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A picture for my boys, Tello, who is 8 and in 3rd grade, and Rico, who's 4 years old. It's very hard being away from them this long... |
12/14/02
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A picture for my boys, Tello, who is 8 and in 3rd grade, and Rico, who's 4 years old. It's very hard being away from them this long... |
What happened today?
a) ride in a PistenBully 20 km out from the
station to collect snow samples: Movie on drachen website, more
on that in tomorrow's journal.
b) Digging out AMANDA cables, heavily drifted over. Movie on drachen
website, more on that in tomorrow's journal as well.
c) made new fake AMANDA website that was very funny, and gave
me giggles (lack of sleep) all afternoon.
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From the fake AMANDUH website :
Back at the station, Chinese uber-physicist Xinhua Bai demonstrates the correct use of the D3 digital supershovel. This shovel has to be very long, because there's a lot of snow shoveling to do at the Pole. |
But today, let's talk about neutrinos.
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Cosmic Gall by John Updike
Neutrinos, they are very small
They have no charge and have no mass
And do not interact at all
The Earth is just a silly ball
To them, through which they simply pass,
Like dustmaids down a drafty hall,
Or photons through a sheet of glass.
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The symbol for the neutrino, carved in ice at the South Pole. |
A delightful poem, much embraced by physicists,
who like everyone, are happy to have anyone show an interest in
what they care about.
But it's inaccurate in a number of ways, though Updike probably
had it right at the time he wrote it. But a lot's changed in neutrinos
recently, so much so that the Nobel prizes in Physics this year
were awarded for two physics experiments which detected a neutrino
signal from a supernova in 1987. Those results gave impetus to
the construction of the many modern neutrino detectors around
the world: AMANDA, ANTARES, Super-K, SNO, and others.
What did Updike get wrong?
a) Neutrinos now do appear to have mass. This
is a recent conclusion, based on the observation that the 3 types
of neutrinos (electron, muon, and tau) oscillate between one type
and another. For a quantum physicist, this "morphing"
can only occur among particles with mass. Neutrino mass oscillation
theory has helped explain a conundrum in physics.
For 50 years, the physics of the sun and other stars has been
worked out in great detail. The sun makes lots of electron neutrinos
as it fuses hydrogen to helium. The exact flux of neutrinos reaching
the earth was precisely predicted by fusion theory, yet the actual
measured flux of electron neutrinos is far less than predicted.
Where are the missing neutrinos?
It now looks as if many electron neutrinos change to muon neutrinos
on their way to the earth, and muon neutrinos were not detected
by earlier experiments. But 2 underground experiments, both large
water tanks surrounded by PMTs in deep mines, can detect muon
neutrinos, which arrive at the earth in greater numbers than fusion
theory would suggest. OK, then. Too few electron neutrinos, too
many muon neutrinos. They change from one kind to another enroute
from Sun to Earth.
Experiments are now underway in Japan & the United States
to manufacture a precisely characterized neutrino beam, send it
several hundred kilometers through the earth to a detector, and
measure the relative proportions of arriving neutrino types. This
will allow physicists to better understand the oscillation process,
and set some limits on neutrino mass.
How much of the universe's mass belongs to neutrinos? Does the
mass of neutrinos account for the so-called dark matter that seems
to be invisible but binds galaxies together through the force
of gravity? We'll certainly have a better idea in 10 years.
b) Neutrinos do interact, with ordinary matter,
but just barely. In physics, we say that neutrinos have a very
small cross-section, a very low probability of "hitting"
something. Hitting isn't even a very good word, because when you
start to talk about very small things, they don't hit, they don't
touch, they just "interact".
Neutrinos are manufactured in certain types of nuclear reactions
involving the so-called nuclear "weak force". They are
electrically neutral, as Updike correctly says, which means they're
not subject to the electromagnetic force which makes protons and
electrons attract. And they're not subject to the strong nuclear
force, which holds the nuclei of atoms together. If they do indeed
have mass, then they're pulled by gravity. So out of the 4 FUNDAMENTAL
FORCES, neutrinos are only involved in 2, the two weakest.
That makes them ideal messengers from distant objects in the universe,
because they come more or less straight to us, unaffected very
much by what's around them. It also makes them hard to detect
at all. That's why you need a BIG detector.
c) Photons, "particles" of light, do pass through glass. But not unaffected. They slow down as they pass through (that's an oversimplification, but this is complicated enough already). That's why light bends as it passes through glass, if it strikes the surface at an angle. This is called refraction, and I for one am happy this occurs, because it produces ice halos and sundogs at the South Pole.
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The AMANDA logo. AMANDA is interested in detecting particles which come into the detector from BELOW. These can only be neutrinos, as they're the only particles that would penetrate the entire Earth. The ice tank detector that Bai & I have been working on is a prototype detector for ICETOP. Nearly 200 of these tanks will sit on the surface of the snow above the ICECUBE array in the deep ice. ICETOP will collect information about all the particles coming from the air above ICECUBE. They're (mostly) not neutrinos, so ICETOP information will be subtracted from the events recorded by ICECUBE, leaving only neutrino events. |