36,421 research outputs found
Charged-Particle Decay at Finite Temperature
Radiative corrections to the decay rate of charged fermions caused by the
presence of a thermal bath of photons are calculated in the limit when
temperatures are below the masses of all charged particles involved. The
cancellation of finite-temperature infrared divergences in the decay rate is
described in detail. Temperature-dependent radiative corrections to a two-body
decay of a hypothetical charged fermion and to electroweak decays of a muon are
given. We touch upon possible implications of these results for charged
particles in the early Universe.Comment: 18 pages, 6 figures. v2: typos corrected, bibliography revised,
content matches published versio
Recommended from our members
Applying Graph Theory to Examine the Dynamics of Student Discussions in Small-Group Learning.
Group work in science, technology, engineering, and mathematics courses is an effective means of improving student outcomes, and many different factors can influence the dynamics of student discussions and, ultimately, the success of collaboration. The substance and dynamics of group discussions are commonly examined using qualitative methods such as discourse analysis. To complement existing work in the literature, we developed a quantitative methodology that uses graph theory to map the progression of talk-turns of discussions within a group. We observed groups of students working with peer facilitators to solve problems in biological sciences, with three iterations of data collection and two major refinements of graph theory calculations. Results include general behaviors based on the turns in which different individuals talk and graph theory parameters to quantify group characteristics. To demonstrate the potential utility of the methodology, we present case studies with distinct patterns: a centralized group in which the peer facilitator behaves like an authority figure, a decentralized group in which most students talk their fair share of turns, and a larger group with subgroups that have implications for equity, diversity, and inclusion. Together, these results demonstrate that our adaptation of graph theory is a viable quantitative methodology to examine group discussions
Probing molecular frame photoionization via laser generated high-order harmonics from aligned molecules
Present photoionization experiments cannot measure molecular frame
photoelectron angular distributions (MFPAD) from the outermost valence
electrons of molecules. We show that details of the MFPAD can be retrieved with
high-order harmonics generated by infrared lasers from aligned molecules. Using
accurately calculated photoionization transition dipole moments for
fixed-in-space molecules, we show that the dependence of the magnitude and
phase of the high-order harmonics on the alignment angle of the molecules
observed in recent experiments can be quantitatively reproduced. This result
provides the needed theoretical basis for ultrafast dynamic chemical imaging
using infrared laser pulses.Comment: 5 pages, 4 figure
Nascent RNA sequencing reveals mechanisms of gene regulation in the human malaria parasite Plasmodium falciparum.
Gene expression in Plasmodium falciparum is tightly regulated to ensure successful propagation of the parasite throughout its complex life cycle. The earliest transcriptomics studies in P. falciparum suggested a cascade of transcriptional activity over the course of the 48-hour intraerythrocytic developmental cycle (IDC); however, the just-in-time transcriptional model has recently been challenged by findings that show the importance of post-transcriptional regulation. To further explore the role of transcriptional regulation, we performed the first genome-wide nascent RNA profiling in P. falciparum. Our findings indicate that the majority of genes are transcribed simultaneously during the trophozoite stage of the IDC and that only a small subset of genes is subject to differential transcriptional timing. RNA polymerase II is engaged with promoter regions prior to this transcriptional burst, suggesting that Pol II pausing plays a dominant role in gene regulation. In addition, we found that the overall transcriptional program during gametocyte differentiation is surprisingly similar to the IDC, with the exception of relatively small subsets of genes. Results from this study suggest that further characterization of the molecular players that regulate stage-specific gene expression and Pol II pausing will contribute to our continuous search for novel antimalarial drug targets
Bias in Matter Power Spectra ?
We review the constraints given by the linear matter power spectra data on
cosmological and bias parameters, comparing the data from the PSCz survey
(Hamilton et al., 2000) and from the matter power spectrum infered by the study
of Lyman alpha spectra at z=2.72 (Croft et al., 2000). We consider
flat-- cosmologies, allowing , and to vary, and we
also let the two ratio factors and () vary independently. Using a simple
minimisation technique, we find confidence intervals on our parameters for each
dataset and for a combined analysis. Letting the 5 parameters vary freely gives
almost no constraints on cosmology, but requirement of a universal ratio for
both datasets implies unacceptably low values of and . Adding
some reasonable priors on the cosmological parameters demonstrates that the
power derived by the PSCz survey is higher by a factor compared to
the power from the Lyman forest survey.Comment: Accepted in A&
A milestone toward understanding PDR properties in the extreme environment of LMC-30Dor
More complete knowledge of galaxy evolution requires understanding the
process of star formation and interaction between the interstellar radiation
field and the interstellar medium in galactic environments traversing a wide
range of physical parameter space. Here we focus on the impact of massive star
formation on the surrounding low metallicity ISM in 30 Doradus in the Large
Magellanic Cloud. A low metal abundance, as is the case of some galaxies of the
early universe, results in less ultra-violet shielding for the formation of the
molecular gas necessary for star formation to proceed. The half-solar
metallicity gas in this region is strongly irradiated by the super star cluster
R136, making it an ideal laboratory to study the structure of the ISM in an
extreme environment. Our spatially resolved study investigates the gas heating
and cooling mechanisms, particularly in the photo-dissociation regions where
the chemistry and thermal balance are regulated by far-ultraviolet photons (6
eV< h\nu <13.6 eV).
We present Herschel observations of far-infrared fine-structure lines
obtained with PACS and SPIRE/FTS. We have combined atomic fine-structure lines
from Herschel and Spitzer observations with ground-based CO data to provide
diagnostics on the properties and the structure of the gas by modeling it with
the Meudon PDR code. We derive the spatial distribution of the radiation field,
the pressure, the size, and the filling factor of the photodissociated gas and
molecular clouds. We find a range of pressure of ~ 10^5 - 1.7x10^6 cm^{-3} K
and a range of incident radiation field G_UV ~ 10^2 - 2.5x10^4 through PDR
modeling. Assuming a plane-parallel geometry and a uniform medium, we find a
total extinction of 1-3 mag , which correspond to a PDR cloud size of 0.2 to
3pc, with small CO depth scale of 0.06 to 0.5pc. We also determine the three
dimensional structure of the gas. (Abridged)Comment: 20 pages, 23 figures, accepted in A&
The Globular Cluster System of M60 (NGC 4649). I. CFHT MOS Spectroscopy and Database
We present the measurement of radial velocities for globular clusters in M60,
giant elliptical galaxy in the Virgo cluster. Target globular cluster
candidates were selected using the Washington photometry based on the deep
16\arcmin \times 16\arcmin images taken at the KPNO 4m and using the
photometry derived from the HST/WFPC2 archive images. The spectra of the target
objects were obtained using the Multi-Object Spectrograph (MOS) at the
Canada-France-Hawaii Telescope (CFHT). We have measured the radial velocity for
111 objects in the field of M60: 93 globular clusters (72 blue globular
clusters with and 21 red globular clusters with
), 11 foreground stars, 6 small galaxies, and the nucleus of
M60. The measured velocities of the 93 globular clusters range from
km s to km s, with a mean value of
km s, which is in good agreement with the velocity of the nucleus of M60
( km s). Combining our results with data in the
literature, we present a master catalog of radial velocities for 121 globular
clusters in M60. The velocity dispersion of the globular clusters in the master
catalog is found to be km s for the entire sample,
km s for 83 blue globular clusters, and
km s for 38 red globular clusters.Comment: 29 pages, 8 figures. To appear in Ap
Anatomy-specific classification of medical images using deep convolutional nets
Automated classification of human anatomy is an important prerequisite for
many computer-aided diagnosis systems. The spatial complexity and variability
of anatomy throughout the human body makes classification difficult. "Deep
learning" methods such as convolutional networks (ConvNets) outperform other
state-of-the-art methods in image classification tasks. In this work, we
present a method for organ- or body-part-specific anatomical classification of
medical images acquired using computed tomography (CT) with ConvNets. We train
a ConvNet, using 4,298 separate axial 2D key-images to learn 5 anatomical
classes. Key-images were mined from a hospital PACS archive, using a set of
1,675 patients. We show that a data augmentation approach can help to enrich
the data set and improve classification performance. Using ConvNets and data
augmentation, we achieve anatomy-specific classification error of 5.9 % and
area-under-the-curve (AUC) values of an average of 0.998 in testing. We
demonstrate that deep learning can be used to train very reliable and accurate
classifiers that could initialize further computer-aided diagnosis.Comment: Presented at: 2015 IEEE International Symposium on Biomedical
Imaging, April 16-19, 2015, New York Marriott at Brooklyn Bridge, NY, US
Chandrasekhar's Dynamical Friction and non-extensive statistics
The motion of a point like object of mass passing through the background
potential of massive collisionless particles () suffers a steady
deceleration named dynamical friction. In his classical work, Chandrasekhar
assumed a Maxwellian velocity distribution in the halo and neglected the self
gravity of the wake induced by the gravitational focusing of the mass . In
this paper, by relaxing the validity of the Maxwellian distribution due to the
presence of long range forces, we derive an analytical formula for the
dynamical friction in the context of the -nonextensive kinetic theory. In
the extensive limiting case (), the classical Gaussian Chandrasekhar
result is recovered. As an application, the dynamical friction timescale for
Globular Clusters spiraling to the galactic center is explicitly obtained. Our
results suggest that the problem concerning the large timescale as derived by
numerical -body simulations or semi-analytical models can be understood as a
departure from the standard extensive Maxwellian regime as measured by the
Tsallis nonextensive -parameter.Comment: 16pp 5 figs, revised and extended version of arXiv:1202.1873 .
Accepted for publication by JCA
A model for atomic and molecular interstellar gas: The Meudon PDR code
We present the revised ``Meudon'' model of Photon Dominated Region (PDR
code), presently available on the web under the Gnu Public Licence at:
http://aristote.obspm.fr/MIS. General organisation of the code is described
down to a level that should allow most observers to use it as an interpretation
tool with minimal help from our part. Two grids of models, one for low
excitation diffuse clouds and one for dense highly illuminated clouds, are
discussed, and some new results on PDR modelisation highlighted.Comment: accepted in ApJ sup
- …