10,635 research outputs found
Amplitudes at Infinity
We investigate the asymptotically large loop-momentum behavior of multi-loop
amplitudes in maximally supersymmetric quantum field theories in four
dimensions. We check residue-theorem identities among color-dressed leading
singularities in supersymmetric Yang-Mills theory to
demonstrate the absence of poles at infinity of all MHV amplitudes through
three loops. Considering the same test for supergravity leads
us to discover that this theory does support non-vanishing residues at infinity
starting at two loops, and the degree of these poles grow arbitrarily with
multiplicity. This causes a tension between simultaneously manifesting
ultraviolet finiteness---which would be automatic in a representation obtained
by color-kinematic duality---and gauge invariance---which would follow from
unitarity-based methods.Comment: 4+1+1 pages; 15 figures; details provided in ancillary Mathematica
file
Black Hole Formation in Fallback Supernova and the Spins of LIGO Sources
Here we investigate within the context of field binary progenitors how the
the spin of LIGO sources vary when the helium star-descendent black hole (BH)
is formed in a failed supernova (SN) explosion rather than by direct collapse.
To this end, we make use of 3d hydrodynamical simulations of fallback supernova
in close binary systems with properties designed to emulate LIGO sources. By
systematically varying the explosion energy and the binary properties, we are
able to explore the effects that the companion has on redistributing the
angular momentum of the system. We find that, unlike the mass, the spin of the
newly formed BH varies only slightly with the currently theoretically
unconstrained energy of the SN and is primarily determined by the initial
binary separation. In contrast, variations in the initial binary separation
yield sizable changes on the resultant effective spin of the system. This
implies that the formation pathways of LIGO sources leading to a particular
effective spin might be far less restrictive than the standard direct collapse
scenario suggests.Comment: 7 pages, 5 figures, submitted to ApJ Letter
Zipping and collapse of diblock copolymers
Using exact enumeration methods and Monte Carlo simulations we study the
phase diagram relative to the conformational transitions of a two dimensional
diblock copolymer. The polymer is made of two homogeneous strands of monomers
of different species which are joined to each other at one end. We find that
depending on the values of the energy parameters in the model, there is either
a first order collapse from a swollen to a compact phase of spiral type, or a
continuous transition to an intermediate zipped phase followed by a first order
collapse at lower temperatures. Critical exponents of the zipping transition
are computed and their exact values are conjectured on the basis of a mapping
onto percolation geometry, thanks to recent results on path-crossing
probabilities.Comment: 12 pages, RevTeX and 14 PostScript figures include
Did GW170817 harbor a pulsar?
If the progenitor of GW170817 harbored a pulsar, then a Poynting flux
dominated bow-shock cavity would have been expected to form around the
traveling binary. The characteristic size of this evacuated region depends
strongly on the spin-down evolution of the pulsar companion, which in turn
depends on the merging timescale of the system. If this evacuated region is
able to grow to a sufficiently large scale, then the deceleration of the jet,
and thus the onset of the afterglow, would be noticeably delayed. The first
detection of afterglow emission, which was uncovered 9.2 days after the
-ray burst trigger, can thus be used to constrain the size of a
pre-existing pulsar-wind cavity. We use this information, together with a model
of the jet to place limits on the presence of a pulsar in GW170817 and discuss
the derived constraints in the context of the observed double neutron star
binary population. We find that the majority of Galactic systems that are close
enough to merge within a Hubble time would have carved a discernibly large
pulsar-wind cavity, inconsistent with the onset timescale of the X-ray
afterglow of GW170817. Conversely, the recently detected system J1913+1102,
which host a low-luminosity pulsar, provides a congruous Milky Way analog of
GW170817's progenitor model. This study highlights the potential of the
proposed observational test for gaining insight into the origin of double
neutron star binaries, in particular if the properties of Galactic systems are
representative of the overall merging population.Comment: Accepted for publication in ApJL, 6 pages, 5 figure
A simple model of DNA denaturation and mutually avoiding walks statistics
Recently Garel, Monthus and Orland (Europhys. Lett. v 55, 132 (2001))
considered a model of DNA denaturation in which excluded volume effects within
each strand are neglected, while mutual avoidance is included. Using an
approximate scheme they found a first order denaturation. We show that a first
order transition for this model follows from exact results for the statistics
of two mutually avoiding random walks, whose reunion exponent is c > 2, both in
two and three dimensions. Analytical estimates of c due to the interactions
with other denaturated loops, as well as numerical calculations, indicate that
the transition is even sharper than in models where excluded volume effects are
fully incorporated. The probability distribution of distances between
homologous base pairs decays as a power law at the transition.Comment: 7 Pages, RevTeX, 8 Figure
Measuring the Temperature of a Mesoscopic Quantum Electron System by means of Single Electron Statistics
We measure the temperature of a mesoscopic system consisting of an
ultra-dilute two dimensional electron gas at the interface in a
metal-oxide-semiconductor field effect transistor (MOSFET) quantum dot by means
of the capture and emission of an electron in a point defect close to the
interface. Contrarily to previous reports, we show that the capture and
emission by point defects in Si n-MOSFETs can be temperature dependent down to
800 mK. As the finite quantum grand canonical ensemble model applies, the time
domain charge fluctuation in the defect is used to determine the temperature of
the few electron gas in the channel.Comment: 4 Figures (color
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