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R. N. Mohapatra
electrically charged constituent (in fact, most likely no constituent at all). It
turns out that in the Quantum Field Theory framework, a neutrino can break
up to a “virtual electron” and a virtual W boson and recombine. These are
virtual processes but are constantly occurring according to Quantum Field
Theory. Since both the W and the electron have electric charge and are also
moving around, they create a magnetic moment for the neutrino, making it
behave like a very tiny magnet. Estimates show that this induced magnetic
moment is likely about 10
−19
− 10
−11 times the magnetic moment of the
electron, depending on different theoretical possibilities [51]. It also depends
on the mass of the neutrino. If the neutrino was massless like in the standard
model, it would have no magnetic moment. So, observation of a magnetic
moment would be a new way to search for neutrino related physics beyond the
standard model. Discovery of a neutrino magnetic moment will therefore be
revolutionary with all kinds of implications for new forces and matter beyond
the standard model.
If the magnetic moment of the electron neutrino was large, it would create
an 11-year modulation of the solar neutrino signal, since the large magnetic
field inside the sun changes every 11 years. The solar magnetic field can flip
the electron neutrino into a sterile neutrino, which will give no signal in
the solar neutrino detector on Earth. The 11-year cycle has to do with the
appearance of the sunspots every 11 years, which is when the magnetic field
in the Sun becomes larger. In the beginning of the Ray Davis observation of
solar neutrinos, there appeared to be some variation of the solar neutrino signal
correlated with the sunspots, but that evidence seems to have gone away. There
are however several laboratory experiments searching for the magnetic moment
of the electron neutrino, using scattering of reactor neutrinos off electrons and
nucleons. The magnetic moment increases the scattering rate of the neutrinos.
So far, no evidence for neutrino magnetic moment has been found.
14.8 What Oscillation Experiments Do Not Tell Us
While the oscillation experiments have provided a wealth of information about
the neutrinos, there is one important piece of information it cannot give
us. That is the absolute mass of the neutrinos. As emphasized earlier, the
oscillation frequency of neutrinos from the sun, atmosphere, or any other
source is always a measure of the difference in their masses. It cannot therefore
tell us what the absolute mass of the neutrinos is. There are however ways
to find the absolute mass, and experiments are under way to do that. For
example, if the neutrinos are Majorana fermions, then the neutrinoless double
R. N. Mohapatra
electrically charged constituent (in fact, most likely no constituent at all). It
turns out that in the Quantum Field Theory framework, a neutrino can break
up to a “virtual electron” and a virtual W boson and recombine. These are
virtual processes but are constantly occurring according to Quantum Field
Theory. Since both the W and the electron have electric charge and are also
moving around, they create a magnetic moment for the neutrino, making it
behave like a very tiny magnet. Estimates show that this induced magnetic
moment is likely about 10
−19
− 10
−11 times the magnetic moment of the
electron, depending on different theoretical possibilities [51]. It also depends
on the mass of the neutrino. If the neutrino was massless like in the standard
model, it would have no magnetic moment. So, observation of a magnetic
moment would be a new way to search for neutrino related physics beyond the
standard model. Discovery of a neutrino magnetic moment will therefore be
revolutionary with all kinds of implications for new forces and matter beyond
the standard model.
If the magnetic moment of the electron neutrino was large, it would create
an 11-year modulation of the solar neutrino signal, since the large magnetic
field inside the sun changes every 11 years. The solar magnetic field can flip
the electron neutrino into a sterile neutrino, which will give no signal in
the solar neutrino detector on Earth. The 11-year cycle has to do with the
appearance of the sunspots every 11 years, which is when the magnetic field
in the Sun becomes larger. In the beginning of the Ray Davis observation of
solar neutrinos, there appeared to be some variation of the solar neutrino signal
correlated with the sunspots, but that evidence seems to have gone away. There
are however several laboratory experiments searching for the magnetic moment
of the electron neutrino, using scattering of reactor neutrinos off electrons and
nucleons. The magnetic moment increases the scattering rate of the neutrinos.
So far, no evidence for neutrino magnetic moment has been found.
14.8 What Oscillation Experiments Do Not Tell Us
While the oscillation experiments have provided a wealth of information about
the neutrinos, there is one important piece of information it cannot give
us. That is the absolute mass of the neutrinos. As emphasized earlier, the
oscillation frequency of neutrinos from the sun, atmosphere, or any other
source is always a measure of the difference in their masses. It cannot therefore
tell us what the absolute mass of the neutrinos is. There are however ways
to find the absolute mass, and experiments are under way to do that. For
example, if the neutrinos are Majorana fermions, then the neutrinoless double