Though trillions of naturally occurring neutrinos pass through us each second, they interact so rarely with other particles that they are very difficult to detect. More than 10 trillion man-made neutrinos pass through the 6,000-ton MINOS far detector, located in the Soudan Underground Laboratory in Minnesota, each year. But only about 1,500 collide with atoms inside the detector.
That is why researchers strive to create intense beams packed with as many neutrinos as they can produce and to build large, precise detectors that can spot them when they interact.
One reason researchers study neutrinos is to try to explain why we exist. Physicists theorize that the big bang created equal amounts of matter and antimatter. When corresponding particles of matter and antimatter meet, they annihilate one another. But somehow we're still here, and antimatter, for the most part, has vanished without a trace.
If this is true, it seems that at some point, matter and antimatter must have behaved differently from one another.
Physicists had held to the decree that nothing would change about the laws of physics if every particle were twisted around its axis and replaced with its antiparticle. This is called charge-parity symmetry. But it turns out that matter and antimatter are not exactly equal opposites, and this could explain why they exist in unbalanced quantities. Breaking charge-parity symmetry is called CP violation.
In order to advance the theory that CP violation caused the imbalance between matter and antimatter, physicists need to observe it in action. They observe decays of particles that can result in either matter or antimatter. If the decays produce the two in unequal amounts, that could signify the new physics researchers hope to discover.
Researchers at Fermilab use the NuMI beam to study neutrino oscillations, when neutrinos change from one flavor of neutrino to another. If antineutrinos do not follow the same pattern as neutrinos when they change from one flavor to another, this is a signal of CP violation. The same mechanism that could cause neutrinos and antineutrinos to oscillate differently can cause decays that would create more matter than antimatter and help explain the dominance of matter in the universe.
When certain subatomic particles called kaons decay, they break into a charged pion, a neutrino and either an electron or its antimatter counterpart, a positron. In the absence of CP violation, the number of electrons and positrons created in these decays would be equal. However, scientists have observed that the scales tip slightly toward decay into electrons. This provides proof that CP violation can lead to an excess of matter over antimatter. If this process occurred in the early universe, all of the positrons would annihilate upon encountering electrons. But after all of the positrons had disappeared, some matter would remain.
This result gives credence to the theory that CP violation allowed all of us to exist. But the effects of the process that causes an excess of matter from kaon decays are too small to complete the picture. The observed difference is orders of magnitude away from explaining asymmetry in the universe. This is one of the reasons that scientists are so interested in observing CP violation in other places, like neutrinos.
The Nu Tau or DONUT collaboration at Fermilab announced on July 21, 2000, the first direct evidence for the subatomic particle called the tau neutrino, the third kind of neutrino known to particle physicists.
Although earlier experiments had produced convincing indirect evidence for the particle's existence, no one had directly observed a tau neutrino, an almost massless particle carrying no electric charge and barely interacting with surrounding matter.
The collaboration reported instances of a neutrino interacting with an atomic nucleus to produce a charged particle called a tau lepton, the signature of a tau neutrino. To make this find, they aimed Fermilab's intense beam of neutrinos across a 3-foot-long target of iron plates sandwiched with emulsion, similar to photographic film, which recorded the particle interactions. In the target, one out of 1 trillion tau neutrinos interacted with an iron nucleus and produced a tau lepton, which left its 1-millimeter-long tell-tale track in the emulsion. Physicists needed about three years of painstaking work to identify the tracks revealing a tau lepton and its decay, the key to exposing the tau neutrino's secret existence.
Experimenters at Fermilab's NuTeV, Neutrinos at the Tevatron, experiment discovered an imbalance of neutrinos and muons emerging from high-energy collisions of neutrinos with target nuclei in a 700-ton detector.
The results of generations of particle experiments with other particles have yielded precise predictions for the value of this ratio, which characterizes the interactions of particles with the weak force, one of the four fundamental forces of nature. But neutrinos did not fall into line with those expectations.
Experimenters using the Large Electron Positron at CERN, the European particle physics laboratory, measured the same neutrino interaction in a different particle reaction. They saw the same discrepancy, although with less precision. If the discrepancy is real, it could be another indication that neutrinos truly are different.