14
What Have We Learned about Neutrinos
from Neutrino Oscillation Experiments?
The discovery of neutrino oscillations was a ground breaking discovery, and
the first thing we learned from this discovery is that the neutrinos have mass,
unlike what we were led to believe by the standard model. This is already a
profound new lesson, which says that the standard model needs modification.
What else have we learned? The next thing we learnt is that the three neutrinos
have different masses. This is because the neutrino oscillation requires that the
mass difference between the neutrinos that oscillate into each other cannot be
zero. The frequency of neutrino oscillations depends on the mass difference
square divided by the neutrino energy and does not say what the individual
masses are.
14.1 Who Is Heavy and Who Is Light: Mass
Ordering of Neutrinos
To see what we actually learned, note that there are two classes of oscillations:
one, where the neutrinos from the atmosphere oscillate, and another where
the neutrinos from the sun oscillate. In the first case, it is primarily the muon
type neutrinos that oscillate. They outnumber the electron type neutrinos by
a factor of two. In the solar case, it is the electron type neutrinos that oscillate,
since no other types of neutrinos can be produced in the sun due to lepton
number conservation. Thus there are two separate mass differences involved.
With three neutrinos, one can only have two differences. In fact, what we have
learned from the atmospheric oscillation observations is that there is a mass
© The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer
Nature Switzerland AG 2021
R. N. Mohapatra, The Neutrino Story: One Tiny Particle’s Grand Role in the Cosmos,
https://doi.org/10.1007/978-3-030-51846-2_14
107
What Have We Learned about Neutrinos
from Neutrino Oscillation Experiments?
The discovery of neutrino oscillations was a ground breaking discovery, and
the first thing we learned from this discovery is that the neutrinos have mass,
unlike what we were led to believe by the standard model. This is already a
profound new lesson, which says that the standard model needs modification.
What else have we learned? The next thing we learnt is that the three neutrinos
have different masses. This is because the neutrino oscillation requires that the
mass difference between the neutrinos that oscillate into each other cannot be
zero. The frequency of neutrino oscillations depends on the mass difference
square divided by the neutrino energy and does not say what the individual
masses are.
14.1 Who Is Heavy and Who Is Light: Mass
Ordering of Neutrinos
To see what we actually learned, note that there are two classes of oscillations:
one, where the neutrinos from the atmosphere oscillate, and another where
the neutrinos from the sun oscillate. In the first case, it is primarily the muon
type neutrinos that oscillate. They outnumber the electron type neutrinos by
a factor of two. In the solar case, it is the electron type neutrinos that oscillate,
since no other types of neutrinos can be produced in the sun due to lepton
number conservation. Thus there are two separate mass differences involved.
With three neutrinos, one can only have two differences. In fact, what we have
learned from the atmospheric oscillation observations is that there is a mass
© The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer
Nature Switzerland AG 2021
R. N. Mohapatra, The Neutrino Story: One Tiny Particle’s Grand Role in the Cosmos,
https://doi.org/10.1007/978-3-030-51846-2_14
107