Through a century of work, physicists have refined a model to describe all fundamental particles, the forces they share, and their interactions on a microscopic scale. This masterpiece of science is called the Standard Model. While this theory is incredibly powerful, we know of at least one particle that exhibits behaviors that are outside of its scope and remain unexplained. These particles are called neutrinos and they are the enigmatic ghosts of the quantum world. Interacting only via the weak nuclear force, literally billions of them pass through you undetected every second. While we understand that particular spooky behavior, we do not understand in any fundamental way how it is that neutrinos can literally change their identity, much as if a house cat could turn into a lion and then a tiger before transitioning back into a house cat again.

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This name is misleading from a historical perspective. Chadwick discovered the particle we now call a neutron about two years later, with properties (other than its charge) quite different from Pauli's proposed particle.
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Note that negatively charged muons and electrons are matter, while the positive ones are antimatter. Further, the line over the neutrino indicates antimatter. Finally, while the decay of negatively charged muons is shown here, the decay of positively charged muons is identical, except that all matter particles are replaced by antimatter and vice versa.
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We also had to consider the four other cases (m2 > m1 > m3), (m3 > m2 > m1), (m1 > m3 > m2), and (m2 > m3 > m1). The first two are ruled out from measuring solar neutrino oscillations, and the last two are ruled out from measuring atmospheric neutrino oscillations.
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