10,900 research outputs found
Fate of global symmetries in the Universe: QCD axion, quintessential axion and trans-Planckian inflaton decay-constant
Pseudoscalars appearing in particle physics are reviewd systematically. From
the fundamental point of view at an ultra-violat completed theory, they can be
light if they are realized as pseudo-Goldstone bosons of some spontaneously
broken global symmetries. The spontaneous breaking scale is parametrized by the
decay constant . The global symmetry is defined by the lowest order terms
allowed in the effective theory consistent with the gauge symmetry in question.
Since any global symmetry is known to be broken at least by quantum
gravitational effects, all pseudoscalars should be massive. The mass scale is
determined by and the explicit breaking terms in the effective
potential and also anomaly terms for some non-Abelian gauge
groups . The well-known example by non-Abelian gauge group breaking is the
potential for the "invisible" QCD axion, via the Peccei-Quinn symmetry, which
constitutes a major part of this review. Even if there is no breaking terms
from gauge anomalies, there can be explicit breaking terms in the
potential in which case the leading term suppressed by determines the
pseudoscalar mass scale. If the breaking term is extremely small and the decay
constant is trans-Planckian, the corresponding pseudoscalar can be a candidate
for a `quintessential axion'. In general, is considered to
be smaller than , and hence the pseudo-Goldstone boson mass scales are
considered to be smaller than the decay constants. In such a case, the
potential of the pseudo-Goldstone boson at the grand unification scale is
sufficiently flat near the top of the potential that it can be a good candidate
for an inflationary model, which is known as `natural inflation'. We review all
these ideas in the bosonic collective motion framework.Comment: 41 pages with 27 figure
Evidence for the two pole structure of the Lambda(1405) resonance
The K^- p --> pi^0 pi^0 Sigma^0 reaction is studied within a chiral unitary
model. The distribution of pi^0 Sigma^0 states forming the Lambda(1405) shows,
in agreement with a recent experiment, a peak at 1420 MeV and a relatively
narrow width of Gamma = 38 MeV. The mechanism for the reaction is largely
dominated by the emission of a pi^0 prior to the K^- p interaction leading to
the Lambda(1405). This ensures the coupling of the Lambda(1405) to the K^- p
channel, thus maximizing the contribution of the second state found in chiral
unitary theories, which is narrow and of higher energy than the nominal
Lambda(1405). This is unlike the pi^- p --> K^0 \pi Sigma reaction, which gives
more weight to the pole at lower energy and with a larger width. The data of
these two experiments, together with the present theoretical analysis, provides
a firm evidence of the two pole structure of the Lambda(1405).Comment: 4 pages, 6 figure
Getting Jobs, Keeping Jobs, and Earning a Living Wage: Can Welfare Reform Work?
Most discussions of welfare and work have focused on how demographic characteristics, schooling, training, and work experience limit welfare mothers’ employment and wages, but they have largely ignored factors such as inappropriate workplace behaviors, expectations of discrimination and harassment, depression, alcoholism, and domestic violence, all of which may affect welfare mothers and make employment difficult. In this paper we review the prevalence of these individual-level barriers and argue that they, in combination with an economy which does not pay low-skill workers well, are likely to impede employment and self-sufficiency for a large proportion of welfare mothers. At the end of the review, we summarize the current state of knowledge about barriers to the employment of welfare recipients and suggest several ways in which welfare-to-work programs might address these barriers.
Memory and superposition in a spin glass
Non-equilibrium dynamics in a Ag(Mn) spin glass are investigated by
measurements of the temperature dependence of the remanent magnetisation. Using
specific cooling protocols before recording the thermo- or isothermal remanent
magnetisations on re-heating, it is found that the measured curves effectively
disclose non-equilibrium spin glass characteristics such as ageing and memory
phenomena as well as an extended validity of the superposition principle for
the relaxation. The usefulness of this "simple" dc-method is discussed, as well
as its applicability to other disordered magnetic systems.Comment: REVTeX style; 8 pages, 4 figure
Embryonic Precursor Cells from the Rhombic Lip Are Specified to a Cerebellar Granule Neuron Identity
AbstractThe specification of diverse classes of neurons is critical to the development of the cerebellar cortex. Here, we describe the purification of early embryonic precursors of cerebellar granule neurons from the rhombic lip, the dorsal aspect of the midbrain/hindbrain region. Isolation of rhombic lip cells reveals a homogenous population of precursor cells that express general neuronal markers and the granule cell marker RU49, but fail to extend neurites or express differentiation markers. Differentiation is induced by coculture with external germinal layer (EGL) cells, or their membranes, suggesting that a local inducing factor acts after formation of the EGL. Thus, proliferating precursors within the rhombic lip are specified to be granule cells very early, with the availability of an inducing factor increasing over the course of development
Finite temperature Casimir effect for massive scalar field in spacetime with extra dimensions
We compute the finite temperature Casimir energy for massive scalar field
with general curvature coupling subject to Dirichlet or Neumann boundary
conditions on the walls of a closed cylinder with arbitrary cross section,
located in a background spacetime of the form ,
where is the -dimensional Minkowski spacetime and
is an -dimensional internal manifold. The Casimir energy is
regularized using the criteria that it should vanish in the infinite mass
limit. The Casimir force acting on a piston moving freely inside the closed
cylinder is derived and it is shown that it is independent of the
regularization procedure. By letting one of the chambers of the cylinder
divided by the piston to be infinitely long, we obtain the Casimir force acting
on two parallel plates embedded in the cylinder. It is shown that if both the
plates assume Dirichlet or Neumann boundary conditions, the strength of the
Casimir force is reduced by the increase in mass. Under certain conditions, the
passage from massless to massive will change the nature of the force from long
range to short range. Other properties of the Casimir force such as its sign,
its behavior at low and high temperature, and its behavior at small and large
plate separations, are found to be similar to the massless case. Explicit exact
formulas and asymptotic behaviors of the Casimir force at different limits are
derived. The Casimir force when one plate assumes Dirichlet boundary condition
and one plate assumes Neumann boundary condition is also derived and shown to
be repulsive.Comment: 28 pages, 4 figure
SPEAR Far Ultraviolet Spectral Images of the Cygnus Loop
We present far-ultraviolet (FUV) spectral images, measured at C IV 1550, He
II 1640, Si IV+O IV] 1400, and O III] 1664, of the entire Cygnus Loop, observed
with the Spectroscopy of Plasma Evolution from Astrophysical Radiation (SPEAR)
instrument, also known as FIMS. The spatial distribution of FUV emission
generally corresponds with a limb-brightened shell, and is similar to optical,
radio and X-ray images. The features found in the present work include a
``carrot'', diffuse interior, and breakout features, which have not been seen
in previous FUV studies. Shock velocities of 140-160 km/s is found from a line
ratio of O IV] to O III], which is insensitive not only to resonance scattering
but also to elemental abundance. The estimated velocity indicates that the fast
shocks are widespread across the remnant. By comparing various line ratios with
steady-state shock models, it is also shown that the resonance scattering is
widespread.Comment: 13 pages, 3 figures, 1 table, accepted for publication in ApJ
A simplified charge projection scheme for long-range electrostatics in ab initio QM/MM calculations
In a previous work [Pan et al., Molecules 23, 2500 (2018)], a charge projection scheme was reported, where outer molecular mechanical (MM) charges [>10 Å from the quantum mechanical (QM) region] were projected onto the electrostatic potential (ESP) grid of the QM region to accurately and efficiently capture long-range electrostatics in ab initio QM/MM calculations. Here, a further simplification to the model is proposed, where the outer MM charges are projected onto inner MM atom positions (instead of ESP grid positions). This enables a representation of the long-range MM electrostatic potential via augmentary charges (AC) on inner MM atoms. Combined with the long-range electrostatic correction function from Cisneros et al. [J. Chem. Phys. 143, 044103 (2015)] to smoothly switch between inner and outer MM regions, this new QM/MM-AC electrostatic model yields accurate and continuous ab initio QM/MM electrostatic energies with a 10 Å cutoff between inner and outer MM regions. This model enables efficient QM/MM cluster calculations with a large number of MM atoms as well as QM/MM calculations with periodic boundary conditions
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