One particle, the Z boson, is the carrier of the weak force, and how quickly it decays depends strongly on the number of particles that it couples to. Credit: CERNĪt the Large Electron-Positron (LEP) collider at CERN, scientists measured the particles that emerged from collisions between electrons and positrons. Technicians make delicate adjustments to one of LEPs thousands of magnets in 1999. Much more data is needed before anything can be decided definitively. If neutrinos oscillate into this fourth kind of neutrino, that could explain the rapid changes and the anomalies seen in experiments. And some experiments have seen neutrinos appearing or disappearing over much shorter distances than the experiments on neutrinos from more distant locations, such as the atmosphere or sun. Some experiments have seen an excess neutrino oscillation where theory predicted they shouldn’t be. One way to discover these secretive particles involves oscillation. Because the weak force would ignore them, sterile (right-handed) neutrinos would interact only through gravity, making them borderline invisible. But if there are right-handed neutrinos, they could be the predicted sterile neutrinos. So far, scientists have found only left-handed neutrinos. The weak force strongly prefers to interact with left-handed particles. These are right-handed and left-handed particles, and they’re important because one of nature’s four forces-the weak force-does not treat them equally. The way your fingers curl represents a particle’s spin, and your thumb points in the direction of travel. Helicity refers to how the spin relates to the movement of the particle, and it’s analogous to the idea of someone being left handed or right handed. The particles don’t literally spin like a top, but this is still a good way of thinking about it. Particles (including the wacky neutrino) have properties called spin and helicity. Is there just one to add in, or perhaps a parallel three? Or maybe there are even more! While they know of the three flavors of neutrinos, scientists aren’t sure how many kinds of sterile neutrinos there might be. Finding slight signals amidst the chaos of the universe will be tough, but not impossible. Gravity is the weakest of all the forces, and neutrinos are very light-so they don’t give gravity much to work with. This would make them even harder to spot than the tricky “regular” neutrinos. While the standard electron, muon, and tau neutrinos (and antineutrinos) interact with matter through two forces (the weak force and gravity), scientists think sterile neutrinos might interact only through gravity. Scientists are looking hard for them in many different experiments. Sterile neutrinos are a special kind of neutrino that has been proposed to explain some unexpected experimental results, but they have not been definitively discovered.
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