Neutrinos are strange particles, and scientists were quite surprised to find that the flavor of a neutrino changes as it travels. Imagine purchasing a carton of chocolate ice cream at the store, driving home, and opening it only to find it was vanilla! So you put a scoop of vanilla in your bowl and walk into the other room to eat it, where you are surprised to find it is now strawberry.
A particle might start out as an electron neutrino, but as it moves, it morphs into a muon neutrino or a tau neutrino, changing flavors as it goes. Looking at how neutrinos change as they travel gives scientists valuable information about the ghostly particles. Neutrinos were originally theorized in by Wolfgang Pauli as a way to balance out the math and the energy in a reaction called beta decay , something that happens in the nucleus of an atom. But because of the dictates of various laws—the conservation of momentum, conservation of energy, and conservation of angular momentum, or spin—there had be an invisible particle that played a role.
Neutrinos were experimentally discovered in a reactor experiment by Frederick Reines and Clyde Cowan. This antimatter quickly annihilated with regular matter, producing gamma rays. The Project Poltergeist team led by Reines holding sign and Cowan far right was the first to experimentally detect the neutrino. Credit: Los Alamos National Laboratory. The neutrinos that were produced in the accelerator created muons when they interacted, as contrasted with neutrinos produced in reactors, which made antielectrons.
The neutrinos were clearly related to their charged partners. They had discovered muon neutrinos. It only takes a minute to sign up. Connect and share knowledge within a single location that is structured and easy to search. Moreover, how-come scientist know that muon-neutrino are different from electron-neutrino when they didn't even know what the difference was? Did they interact differently with other particles?
The Standard Model of paticle physics is a shorthand description of data gathered laboriously over half a century.
From the first decays observed, in cosmic rays and cloud chambers, it was obvious that the mediating force was different than the electromagnetic or the nuclear force. Decays are mediated by weak interactions , a conclusion that explained in a single format all observed decays. This image of neutron decay shows the process. It is a three body decay, and that is an experimental point because the momentum distributions of two body and three body decays are different. Using energy conservation and four momentum balance at first it seemed that the neutrinos had zero mass, and for many years that was the hypothesis, until neutrino oscillations were observed , but that is another story.
Eventually beams of neutrinos were created and it was affirmed that muon neutrinos scattering on protons created muons, whereas electron neutrinos created electrons. Yes they interact differently for each species, muon, electron , tau.
Thus the ground was laid for the SU 3 xSU 2 xU 1 symmetry which encompasses all the quantum number data of the observed up to now particles in a single standard model. How do they affect Big Bang cosmology? Do neutrinos oscillate? Or can neutrinos of one type change into another type as they travel through matter and space? Are neutrinos fundamentally distinct from their anti-particles?
How do stars collapse and form supernovae? What is the role of the neutrino in cosmology? One long-standing issue of particular interest is the so-called solar neutrino problem.
This name refers to the fact that several terrestrial experiments, spanning the past three decades, have consistently observed fewer solar neutrinos than would be necessary to produce the energy emitted from the sun. One possible solution is that neutrinos oscillate--that is, the electron neutrinos created in the sun change into muon- or tau-neutrinos as they travel to the earth.
Because it is much more difficult to measure low-energy muon- or tau-neutrinos, this sort of conversion would explain why we have not observed the correct number of neutrinos on Earth. Newsletter Get smart. Sign up for our email newsletter. Already a subscriber? Sign in. In this case, the difference is about one part in Or, to say it another way, mass 3 is very different from mass 1 and mass 2. What scientists have not yet determined is whether the mass 3 neutrino is heavier or lighter than the other two.
To figure it out, they are performing experiments where neutrinos are created at known energies and then allowed to propagate through enough of Earth, producing a similar effect to what happens when electron neutrinos travel through the sun.
If the third mass state is lighter than the others, then the oscillation probability will be lower than expected when the neutrinos travel through vacuum, while if it is heavier, the oscillation probability will be higher than expected in vacuum.
The opposite relationship is true for antineutrinos, and experiments compare the differences between the two to look for differences between matter and antimatter.
Physicists are not yet sure what the ordering of the neutrino masses tells us about the universe. There are many possible Grand Unified Theories , which seek to explain particle interactions as different manifestations of a single force, that predict the mass 3 neutrino is heavier than the other two. On the other hand, if the third mass state were lighter, a host of other theories about the nature of matter would become possible.
In the end, scientists have to measure the ordering of the masses first, before they can use that information as a piece of the puzzle to learn more about the nature of matter and the universe. We hope this site will serve as a resource for all those intrigued by the mysterious neutrinos that are traveling above, below, and through us.
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