697 research outputs found
Neutrinos Have Mass - So What?
In this brief review, I discuss the new physics unveiled by neutrino
oscillation experiments over the past several years, and discuss several
attempts at understanding the mechanism behind neutrino masses and lepton
mixing. It is fair to say that, while significant theoretical progress has been
made, we are yet to construct a coherent picture that naturally explains
non-zero, yet tiny, neutrino masses and the newly revealed, puzzling patterns
of lepton mixing. I discuss what the challenges are, and point to the fact that
more experimental input (from both neutrino and non-neutrino experiments) is
dearly required - and that new data is expected to reveal, in the next several
years, new information. Finally, I draw attention to the fact that neutrinos
may have only just begun to reshape fundamental physics, given the fact that we
are still to explain the LSND anomaly and because the neutrino oscillation
phenomenon is ultimately sensitive to very small new-physics effects.Comment: invited brief review, 15 pages, 1 eps figure, typo corrected,
reference adde
Addressing the Majorana vs. Dirac Question with Neutrino Decays
The Majorana versus Dirac nature of neutrinos remains an open question. This
is due, in part, to the fact that virtually all the experimentally accessible
neutrinos are ultra-relativistic. Noting that Majorana neutrinos can behave
quite differently from Dirac ones when they are non-relativistic, we show that,
at leading order, the angular distribution of the daughters in the decay of a
heavy neutrino into a lighter one and a self-conjugate boson is isotropic in
the parent's rest frame if the neutrinos are Majorana, independent of the
parent's polarization. If the neutrinos are Dirac fermions, this is, in
general, not the case. This result follows from CPT invariance and is
independent of the details of the physics responsible for the decay. We explore
the feasibility of using these angular distributions -- or, equivalently, the
energy distributions of the daughters in the laboratory frame -- in order to
address the Majorana versus Dirac nature of neutrinos if a fourth, heavier
neutrino mass eigenstate reveals itself in the current or next-generation of
high-energy colliders, intense meson facilities, or neutrino beam experiments.Comment: 11 pages, 3 figure
The Physical Range of Majorana Neutrino Mixing Parameters
If neutrinos are Majorana fermions, the lepton mixing parameter space
consists of six mixing parameters: three mixing angles and three CP-odd phases.
A related issue concerns the physical range of the mixing parameters. What
values should these take so that all physically distinguishable mixing
scenarios are realized? We present a detailed discussion of the lepton mixing
parameter space in the case of two and three active neutrinos, and in the case
of three active and N sterile neutrinos. We emphasize that this question, which
has been a source of confusion even among "neutrino" physicists, is connected
to an unambiguous definition of the neutrino mass eigenstates. We find that all
Majorana phases can always be constrained to lie between 0 and pi, and that all
mixing angles can be chosen positive and at most less than or equal to pi/2
provided the Dirac phases are allowed to vary between -pi and pi. We illustrate
our results with several examples. Finally, we point out that, in the case of
new flavor-changing neutrino interactions, the lepton mixing parameter space
may need to be enlarged. We properly qualify this statement, and offer concrete
examples.Comment: 16 pages, 2 .eps figures, references added, minor typos correcte
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