110
R. N. Mohapatra
different. The first and second families mix almost as strongly as the second
and third family. Why is this so? If quarks and leptons are to be part of the
same kind of matter as grand unified theories would want them to be, how
do we understand such a diverse pattern in their mixing? This has remained
a challenge to theorists working on building unified models for neutrinos
with quarks. There are speculations that they may owe their origin to new
symmetries of nature or some particular kind of grand unified theory [66].
14.2 Refraction of Neutrinos in Matter
The phenomenon of light refraction is well known from high school physics.
When light passes through a transparent medium such as glass or water, the
ray of light gets bent. The reason for this is, as light travels from one medium
to another, its speed becomes less if the second medium is denser. Since
according to quantum mechanics, all matter also behave like waves, could it
be that neutrinos passing through matter undergo refraction, just like light?
The fact that this indeed happens to neutrinos was first suggested by Lincoln
Wolfenstein in 1978 [104]. As the neutrinos pass from a less dense to a more
dense medium, their speed goes down. How much the speed goes down also
depends on their original speed. And if the neutrinos mix, the oscillation from
one to the other neutrino takes place at a different rate in a medium than in an
empty space. This has significant implications for neutrinos traveling through
the sun or the Earth, or other dense environments, like supernovae.
After this suggestion was made by Wolfenstein, it was applied to neutrinos
coming from the core of the sun to the Earth by Stanislaw Mikheyev and
Alexei Smirnov [74], to see how it manifests in the solar neutrino signal on
Earth. Since the sun has a lot of matter and neutrinos produced in the sun’s
core have to travel through it, there has to be some effect on them. Mikheyev
and Smirnov studied this in detail and showed how it affects the oscillation
pattern of the neutrinos of different energies. This effect is called the MSW
effect after them. Since the various solar neutrino experiments explore different
energy neutrinos from the sun (and the MSW effect depends on the neutrino
energy), this kind of effect should be observable by a different final energy
distribution of neutrinos on Earth compared to what was expected from the
core. It indeed appears that solar neutrinos exhibit matter-induced refractive
effects due to the core of the sun.
A confirmation of this effect is expected to come from the observation of
other solar neutrino effects. For instance, when the solar neutrinos come to
the Earth during the day time, they do not go through the Earth matter,
R. N. Mohapatra
different. The first and second families mix almost as strongly as the second
and third family. Why is this so? If quarks and leptons are to be part of the
same kind of matter as grand unified theories would want them to be, how
do we understand such a diverse pattern in their mixing? This has remained
a challenge to theorists working on building unified models for neutrinos
with quarks. There are speculations that they may owe their origin to new
symmetries of nature or some particular kind of grand unified theory [66].
14.2 Refraction of Neutrinos in Matter
The phenomenon of light refraction is well known from high school physics.
When light passes through a transparent medium such as glass or water, the
ray of light gets bent. The reason for this is, as light travels from one medium
to another, its speed becomes less if the second medium is denser. Since
according to quantum mechanics, all matter also behave like waves, could it
be that neutrinos passing through matter undergo refraction, just like light?
The fact that this indeed happens to neutrinos was first suggested by Lincoln
Wolfenstein in 1978 [104]. As the neutrinos pass from a less dense to a more
dense medium, their speed goes down. How much the speed goes down also
depends on their original speed. And if the neutrinos mix, the oscillation from
one to the other neutrino takes place at a different rate in a medium than in an
empty space. This has significant implications for neutrinos traveling through
the sun or the Earth, or other dense environments, like supernovae.
After this suggestion was made by Wolfenstein, it was applied to neutrinos
coming from the core of the sun to the Earth by Stanislaw Mikheyev and
Alexei Smirnov [74], to see how it manifests in the solar neutrino signal on
Earth. Since the sun has a lot of matter and neutrinos produced in the sun’s
core have to travel through it, there has to be some effect on them. Mikheyev
and Smirnov studied this in detail and showed how it affects the oscillation
pattern of the neutrinos of different energies. This effect is called the MSW
effect after them. Since the various solar neutrino experiments explore different
energy neutrinos from the sun (and the MSW effect depends on the neutrino
energy), this kind of effect should be observable by a different final energy
distribution of neutrinos on Earth compared to what was expected from the
core. It indeed appears that solar neutrinos exhibit matter-induced refractive
effects due to the core of the sun.
A confirmation of this effect is expected to come from the observation of
other solar neutrino effects. For instance, when the solar neutrinos come to
the Earth during the day time, they do not go through the Earth matter,