5,057 research outputs found
Subfactor realisation of modular invariants
We study the problem of realising modular invariants by braided subfactors
and the related problem of classifying nimreps. We develop the fusion rule
structure of these modular invariants. This structure is useful tool in the
analysis of modular data from quantum double subfactors, particularly those of
the double of cyclic groups, the symmetric group on 3 letters and the double of
the subfactors with principal graph the extended Dynkin diagram D_5^(1). In
particular for the double of S_3, 14 of the 48 modular modular invariants are
nimless, and only 28 of the remaining 34 nimble invariants can be realised by
subfactors
Spectral densities from the lattice
We discuss a method to extract the K\"all\'{e}n-Lehmann spectral density of a
particle (be it elementary or bound state) propagator by means of 4d lattice
data. We employ a linear regularization strategy, commonly known as the
Tikhonov method with Morozov discrepancy principle. An important virtue over
the popular maximum entropy method is the possibility to also probe unphysical
spectral densities, as, for example, of a confined gluon. We apply our proposal
to the SU(3) glue sector.Comment: 7 pages, 9 figures, talk given at the 31st International Symposium on
Lattice Field Theory (LATTICE 2013), July 29-August 3 2013, Mainz, German
Impact of Geo-distribution and Mining Pools on Blockchains: A Study of Ethereum
Given the large adoption and economical impact of permissionless blockchains,
the complexity of the underlying systems and the adversarial environment in
which they operate, it is fundamental to properly study and understand the
emergent behavior and properties of these systems. We describe our experience
on a detailed, one-month study of the Ethereum network from several
geographically dispersed observation points. We leverage multiple geographic
vantage points to assess the key pillars of Ethereum, namely geographical
dispersion, network efficiency, blockchain efficiency and security, and the
impact of mining pools. Among other new findings, we identify previously
undocumented forms of selfish behavior and show that the prevalence of powerful
mining pools exacerbates the geographical impact on block propagation delays.
Furthermore, we provide a set of open measurement and processing tools, as well
as the data set of the collected measurements, in order to promote further
research on understanding permissionless blockchains.Comment: To appear in 50th IEEE/IFIP International Conference on Dependable
Systems and Networks (DSN), 202
Spectral representation of lattice gluon and ghost propagators at zero temperature
We consider the analytic continuation of Euclidean propagator data obtained
from 4D simulations to Minkowski space. In order to perform this continuation,
the common approach is to first extract the K\"all\'en-Lehmann spectral density
of the field. Once this is known, it can be extended to Minkowski space to
yield the Minkowski propagator. However, obtaining the K\"all\'en-Lehmann
spectral density from propagator data is a well known ill-posed numerical
problem. To regularize this problem we implement an appropriate version of
Tikhonov regularization supplemented with the Morozov discrepancy principle. We
will then apply this to various toy model data to demonstrate the conditions of
validity for this method, and finally to zero temperature gluon and ghost
lattice QCD data. We carefully explain how to deal with the IR singularity of
the massless ghost propagator. We also uncover the numerically different
performance when using two ---mathematically equivalent--- versions of the
K\"all\'en-Lehmann spectral integral.Comment: 33 pages, 18 figure
Cosmology with a Continuous Tower of Scalar Fields
We study the cosmological evolution for a universe in the presence of a
continuous tower of massive scalar fields which can drive the current phase of
accelerated expansion of the universe and, in addition, can contribute as a
dark matter component. The tower consists of a continuous set of massive scalar
fields with a gaussian mass distribution. We show that, in a certain region of
the parameter space, the {\it heavy} modes of the tower (those with masses much
larger than the Hubble expansion rate) dominate at early times and make the
tower behave like the usual single scalar field whose coherent oscillations
around the minimum of the potential give a matter-like contribution. On the
other hand, at late times, the {\it light} modes (those with masses much
smaller than the Hubble expansion rate) overcome the energy density of the
tower and they behave like a perfect fluid with equation of state ranging from
0 to -1, depending on the spectral index of the initial spectrum. This is a
distinctive feature of the tower with respect to the case of quintessence
fields, since a massive scalar field can only give acceleration with equation
of state close to -1. Such unique property is the result of a synergy effect
between the different mass modes. Interestingly, we find that, for some choices
of the spectral index, the tower tracks the matter component at high redshifts
(or it can even play the role of the dark matter) and eventually becomes the
dominant component of the universe and give rise to an accelerated expansion.Comment: 13 pages, 8 figures. V2: minor changes to match published versio
Impulsive Heating of Solar Flare Ribbons Above 10 MK
The chromospheric response to the input of flare energy is marked by extended
extreme ultraviolet (EUV) ribbons and hard X-ray (HXR) footpoints. These are
usually explained as the result of heating and bremsstrahlung emission from
accelerated electrons colliding in the dense chromospheric plasma. We present
evidence of impulsive heating of flare ribbons above 10 MK in a two-ribbon
flare. We analyse the impulsive phase of SOL2013-11-09T06:38, a C2.6 class
event using data from Atmospheric Imaging Assembly (AIA) on board of Solar
Dynamics Observatory (SDO) and the Reuven Ramaty High Energy Solar
Spectroscopic Imager (RHESSI) to derive the temperature, emission measure and
differential emission measure of the flaring regions and investigate the
evolution of the plasma in the flaring ribbons. The ribbons were visible at all
SDO/AIA EUV/UV wavelengths, in particular, at 94 and 131 \AA\ filters,
sensitive to temperatures of 8 MK and 12 MK. Time evolution of the emission
measure of the plasma above 10 MK at the ribbons has a peak near the HXR peak
time. The presence of hot plasma in the lower atmosphere is further confirmed
by RHESSI imaging spectroscopy analysis, which shows resolved sources at 11-13
MK associated with at least one ribbon. We found that collisional beam heating
can only marginally explain the necessary power to heat the 10 MK plasma at the
ribbons.Comment: 21 pages, 15 figure
Undulatory swimming in shear-thinning fluids: Experiments with C. elegans
The swimming behaviour of microorganisms can be strongly influenced by the
rheology of their fluid environment. In this manuscript, we experimentally
investigate the effects of shear-thinning viscosity on the swimming behaviour
of an undulatory swimmer, the nematode Caenorhabditis elegans. Tracking methods
are used to measure the swimmer's kinematic data (including propulsion speed)
and velocity fields. We find that shear-thinning viscosity modifies the
velocity fields produced by the swimming nematode but does not modify the
nematode's speed and beating kinematics. Velocimetry data show significant
enhancement in local vorticity and circulation and an increase in fluid
velocity near the nematode's tail compared to Newtonian fluids of similar
effective viscosity. These findings are compared to recent theoretical and
numerical results
Finite temperature gluon propagator in Landau gauge: non-zero Matsubara frequencies and spectral densities
We report on the lattice computation of the Landau gauge gluon propagator at
finite temperature, including the non-zero Matsubara frequencies. Moreover, the
corresponding K\"all\'en-Lehmann spectral density is computed, using a Tikhonov
regularisation together with the Morozov discrepancy principle. Implications
for gluon confinement are also discussed.Comment: 5 pages, 5 figures, Lattice 2017 proceeding
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