2,206 research outputs found
Neutrino spin relaxation in medium with stochastic characteristics
The helicity evolution of a neutrino interacting with randomly moving and
polarized matter is studied. We derive the equation for the averaged neutrino
helicity. The type of the neutrino interaction with background fermions is not
fixed. In the particular case of a tau-neutrino interacting with
ultrarelativistic electron-positron plasma we obtain the expression for the
neutrino helicity relaxation rate in the explicit form. We study the neutrino
spin relaxation in the relativistic primordial plasma. Supposing that the
conversion of left-handed neutrinos into right-handed ones is suppressed at the
early stages of the Universe evolution we get the upper limit on the
tau-neutrino mass.Comment: 6 pages, RevTeX4; 2 references added; more detailed discussion of
correlation functions and cosmological neutrinos is presented; version to be
published in Int. J. Mod. Phys.
Bootstrap tomography of high-precision pulses for quantum control
Long-time dynamical decoupling and quantum control of qubits require
high-precision control pulses. Full characterization (quantum tomography) of
imperfect pulses presents a bootstrap problem: tomography requires initial
states of a qubit which can not be prepared without imperfect pulses. We
present a protocol for pulse error analysis, specifically tailored for a wide
range of the single solid-state electron spins. Using a single electron spin of
a nitrogen-vacancy (NV) center in diamond, we experimentally verify the
correctness of the protocol, and demonstrate its usefulness for quantum control
tasks
The role of the slope of `realistic' potential barriers in preventing relativistic tunnelling in the Klein zone
The transmission of fermions of mass m and energy E through an electrostatic
potential barrier of rectangular shape (i.e. supporting an infinite electric
field), of height U> E + m - due to the many-body nature of the Dirac equation
evidentiated by the Klein paradox - has been widely studied. We exploit here
the analytical solution, given by Sauter for the linearly rising potential
step, to show that the tunnelling rate through a more realistic trapezoidal
barrier is exponentially depressed, as soon as the length of the regions
supporting a finite electric field exceeds the Compton wavelenght of the
particle - the latter circumstance being hardly escapable in most realistic
cases
Anisotropy of the Cosmic Neutrino Background
The cosmic neutrino background (CNB) consists of low-energy relic neutrinos
which decoupled from the cosmological fluid at a redshift z ~ 10^{10}. Despite
being the second-most abundant particles in the universe, direct observation
remains a distant challenge. Based on the measured neutrino mass differences,
one species of neutrinos may still be relativistic with a thermal distribution
characterized by the temperature T ~ 1.9K. We show that the temperature
distribution on the sky is anisotropic, much like the photon background,
experiencing Sachs-Wolfe and integrated Sachs-Wolfe effects.Comment: 5 pages, 2 figures / updated references, discussion of earlier wor
Modeling ChIP Sequencing In Silico with Applications
ChIP sequencing (ChIP-seq) is a new method for genomewide mapping of protein binding sites on DNA. It has generated much excitement in functional genomics. To score data and determine adequate sequencing depth, both the genomic background and the binding sites must be properly modeled. To develop a computational foundation to tackle these issues, we first performed a study to characterize the observed statistical nature of this new type of high-throughput data. By linking sequence tags into clusters, we show that there are two components to the distribution of tag counts observed in a number of recent experiments: an initial power-law distribution and a subsequent long right tail. Then we develop in silico ChIP-seq, a computational method to simulate the experimental outcome by placing tags onto the genome according to particular assumed distributions for the actual binding sites and for the background genomic sequence. In contrast to current assumptions, our results show that both the background and the binding sites need to have a markedly nonuniform distribution in order to correctly model the observed ChIP-seq data, with, for instance, the background tag counts modeled by a gamma distribution. On the basis of these results, we extend an existing scoring approach by using a more realistic genomic-background model. This enables us to identify transcription-factor binding sites in ChIP-seq data in a statistically rigorous fashion
Communications Biophysics
Contains reports on three research projects.United States Air Force (Contract AF19(604)-4112
Identification and analysis of unitary pseudogenes: historic and contemporary gene losses in humans and other primates
Novel human pseudogenes are identified that had previous functionality and their age is estimated. The rate of loss-of-function occurred uniformly
Direct Detection of Warm Dark Matter in the X-ray
We point out a serendipitous link between warm dark matter (WDM) models for
structure formation on the one hand and the high sensitivity energy range (1-10
keV) for x-ray photon detection on the Chandra and XMM-Newton observatories on
the other. This fortuitous match may provide either a direct detection of the
dark matter or exclusion of many candidates. We estimate expected x-ray fluxes
from field galaxies and clusters of galaxies if the dark matter halos of these
objects are composed of WDM candidate particles with rest masses in the
structure formation-preferred range (~1 keV to ~20 keV) and with small
radiative decay branches. Existing observations lead us to conclude that for
singlet neutrinos (possessing a very small mixing with active neutrinos) to be
a viable WDM candidate they must have rest masses < 5 keV in the zero lepton
number production mode. Future deeper observations may detect or exclude the
entire parameter range for the zero lepton number case, perhaps restricting the
viability of singlet neutrino WDM models to those where singlet production is
driven by a significant lepton number. The Constellation X project has the
capability to detect/exclude singlet neutrino WDM for lepton number values up
to 10% of the photon number. We also consider diffuse x-ray background
constraints on these scenarios. These same x-ray observations additionally may
constrain parameters of active neutrino and gravitino WDM candidates.Comment: 11 pages, 6 figures, replacement to match ApJ versio
Long-time electron spin storage via dynamical suppression of hyperfine-induced decoherence in a quantum dot
The coherence time of an electron spin decohered by the nuclear spin
environment in a quantum dot can be substantially increased by subjecting the
electron to suitable dynamical decoupling sequences. We analyze the performance
of high-level decoupling protocols by using a combination of analytical and
exact numerical methods, and by paying special attention to the regimes of
large inter-pulse delays and long-time dynamics, which are outside the reach of
standard average Hamiltonian theory descriptions. We demonstrate that dynamical
decoupling can remain efficient far beyond its formal domain of applicability,
and find that a protocol exploiting concatenated design provides best
performance for this system in the relevant parameter range. In situations
where the initial electron state is known, protocols able to completely freeze
decoherence at long times are constructed and characterized. The impact of
system and control non-idealities is also assessed, including the effect of
intra-bath dipolar interaction, magnetic field bias and bath polarization, as
well as systematic pulse imperfections. While small bias field and small bath
polarization degrade the decoupling fidelity, enhanced performance and temporal
modulation result from strong applied fields and high polarizations. Overall,
we find that if the relative errors of the control parameters do not exceed 5%,
decoupling protocols can still prolong the coherence time by up to two orders
of magnitude.Comment: 16 pages, 10 figures, submitted to Phys. Rev.
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