6,602 research outputs found
Consequences of t-channel unitarity for the interaction of real and virtual photons at high energies
We analyze the consequences of t-channel unitarity for photon cross sections
and show what assumptions are necessary to allow for the existence of new
singularities at for the and total
cross sections. For virtual photons, such singularities can in general be
present, but we show that, apart from the perturbative singularity associated
with , no new ingredient is needed to
reproduce the data from LEP and HERA, in the Regge region.Comment: 10 pages, LaTeX2e with kluwer.sty, 7 figures. Talk presented at the
Second International "Cetraro" Workshop & NATO Advanced Research Workshop
"Diffraction 2002", Alushta, Crimea, Ukraine, August 31 - September 6, 200
The KLN Theorem and Soft Radiation in Gauge Theories: Abelian Case
We present a covariant formulation of the Kinoshita, Lee, Nauenberg (KLN)
theorem for processes involving the radiation of soft particles. The role of
the disconnected diagrams is explored and a rearrangement of the perturbation
theory is performed such that the purely disconnected diagrams are factored
out. The remaining effect of the disconnected diagrams results in a simple
modification of the usual Feynman rules for the S-matrix elements. As an
application, we show that when combined with the Low theorem, this leads to a
proof of the absense of the corrections to inclusive processes (like the
Drell-Yan process). In this paper the abelian case is discussed to all orders
in the coupling.Comment: 27 pages, LaTeX, 14 figure
Genes Associated with Honey Bee Behavioral Maturation Affect Clock-Dependent and -Independent Aspects of Daily Rhythmic Activity in Fruit Flies
BACKGROUND: In the honey bee, the age-related and socially regulated transition of workers from in-hive task performance (e.g., caring for young) to foraging (provisioning the hive) is associated with changes in many behaviors including the 24-hour pattern of rhythmic activity. We have previously shown that the hive-bee to forager transition is associated with extensive changes in brain gene expression. In this study, we test the possible function of a subset of these genes in daily rhythmic activity pattern using neural-targeted RNA interference (RNAi) of an orthologous gene set in Drosophila melanogaster. PRINCIPAL FINDINGS: Of 10 genes tested, knockdown of six affected some aspect of locomotor activity under a 12 h:h light:dark regime (LD). Inos affected anticipatory activity preceding lights-off, suggesting a possible clock-dependent function. BM-40-SPARC, U2af50 and fax affected peak activity at dawn without affecting anticipation or overall inactivity (proportion of 15-min intervals without activity), suggesting that these effects may depend on the day-night light cycle. CAH1 affected overall inactivity. The remaining gene, abl, affected peak activity levels but was not clearly time-of-day-specific. No gene tested affected length of period or strength of rhythmicity in constant dark (DD), suggesting that these genes do not act in the core clock. SIGNIFICANCE: Taking advantage of Drosophila molecular genetic tools, our study provides an important step in understanding the large set of gene expression changes that occur in the honey bee transition from hive bee to forager. We show that orthologs of many of these genes influence locomotor activity in Drosophila, possibly through both clock-dependent and -independent pathways. Our results support the importance of both circadian clock and direct environmental stimuli (apart from entrainment) in shaping the bee's 24-hour pattern of activity. Our study also outlines a new approach to dissecting complex behavior in a social animal
Quantum-fluid dynamics of microcavity polaritons
Semiconductor microcavities offer a unique system to investigate the physics
of weakly interacting bosons. Their elementary excitations, polaritons--a
mixture of excitons and photons--behave, in the low density limit, as bosons
that can undergo a phase transition to a regime characterised by long range
coherence. Condensates of polaritons have been advocated as candidates for
superfluidity; and the formation of vortices as well as elementary excitations
with a linear dispersion are actively sought after. In this work, we have
created and set in motion a macroscopically degenerate state of polaritons and
let it collide with a variety of defects present in the sample. Our experiments
show striking manifestations of a coherent light-matter packet that displays
features of a superfluid, although one of a highly unusual character as it
involves an out-of-equilibrium dissipative system where it travels at
ultra-fast velocity of the order of 1% the speed of light. Our main results are
the observation of i) a linear polariton dispersion accompanied with
diffusion-less motion, ii) flow without resistance when crossing an obstacle,
iii) suppression of Rayleigh scattering and iv) splitting into two fluids when
the size of the obstacle is comparable with the size of the wavepacket. This
work opens the way to the investigation of new phenomenology of
out-of-equilibrium condensates.Comment: 22 pages, 5 figure
Proposal of an extended t-J Hamiltonian for high-Tc cuprates from ab initio calculations on embedded clusters
A series of accurate ab initio calculations on Cu_pO-q finite clusters,
properly embedded on the Madelung potential of the infinite lattice, have been
performed in order to determine the local effective interactions in the CuO_2
planes of La_{2-x}Sr_xCuO_4 compounds. The values of the first-neighbor
interactions, magnetic coupling (J_{NN}=125 meV) and hopping integral
(t_{NN}=-555 meV), have been confirmed. Important additional effects are
evidenced, concerning essentially the second-neighbor hopping integral
t_{NNN}=+110meV, the displacement of a singlet toward an adjacent colinear
hole, h_{SD}^{abc}=-80 meV, a non-negligible hole-hole repulsion
V_{NN}-V_{NNN}=0.8 eV and a strong anisotropic effect of the presence of an
adjacent hole on the values of the first-neighbor interactions. The dependence
of J_{NN} and t_{NN} on the position of neighbor hole(s) has been rationalized
from the two-band model and checked from a series of additional ab initio
calculations. An extended t-J model Hamiltonian has been proposed on the basis
of these results. It is argued that the here-proposed three-body effects may
play a role in the charge/spin separation observed in these compounds, that is,
in the formation and dynamic of stripes.Comment: 24 pages, 4 figures, submitted to Phys. Rev.
Association between Genotype of the Serotonin Transporter-Linked Polymorphic Region of the Serotonin Transporter Gene and Age of Onset of Methamphetamine Use: a Preliminary Analysis
Early-onset methamphetamine use increases the lifetime prevalence of methamphetamine dependence. An earlier onset of methamphetamine use leads to greater damage to the terminal ends of serotonin neurons, more reduction in serotonin transporter (5-HTT) density, and an increased propensity toward further methamphetamine use. Because the 5-HTT-linked polymorphic region (5′-HTTLPR) within the promoter region of the 5-HTT gene leads to differential expression of the 5-HTT, we examined, for the first time, whether there is a differential association between the long (L) and short (S) alleles of the 5′-HTTLPR and the age of first methamphetamine use (AMU). The study included 120 methamphetamine-dependent adults of European descent. Diagnosis of methamphetamine dependence and AMU were collected using structured questionnaires, and the 5′-HTTLPR genotypes were determined using the polymerase chain reaction–restriction fragment length polymorphism method. Statistical analysis with the general linear model detected a significant interactive effect of 5′-HTTLPR genotypes (SS vs. L-carriers) and gender, associated with AMU (F = 3.99; p = 0.048). Further analysis of 5′-HTTLPR effects on AMU in males and females separately showed that the SS genotype compared with L-carriers had about two times greater risk of an earlier onset of methamphetamine use in men (hazard ratio = 1.839; 95% confidence interval = 1.042–3.246; p = 0.036) but not in women. Together, our findings in this preliminary study suggest a greater risk for earlier onset methamphetamine use associated with the SS genotype of the 5′-HTTLPR among methamphetamine-dependent Caucasian males
The development and application of a new tool to assess the adequacy of the content and timing of antenatal care
Abstract
Background: Current measures of antenatal care use are limited to initiation of care and number of visits. This
study aimed to describe the development and application of a tool to assess the adequacy of the content and
timing of antenatal care.
Methods: The Content and Timing of care in Pregnancy (CTP) tool was developed based on clinical relevance for
ongoing antenatal care and recommendations in national and international guidelines. The tool reflects minimal
care recommended in every pregnancy, regardless of parity or risk status. CTP measures timing of initiation of care,
content of care (number of blood pressure readings, blood tests and ultrasound scans) and whether the
interventions were received at an appropriate time. Antenatal care trajectories for 333 pregnant women were then
described using a standard tool (the APNCU index), that measures the quantity of care only, and the new CTP tool.
Both tools categorise care into 4 categories, from ‘Inadequate’ (both tools) to ‘Adequate plus’ (APNCU) or
‘Appropriate’ (CTP). Participants recorded the timing and content of their antenatal care prospectively using diaries.
Analysis included an examination of similarities and differences in categorisation of care episodes between the
tools.
Results: According to the CTP tool, the care trajectory of 10,2% of the women was classified as inadequate, 8,4%
as intermediate, 36% as sufficient and 45,3% as appropriate. The assessment of quality of care differed significantly
between the two tools. Seventeen care trajectories classified as ‘Adequate’ or ‘Adequate plus’ by the APNCU were
deemed ‘Inadequate’ by the CTP. This suggests that, despite a high number of visits, these women did not receive
the minimal recommended content and timing of care.
Conclusions: The CTP tool provides a more detailed assessment of the adequacy of antenatal care than the
current standard index. However, guidelines for the content of antenatal care vary, and the tool does not at the
moment grade over-use of interventions as ‘Inappropriate’. Further work needs to be done to refine the content
items prior to larger scale testing of the impact of the new measure
Bloch oscillations of magnetic solitons in anisotropic spin-1/2 chains
We study the quantum dynamics of soliton-like domain walls in anisotropic
spin-1/2 chains in the presence of magnetic fields. In the absence of fields,
domain walls form a Bloch band of delocalized quantum states while a static
field applied along the easy axis localizes them into Wannier wave packets and
causes them to execute Bloch oscillations, i.e. the domain walls oscillate
along the chain with a finite Bloch frequency and amplitude. In the presence of
the field, the Bloch band, with a continuum of extended states, breaks up into
the Wannier-Zeeman ladder -- a discrete set of equally spaced energy levels. We
calculate the dynamical structure factor in the one-soliton sector at finite
frequency, wave vector, and temperature, and find sharp peaks at frequencies
which are integer multiples of the Bloch frequency. We further calculate the
uniform magnetic susceptibility and find that it too exhibits peaks at the
Bloch frequency. We identify several candidate materials where these Bloch
oscillations should be observable, for example, via neutron scattering
measurements. For the particular compound CoCl_2.2H_2O we estimate the Bloch
amplitude to be on the order of a few lattice constants, and the Bloch
frequency on the order of 100 GHz for magnetic fields in the Tesla range and at
temperatures of about 18 Kelvin.Comment: 31 single-spaced REVTeX pages, including 7 figures embedded with eps
Proton Spin Structure in the Resonance Region
We have examined the spin structure of the proton in the region of the
nucleon resonances (1.085 GeV < W < 1.910 GeV) at an average four momentum
transfer of Q^2 = 1.3 GeV^2. Using the Jefferson Lab polarized electron beam, a
spectrometer, and a polarized solid target, we measured the asymmetries
A_parallel and A_perp to high precision, and extracted the asymmetries A_1 and
A_2, and the spin structure functions g_1 and g_2. We found a notably non-zero
A_perp, significant contributions from higher-twist effects, and only weak
support for polarized quark--hadron duality.Comment: 6 pages, 4 figures, REVTeX4, similar to PRL submission, plots
colorized and appenix added, v3: minor edit, matches PR
Universal energy fluctuations in thermally isolated driven systems
When an isolated system is brought in contact with a heat bath its final
energy is random and follows the Gibbs distribution -- a cornerstone of
statistical physics. The system's energy can also be changed by performing
non-adiabatic work using a cyclic process. Almost nothing is known about the
resulting energy distribution in this setup, which is especially relevant to
recent experimental progress in cold atoms, ions traps, superconducting qubits
and other systems. Here we show that when the non-adiabatic process comprises
of many repeated cyclic processes the resulting energy distribution is
universal and different from the Gibbs ensemble. We predict the existence of
two qualitatively different regimes with a continuous second order like
transition between them. We illustrate our approach performing explicit
calculations for both interacting and non-interacting systems
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