2,729 research outputs found
Neutrino Mass Hierarchy, Vacuum Oscillations, and Vanishing U_e3
Is the relatively isolated member of the neutrino mass spectrum heavier or
lighter than the two closely-spaced members? This question - the character of
the neutrino mass hierarchy - is of great theoretical interest. All previously
identified experiments for addressing it via neutrino oscillations require that
the currently unknown size of the U_e3 element of the leptonic mixing matrix
(parameterized by the unknown theta_13 mixing angle) be sufficiently large, and
will utterly fail in the limit theta_13=0. For this reason, we explore
alternative oscillation approaches that would still succeed even if theta_13
vanishes. We identify several alternatives that require neither a nonzero
|U_e3| nor the presence of significant matter effects. All include multiple
percent-level neutrino oscillation measurements, usually involving
muon-neutrino (or antineutrino) disappearance and very long baselines. We
comment on the degree of promise that these alternative approaches show.Comment: 15 pages revtex, five eps figures, references and acknowledgments
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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
CP violation in the lepton sector with Majorana neutrinos
We study CP violation in the lepton sector in extended models with
right-handed neutrinos, without and with left-right symmetry, and with
arbitrary mass terms. We find the conditions which must be satisfied by the
neutrino and charged lepton mass matrices for CP conservation. These
constraints, which are independent of the choice of weak basis, are proven to
be also sufficient in simple cases. This invariant formulation makes apparent
the necessary requirements for CP violation, as well as the size of CP
violating effects. As an example, we show that CP violation can be much larger
in left-right symmetric models than in models with only additional right-handed
neutrinos, {\it i.e.}, without right-handed currents.Comment: 19 page
Constraints on dark and visible mass in galaxies from strong gravitational lensing
We give a non-exhaustive review of the use of strong gravitational lensing in
placing constraints on the quantity of dark and visible mass in galaxies. We
discuss development of the methodology and summarise some recent results.Comment: To appear in proceedings of IAU Symposium 244, 'Dark Galaxies and
Lost Baryons', 25th - 29th June 2007. Nine pages, five figures. Version 2
updates bibliograph
Flavor and chiral oscillations with Dirac wave packets
We report about recent results on Dirac wave packets in the treatment of
neutrino flavor oscillation where the initial localization of a spinor state
implies an interference between positive and negative energy components of
mass-eigenstate wave packets. A satisfactory description of fermionic particles
requires the use of the Dirac equation as evolution equation for the
mass-eigenstates. In this context, a new flavor conversion formula can be
obtained when the effects of chiral oscillation are taken into account. Our
study leads to the conclusion that the fermionic nature of the particles, where
chiral oscillations and the interference between positive and negative
frequency components of mass-eigenstate wave packets are implicitly assumed,
modifies the standard oscillation probability. Nevertheless, for
ultra-relativistic particles and sharply peaked momentum distributions, we can
analytically demonstrate that these modifications introduce correction factors
proportional to (m12/p0) square which are practically un-detectable by any
experimental analysisComment: 16 pages, 2 figure
Would Relaxation of the Anti-doping Rule Lead to Red Queen Effects?
status: Published onlin
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
Lensing and the Centers of Distant Early-Type Galaxies
Gravitational lensing provides a unique probe of the inner 10-1000 pc of
distant galaxies (z=0.2-1). Lens theory predicts that every strong lens system
should have a faint image near the center of the lens galaxy, which should be
visible in radio lenses but have not been observed. We study these ``core''
images using models derived from the stellar distributions in nearby early-type
galaxies. We find that realistic galaxies predict a remarkably wide range of
core images, with lensing magnifications spanning some six orders of magnitude.
More concentrated galaxies produce fainter core images, although not with any
simple, quantitative, model independent relation. Some real galaxies have
diffuse cores and predict bright core images (magnification mu>~0.1), but more
common are galaxies that predict faint core images (mu<~0.001). Thus, stellar
mass distributions alone are probably concentrated enough to explain the lack
of observed core images, and may require observational sensitivity to improve
by an order of magnitude before detections of core images become common.
Two-image lenses will tend to have brighter core images than four-image lenses,
so they will be the better targets for finding core images and exploiting these
tools for studying the central mass distributions of distant galaxies.Comment: 13 pages, emulateapj; submitted to Ap
An Analytic Approach to the Wave Packet Formalism in Oscillation Phenomena
We introduce an approximation scheme to perform an analytic study of the
oscillation phenomena in a pedagogical and comprehensive way. By using Gaussian
wave packets, we show that the oscillation is bounded by a time-dependent
vanishing function which characterizes the slippage between the mass-eigenstate
wave packets. We also demonstrate that the wave packet spreading represents a
secondary effect which plays a significant role only in the non-relativistic
limit. In our analysis, we note the presence of a new time-dependent phase and
calculate how this additional term modifies the oscillating character of the
flavor conversion formula. Finally, by considering Box and Sine wave packets we
study how the choice of different functions to describe the particle
localization changes the oscillation probability.Comment: 16 pages, 7 figures, AMS-Te
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