19,989 research outputs found
Astrophysical constraints on the proton-to-electron mass ratio with FAST
That the laws of physics are the same at all times and places throughout the
Universe is one of the basic assumptions of physics. Astronomical observations
provide the only means to test this basic assumption on cosmological time and
distance scales. The possibility of variations in the dimensionless physical
constant {\mu} - the proton-to-electron mass ratio, can be tested by comparing
astronomical measurements of the rest frequency of certain spectral lines at
radio wavelengths with laboratory determinations. Different types of molecular
transitions have different dependencies on {\mu} and so observations of two or
more spectral lines towards the same astronomical source can be used to test
whether there is any evidence for either temporal or spatial changes in the
physical fundamental constants. {\mu} will change if the relative strength of
the strong nuclear force compared to the electromagnetic force varies.
Theoretical studies have shown that the rotational transitions of some
molecules which have transitions in the frequency range that will be covered by
FAST (e.g., CH3OH, OH and CH) are sensitive to changes in {\mu}. A number of
studies looking for possible variations in {\mu} have been undertaken with
existing telescopes, however, the greater sensitivity of FAST means it will
open new opportunities to significantly improve upon measurements made to date.
In this paper, we discuss which molecular transitions and sources (both in the
Galaxy and external galaxies) are likely targets for providing improved
constraints on {\mu} with FAST
Does a proton "bubble" structure exist in the low-lying states of 34Si?
The possible existence of a "bubble" structure in the proton density of
Si has recently attracted a lot of research interest. To examine the
existence of the "bubble" structure in low-lying states, we establish a
relativistic version of configuration mixing of both particle number and
angular momentum projected quadrupole deformed mean-field states and apply this
state-of-the-art beyond relativistic mean-field method to study the density
distribution of the low-lying states in Si. An excellent agreement with
the data of low-spin spectrum and electric multipole transition strengths is
achieved without introducing any parameters. We find that the central
depression in the proton density is quenched by dynamic quadrupole shape
fluctuation, but not as significantly as what has been found in a beyond
non-relativistic mean-field study. Our results suggest that the existence of
proton "bubble" structure in the low-lying excited and states
is very unlikely.Comment: 6 pages, 8 figures and 1 table, accepted for publication in Physics
Letters
Spin-roton excitations in the cuprate superconductors
We identify a new kind of elementary excitations, spin-rotons, in the doped
Mott insulator. They play a central role in deciding the superconducting
transition temperature Tc, resulting in a simple Tc formula,Tc=Eg/6, with Eg as
the characteristic energy scale of the spin rotons. We show that the degenerate
S=1 and S=0 rotons can be probed by neutron scattering and Raman scattering
measurements, respectively, in good agreement with the magnetic resonancelike
mode and the Raman A1g mode observed in the high-Tc cuprates.Comment: 10 pages, 9 figure
A Monte-Carlo simulation of the equilibrium beam polarization in ultra-high energy electron (positron) storage rings
With the recently emerging global interest in building a next generation of
circular electron-positron colliders to study the properties of the Higgs
boson, and other important topics in particle physics at ultra-high beam
energies, it is also important to pursue the possibility of implementing
polarized beams at this energy scale. It is therefore necessary to set up
simulation tools to evaluate the beam polarization at these ultra-high beam
energies. In this paper, a Monte-Carlo simulation of the equilibrium beam
polarization based on the Polymorphic Tracking Code(PTC) (Schmidt et al., 2002
[1]) is described. The simulations are for a model storage ring with parameters
similar to those of proposed circular colliders in this energy range, and they
are compared with the suggestion (Derbenev et al., 1978 [2]) that there are
different regimes for the spin dynamics underlying the polarization of a beam
in the presence of synchrotron radiation at ultra-high beam energies. In
particular, it has been suggested that the so-called "correlated" crossing of
spin resonances during synchrotron oscillations at current energies, evolves
into "uncorrelated" crossing of spin resonances at ultra-high energies.Comment: submitted to and accepted by Nucl. Instrum. Meth.
Complex Agent Networks explaining the HIV epidemic among homosexual men in Amsterdam
Simulating the evolution of the Human Immunodeficiency Virus (HIV) epidemic
requires a detailed description of the population network, especially for small
populations in which individuals can be represented in detail and accuracy. In
this paper, we introduce the concept of a Complex Agent Network(CAN) to model
the HIV epidemics by combining agent-based modelling and complex networks, in
which agents represent individuals that have sexual interactions. The
applicability of CANs is demonstrated by constructing and executing a detailed
HIV epidemic model for men who have sex with men (MSM) in Amsterdam, including
a distinction between steady and casual relationships. We focus on MSM contacts
because they play an important role in HIV epidemics and have been tracked in
Amsterdam for a long time. Our experiments show good correspondence between the
historical data of the Amsterdam cohort and the simulation results.Comment: 21 pages, 4 figures, Mathematics and Computers in Simulation, added
reference
Simple one-dimensional quantum-mechanical model for a particle attached to a surface
We present a simple one-dimensional quantum-mechanical model for a particle
attached to a surface. We solve the Schr\"odinger equation in terms of Weber
functions and discuss the behavior of the eigenvalues and eigenfunctions. We
derive the virial theorem and other exact relationships as well as the
asymptotic behaviour of the eigenvalues. We calculate the zero-point energy for
model parameters corresponding to H adsorbed on Pd(100) and also outline the
application of the Rayleigh-Ritz variational method
Configuration mixing of angular-momentum projected triaxial relativistic mean-field wave functions. II. Microscopic analysis of low-lying states in magnesium isotopes
The recently developed structure model that uses the generator coordinate
method to perform configuration mixing of angular-momentum projected wave
functions, generated by constrained self-consistent relativistic mean-field
calculations for triaxial shapes (3DAMP+GCM), is applied in a systematic study
of ground states and low-energy collective states in the even-even magnesium
isotopes Mg. Results obtained using a relativistic point-coupling
nucleon-nucleon effective interaction in the particle-hole channel, and a
density-independent -interaction in the pairing channel, are compared
to data and with previous axial 1DAMP+GCM calculations, both with a
relativistic density functional and the non-relativistic Gogny force. The
effects of the inclusion of triaxial degrees of freedom on the low-energy
spectra and E2 transitions of magnesium isotopes are examined.Comment: 28 pages, 11 figures and 1 tabl
Rapid structural change in low-lying states of neutron-rich Sr and Zr isotopes
The rapid structural change in low-lying collective excitation states of
neutron-rich Sr and Zr isotopes is tudied by solving a five-dimensional
collective Hamiltonian with parameters determined by both relativistic
mean-field and non-relativistic Skyrme-Hartree-Fock calculations using the
PC-PK1 and SLy4 forces respectively. Pair correlations are treated in BCS
method with either a separable pairing force or a density-dependent zero-range
force. The isotope shifts, excitation energies, electric monopole and
quadrupole transition strengths are calculated and compared with corresponding
experimental data. The calculated results with both the PC-PK1 and SLy4 forces
exhibit a picture of spherical-oblate-prolate shape transition in neutron-rich
Sr and Zr isotopes. Compared with the experimental data, the PC-PK1 (or SLy4)
force predicts a more moderate (or dramatic) change in most of the collective
properties around N=60. The underlying microscopic mechanism responsible for
the rapid transition is discussed.Comment: 10 pages (twocolumn), 10 figure
Spatial particle correlations in light nuclei. I Two-particle systems
Expressions for spatial two-particle correlations in an LS-coupled basis of the harmonic oscillator are used to display the probability distribution of two identical nucleons as a function of their relative distance and their distance from the center of the nucleus. It is shown that a two-nucleon state in the p shell with total orbital angular momentum L = 0 and total spin S = 0 contains a di-neutron and a cigarlike component with equal probability. This result can also be proven analytically with the use of angular correlation functions. Scattering of the nucleons from the p shell to other shells leads to the enhancement of the di-neutron con guration. A semi-quantitative application to 6He is presented which shows that the probability of the di-neutron con guration in the ground state is of the order of 60%. The longterm goal of this work is to obtain a geometric insight into the properties of nuclei with several nucleons in a valence shell
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