An essay on why I am a Neutrino Physicist, written for a book of portraits of scientists by Mariana Cooke, in publication

You can't see them but they're everywhere. There are 60 billion in front of your nose this second. You can't smell them. You can't taste them when they touch your tongue. You can't hear them. You can't feel them when they pass through you. 10,000,000,000,000,000 will do it while you read this page and you will never know.

They are neutrinos, the ``little neutral ones'' in the family of subatomic particles. Neutrinos hold secrets from the earliest days of the universe. They bring us information from deep inside exploding stars and from high energy particle collisions. Their presence may signal unexpected phenomena. Measuring their properties will help us understand how the universe will evolve.

We search for neutrinos using detectors all around the world. My experiment is at Fermi National Accelerator Laboratory, located just outside of Chicago. Our detector, MiniBooNE, is a forty foot high sphere, filled with mineral oil. When the neutrinos interact in the oil, they make small flashes of light which are seen by phototubes, which work essentially like inverse light bulbs: light goes in, an electrical pulse comes out. The phototubes are 8-inch diameter circles that are beautiful dark amber. When we installed them on the ceiling of the detector, MiniBooNE looked like a bizarre, beautiful planetarium with many moons and tiny stars that were the screws holding the tubes onto their black support structure. Despite being made of 800 tons of oil, MiniBooNE is actually quite mini compared to other neutrino experiments. The most awesomely large neutrino detector, Super-K, is located in Japan. A fifteen story building can fit comfortably within that detector!

We need such large detectors because neutrinos don't interact with matter very often. Most subatomic particles are very interactive. For example, quarks, which make up most of ordinary matter, are so active in our detectors that it is difficult to sort out the patterns that they leave. The electron is another highly evident particle -- and reliable, too. You can count on finding electrons inside your typical wall outlet and also inside your typical particle interaction. But the neutrino is different from the rest. Their interactions occur far more rarely. At the highest energy accelerator in the world, Fermilab, we observe neutrino reactions 10,000,000,000 times less often than those of quarks. They just quietly zip through the detector and go on their merry way.

Neutrino research is fascinating today because the results are full of contradictions. For fifty years, all of the evidence pointed to neutrinos being bundles of moving energy that had no mass -- a pretty weird concept for a particle. But recently we discovered a novel behavior which can only be explained if neutrinos do have mass. How do resolve this conflict?

If the neutrino has mass, it must be very, very small. It would take at least half a million neutrinos to tip the scales on the electron. Still, such a wispy particle will have a big effect in the universe. The collective mass of the neutrinos rivals the mass of all the stars! Given the discovery of mass, we can begin asking even more exciting questions. The Big Bang, for example, produced a million neutrinos in every gallon of space. The holy grail of neutrino physics is to detect these relics. Their mass may hold the key.

All of that sounds pretty esoteric, and you may ask: ``What have neutrinos done for me lately?'' Actually, they matter a lot to you. They are part of the ignition process of the sun. They play a role in heating the center of the earth, causing continental drift. So the next time you see a koala, whose evolution depended on living on an isolated continent, thank a neutrino! The tools that physicists use to create and study neutrinos have direct benefit to every one of us. One fork of the beam line for our neutrino experiment at the Fermilab goes to Neutron Therapy, a very successful cancer treatment method. The extremely clean laboratory environment of state-of-the-art solar neutrino experiments can be used for sensitive tests to monitor violations of the nuclear test ban treaty.

I love this particle because it always reminds me that even the smallest among us can change the universe.