1,720,979 research outputs found
Aspects of the electroweak phase transition and baryogenesis
Get PDFIn this thesis we study aspects of the cosmological electroweak phase transition which are relevant to the possibility of baryogenesis at this epoch. We focus on two issues: first, requiring that the observed baryon number be of electroweak origin places strong constraints on electroweak physics, and second, baryogenesis at the electroweak scale may be driven by an asymmetry generated at the GUT scale. We use the effective potential at finite temperature as a means of analyzing phase transitions associated with spontaneous symmetry breaking. We develop the theory with two basic examples: the scalar and Abelian Higgs models. Infrared divergences near the phase transition make the one-loop description unreliable, and indeed invalidate conventional perturbation theory. Borrowing a method from studies of QCD at high temperatures, we demonstrate that the summation of ring diagrams cures the leading infrared divergences and achieves a more reliable perturbative expansion. We then apply this formalism to the minimal Standard Model, following previous work, and confirm weak first-order behavior at the phase transition. We show that requiring the baryon number not be erased by sphaleron processes after the phase transition places a stringent bound on the Higgs mass, which is incompatible with experiment. This cosmological bound, however, may be relaxed by extending the scalar sector of the Standard Model. We consider the two simplest such extensions, the addition of a gauge singlet and of a second doublet. We demonstrate that ring-improvement in the singlet extension alters previous arguments at the one-loop level and yields a more restrictive bound on the Higgs mass. While ring-improvement in the two-doublet model, in principle, also reduces the Higgs mass bound found earlier at one loop, the multitude of new couplings in this model does not permit a definitive statement. We then investigate a mechanism for generating the observed baryon asymmetry (nB/S~ 10-10) at the electroweak phase transition from a pre-existing leptonic asymmetry (LT/s~ 10-5) produced at the GUT scale. This mechanism works by charge transport in a strongly first-order phase transition and avoids the need for large CP-violation at the electroweak scale
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Degradation of photon distribution and its effect on high frequency performance of 2D hot electrons in CdS quantum wells
No full text52-56<span style="font-size:
15.0pt;mso-bidi-font-size:8.0pt;font-family:" arial","sans-serif""="">The high
frequency performance of two-dimensional hot electron in CdS quantum wells is
studied in the framework of the heated, drifted Fermi -Dirac distribution
function, incorporating the relevant scattering mechanisms. The effects of non-equilibrium
longitudinal optic (LO) phonons are also included in the calculations. The influences
of large- q LO phonon and electron- LO phonon interactions at low qL%
<span style="font-size:15.0pt;mso-bidi-font-size:8.0pt;font-family:
" arial","sans-serif""="">are also studied. The ac mobility normalized by its de
value is found to decrease with increasing frequency of the applied field
beyond about 100 GHz while the drift velocity lags behind the applied field
above, about 10 GHz. The degradation of phonon distribution enhances the 3 dB
cut-off frequency at which the ac mobility drops <span style="font-size:
14.5pt;mso-bidi-font-size:7.5pt;font-family:" times="" new="" roman","serif""="">to 0.707
of its low frequency value.
</span
GA Based Parameter Optimizations for Better Millimeter and Sub-Millimeter Wave Response Characteristics of GaAs Quantum Wells
Get PDFAd hoc mobile wireless networks: principles, protocols, and applications
No full textThe military, the research community, emergency services, and industrial environments all rely on ad hoc mobile wireless networks because of their simple infrastructure and minimal central administration. Now in its second edition, Ad Hoc Mobile Wireless Networks: Principles, Protocols, and Applications explains the concepts, mechanism, design, and performance of these highly valued systems. Following an overview of wireless network fundamentals, the book explores MAC layer, routing, multicast, and transport layer protocols for ad hoc mobile wireless networks. Next, it examines quality of ser
Genetic approach towards the optimization of the system parameters of polar semiconductor quantum wells for better high frequency response
No full text691-696The optimized system
parameters of polar semiconductor quantum wells have been calculated to get
desired high frequency response characterized by 3dB cut-off frequency at which
the ac mobility drops to 0.707 of its
low frequency value. The ac mobility
of two-dimensional hot electrons in square quantum wells of GaAs and (In, Ga)As
are computed numerically using heated drifted Fermi-Dirac distribution function
and prevalent scattering mechanisms. Genetic algorithm has been employed to get
the optimized system parameters for getting the desired high-frequency, small
power consuming and low dimensional devices, thereby saving significantly the
search time for the technologists involved in the fabrication of such devices
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