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.

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

Scaling laws of human interaction activity

Even though people in our contemporary, technological society are depending on communication, our understanding of the underlying laws of human communicational behavior continues to be poorly understood. Here we investigate the communication patterns in two social Internet communities in search of statistical laws in human interaction activity. This research reveals that human communication networks dynamically follow scaling laws that may also explain the observed trends in economic growth. Specifically, we identify a generalized version of Gibrat's law of social activity expressed as a scaling law between the fluctuations in the number of messages sent by members and their level of activity. Gibrat's law has been essential in understanding economic growth patterns, yet without an underlying general principle for its origin. We attribute this scaling law to long-term correlation patterns in human activity, which surprisingly span from days to the entire period of the available data of more than one year. Further, we provide a mathematical framework that relates the generalized version of Gibrat's law to the long-term correlated dynamics, which suggests that the same underlying mechanism could be the source of Gibrat's law in economics, ranging from large firms, research and development expenditures, gross domestic product of countries, to city population growth. These findings are also of importance for designing communication networks and for the understanding of the dynamics of social systems in which communication plays a role, such as economic markets and political systems.Comment: 20+7 pages, 4+2 figure

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

Investigation of the electroplastic effect using nanoindentation

A promising approach to deform metallic-intermetallic composite materials is the application of electric current pulses during the deformation process to achieve a lower yield strength and enhanced elongation to fracture. This is known as the electroplastic effect. In this work, a novel setup to study the electroplastic effect during nanoindentation on individual phases and well-defined interfaces was developed. Using a eutectic Al-Al2Cu alloy as a model material, electroplastic nanoindentation results were directly compared with macroscopic electroplastic compression tests. The results of the micro- and macroscopic investigations reveal current induced displacement shifts and stress drops, respectively, with the first displacement shift/stress drop being higher than the subsequent ones. A higher current intensity, higher loading rate and larger pulsing interval all cause increased displacement shifts. This observation, in conjunction with the fact that the first displacement shift is highest, strongly indicates that de-pinning of dislocations from obstacles dominates the mechanical response, rather than solely thermal effects

Communications Biophysics

Contains reports on three research projects.United States Air Force (Contract AF19(602)-4112

Spectra of Discrete Schr\"odinger Operators with Primitive Invertible Substitution Potentials

We study the spectral properties of discrete Schr\"odinger operators with potentials given by primitive invertible substitution sequences (or by Sturmian sequences whose rotation angle has an eventually periodic continued fraction expansion, a strictly larger class than primitive invertible substitution sequences). It is known that operators from this family have spectra which are Cantor sets of zero Lebesgue measure. We show that the Hausdorff dimension of this set tends to $1$ as coupling constant $\lambda$ tends to $0$. Moreover, we also show that at small coupling constant, all gaps allowed by the gap labeling theorem are open and furthermore open linearly with respect to $\lambda$. Additionally, we show that, in the small coupling regime, the density of states measure for an operator in this family is exact dimensional. The dimension of the density of states measure is strictly smaller than the Hausdorff dimension of the spectrum and tends to $1$ as $\lambda$ tends to $0$

Communications Biophysics

Contains reports on five research projects.National Science Foundation (Grant G-16526)National Institutes of Health (Grant MH-04737-02)