September 7, 2006

Physicists Play Tag With Neutrinos

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Watch out! As you are reading this article you are being bombarded and penetrated by approximately 100 billion particles every second. Yet you don’t feel or see them and the bombardiers don’t really feel or sense you either. That’s because they are exceptionally insensitive particles called neutrinos. Although neutrinos (an Italian term coined by Enrico Fermi, meaning small and neutral) are tiny, if you were being bombarded at the same rate by other particles, such as electrons, the radiation would be devastating. To give an idea of how many neutrinos exist, if the number of objects in the universe was divided evenly amongst its entire volume, for each cubic meter there would exist approximately one hydrogen atom but 300 million neutrinos.
The neutrino is extremely small, approximately 3.5 billion times less massive than that single hydrogen atom. Even though neutrinos weigh so little, they may contribute as much mass to the universe as all the stars due to their sheer number. Neutrinos are created through a number of natural processes (like radioactive decay) and recently scientists have created neutrinos in particle accelerators. Since the human body contains about 20 milligrams of radioactive potassium, you are actually emitting about 340 million neutrinos a day.
Going back to the bombardment, if this many particles are passing through you, why can’t you feel them? Their weak interaction is the neutrino’s most dominating feature and historically a source of frustration. The Nobel Prize winner Wolfgang Pauli predicted the neutrino in 1930 and said, “I have done something very bad today by proposing a particle that cannot be detected; it is something no theorist should ever do.” An inability to detect the right number of neutrinos emitted by the sun exasperated scientists from the 1960s into the 21st century, until detectors that could measure different types of neutrinos were built.
Neutrino detection is a challenge because they do not interact through the electromagnetic force, unlike electrons that can be easily detected through this force. Electromagnetism isn’t just circuits and compasses. It’s much more than that; it provides the force that gives objects shapes, prevents objects from falling through each other, and dominates interactions in almost all of classical physics and chemistry. Scientists rely almost exclusively on this force for observation in research. Neutrinos do interact through its gravitational pull but the force is minuscule because each neutrino is so small. Most research uses the weak force to detect neutrinos, which require small distances on the order of a thousand times less than the diameter of the hydrogen nucleus.
Your body appears to be a solid surface with well-defined edges. However, the neutrino sees only a collection of tiny points representing the individual atomic nuclei inside each atom. The nucleus of the hydrogen atom is a hundred thousand times smaller than the diameter of the atom. This means that more than 99 percent of ‘solid’ matter is really empty space and this explains why neutrinos pass so easily through matter. It takes an almost direct hit — the neutrino must pass very close to the nucleus or the even smaller electron in order to interact and be detected by scientists. This is why most neutrinos just pass right through everything. Despite 100 billion neutrinos passing through you every second, your body will interact with only one neutrino in your entire lifetime (and not really affect you at all).
Neutrinos are also complicated because they come in three types and can oscillate between types over time. It is these oscillations that allow their masses to be measured and the different types to be determined. Last week Dr. William Louis of the Los Alamos National Laboratories, a neutrino physicist, spoke at the physics colloquium. He is one of the scientists working on the MiniBooNE experiment designed to detect neutrinos and their properties better than has been previously done. The experiment uses 800 tons of mineral oil to serve as a target for incoming neutrinos generated in huge numbers by the Fermi National Lab accelerator. There are 1,520 detectors surrounding the mineral oil in order to record each collision event. Using knowledge of how neutrinos oscillate between known types, the MiniBooNE data may someday indicate that a new type of neutrino exists. These new “sterile” neutrinos will interact even less than the known types. If such sterile neutrinos exist, they would increase the mass contribution of neutrinos to the universe and may help explain science phenomena such as dark matter.
The weekly physics colloquium is held on Mondays in the Schwartz Auditorium, Rockefeller Hall at 4 pm.