102 research outputs found
Production of a sterile species via active-sterile mixing: an exactly solvable model
The production of a sterile species via active-sterile mixing in a thermal
medium is studied in an exactly solvable model. The \emph{exact} time evolution
of the sterile distribution function is determined by the dispersion relations
and damping rates for the quasiparticle modes. These depend on
\wtg = \Gamma_{aa}/2\Delta E, with the interaction rate of the
active species in absence of mixing and the oscillation frequency in
the medium without damping. \wtg \ll1,\wtg \gg 1 describe the weak and strong
damping limits respectively. For \wtg\ll1, \Gamma_1 = \Gamma_{aa}\cos^2\tm ;
\Gamma_{2}=\Gamma_{aa}\sin^2\tm where \tm is the mixing angle in the medium
and the sterile distribution function \emph{does not} obey a simple rate
equation. For \wtg \gg 1, and \Gamma_2 = \Gamma_{aa}
\sin^22\tm/4\wtg^2, is the sterile production rate. In this regime sterile
production is suppressed and the oscillation frequency \emph{vanishes} at an
MSW resonance, with a breakdown of adiabaticity. These are consequences of
quantum Zeno suppression. For active neutrinos with standard model interactions
the strong damping limit is \emph{only} available near an MSW resonance
\emph{if} with the vacuum mixing angle.
The full set of quantum kinetic equations for sterile production for arbitrary
\wtg are obtained from the quantum master equation. Cosmological resonant
sterile neutrino production is quantum Zeno suppressed relieving potential
uncertainties associated with the QCD phase transition.Comment: To appear in Phys. Rev.
Cosmological constraints on neutrino plus axion hot dark matter: Update after WMAP-5
We update our previous constraints on two-component hot dark matter (axions
and neutrinos), including the recent WMAP 5-year data release. Marginalising
over sum m_nu provides m_a < 1.02 eV (95% C.L.) for the axion mass. In the
absence of axions we find sum m_nu < 0.63 eV (95% C.L.).Comment: 4 pages, 1 figure, uses iopart.cls; v2 matches published versio
Cosmological constraints on neutrino plus axion hot dark matter
We use observations of the cosmological large-scale structure to derive
limits on two-component hot dark matter consisting of mass-degenerate neutrinos
and hadronic axions, both components having velocity dispersions corresponding
to their respective decoupling temperatures. We restrict the data samples to
the safely linear regime, in particular excluding the Lyman-alpha forest. Using
standard Bayesian inference techniques we derive credible regions in the
two-parameter space of m_a and sum(m_nu). Marginalising over sum(m_nu) provides
m_a < 1.2 eV (95% C.L.). In the absence of axions the same data and methods
give sum(m_nu) < 0.65 eV (95% C.L.). We also derive limits on m_a for a range
of axion-pion couplings up to one order of magnitude larger or smaller than the
hadronic value.Comment: 13 pages, 2 figures, uses iopart.cl
New PVLAS results and limits on magnetically induced optical rotation and ellipticity in vacuum
IIn 2006 the PVLAS collaboration reported the observation of an optical
rotation generated in vacuum by a magnetic field. To further check against
possible instrumental artifacts several upgrades to the PVLAS apparatus have
been made during the last year. Two data taking runs, at the wavelength of 1064
nm, have been performed in the new configuration with magnetic field strengths
of 2.3 T and 5 T. The 2.3 T field value was chosen in order to avoid stray
fields. The new observations do not show the presence of a rotation signal down
to the levels of rad at 5 T and rad at
2.3 T (at 95% c.l.) with 45000 passes in the magnetic field zone. In the same
conditions no ellipticity signal was detected down to at 2.3
T (at 95% c.l.), whereas at 5 T a signal is still present. The physical nature
of this ellipticity as due to an effect depending on can be excluded by
the measurement at 2.3 T. These new results completely exclude the previously
published magnetically induced vacuum dichroism results, indicating that they
were instrumental artifacts. These new results therefore also exclude the
particle interpretation of the previous PVLAS results as due to a spin zero
boson. The background ellipticity at 2.3 T can be used to determine a new limit
on the total photon-photon scattering cross section of barn at 95% c.l..Comment: 25 pages, 7 figures Main changes rel. to v.2: minor changes to
abstract, replaced Figures 4,5,6, corrected typographical errors. Paper
submitted to Physical Review
Collider signals from slow decays in supersymmetric models with an intermediate-scale solution to the mu problem
The problem of the origin of the mu parameter in the Minimal Supersymmetric
Standard Model can be solved by introducing singlet supermultiplets with
non-renormalizable couplings to the ordinary Higgs supermultiplets. The
Peccei-Quinn symmetry is broken at a scale which is the geometric mean between
the weak scale and the Planck scale, yielding a mu term of the right order of
magnitude and an invisible axion. These models also predict one or more singlet
fermions which have electroweak-scale masses and suppressed couplings to MSSM
states. I consider the case that such a singlet fermion, containing the axino
as an admixture, is the lightest supersymmetric particle. I work out the
relevant couplings in several of the simplest models of this type, and compute
the partial decay widths of the next-to-lightest supersymmetric particle
involving leptons or jets. Although these decays will have an average proper
decay length which is most likely much larger than a typical collider detector,
they can occasionally occur within the detector, providing a striking signal.
With a large sample of supersymmetric events, there will be an opportunity to
observe these decays, and so gain direct information about physics at very high
energy scales.Comment: 24 pages, LaTeX, 4 figure
Observational bounds on the cosmic radiation density
We consider the inference of the cosmic radiation density, traditionally
parameterised as the effective number of neutrino species N_eff, from precision
cosmological data. Paying particular attention to systematic effects, notably
scale-dependent biasing in the galaxy power spectrum, we find no evidence for a
significant deviation of N_eff from the standard value of N_eff^0=3.046 in any
combination of cosmological data sets, in contrast to some recent conclusions
of other authors. The combination of all available data in the linear regime
prefers, in the context of a ``vanilla+N_eff'' cosmological model,
1.1<N_eff<4.8 (95% C.L.) with a best-fit value of 2.6. Adding data at smaller
scales, notably the Lyman-alpha forest, we find 2.2<N_eff<5.8 (95% C.L.) with
3.8 as the best fit. Inclusion of the Lyman-alpha data shifts the preferred
N_eff upwards because the sigma_8 value derived from the SDSS Lyman-alpha data
is inconsistent with that inferred from CMB. In an extended cosmological model
that includes a nonzero mass for N_eff neutrino flavours, a running scalar
spectral index and a w parameter for the dark energy, we find 0.8<N_eff<6.1
(95% C.L.) with 3.0 as the best fit.Comment: 23 pages, 3 figures, uses iopart.cls; v2: 1 new figure, references
added, matches published versio
Neutrino masses and cosmic radiation density: Combined analysis
We determine the range of neutrino masses and cosmic radiation content
allowed by the most recent CMB and large-scale structure data. In contrast to
other recent works, we vary these parameters simultaneously and provide
likelihood contours in the two-dimensional parameter space of N_eff}, the usual
effective number of neutrino species measuring the radiation density, and \sum
m_nu. The allowed range of \sum m_nu and N_eff has shrunk significantly
compared to previous studies. The previous degeneracy between these parameters
has disappeared, largely thanks to the baryon acoustic oscillation data. The
likelihood contours differ significantly if \sum m_nu resides in a single
species instead of the standard case of being equally distributed among all
flavors. For \sum m_nu=0 we find 2.7 < N_eff < 4.6 at 95% CL while \sum m_nu <
0.62 eV at 95% CL for the standard radiation content.Comment: 8 pages, 2 figure
Probing neutrino decays with the cosmic microwave background
We investigate in detail the possibility of constraining neutrino decays with
data from the cosmic microwave background radiation (CMBR). Two generic decays
are considered \nu_H -> \nu_L \phi and \nu_H -> \nu_L \nu_L_bar \nu_L. We have
solved the momentum dependent Boltzmann equation in order to account for
possible relativistic decays. Doing this we estimate that any neutrino with
mass m > 1 eV decaying before the present should be detectable with future CMBR
data. Combining this result with other results on stable neutrinos, any
neutrino mass of the order 1 eV should be detectable.Comment: 8 pages, 4 figures, to appear in Phys. Rev.
S_3-flavour symmetry as realized in lepton flavour violating processes
A variety of lepton flavour violating effects related to the recent discovery
of neutrino oscillations and mixings is here systematically discussed in terms
of an S_3-flavour permutational symmetry. After a brief review of some relevant
results on lepton masses and mixings, that had been derived in the framework of
a Minimal S_3-Invariant Extension of the Standard Model, we derive explicit
analytical expressions for the matrices of the Yukawa couplings and compute the
branching ratios of some selected flavour changing neutral current (FCNC)
processes, as well as, the contribution of the exchange of neutral flavour
changing scalars to the anomaly of the muon's magnetic moment as functions of
the masses of the charged leptons and the neutral Higgs bosons. We find that
the S_3 x Z_2 flavour symmetry and the strong mass hierarchy of the charged
leptons strongly suppress the FCNC processes in the leptonic sector well below
the present experimental upper bounds by many orders of magnitude. The
contribution of FCNC to the anomaly of the muon's magnetic moment is small but
non-negligible.Comment: 23 pages, one figure. To appear in J. Phys A: Mathematical and
Theoretical (SPE QTS5
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