14 What Have We Learned about Neutrinos...
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whereas, when they reach the detector at night, they do. If the neutrinos indeed
undergo matter refraction, there should be a difference between the day and
night amounts of solar neutrinos at the Earth based detector. That is called the
day–night effect. Experiments like Super-Kamiokande and SNO have looked
for it. At the moment, there is only some weak evidence for it from the Super-K
observations.
14.3 Do Neutrinos Decay?
Since the neutrino puzzles discovered in the solar and atmospheric neutrino
searches involved missing neutrinos, a logical possibility is that the neutrinos
on their way could be decaying to some other particles. This possibility has
been extensively studied [48]. The first question that arises in the discussion is
how the neutrinos which are such tiny, ultra-light particles could decay. The
heavier neutrinos could decay to a lighter one together with a photon or three
light neutrinos. For a review of the current knowledge about these decays,
see [48].
An interesting situation arises when the decay to final states are invisible.
A possible theory for such an invisible decay was discussed in 1980 by Yuichi
Chikashige, Rabindra Mohapatra (the author), and Roberto Peccei [29]. It was
suggested in [29] that if the neutrinos are their own anti-particles, there could
exist a massless particle called the Majoron to which the neutrinos connect.
Therefore a heavier neutrino could decay into lighter ones by emitting a
Majoron. If we denote the Majoron particle by the symbol J , then decay could
be something like ν i → ν j + J . Since we do not know which neutrino is
heavier among the three, we could have either the solar or the atmospheric
muon neutrino or both involve in this decay. That could explain the observed
deficits since a decay makes the particle disappear. Such decays however have
other manifestations which constrain their strength to such an extent that they
are not able to explain the solar [5] and atmospheric neutrino deficits fully.
They could be contributing to the fluxes partially, though. There are however
ongoing experiments to search for such invisible decays, as well as the presence
of the Majoron particle in the universe, using the same neutrinoless double
beta decay experiments mentioned earlier (see Chap. 11). Majoron decays have
other implications for cosmology as well [30].
111
whereas, when they reach the detector at night, they do. If the neutrinos indeed
undergo matter refraction, there should be a difference between the day and
night amounts of solar neutrinos at the Earth based detector. That is called the
day–night effect. Experiments like Super-Kamiokande and SNO have looked
for it. At the moment, there is only some weak evidence for it from the Super-K
observations.
14.3 Do Neutrinos Decay?
Since the neutrino puzzles discovered in the solar and atmospheric neutrino
searches involved missing neutrinos, a logical possibility is that the neutrinos
on their way could be decaying to some other particles. This possibility has
been extensively studied [48]. The first question that arises in the discussion is
how the neutrinos which are such tiny, ultra-light particles could decay. The
heavier neutrinos could decay to a lighter one together with a photon or three
light neutrinos. For a review of the current knowledge about these decays,
see [48].
An interesting situation arises when the decay to final states are invisible.
A possible theory for such an invisible decay was discussed in 1980 by Yuichi
Chikashige, Rabindra Mohapatra (the author), and Roberto Peccei [29]. It was
suggested in [29] that if the neutrinos are their own anti-particles, there could
exist a massless particle called the Majoron to which the neutrinos connect.
Therefore a heavier neutrino could decay into lighter ones by emitting a
Majoron. If we denote the Majoron particle by the symbol J , then decay could
be something like ν i → ν j + J . Since we do not know which neutrino is
heavier among the three, we could have either the solar or the atmospheric
muon neutrino or both involve in this decay. That could explain the observed
deficits since a decay makes the particle disappear. Such decays however have
other manifestations which constrain their strength to such an extent that they
are not able to explain the solar [5] and atmospheric neutrino deficits fully.
They could be contributing to the fluxes partially, though. There are however
ongoing experiments to search for such invisible decays, as well as the presence
of the Majoron particle in the universe, using the same neutrinoless double
beta decay experiments mentioned earlier (see Chap. 11). Majoron decays have
other implications for cosmology as well [30].