1,257 research outputs found
Muon production in low-energy electron-nucleon and electron-nucleus scattering
Recently, muon production in electron-proton scattering has been suggested as
a possible candidate reaction for the identification of lepton-flavor violation
due to physics beyond the Standard Model. Here we point out that the
Standard-Model processes and can cloud potential beyond-the-Standard-Model signals in
electron-proton collisions. We find that Standard-Model cross
sections exceed those from lepton-flavor-violating operators by several orders
of magnitude. We also discuss the possibility of using a nuclear target to
enhance the signal.Comment: 24 pages. Additional figure showing energy-dependence of total cross
section, minor changes to text. Conclusions unaltered. This version to appear
in Physical Review
Internal Heating of Old Neutron Stars: Contrasting Different Mechanisms
Context: The standard cooling models of neutron stars predict temperatures
yr. However, the likely thermal emission
detected from the millisecond pulsar J0437-4715, of spin-down age yr, implies a temperature K. Thus, a heating
mechanism needs to be added to the cooling models in order to obtain agreement
between theory and observation. Aims: Several internal heating mechanisms could
be operating in neutron stars, such as magnetic field decay, dark matter
accretion, crust cracking, superfluid vortex creep, and non-equilibrium
reactions ("rotochemical heating"). We study these mechanisms in order to
establish which could be the dominant source of thermal emission from old
pulsars. Methods: We show by simple estimates that magnetic field decay, dark
matter accretion, and crust cracking mechanism are unlikely to have a
significant effect on old neutron stars. The thermal evolution for the other
mechanisms is computed using the code of Fern\'andez and Reisenegger. Given the
dependence of the heating mechanisms on the spin-down parameters, we study the
thermal evolution for two types of pulsars: young, slowly rotating "classical"
pulsars and old, fast rotating millisecond pulsars. Results: We find that
magnetic field decay, dark matter accretion, and crust cracking do not produce
detectable heating of old pulsars. Rotochemical heating and vortex creep can be
important both for classical pulsars and millisecond pulsars. More restrictive
upper limits on the surface temperatures of classical pulsars could rule out
vortex creep as the main source of thermal emission. Rotochemical heating in
classical pulsars is driven by the chemical imbalance built up during their
early spin-down, and therefore strongly sensitive to their initial rotation
period.Comment: 7 pages, 5 figures, accepted version to be published in A&
Surface structure of Quark stars with magnetic fields
We investigate the impact of magnetic fields on the electron distribution in
the electrosphere of quark stars. For moderately strong magnetic fields G, quantization effects are generally weak due to the large number
density of electrons at surface, but can nevertheless affect the spectral
features of quark stars. We outline the main observational characteristics of
quark stars as determined by their surface emission, and briefly discuss their
formation in explosive events termed Quark-Novae, which may be connected to the
-process.Comment: 9 pages, 3 figures. Contribution to the proceedings of the IXth
Workshop on High Energy Physics Phenomenology (WHEPP-9), Bhubaneswar, India,
3-14 Jan. 200
Removal of Mercury by Foam Fractionation Using Surfactin, a Biosurfactant
The separation of mercury ions from artificially contaminated water by the foam fractionation process using a biosurfactant (surfactin) and chemical surfactants (SDS and Tween-80) was investigated in this study. Parameters such as surfactant and mercury concentration, pH, foam volume, and digestion time were varied and their effects on the efficiency of mercury removal were investigated. The recovery efficiency of mercury ions was highly sensitive to the concentration of the surfactant. The highest mercury ion recovery by surfactin was obtained using a surfactin concentration of 10 × CMC, while recovery using SDS required < 10 × CMC and Tween-80 >10 × CMC. However, the enrichment of mercury ions in the foam was superior with surfactin, the mercury enrichment value corresponding to the highest metal recovery (10.4%) by surfactin being 1.53. Dilute solutions (2-mg L−1 Hg2+) resulted in better separation (36.4%), while concentrated solutions (100 mg L−1) enabled only a 2.3% recovery using surfactin. An increase in the digestion time of the metal solution with surfactin yielded better separation as compared with a freshly-prepared solution, and an increase in the airflow rate increased bubble production, resulting in higher metal recovery but low enrichment. Basic solutions yielded higher mercury separation as compared with acidic solutions due to the precipitation of surfactin under acidic conditions
The Agrobacterium-mediated transformation of common wheat (Triticum aestivum L.) and triticale (x Triticosecale Wittmack): role of the binary vector system and selection cassettes
The influence of two binary vector systems, pGreen and pCAMBIA, on the Agrobacterium-mediated transformation ability of wheat and triticale was studied. Both vectors carried selection cassettes with bar or nptII driven by different promoters. Two cultivars of wheat, Kontesa and Torka, and one cultivar of triticale, Wanad, were tested. The transformation rates for the wheat cultivars ranged from 0.00 to 3.58% and from 0.00 to 6.79% for triticale. The best values for wheat were 3.58% for Kontesa and 3.14% for Torka, and these were obtained after transformation with the pGreen vector carrying the nptII selection gene under the control of 35S promoter. In the case of the bar selection system, the best transformation rates were, respectively, 1.46 and 1.79%. Such rates were obtained when the 35S::bar cassette was carried by the pCAMBIA vector; they were significantly lower with the pGreen vector. The triticale cultivar Wanad had its highest transformation rate after transformation with nptII driven by 35S in pCAMBIA. The bar selection system for the same triticale cultivar was better when the gene was driven by nos and the selection cassette was carried by pGreen. The integration of the transgenes was confirmed with at least three pairs of specific starters amplifying the fragments of nptII, bar, or gus. The expression of selection genes, measured by reverse transcriptase polymerase chain reaction (RT-PCR) in relation to the actin gene, was low, ranging from 0.00 to 0.63 for nptII and from 0.00 to 0.33 for bar. The highest relative transcript accumulation was observed for nptII driven by 35S and expressed in Kontesa that had been transformed with pGreen
Is SAX J1808.4-3658 a Strange Star ?
One of the most important questions in the study of compact objects is the
nature of pulsars, including whether they are composed of -stable
nuclear matter or strange quark matter. Observations of the newly discovered
millisecond X-ray pulsar \sax with the Rossi X-Ray Timing Explorer place firm
constraint on the radius of the compact star. Comparing the mass - radius
relation of \sax with the theoretical mass - radius relation for neutron stars
and for strange stars, we find that a strange star model is more consistent
with SAX J1808.4-3658, and suggest that it is a likely strange star candidate.Comment: 5 pages, Latex, 1 figure, corrected for some typo
Synthesis of Brushite Particles in Reverse Microemulsions of the Biosurfactant Surfactin
In this study the “green chemistry” use of the biosurfactant surfactin for the synthesis of calcium phosphate using the reverse microemulsion technique was demonstrated. Calcium phosphates are bioactive materials that are a major constituent of human teeth and bone tissue. A reverse microemulsion technique with surfactin was used to produce nanocrystalline brushite particles. Structural diversity (analyzed by SEM and TEM) resulted from different water to surfactin ratios (W/S; 250, 500, 1000 and 40,000). The particle sizes were found to be in the 16–200 nm range. Morphological variety was observed in the as-synthesized microemulsions, which consisted of nanospheres (~16 nm in diameter) and needle-like (8–14 nm in diameter and 80–100 nm in length) noncalcinated particles. However, the calcinated products included nanospheres (50–200 nm in diameter), oval (~300 nm in diameter) and nanorod (200–400 nm in length) particles. FTIR and XRD analysis confirmed the formation of brushite nanoparticles in the as-synthesized products, while calcium pyrophosphate was produced after calcination. These results indicate that the reverse microemulsion technique using surfactin is a green process suitable for the synthesis of nanoparticles
Neutrino-Nucleon Interactions in Magnetized Neutron-Star Matter: The Effects of Parity Violation
We study neutrino-nucleon scattering and absorption in a dense, magnetized
nuclear medium. These are the most important sources of neutrino opacity
governing the cooling of a proto-neutron star in the first tens of seconds
after its formation. Because the weak interaction is parity violating, the
absorption and scattering cross-sections depend asymmetrically on the
directions of the neutrino momenta with respect to the magnetic field. We
develop the moment formalism of neutrino transport in the presence of such
asymmetric opacities and derive explicit expressions for the neutrino flux and
other angular moments of the Boltzmann transport equation. For a given neutrino
species, there is a drift flux of neutrinos along the magnetic field in
addition to the usual diffusive flux. This drift flux depends on the deviation
of the neutrino distribution function from thermal equilibrium. Hence, despite
the fact that the neutrino cross-sections are asymmetric throughout the star,
asymmetric neutrino flux can be generated only in the outer region of the
proto-neutron star where the neutrino distribution deviates significantly from
thermal equilibrium. In addition to the asymmetric absorption opacity arising
from nucleon polarization, we find the contribution of the electron (or
positron) ground state Landau level. For neutrinos of energy less than a few
times the temperature, this is the dominant source of asymmetric opacity.
Lastly, we discuss the implication of our result to the origin of pulsar kicks:
in order to generate kick velocity of a few hundred km/s from asymmetric
neutrino emission using the parity violation effect, the proto-neutron star
must have a dipole magnetic field of at least G.Comment: 35 pages, no figures, submitted to Phys.Rev.
Neutrino Propagation in a Strongly Magnetized Medium
We derive general expressions at the one-loop level for the coefficients of
the covariant structure of the neutrino self-energy in the presence of a
constant magnetic field. The neutrino energy spectrum and index of refraction
are obtained for neutral and charged media in the strong-field limit () using the lowest Landau level
approximation. The results found within the lowest Landau level approximation
are numerically validated, summing in all Landau levels, for strong and weakly-strong fields. The neutrino energy in
leading order of the Fermi coupling constant is expressed as the sum of three
terms: a kinetic-energy term, a term of interaction between the magnetic field
and an induced neutrino magnetic moment, and a rest-energy term. The leading
radiative correction to the kinetic-energy term depends linearly on the
magnetic field strength and is independent of the chemical potential. The other
two terms are only present in a charged medium. For strong and weakly-strong
fields, it is found that the field-dependent correction to the neutrino energy
in a neutral medium is much larger than the thermal one. Possible applications
to cosmology and astrophysics are considered.Comment: 23 pages, 4 figures. Corrected misprints in reference
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