen tested to a reasonable degree of
accuracy. Martin Perl, co-recipient of
the 1995 Nobel prize in physics and his
colleagues differ.
Ped's objective: upsetting the
Standard Model. His experimental
arrangement: tiny droplets of silicone oil, about seven micrometres (one
inicrometre- 10-6 metre) in diameter,
falling between two electrically charged
plates lit by a strobe light and a charge-
coupled device camera to record the
images at the Stanford Linear
Accelerator Centre, us. His principle:
the same that Robert Millikan applied in
his classical oil drop method to evaluate
the charge on a single electron, that is,
the rate at which each drop falls through
the varying electric field indicates the
charge it carries. The success indicator is
finding a droplet with charge less than
that on an electron, the amount that a
quark, fundamental particles that make
up electrons and protons, would carry
on its own.
There has been no success until now
which is in confirmation with the concept of quantum chromodynamics
(QCD), one of the components of the
Standard Model. The basis of QCD is the
existence of quarks which are bound
together by strong forces. Quarks differ
from other particles because they carry
fractional charge and exist only in
groups of two's or three's. A free quark
cannot exist. Or so says QCD.
Ever since Murray Gell Mann and
George Zwieg proposed the quark
model in 1964, several searches have
been carried out for free quarks. The
techniques used have varied from a
modified Millikans oil drop experiment
to magnetic levitation, particle accelerator experiments and searches in cosmic
ray tracks. The materials searched have
ranged from niobium to moon rocks
brought back by the astronauts! But all
these experiments, barring one, have
come up with a negative result; none of
them have seen a free quark.
'In 1968, Brian McCusker first
reported a track in a cloud chamber (a
kind of particle detector) which looked
like it was produced by an unusual particle. Its density was much less than
would be expected from a conventional
particle like a proton. This report
remained unconfirmed and the track
was not found in the many cloud chamber pictures taken subsequently.
In the late '70s, William Fairbank
reported seeing fractionally charged
particles on niobium-coated tungsten
balls in a repeat of the Millikan oil drop
experiment. There was a lot of excitemerit in the scientific community but it
fizzled out because the results could not
be replicated by many others, who
carried out similar experiments. The
result was that most scientists gave up
the search and were brought around to
believing QCD.
Though no evidence of a fractional
charge has been seen, Perl and his
co-workers are now improving their apparatus to examine much more material in a much shorter time. They plan to
use the modified arrangement to study
other materials later this year. If successful, it could be a major blow to QCD and
will send the theorists back to their calculations (Science, Vol 2 71, No 5254).
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