523 research outputs found
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
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
Low Temperature Static and Dynamic Behavior of the Two-Dimensional Easy-Axis Heisenberg Model
We apply the self-consistent harmonic approximation (SCHA) to study static
and dynamic properties of the two-dimensional classical Heisenberg model with
easy-axis anisotropy. The static properties obtained are magnetization and spin
wave energy as functions of temperature, and the critical temperature as a
function of the easy-axis anisotropy. We also calculate the dynamic correlation
functions using the SCHA renormalized spin wave energy. Our analytical results,
for both static properties and dynamic correlation functions, are compared to
numerical simulation data combining cluster-Monte Carlo algorithms and Spin
Dynamics. The comparison allows us to conclude that far below the transition
temperature, where the SCHA is valid, spin waves are responsible for all
relevant features observed in the numerical simulation data; topological
excitations do not seem to contribute appreciably. For temperatures closer to
the transition temperature, there are differences between the dynamic
correlation functions from SCHA theory and Spin Dynamics; these may be due to
the presence of domain walls and solitons.Comment: 12 pages, 14 figure
Parameterizing Majorana Neutrino Couplings in the Higgs Sector
Nonzero masses for the active neutrinos - regardless of their nature or
origin - arise only after electroweak symmetry breaking. We discuss the
parameterization of neutrino couplings to a Higgs sector consisting of one
SU(2)_L scalar doublet and one SU(2)_L scalar triplet, and allow for
right-handed neutrinos whose Majorana mass parameters arise from the vacuum
expectation value of a Standard Model scalar singlet. If the neutrinos are
Majorana fermions, all Yukawa couplings can be expressed as functions of the
neutrino mass eigenvalues and a subset of the elements of the neutrino mixing
matrix. In the mass basis, the Yukawa couplings are, in general, not diagonal.
This is to be contrasted to the case of charged-fermions or Dirac neutrinos,
where couplings to the Higgs-boson are diagonal in the mass basis and
proportional only to the fermion masses. Nonetheless, all physically
distinguishable parameters can be reached if all neutrino masses are
constrained to be positive, all mixing angles constrained to lie in the first
quadrant (theta in [0,pi/2]), and all Majorana phases to lie in the first two
quadrants (phi in [0,pi]), as long as all Dirac phases vary within the entire
unit circle (delta in [0,2pi}). We discuss several concrete examples and
comment on the Casas-Ibarra parameterization for the neutrino Yukawa couplings
in the case of the type-I Seesaw Lagrangian.Comment: 13 pages, 2 eps figure
- âŠ