Meet Professor Kara Hoffman,
Looking To the Heavens For Neutrinos

Kara Hoffman knows about speeding particles. After all, she has worked on CDF, a giant microscope used to analyze the debris from protons colliding with their antimatter analog, the antiproton, at the Tevatron, currently the world's high energy "atom-smasher". She also lived in Switzerland for a few years while she collaborated with scientists working on the Large Electron Positron Collider (LEP), which hurtled electrons at it's antimatter partner, the positron, at energies infinitesimally close to the speed of light. She even tried her hand at designing the machines which accelerate subatomic particles to such mind-boggling speeds: as a member of the Muon Collaboration, she worked on designing fanciful machines which might one day accelerate muons, a much heavier cousin of the electron, to such a clip. Such an accelerator is made even more intriguing because muons are short lived, and decay to a beam of neutrinos, ephemeral particles so lightweight that they have been only recently discovered to have any mass at all. So why has Professor Hoffman now turned her attention to the heavens?

"There are objects out there in space that can accelerate subatomic particles to energies much greater than any man made accelerator here on earth," she explains. "Scientists have been aware of this for decades, but no one can explain where they're coming from...and instances have been recorded of particles reaching earth at billions of times the energies achieved at even the Tevatron."

The search for the origins of these so-called "cosmic rays" has previously focused on messengers such as photons (light), and more recently protons. These particles are launched on their journey through space by violent collisions and nuclear reactions that take place within stars and galaxies, for example. Professor Hoffman explains that modern day astrophysicists can use radiation detectors to "see" these particles radiating from various points in the sky.

"This is not so different from the methods of famous astronomers of antiquity," she says. "Galileo, in fact, used his eyes as radiation detectors to sense photons -light, that is- emanating from points in the sky. Your eye is a radiation detector, of sorts, but it's sensitivity is limited to radiation of a particular type and frequency- visible light. You can supplement this information by looking at other types of radiation."

Protons, like photons, are also relatively easy to detect. That's because protons have a mass and electrical charge and therefore make a mark when they collide with a radiation detector. The energy from the impact allows the detector to sense their presence. Therein lies the problem, explains Professor Hoffman: "if a particle can deposit enough energy in your detector for you to see it, how do you know it hasn't been depositing energy and slowing down as it travels through space to reach us? That means the information we receive from photons and protons may be somewhat misleading. For example, if they ricochet in a collision, they may appear to be coming from somewhere they're not."

The solution to this ambiguity may lie in those ghostly neutrinos mentioned earlier. Neutrinos often travel clean through the earth without slowing down or deviating from their original path. It is hypothesized that neutrinos might be copiously produced by these enigmatic cosmic accelerators in much the same way that Professor Hoffman and her former colleagues proposed to make a neutrino beam here on earth. The problem is, if neutrinos rarely leave any trace, then how do you build a telescope study them?

Professor Hoffman and her new colleagues on the IceCube collaboration, which also includes Professors Sullivan and Goodman, are banking on a new idea--building a telescope that instead of looking up into the sky, looks down into the earth. The earth is not much of an obstacle to a neutrino, but it will screen out other, more easily detected types of radiation. Otherwise, looking for neutrinos would be like stargazing on a sunny day. "Although most neutrinos travel through the earth undisturbed, there is a chance, a tiny chance, that they will interact with the material in the earth or your detector. When they do, they produce a particle such as a muon or an electron that we CAN detect. Muons leave a faint blue glow, called Cerenkov radiation, when traveling through water, so your telescope is essentially a huge tank." The trouble is, with such small odds, how do you catch enough of them to form an image of what's going on out there? "You build a VERY big telescope", says Professor Hoffman.

The IceCube telescope will be very big indeed. It will be shaped like a cube that is 1 km (2/3 of a mile) on each side. Luckily, nature has provided the huge tank of water needed-- in the form of ice. The South Pole is covered with a solid block of ice several miles thick and largely unpolluted by human activity.

All scientists from the IceCube collaboration need to do is embed instruments called phototubes into the ice to detect the faint blue glow that results from neutrino interactions. Phototubes are essentially a light bulb, but in reverse. Instead of converting electricity into light, they convert light into electricity, which is then used by a computer at the South Pole to gather information about the sky--over the Northern hemisphere!
"It's a simple idea, in principle, just add phototubes and--viola!"

The experiment, however, is complicated by the fact that IceCube scientists are working in perhaps the most hostile environment on earth. Will the ghostly neutrinos finally bring our extreme universe into focus? "It's quite a challenging project", says Professor Hoffman, "but one we hope will pay off with some very exciting discoveries. If we 'see' something, I'll make sure that readers of The Photon will be among the first to know!"

Well, in that case, I guess we here at The Photon should wish her luck...

**Editor's Note: On January 27, 2005 (after this article was completed), the IceCube team successfully deployed its first string of 60 phototubes into the ice of the South Pole. Many congratulations to Professor Hoffman and the many hard-working scientists involved with the project.