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The Neutrino Physics Motivation

What Are Neutrinos, Anyway?

Neutrinos are one of nature's elementary particles. They are neutrally-charged, extremely light, and can travel at speeds very close to the speed of light. Neutrinos are also hard to detect, as they pass straight through everyday matter. In fact, about a hundred billion neutrinos just went through your thumbnail in the last second! Detecting neutrinos is a challenge for experiments like IsoDAR and DAEδALUS, but it can be done.

Neutrinos come in three flavors: electron neutrino, muon neutrino, and tau neutrino.

They also have corresponding antiparticles, collectively called antineutrinos.

Sometimes, the term "neutrinos" refers to both neutrinos and antineutrinos.

So, we have three flavors of neutrinos, and three corresponding flavors of antineutrinos. The neutrino story sounds straightforward so far, but it gets more puzzling from here.

Neutrino Oscillations

Neutrinos gained their flavor state names because electron neutrinos interact with electrons, muon neutrinos interact with muons, and tau neutrinos interact with tau particles (electrons, muons, and tau particles are all elementary particles).

However, neutrinos travel as a mixture of the three flavor states which are known as the mass eigenstates. There are currently only three known mass eigenstates. They are called ν1, ν2, and ν3 and their proportion of mixing is shown below.

This "neutrino mixing" causes neutrinos to oscillate between flavor states as they travel. Through neutrino oscillations, all three flavors of neutrinos can change into one another.

For example, an electron neutrino is a mixture of the mass eigenstates of ν1, ν2, and a little bit of ν3. (You can see this graphically on the mixing chart above; the electron neutrino is represented by the green.) Even if you are sure the neutrino you started off with was an electron neutrino, when it travels, it evolves as a mixture of the ν1, ν2, and ν3 neutrinos. Since each of these have different masses, they evolve with a different quantum (de Broglie) wavelength and, as they propogate, become different mixutres of the electron, muon, and tau neutrinos. (Just as the flavor states are a mixture of the mass states, the mass states are a mixture of the flavor states.)

Later, when you check up on the flavor of what was originally an electron neutrino, you are actually looking at a neutrino that is a mix of three flavors. Therefore, the neutrino previously observed as having electron flavor now has nonzero probabilities of being observed with either electron, muon, or tau flavor.

The proportions of flavor mixing in mass eigenstates are fixed quantities, and these proportions are described by a set of fixed parameters. Neutrino oscillations are fundamentally described by these parameters. These parameters, if known, can be used to calculate oscillation probabilities in neutrino oscillations.

Great headway has been made in the relatively short history of neutrino research in narrowing uncertainties on the numerical values of the neutrino mixing parameters. However, two parameters remain ill-defined in value. One of them is δcp, a mathematical term that describes how flavors mix, and it is the principal subject of scrutiny for DAEδALUS. Another issue is that these mixing angles do not always give the same oscillation probabilities as those observed through experiment. This could be evidence of sterile neutrinos, which is a focus of IsoDAR.

Shattered Symmetries in CP Violation

This δcp is an especially interesting parameter. It is linked to something known as CP violation, a phenomenon that may be able to explain the universe's matter-antimatter asymmetry. In particular, if DAEδALUS can find CP violation in neutrinos, we may be able to understand why we exist.

To learn more about CP violation and how it relates to neutrinos and our existence, read on in the CP Violation section.