14 December, 2002

14 December, 2002

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.

But today, let's talk about neutrinos.

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.

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 parcticles 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, "parcticles" 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 ha

The symbol for the neutrino, carved in ice at the South Pole.

The AMANDA logo. AMANDA is interested in detecting parcticles which come into the detector from BELOW. These can only be neutrinos, as they're the only parcticles 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 parcticles 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.