arXiv subjects

Kevork Abazajian

Publications and source records attributed to Kevork Abazajian.

32 records · Page 2Linked to original sources

arXiv2003

The Cosmological Energy Density of Neutrinos from Oscillation Measurements

The emerging structure of the neutrino mass matrix, when combined with the primordial element abundances, places the most stringent constraint on the flavor asymmetries in the cosmological neutrino background and therefore its energy density. I review the mechanism of synchronized neutrino oscillations in the an early universe with degenerate (asymmetric) neutrino and antineutrino densities and the implications of refined measurements of neutrino parameters.

hep-ph↗

arXiv2003

The First Data Release of the Sloan Digital Sky Survey

The Sloan Digital Sky Survey has validated and made publicly available its First Data Release. This consists of 2099 square degrees of five-band (u, g, r, i, z) imaging data, 186,240 spectra of galaxies, quasars, stars and calibrating blank sky patches selected over 1360 square degrees of this area, and tables of measured parameters from these data. The imaging data go to a depth of r ~ 22.6 and are photometrically and astrometrically calibrated to 2% rms and 100 milli-arcsec rms per coordinate, respectively. The spectra cover the range 3800--9200 A, with a resolution of 1800--2100. Further characteristics of the data are described, as are the data products themselves.

astro-ph↗

arXiv2003

Neutrino Mass and Dark Energy from Weak Lensing

Weak gravitational lensing of background galaxies by intervening matter directly probes the mass distribution in the universe. This distribution, and its evolution at late times, is sensitive to both the dark energy, a negative pressure energy density component, and neutrino mass. We examine the potential of lensing experiments to measure features of both simultaneously. Focusing on the radial information contained in a future deep 4000 square degree survey, we find that the expected (1-sigma) error on a neutrino mass is 0.1 eV, if the dark energy parameters are allowed to vary. The constraints on dark energy parameters are similarly restrictive, with errors on w of 0.09. Much of the restrictive power on the dark energy comes not from the evolution of the gravitational potential but rather from how distances vary as a function of redshift in different cosmologies.

astro-ph↗

arXiv2003

Cosmological Constraints on Bulk Neutrinos

Recent models invoking extra spacelike dimensions inhabited by (bulk) neutrinos are shown to have significant cosmological effects if the size of the largest extra dimension is R > 1 fm. We consider effects on cosmic microwave background anisotropies, big bang nucleosynthesis, deuterium and Li-6 photoproduction, diffuse photon backgrounds, and structure formation. The resulting constraints can be stronger than either bulk graviton overproduction constraints or laboratory constraints.

hep-ph↗

arXiv2001

Testing the Cosmic Coincidence Problem and the Nature of Dark Energy

Dark energy models which alter the relative scaling behavior of dark energy and matter could provide a natural solution to the cosmic coincidence problem - why the densities of dark energy and dark matter are comparable today. A generalized class of dark energy models is introduced which allows non-canonical scaling of the ratio of dark matter and dark energy with the Robertson-Walker scale factor a(t). Upcoming observations, such as a high redshift supernova survey, application of the Alcock-Paczynski test to quasar pairs, and cluster evolution, will strongly constrain the relative scaling of dark matter and dark energy as well as the equation of state of the dark energy. Thus, whether there actually is a coincidence problem, and the extent of cosmic coincidence in the universe's recent past can be answered observationally in the near future. Determining whether today is a special time in the history of the universe will be a SNAP.

astro-ph↗

arXiv2001

A New Connection between Central Engine Weak Physics and the Dynamics of Gamma-Ray Burst Fireballs

We demonstrate a qualitatively new aspect of the dynamics of Gamma-Ray Burst (GRB) fireballs: the development of a substantial dispersion in the proton component in fireballs in which neutron decoupling occurs and is sufficiently pronounced. This effect depends sensitively on the neutron to proton ratio in the fireball, becoming more dramatic with increasing neutron excess. Simple physical arguments and transport calculations indicate that the dispersion in Lorentz factor of the protons can be of order the final mean Lorentz factor of the fireball. We show how plasma instabilities could play an important role in the evolution of the fireball and how they might ultimately govern the development of such a velocity dispersion in the proton component. The role of these instabilities in setting/diminishing a proton Lorentz factor dispersion represents a new and potentially important venue for the study of plasma instabilities. Significant dispersion in the proton velocities translates into fewer protons attaining the highest Lorentz factors. This is tantamount to a reduction in the total energy required to attain a given Lorentz factor for the highest energy protons. As well, a proton component dispersion can have consequences for the electromagnetic and neutrino signature of GRBs.

astro-ph↗

arXiv2000

Can a Large Neutron Excess Help Solve the Baryon Loading Problem in Gamma-Ray Burst Fireballs?

We point out that the baryon-loading problem in Gamma-Ray Burst (GRB) models can be amelioriated if a significant fraction of the baryons which inertially confine the fireball are converted to neutrons. A high neutron fraction in some circumstances can result in a reduced transfer of energy from relativistic light particles in the fireball to baryons. The energy needed to produce the required relativistic flow in the GRB is consequently reduced, in some cases by orders of magnitude. This could be relevant to GRB models because a high neutron-to-proton ratio has been calculated in neutron star-merger fireball environments. Significant neutron excess also could occur near compact objects with high neutrino fluxes.

astro-ph↗

arXiv2000

The Increase in the Primordial He-4 Yield in the Two-Doublet Four-Neutrino Mixing Scheme

We assess the effects on Big Bang Nucleosynthesis (BBN) of lepton number generation in the early universe resulting from the two-doublet four-neutrino mass/mixing scheme. It has been argued that this neutrino mass/mixing arrangement gives the most viable fit to the existing data. We study full 4 x 4 mixing matrices and show how possible symmetries in these can affect the BBN He-4 abundance yields. Though there is as yet no consensus on the reliability of BBN calculations with neutrino flavor mixing, we show that, in the case where the sign of the lepton number asymmetry is unpredictable, BBN considerations may pick out specific relationships between mixing angles. In particular, reconciling the observed light element abundances with a \barν_μ<-> \barν_e oscillation interpretation of LSND would allow unique new constraints on the neutrino mixing angles in this model.

astro-ph↗

arXiv1999

Transformation-Induced Nonthermal Neutrino Spectra and Primordial Nucleosynthesis

We examine in detail the changes in the production of primordial helium resulting from nonthermal neutrino momentum distributions produced by resonant transformation of electron-type neutrinos to steriles. These transformations, anti-nu_e -> anti-nu_s (nu_e->nu_s), amplify a positive (negative) lepton number asymmetry. We find that the resulting supression relative to a thermal distribution of low energy nu_e reduces n -> p conversion to a greater extent than does the enhancement of n -> p from an identical suppression of anti-nu_e. Thus, equal lepton-number asymmetries of opposite sign have unequal effects on the resulting helium yield in primordial nucleosynthesis.

astro-ph↗

arXiv1999

Comments Regarding "On Neutrino-Mixing-Generated Lepton Asymmetry and the Primordial Helium-4 Abundance"

This is a reply to the preprint "On Neutrino-Mixing-Generated Lepton Asymmetry and the Primordial Helium-4 Abundance" by M. V. Chizhov and D. P. Kirilova (hep-ph/9908525), which criticised our recent publication (X. Shi, G. M. Fuller and K. Abazajian Phys. Rev. D 60, 063002 (1999)). Here we point out factual errors in their description of what our paper says. We also show that their criticisms of our work have no merit.

astro-ph↗

arXiv1999

Neutrino-Mixing-Generated Lepton Asymmetry and the Primordial $^4$He Abundance

It has been proposed that an asymmetry in the electron neutrino sector may be generated by resonant active-sterile neutrino transformations during Big Bang Nucleosynthesis (BBN). We calculate the change in the primordial $^4$He yield $Y$ resulting from this asymmetry, taking into account both the time evolution of the $ν_e$ and $\barν_e$ distribution function and the spectral distortions in these. We calculate this change in two schemes: (1) a lepton asymmetry directly generated by $ν_e$ mixing with a lighter right-handed sterile neutrino $ν_s$; and (2) a lepton asymmetry generated by a $ν_τ\leftrightarrowν_s$ or $ν_μ\leftrightarrowν_s$ transformation which is subsequently partially converted to an asymmetry in the $ν_e\barν_e$ sector by a matter-enhanced active-active neutrino transformation. In the first scheme, we find that the percentage change in $Y$ is between -1% and 9% (with the sign depending on the sign of the asymmetry), bounded by the Majorana mass limit $m_{ν_e}\la 1$ eV. In the second scheme, the maximal percentage reduction in $Y$ is 2%, if the lepton number asymmetry in neutrinos is positive; Otherwise, the percentage increase in $Y$ is $\la 5%$ for $m^2_{ν_μ,ν_τ}-m^2_{ν_s}\la 10^4$ eV. We conclude that the change in the primordial $^4$He yield induced by a neutrino-mixing-generated lepton number asymmetry can be substantial in the upward direction, but limited in the downward direction.

astro-ph↗

arXiv1999

Active-Sterile Neutrino Mixing in the Early Universe and Primordial Nucleosynthesis

We investigate the effects of matter-enhanced (MSW) transformation of neutrinos in the Early Universe on the primordial helium yield (Y_p). We find that Y_p is affected much more by the MSW-induced alterations in the neutrino energy spectra than by the associated change in expansion rate. Specifically, the absence due to transformation of low energy electron neutrinos can significantly affect neutron-proton weak interconversion rates through the lifting of Fermi-blocking of neutron decay at low energies and through halting low-energy neutrino capture on neutrons. We find that the change of Y_p within a causal horizon is -0.005\le δY_p \leq 0.013 for m_{ν_{μ,τ}}^2 - m_{ν_s}^2 \le 10^4 eV^2 in the case of ν_{μ,τ}-ν_s-ν_e mixing, with the lower limit at m_{ν_{μ,τ}}^2 - m_{ν_s}^2 \approx 100 eV^2, and -0.002\le δY_p \leq 0.020 for m_{ν_e}^2 - m_{ν_s}^2 \le 1 eV^2 in the case of ν_e-ν_s mixing.

astro-ph↗

arXiv1998

Are Gamma-Ray Bursts Signals of Supermassive Black Hole Formation?

The formation of supermassive black holes through the gravitational collapse of supermassive objects ($M \ga 10^4 M_\odot$) has been proposed as a source of cosmological gamma-ray bursts. The major advantage of this model is that such collapses are far more energetic than stellar-remnant mergers. The major drawback of this idea is the severe baryon loading problem in one-dimensional models. We can show that the observed log N - log P (number vs. peak flux) distribution for gamma-ray bursts in the BATSE database is not inconsistent with an identification of supermassive object collapse as the origin of the gamma-ray bursts. This conclusion is valid for a range of plausible cosmological and gamma-ray burst spectral parameters.

astro-ph↗

arXiv1998

Baryon/anti-baryon inhomogeneity and big bang nucleosynthesis

We investigate the effects of baryon/anti-baryon inhomogeneity on primordial nucleosynthesis. Recent work claims that electroweak baryogenesis could give rise to distinct regions of net baryon and anti-baryon number, which could survive until the nucleosynthesis epoch. We discuss neutron diffusion effects on nucleosynthesis yields in these models and speculate on the prospects for obtaining constraints.

astro-ph↗