From: LARRY KLAES (ljk4_at_msn.com)
Date: Wed Feb 05 2003 - 09:28:49 PST
>From Wired Magazine, available online at:
http://www.wired.com/wired/archive/11.02/neutrino.html
The Neutrino Has Landed
Using the moon as a supercollider
By Oliver Morton
Earth is not the only planetary body that Superconducting Super
Collider refugees are turning into a particle detector.
Chuck Naudet is one of the physicists who, if everything had gone
according to plan, would now be spending a lot of his time in that
great big hole under Texas. But when I met him last year, I was soon
following him up to the topmost rungs of a cascade of ladders,
companionways, and staircases high above the Mojave Desert, where he
was checking up on the workings of one of the best radio telescopes in
the world. He's still looking for particles with energies far greater
than any seen before. He's just no longer looking at Earth. He's
looking at the moon.
Naudet is one of the five people who make up the Goldstone Lunar Ultra
High Energy Neutrino Experiment, aka GLUE. Neutrinos are
almost-massless particles produced during various nuclear reactions
and decay; since the discovery of the particles half a century ago,
physicists have searched for them in myriad ways, from monitoring
chemical changes in underground tanks of dry-cleaning fluid to looking
for sparks deep inside the Antarctic ice. These experiments have seen
neutrinos from the humdrum nuclear reactions at the core of the sun
and glimpsed their higher-energy cousins from the more spectacular
explosions at the hearts of supernovas. But GLUE is looking for
something else - neutrinos that no well-understood process could
conceivably have made, neutrinos with energies that dwarf anything
seen before. GLUE is looking for individual subatomic particles that
pack as much punch as a tennis champion's serve.
In some ways, the project is easy. Low-energy neutrinos are
notoriously hard to detect because they almost never interact with
ordinary matter. But neutrinos with the sort of energies that Naudet
and his colleagues are interested in can be stopped pretty reliably by
just a few kilometers of rock. As a bonus, they can be expected to
give off a reasonably plaintive yelp of electromagnetic radiation as
they do so. Unfortunately, these more obtrusive neutrinos are also
far, far more rare; at the surface of Earth, you can expect fewer than
one per square kilometer per year. Picking up a signal like that takes
a very big detector, indeed.
As Naudet tests out the systems at the focus of NASA's best radio
antenna, the neutrino-stopping part of his experiment is dragging
itself over the eastern horizon, big and yellow. In a couple of hours,
after exhaustive tests, the GLUE team - two professors, two NASA staff
researchers, and a graduate student - will settle down to listen for
radio squawks from dying neutrinos. It's a long way from the
thousand-person teams that would have worked the detectors beneath the
farmland south of Dallas. But if the GLUE team is small by Texan
standards, its detector is not - lighting up the night sky from a
quarter-million miles away.
GLUE's biggest challenge is showing that any neutrino-ish signals it
picks up actually come from the moon, rather than from local radio
interference. It was Peter Gorham, then a colleague of Naudet's at
NASA's Jet Propulsion Lab, now a professor at the University of
Hawaii, who saw that JPL's Goldstone interplanetary communications
facility in the Mojave Desert offered a way to minimize the problem.
At Goldstone, they can use two dishes at once, 22 kilometers apart. At
any given time, one of the dishes will be a bit nearer to the moon,
and so will receive any lunar radio burst a few microseconds earlier.
The time lag will change continuously as the moon moves through the
sky, in a way any astronomer can easily calculate. In just over a
hundred hours of operation spread out over a few years (the Goldstone
dishes are only rarely spared for listening to anything other than
spacecraft), GLUE has yet to see a signal that matches exactly what it
would expect from a neutrino in the moon. But it has seen something
odd. Gorham and his colleagues keep on receiving intriguing signals
that have almost, but not quite, the offset necessary for something
coming from the moon. These signals are consistently 1 microsecond off
- which suggests that they are coming from an empty piece of space
that sticks close to the moon as it passes across the sky.
This is exquisitely infuriating to the GLUEsters. If the
1-microsecond-off signals represent a statistical fluke, then it's a
big one, which will take years for new findings to dilute away. If
they represent a real signal, either a radio source is traveling a
little ahead of the moon in its orbit around Earth - which is pretty
unlikely - or the signals really are from the moon, but some sort of
delay is built into the observing system that the GLUE team doesn't
quite yet grasp. If the latter is true, then the team is looking at an
amazing discovery - but doesn't understand it well enough to say so.
The 1-microsecond differential in the data is all the more frustrating
because of the wonders it hints at. No process so far discovered could
generate neutrinos powerful enough to strike these radio sparks off
the moon, but there are a wide range of possible causes at the
speculative end of particle physics and cosmology. Perhaps the
neutrinos come from the decay of wimpzillas - a generic name for very
massive but almost stable particles - or cryptons, a breed of
wimpzilla favored by superstring theorists. Maybe they mark the demise
of previously undiscovered ur-neutrinos as they plough into the clouds
of everyday neutrinos that take up the space between galaxies. Perhaps
they mark the unwinding of little knots of space and time tangled up
when the universe first arose from the primordial quantum foam.
During the long, dark hours in the Goldstone control room, all this
and more seems possible - but none of it proven. And outside,
regardless of what it may be doing in the radio spectrum, the moon
shines down desert-bright, the heart of a particle-physics experiment
30 times the size of Texas.
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