July 11, 2008
Five Questions on Catching "Ghost Particles" with Kate Scholberg
Ultra-tiny, ultra-fast neutrinos are zinging around in every direction
By Kate Scholberg
Duke physicist Kate Scholberg has been trying to catch neutrinos, the smallest, fastest most ghostly particles you can imagine. She's an associate professor at Duke, and spends a lot of time in Japan at the "Super-K" neutrino detector. Learn more about her work and see some pictures here.
Q - What is a neutrino?
Neutrinos, sometimes known as "ghost particles," are among the known "elementary" particles: unlike atoms, they are not made up of anything smaller. Neutrinos are special because they are neutral (have no electric charge) and they interact extremely weakly with matter. They also have very tiny masses: a neutrino has no more than about 1/500,000 the mass of an electron. Because of their tiny masses, neutrinos also travel at speeds very close to the speed of light. It's been only a bit more than a decade since we've learned that neutrino masses are not zero, but have small but finite values. Neutrinos come in three "flavors": electron, muon and tau.
Q- Why are neutrinos so hard to catch?
One of the neutrino's basic properties is that it interacts only via the weak force-- this is one of the four known forces (the others are gravity, electromagnetism, and the strong force which holds atomic nuclei together). As you might guess from the name, the weak force is really feeble, and that means that neutrinos hardly ever interact with matter at all. Mostly they just pass right through things without leaving any trace. Once in a while, they do interact, leaving a charged particle that you can detect. In order to "catch" a neutrino (that is, to detect the interaction), you need either a huge number of neutrinos, or an enormous detector, or preferably both. For example, Super-Kamiokande, the neutrino detector I work on in Japan, is gigantic: it's about forty meters high, and contains 50 kilotons of water. We see only about ten high energy neutrinos per day in the detector. On the surface of the Earth, cosmic radiation can easily swamp a signal this slight, so neutrino detectors are often built underground where they are shielded from cosmic radiation.
Q- When we can catch a neutrino, what do we learn from it?
Particle physicists like me try to understand the basic nature of matter and energy: our goal is to learn what the universe is made of, and how its constituents interact with one another. We're also interested in cosmology-- the history and evolution of the entire universe. It's essential to understand the fundamental physics in order to understand what happened after the Big Bang, and why the universe looks as it does today. For instance, nobody understands why the universe is made primarily of matter and not antimatter (which has properties very much like matter, but opposite charge). The study of neutrinos can give insight into many questions like this one. What specifically we learn with a neutrino detector depends on the source of neutrino, the type of neutrino, and how far the neutrinos travel. For instance, at Super-K we can detect neutrinos that come from collisions of cosmic rays (high energy particles from outer space) with the upper atmosphere. These neutrinos travel through the Earth: some of them go a short distance (if they came from above) and some of them travel all the way from the other side of the Earth. What we observe is that neutrinos change from one flavor to another as they travel-- it turns out that this can only happen if neutrinos have mass. For our next experiment, called T2K, we will send a beam of high-energy neutrinos from an accelerator a distance of 300 km to Super-K. This may tell us more about how neutrinos change flavor. The neutrino properties we will measure are intimately connected to the question of why the universe is made of matter and not antimatter.
Q- Super-K looks like it was really expensive to build. Who's spending all this money and why?
Super-K is funded by the Japanese and US governments. Governments fund particle physics research as a long-term investment in the scientific and technological health of their nations. Research into basic science, like neutrino physics, doesn't always have many obvious short term applications. It's the long term that matters: consider that a century or so ago nobody knew what electrons were good for. But without an understanding of these elementary particles, we'd never have cell phones, lasers, computers-- the list is endless. In addition, often there are tangential technology spin-offs: for example, the World Wide Web was invented at CERN, a particle physics lab in Switzerland.
Q - If neutrinos are a form of space radiation and they're hitting us all the time, do we know whether they're causing cancer or other mutations?
Neutrinos are actually not really "hitting" us all the time-- they mostly just slide right through us without interacting, and do no damage at all. We are constantly being bombarded by other kinds of natural radiation, such as charged particles from space and radioactivity from the Earth: these particles do deposit energy in your body and can disrupt cellular function. However the contribution neutrinos make to this damage is essentially nothing. Neutrinos are the least of our worries as far as radiation damage is concerned.
Christopher G. Willett, M.D. is the Chairman and Leonard R. Prosnitz Professor of Radiation Oncology in the Duke Medical Center. Dr. Willett is a national authority on the treatment of gastrointestinal cancers. Read more about him here and then post your questions about radiation oncology by email to firstname.lastname@example.org