2,871 research outputs found
Quasiclassical Coarse Graining and Thermodynamic Entropy
Our everyday descriptions of the universe are highly coarse-grained,
following only a tiny fraction of the variables necessary for a perfectly
fine-grained description. Coarse graining in classical physics is made natural
by our limited powers of observation and computation. But in the modern quantum
mechanics of closed systems, some measure of coarse graining is inescapable
because there are no non-trivial, probabilistic, fine-grained descriptions.
This essay explores the consequences of that fact. Quantum theory allows for
various coarse-grained descriptions some of which are mutually incompatible.
For most purposes, however, we are interested in the small subset of
``quasiclassical descriptions'' defined by ranges of values of averages over
small volumes of densities of conserved quantities such as energy and momentum
and approximately conserved quantities such as baryon number. The
near-conservation of these quasiclassical quantities results in approximate
decoherence, predictability, and local equilibrium, leading to closed sets of
equations of motion. In any description, information is sacrificed through the
coarse graining that yields decoherence and gives rise to probabilities for
histories. In quasiclassical descriptions, further information is sacrificed in
exhibiting the emergent regularities summarized by classical equations of
motion. An appropriate entropy measures the loss of information. For a
``quasiclassical realm'' this is connected with the usual thermodynamic entropy
as obtained from statistical mechanics. It was low for the initial state of our
universe and has been increasing since.Comment: 17 pages, 0 figures, revtex4, Dedicated to Rafael Sorkin on his 60th
birthday, minor correction
Phenomenology of Dirac Neutrinogenesis in Split Supersymmetry
In Split Supersymmetry scenarios the possibility of having a very heavy
gravitino opens the door to alleviate or completely solve the worrisome
"gravitino problem'' in the context of supersymmetric baryogenesis models. Here
we assume that the gravitino may indeed be heavy and that Majorana masses for
neutrinos are forbidden as well as direct Higgs Yukawa couplings between left
and right handed neutrinos. We investigate the viability of the mechansim known
as Dirac leptogenesis (or neutrinogenesis), both in solving the baryogenesis
puzzle and explaining the observed neutrino sector phenomenology. To
successfully address these issues, the scenario requires the introduction of at
least two new heavy fields. If a hierarchy among these new fields is
introduced, and some reasonable stipulations are made on the couplings that
appear in the superpotential, it becomes a generic feature to obtain the
observed large lepton mixing angles. We show that in this case, it is possible
simultaneously to obtain both the correct neutrino phenomenology and enough
baryon number, making thermal Dirac neutrinogenesis viable. However, due to
cosmological constraints, its ability to satisfy these constraints depends
nontrivially on model parameters of the overall theory, particularly the
gravitino mass. Split supersymmetry with m_{3/2} between 10^{5} and 10^{10} GeV
emerges as a "natural habitat" for thermal Dirac neutrinogenesis.Comment: 37 pages, 8 figure
Neutrino Models of Dark Energy
I consider a scenario proposed by Fardon, Nelson and Weiner where dark energy
and neutrinos are connected. As a result, neutrino masses are not constant but
depend on the neutrino number density. By examining the full equation of state
for the dark sector, I show that in this scenario the dark energy is equivalent
to having a cosmological constant, but one that "runs" as the neutrino mass
changes with temperature. Two examples are examined that illustrate the
principal feautures of the dark sector of this scenario. In particular, the
cosmological constant is seen to be negligible for most of the evolution of the
Universe, becoming inportant only when neutrinos become non-relativistic. Some
speculations on features of this scenario which might be present in a more
realistic theory are also presented.Comment: 12 pages, 6 figures. Added comments on why FNW scenario always leads
to a running cosmological constant and a few references. To be published in
Phys. Rev.
Thermal leptogenesis in brane world cosmology
The thermal leptogenesis in brane world cosmology is studied. In brane world
cosmology, the expansion law is modified from the four-dimensional standard
cosmological one at high temperature regime in the early universe. As a result,
the well-known upper bound on the lightest light neutrino mass induced by the
condition for the out-of-equilibrium decay of the lightest heavy neutrino,
eV, can be moderated to be in the case of with the
lightest heavy neutrino mass () and the ``transition temperature''
(), at which the modified expansion law in brane world cosmology is
smoothly connecting with the standard one. This implies that the degenerate
mass spectrum of the light neutrinos can be consistent with the thermal
leptogenesis scenario. Furthermore, as recently pointed out, the gravitino
problem in supersymmetric case can be solved if the transition temperature is
low enough GeV. Therefore, even in the supersymmetric
case, thermal leptogenesis scenario can be successfully realized in brane world
cosmology.Comment: 9 pages, final versio
Insensitivity of flavoured leptogenesis to low energy CP violation
If the baryon asymmetry of the Universe is produced by leptogenesis, CP
violation is required in the lepton sector. In the seesaw extension of the
Standard Model with three hierarchical right-handed neutrinos, we show that the
baryon asymmetry is insensitive to the PMNS phases: thermal leptogenesis can
work for any value of the observable phases. This result was well-known when
there are no flavour effects in leptogenesis; we show that it remains true when
flavour effects are included.Comment: 4 pages, 1 figure; version accepted for publication, added
explanations, notation clarifie
Signatures from an extra-dimensional seesaw model
We study the generation of small neutrino masses in an extra-dimensional
model, where right-handed neutrinos are allowed to propagate in the extra
dimension, while the Standard Model particles are confined to a brane.
Motivated by the fact that extra-dimensional models are non-renormalizable, we
truncate the Kaluza-Klein towers at a maximal extra-dimensional momentum. The
structure of the bulk Majorana mass term, motivated by the Sherk-Schwarz
mechanism, implies that the right-handed Kaluza-Klein neutrinos pair to form
Dirac neutrinos, except for a number of unpaired Majorana neutrinos at the top
of each tower. These heavy Majorana neutrinos are the only sources of lepton
number breaking in the model, and similarly to the type-I seesaw mechanism,
they naturally generate small masses for the left-handed neutrinos. The lower
Kaluza-Klein modes mix with the light neutrinos, and the mixing effects are not
suppressed with respect to the light-neutrino masses. Compared to conventional
fermionic seesaw models, such mixing can be more significant. We study the
signals of this model at the Large Hadron Collider, and find that the current
low-energy bounds on the non-unitarity of the leptonic mixing matrix are strong
enough to exclude an observation.Comment: 17 pages, 3 figures, REVTeX4. Final version published in Phys. Rev.
B-L Violating Nucleon Decay and GUT Scale Baryogenesis in SO(10)
We show that grand unified theories based on SO(10) generate naturally the
next-to-leading baryon number violating operators of dimension seven. These
operators, which violate B-L, lead to unconventional decays of the nucleon such
as n -> e^-K^+, e^- \pi^+ and p -> \nu \pi^+. In two-step breaking schemes of
non-supersymmetric SO(10), nucleon lifetime for decays into these modes is
found to be within reach of experiments. We also identify supersymmetric
scenarios where these decays may be accessible, consistent with gauge coupling
unification. Further, we show that the (B-L)-asymmetry generated in the decays
of GUT scale scalar bosons and/or gauge bosons can explain consistently the
observed baryon asymmetry of the universe. The induced (B-L)-asymmetry is
sphaleron-proof, and survives down to the weak scale without being erased by
the electroweak interactions. This mechanism works efficiently in a large class
of non-SUSY and SUSY SO(10) models, with either a 126 or a 16 Higgs field
employed for rank reduction. In minimal models the induced baryon asymmetry is
tightly connected to the masses of quarks, leptons and neutrinos and is found
to be compatible with observations.Comment: 26 pages, 9 figure
A model for fluctuating inflaton coupling: (s)neutrino induced adiabatic perturbations and non-thermal leptogenesis
We discuss an unique possibility of generating adiabatic density
perturbations and leptogenesis from the spatial fluctuations of the inflaton
decay rate. The key assumption is that the initial isocurvature perturbations
are created in the right handed sneutrino sector during inflation which is then
converted into adiabatic perturbations when the inflaton decays. We discuss
distinct imprints on the cosmic micro wave background radiation, which can
distinguish non-thermal versus thermal leptogenesis.Comment: 4 pages, version to be published in PR
Multicomponent dense electron gas as a model of Si MOSFET
We solve two-dimensional model of -component dense electron gas in the
limit of large and in a range of the Coulomb interaction parameter:
. The quasiparticle interaction on the Fermi circle
vanishes as 1/N. The ground state energy and the effective mass are found as
series in powers of . In the quantum Hall state on the lowest Landau
level at integer filling: , the charge activation energy gap and the
exchange constant are found.Comment: 10 pages, 4 figure
Flavour-Dependent Type II Leptogenesis
We reanalyse leptogenesis via the out-of-equilibrium decay of the lightest
right-handed neutrino in type II seesaw scenarios, taking into account
flavour-dependent effects. In the type II seesaw mechanism, in addition to the
type I seesaw contribution, an additional direct mass term for the light
neutrinos is present. We consider type II seesaw scenarios where this
additional contribution arises from the vacuum expectation value of a Higgs
triplet, and furthermore an effective model-independent approach. We
investigate bounds on the flavour-specific decay asymmetries, on the mass of
the lightest right-handed neutrino and on the reheat temperature of the early
universe, and compare them to the corresponding bounds in the type I seesaw
framework. We show that while flavour-dependent thermal type II leptogenesis
becomes more efficient for larger mass scale of the light neutrinos, and the
bounds become relaxed, the type I seesaw scenario for leptogenesis becomes more
constrained. We also argue that in general, flavour-dependent effects cannot be
ignored when dealing with leptogenesis in type II seesaw models.Comment: 19 pages, 8 figures; v3: minor additions, typos corrected, results
and conclusions unchange
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