24 Dark Universe
185
will only be significant on the cosmological scale with large masses for mirror
stars. Because of this, if the dark matter particle comes from this sector, it will
not be detectible by conventional dark matter search experiments.
As far as self-interaction of dark matter is concerned, this property is now
obvious if a mirror hydrogen atom or mirror neutron and proton is the dark
matter. This is because the same particles of our sector have self-interaction.
Thus self-interaction is an automatic property of mirror dark matter.
24.4 Are There Sterile Neutrinos?
We have been discussing so far three kinds of neutrinos (ν e , ν μ , ν τ ) in previous
chapters, all of which have been discovered and studied in great detail. As
these experiments were going on, an experiment was carried out in Los Alamos
National Laboratory in the early 1990s, where a new kind of oscillation of
muon neutrinos to electron neutrinos was discovered. This was different from
the corresponding oscillations discovered for solar and atmospheric neutrinos.
What was found in the Los Alamos experiment (called LSND) was that the
mass difference square of the two neutrinos was of the order of one eV
2 . This
is very different from the solar and atmospheric observations where the mass
differences confirmed were much smaller (of order ∼10
−5 eV
2 . So what is
going on?
There were many different theoretical models proposed to explain this
observation, but the only model that fits all other oscillation observations along
with this new one is that there is one or several new kinds of neutrinos which
have masses of about one eV. The above process occurs when ν μ oscillates to
the new state and then the new state (call it ν s ) oscillates back to a ν e [56].
The mixing angles which determine the strength of this oscillation is much
smaller than those controlling solar and atmospheric neutrino oscillations. As
a result, the presence of this new oscillation adds very little to the solar and
atmospheric results, which are well understood without this new particle. This
new neutrino state is called a sterile neutrino, which means it does not have
interactions with W and Z bosons. Because if it did, it should have been seen
in the decay of the Z boson, which is well studied and understood and has no
room for decay to this new neutrino.
There are also other observations which have been pointing towards such an
oscillation: they are from the anti-neutrinos emitted from a reactor. In a reactor, different radioactive nuclei such as
235 U,
238 U,
239 Pu,
241 Pu (U = uranium
and Pu = plutonium) decay, giving rise to electron anti-neutrinos and an
electron. Electron energies from these decays are carefully measured, and from
185
will only be significant on the cosmological scale with large masses for mirror
stars. Because of this, if the dark matter particle comes from this sector, it will
not be detectible by conventional dark matter search experiments.
As far as self-interaction of dark matter is concerned, this property is now
obvious if a mirror hydrogen atom or mirror neutron and proton is the dark
matter. This is because the same particles of our sector have self-interaction.
Thus self-interaction is an automatic property of mirror dark matter.
24.4 Are There Sterile Neutrinos?
We have been discussing so far three kinds of neutrinos (ν e , ν μ , ν τ ) in previous
chapters, all of which have been discovered and studied in great detail. As
these experiments were going on, an experiment was carried out in Los Alamos
National Laboratory in the early 1990s, where a new kind of oscillation of
muon neutrinos to electron neutrinos was discovered. This was different from
the corresponding oscillations discovered for solar and atmospheric neutrinos.
What was found in the Los Alamos experiment (called LSND) was that the
mass difference square of the two neutrinos was of the order of one eV
2 . This
is very different from the solar and atmospheric observations where the mass
differences confirmed were much smaller (of order ∼10
−5 eV
2 . So what is
going on?
There were many different theoretical models proposed to explain this
observation, but the only model that fits all other oscillation observations along
with this new one is that there is one or several new kinds of neutrinos which
have masses of about one eV. The above process occurs when ν μ oscillates to
the new state and then the new state (call it ν s ) oscillates back to a ν e [56].
The mixing angles which determine the strength of this oscillation is much
smaller than those controlling solar and atmospheric neutrino oscillations. As
a result, the presence of this new oscillation adds very little to the solar and
atmospheric results, which are well understood without this new particle. This
new neutrino state is called a sterile neutrino, which means it does not have
interactions with W and Z bosons. Because if it did, it should have been seen
in the decay of the Z boson, which is well studied and understood and has no
room for decay to this new neutrino.
There are also other observations which have been pointing towards such an
oscillation: they are from the anti-neutrinos emitted from a reactor. In a reactor, different radioactive nuclei such as
235 U,
238 U,
239 Pu,
241 Pu (U = uranium
and Pu = plutonium) decay, giving rise to electron anti-neutrinos and an
electron. Electron energies from these decays are carefully measured, and from