34,407 research outputs found
The delayed contribution of low and intermediate mass stars to chemical galactic enrichment: An analytical approach
We find a new analytical solution for the chemical evolution equations,
taking into account the delayed contribution of all low and intermediate mass
stars (LIMS) as one representative star that enriches the interstellar
medium.This solution is built only for star formation rate proportional to the
gas mass in a closed box model. We obtain increasing C/O and N/O ratios with
increasing O/H, behavior impossible to match with the Instantaneous Recycling
Approximation (IRA). Our results, obtained by two analytical equations, are
very similar to those found by numerical models that consider the lifetimes of
each star. This delayed model reproduces successfully the evolution of C/O-O/H
and Y-O relations in the solar vicinity. This analytical approximation is a
useful tool to study the chemical evolution of elements produced by LIMS when a
galactic chemical evolutionary code is not available.Comment: 19 pages, 5 figures, to be published in the RevMexAA in October 200
Quantum phase estimation algorithms with delays: effects of dynamical phases
The unavoidable finite time intervals between the sequential operations
needed for performing practical quantum computing can degrade the performance
of quantum computers. During these delays, unwanted relative dynamical phases
are produced due to the free evolution of the superposition wave-function of
the qubits. In general, these coherent "errors" modify the desired quantum
interferences and thus spoil the correct results, compared to the ideal
standard quantum computing that does not consider the effects of delays between
successive unitary operations. Here, we show that, in the framework of the
quantum phase estimation algorithm, these coherent phase "errors", produced by
the time delays between sequential operations, can be avoided by setting up the
delay times to satisfy certain matching conditions.Comment: 10 pages, no figur
Scheme Independence of the Effective Hamiltonian for and Decays
We present a calculation of the effective weak Hamiltonian which governs and transitions in two different
renormalization schemes (NDR and HV). In the leading logarithmic approximation,
we show that the coefficients of the effective Hamiltonian are scheme
independent only when one takes correctly into account the scheme dependence of
one- and two- loop diagrams. We demonstrate that in NDR there are contributions
which were missed in previous calculations. These contributions are necessary
to obtain scheme independent coefficients in the final results.Comment: 16 pp + 5 figures not included (available by anonymous ftp at
amisan.iss.infn.it (141.108.15.215), directory /ftp/bsgamma), LaTeX, LPTENS
93/28, ROME 93/958, ULB-TH 93/0
Gravitational perturbations of the Higgs field
We study the possible effects of classical gravitational backgrounds on the
Higgs field through the modifications induced in the one-loop effective
potential and the vacuum expectation value of the energy-momentum tensor. We
concentrate our study on the Higgs self-interaction contribution in a perturbed
FRW metric. For weak and slowly varying gravitational fields, a complete set of
mode solutions for the Klein-Gordon equation is obtained to leading order in
the adiabatic approximation. Dimensional regularization has been used in the
integral evaluation and a detailed study of the integration of nonrational
functions in this formalism has been presented. As expected, the regularized
effective potential contains the same divergences as in flat spacetime, which
can be renormalized without the need of additional counterterms. We find that,
in contrast with other regularization methods, even though metric perturbations
affect the mode solutions, they do not contribute to the leading adiabatic
order of the potential. We also obtain explicit expressions of the complete
energy-momentum tensor for general nonminimal coupling in terms of the
perturbed modes. The corresponding leading adiabatic contributions are also
obtained.Comment: 15 pages. Version accepted for publication in PRD. Error corrected in
the angular integration in Appendix B. Conclusions changed. New section
include
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