14,356 research outputs found
Hard parton damping in hot QCD
The gluon and quark collisional widths in hot QCD plasmas are discussed with
emphasis on temperatures near Tc, where the coupling is large. Considering the
effect on the entropy, which is known from lattice calculations, it is argued
that the width of the partons, which in the perturbative limit is given by
gamma ~ g^2 ln(1/g) T, should be sizeable at intermediate temperatures but has
to be small close to Tc. This behavior implies a substantial reduction of the
radiative energy loss of jets near Tc.Comment: invitetd talk given at 'Hot Quarks 04', July 18-24 2004, Taos Valley,
NM, US
The Stability of Branonium
We analyse the orbital motion of a light anti D6-brane in the presence of a
stack of heavy, distant D6-branes in ten dimensions, taking account of possible
time-variations in the background moduli fields. The Coulomb-like central
potential arising through brane-antibrane interactions is then modified to
include time-dependent prefactors, which generally preclude the existence of
stable elliptical orbits.Comment: 13 pages, Latex, 3 eps figure
The performance of socially responsible mutual funds: the role of fees and management companies
In this paper, we shed light on the debate about the financial performance of socially responsible
investment (SRI) mutual funds by separately analyzing the contributions of before-fee performance and
fees to SRI funds' performance and by investigating the role played by fund management companies in
the determination of those variables. We apply the matching estimator methodology to obtain our results
and find that in the period 1997-2005, US SRI funds had significantly higher fees and better before- and
after-fee performance than conventional funds with similar characteristics. Differences, however, were
driven exclusively by SRI funds run by management companies specialized in socially responsible
investment
Symmetric Vacua in Heterotic M-Theory
Symmetric vacua of heterotic M-theory, characterized by vanishing cohomology
classes of individual sources in the three-form Bianchi identity, are analyzed
on smooth Calabi-Yau three-folds. We show that such vacua do not exist for
elliptically fibered Calabi-Yau spaces. However, explicit examples are found
for Calabi-Yau three-folds arising as intersections in both unweighted and
weighted projective space. We show that such symmetric vacua can be combined
with attractive phenomenological features such as three generations of quarks
and leptons. Properties of the low energy effective actions associated with
symmetric vacua are discussed. In particular, the gauge kinetic functions
receive no perturbative threshold corrections, there are no corrections to the
matter field Kahler metric and the associated five-dimensional effective theory
admits flat space as its vacuum.Comment: 22 pages, Late
Multiple protostellar systems. II. A high resolution near-infrared imaging survey in nearby star-forming regions
(abridged) Our project endeavors to obtain a robust view of multiplicity
among embedded Class I and Flat Spectrum protostars in a wide array of nearby
molecular clouds to disentangle ``universal'' from cloud-dependent processes.
We have used near-infrared adaptive optics observations at the VLT through the
H, Ks and L' filters to search for tight companions to 45 Class I and Flat
Spectrum protostars located in 4 different molecular clouds (Taurus-Auriga,
Ophiuchus, Serpens and L1641 in Orion). We complemented these observations with
published high-resolution surveys of 13 additional objects in Taurus and
Ophiuchus. We found multiplicity rates of 32+/-6% and 47+/-8% over the 45-1400
AU and 14-1400 AU separation ranges, respectively. These rates are in excellent
agreement with those previously found among T Tauri stars in Taurus and
Ophiuchus, and represent an excess of a factor ~1.7 over the multiplicity rate
of solar-type field stars. We found no non-hierarchical triple systems, nor any
quadruple or higher-order systems. No significant cloud-to-cloud difference has
been found, except for the fact that all companions to low-mass Orion
protostars are found within 100 AU of their primaries whereas companions found
in other clouds span the whole range probed here. Based on this survey, we
conclude that core fragmentation always yields a high initial multiplicity
rate, even in giant molecular clouds such as the Orion cloud or in clustered
stellar populations as in Serpens, in contrast with predictions of numerical
simulations. The lower multiplicity rate observed in clustered Class II and
Class III populations can be accounted for by a universal set of properties for
young systems and subsequent ejections through close encounters with unrelated
cluster members.Comment: 15 pages, 6 figures, accepted for publication in Astronomy &
Astrophysic
Recent advances on IMF research
Here I discuss recent work on brown dwarfs, massive stars and the IMF in
general. The stellar IMF can be well described by an invariant two-part power
law in present-day star-formation events within the Local Group of galaxies. It
is nearly identical in shape to the pre-stellar core mass function. The
majority of brown dwarfs follow a separate IMF. Evidence from globular clusters
and ultra-compact dwarf galaxies has emerged that IMFs may have been top heavy
depending on the star-formation rate density. The IGIMF then ranges from bottom
heavy at low galaxy-wide star formation rates to being top-heavy in
galaxy-scale star bursts.Comment: 6 pages, LaTeX, to appear in The Labyrinth of Star Formation, 18-22
June 2012, Crete, (eds.) D. Stamatellos, S. Goodwin, and D. Ward-Thompson,
Springer, in press; replaced version: very minor corrections plus the
addition of reference Smith & Lucey (2013) on the bottom-heavy IMF in
elliptical galaxie
Hybrid GMR Sensor Detecting 950 pT/sqrt(Hz) at 1 Hz and Room Temperature.
Advances in the magnetic sensing technology have been driven by the increasing demand for the capability of measuring ultrasensitive magnetic fields. Among other emerging applications, the detection of magnetic fields in the picotesla range is crucial for biomedical applications. In this work Picosense reports a millimeter-scale, low-power hybrid magnetoresistive-piezoelectric magnetometer with subnanotesla sensitivity at low frequency. Through an innovative noise-cancelation mechanism, the 1/f noise in the MR sensors is surpassed by the mechanical modulation of the external magnetic fields in the high frequency regime. A modulation efficiency of 13% was obtained enabling a final device's sensitivity of ~950 pT/Hz1/2 at 1 Hz. This hybrid device proved to be capable of measuring biomagnetic signals generated in the heart in an unshielded environment. This result paves the way for the development of a portable, contactless, low-cost and low-power magnetocardiography device
- …