Particles (Nuclear physics)

The starting point of any general relativistic numerical simulation is a solution
of the Hamiltonian and momentum constraints that (ideally) represents an astrophysically realistic scenario. This dissertation presents a new method to produce initial data sets for binary neutron stars with arbitrary spins and orbital eccentricities. The method only provides approximate solutions to the constraints. However, it was
shown that the corresponding constraint violations subside after a few orbits, becoming
comparable to those found in evolutions of standard conformally flat, helically
symmetric binary initial data. This dissertation presents the first spinning neutron
star binary simulations in circular orbits with a orbital eccentricity less then 0.01. The
initial data sets corresponding to binaries with spins aligned, zero and anti-aligned
with the orbital angular momentum were evolved in time. These simulations show
the orbital “hang-up” effect previously seen in binary black holes. Additionally, they
show orbital eccentricities that can be up to one order of magnitude smaller than
those found in helically symmetric initial sets evolutions.

In this work, we discuss the conceptual framework of quantum mechanics in the Hilbert space formalism from a group representation point of view. After a brief review of the main results of the theory of groups and their representations, we describe mathematical models of the subject, and show the applications of this theory for getting numerical answers to problems in elementary particle physics.

We study the evolution of binary black hole initial data schemes as alternatives to the standard puncture construction. These alternatives are based on post-Newtonian expansions that contain realistic gravitational waves. The first scheme is based on a second order post-Newtonian expansion in Arnowitt, Deser, and Misner transverse-traceless (ADMTT) gauge that has been re-summed to approach standard puncture data at the black holes. The other schemes are based on asymptotic matching of the 4-metrics of two tidally perturbed Schwarzschild solutions to post-Newtonian expansions at different orders. The alternatives are encouraging and lead to quasi-circular orbits and show gravitational radiation from the onset, as well as a reduction of spurious radiation. Deficiencies compared to punctures include more eccentricity during inspiral and larger constraint violations, since the alternative data sets only approximate solutions of Einstein's equations.

A method for estimating the size distribution of magnetite nanoparticles from their magnetic properties is presented. The 10 nm diameter particles were coated with poly(acrylic) acid and prepared as a water-based suspension. A vacuum-dried sample was placed in a transmission electron microscope (TEM) so that the physical sizes of the particles could be estimated. The particle magnetization was measured by a superconducting quantum interference device (SQUID) in magnetic fields up to 25 kiloOersted and temperatures ranging from 5 to 370 Kelvin. The magnetic moments in the sample were estimated by fitting those measurements to a Langevin magnetization model, weighted by a log-normal distribution with unknown parameters.The best-fit procedure yielded particle volumes smaller than those observed by transmission electron microscopy, suggesting the existence of a magnetically inactive layer of atoms. In addition, our particles exhibited stronger spin-wave behavior than expected for particles of similar size, as evidenced by the lower saturation magnetization and higher Bloch coefficient.