2,545 research outputs found
Magnetic-Island Contraction and Particle Acceleration in Simulated Eruptive Solar Flares
The mechanism that accelerates particles to the energies required to produce
the observed high-energy impulsive emission in solar flares is not well
understood. Drake et al. (2006) proposed a mechanism for accelerating electrons
in contracting magnetic islands formed by kinetic reconnection in multi-layered
current sheets. We apply these ideas to sunward-moving flux ropes (2.5D
magnetic islands) formed during fast reconnection in a simulated eruptive
flare. A simple analytic model is used to calculate the energy gain of
particles orbiting the field lines of the contracting magnetic islands in our
ultrahigh-resolution 2.5D numerical simulation. We find that the estimated
energy gains in a single island range up to a factor of five. This is higher
than that found by Drake et al. for islands in the terrestrial magnetosphere
and at the heliopause, due to strong plasma compression that occurs at the
flare current sheet. In order to increase their energy by two orders of
magnitude and plausibly account for the observed high-energy flare emission,
the electrons must visit multiple contracting islands. This mechanism should
produce sporadic emission because island formation is intermittent. Moreover, a
large number of particles could be accelerated in each
magnetohydrodynamic-scale island, which may explain the inferred rates of
energetic-electron production in flares. We conclude that island contraction in
the flare current sheet is a promising candidate for electron acceleration in
solar eruptions.Comment: Accepted for publication in The Astrophysical Journal (2016
On the evolution of flow topology in turbulent Rayleigh-Bénard convection
Copyright 2016 AIP Publishing. This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing.Small-scale dynamics is the spirit of turbulence physics. It implicates many attributes of flow topology evolution, coherent structures, hairpin vorticity dynamics, and mechanism of the kinetic energy cascade. In this work, several dynamical aspects of the small-scale motions have been numerically studied in a framework of Rayleigh-Benard convection (RBC). To do so, direct numerical simulations have been carried out at two Rayleigh numbers Ra = 10(8) and 10(10), inside an air-filled rectangular cell of aspect ratio unity and pi span-wise open-ended distance. As a main feature, the average rate of the invariants of the velocity gradient tensor (Q(G), R-G) has displayed the so-calledPeer ReviewedPostprint (author's final draft
A geometrically motivated coordinate system for exploring spacetime dynamics in numerical-relativity simulations using a quasi-Kinnersley tetrad
We investigate the suitability and properties of a quasi-Kinnersley tetrad
and a geometrically motivated coordinate system as tools for quantifying both
strong-field and wave-zone effects in numerical relativity (NR) simulations. We
fix the radial and latitudinal coordinate degrees of freedom of the metric,
using the Coulomb potential associated with the quasi-Kinnersley transverse
frame. These coordinates are invariants of the spacetime and can be used to
unambiguously fix the outstanding spin-boost freedom associated with the
quasi-Kinnersley frame (resulting in a preferred quasi-Kinnersley tetrad
(QKT)). In the limit of small perturbations about a Kerr spacetime, these
coordinates and QKT reduce to Boyer-Lindquist coordinates and the Kinnersley
tetrad, irrespective of the simulation gauge choice. We explore the properties
of this construction both analytically and numerically, and we gain insights
regarding the propagation of radiation described by a super-Poynting vector. We
also quantify in detail the peeling properties of the chosen tetrad and gauge.
We argue that these choices are particularly well suited for a rapidly
converging wave-extraction algorithm as the extraction location approaches
infinity, and we explore numerically the extent to which this property remains
applicable on the interior of a computational domain. Using a number of
additional tests, we verify that the prescription behaves as required in the
appropriate limits regardless of simulation gauge. We explore the behavior of
the geometrically motivated coordinate system in dynamical binary-black-hole NR
mergers, and find them useful for visualizing features in NR simulations such
as the spurious "junk" radiation. Finally, we carefully scrutinize the head-on
collision of two black holes and, for example, the way in which the extracted
waveform changes as it moves through the computational domain.Comment: 30 pages, 17 figures, 2 table
The Steinmann Cluster Bootstrap for N=4 Super Yang-Mills Amplitudes
We review the bootstrap method for constructing six- and seven-particle
amplitudes in planar super Yang-Mills theory, by exploiting
their analytic structure. We focus on two recently discovered properties which
greatly simplify this construction at symbol and function level, respectively:
the extended Steinmann relations, or equivalently cluster adjacency, and the
coaction principle. We then demonstrate their power in determining the
six-particle amplitude through six and seven loops in the NMHV and MHV sectors
respectively, as well as the symbol of the NMHV seven-particle amplitude to
four loops.Comment: 36 pages, 4 figures, 5 tables, 1 ancillary file. Contribution to the
proceedings of the Corfu Summer Institute 2019 "School and Workshops on
Elementary Particle Physics and Gravity" (CORFU2019), 31 August - 25
September 2019, Corfu, Greec
Novel Aspects of QCD in Leptoproduction
I review several topics in electroproduction which test fundamental aspects
of QCD. These include the role of final-state interactions in producing
diffractive leptoproduction processes, the shadowing of nuclear structure
functions, and target-spin asymmetries. The antishadowing of nuclear structure
functions is shown to be quark-flavor specific, suggesting that some part of
the anomalous NuTeV result for could be due to the
non-universality of nuclear antishadowing for charged and neutral currents. I
also discuss the physics of the heavy-quark sea, hidden color in nuclear
wavefunctions, and evidence for color transparency for nuclear processes. The
AdS/CFT correspondence connecting superstring theory to superconformal gauge
theory has important implications for hadron phenomenology in the conformal
limit, including an all-orders demonstration of counting rules for hard
exclusive processes, as well as determining essential aspects of hadronic
light-front wavefunctions.Comment: Presented at the conference, Electron-Nucleus Scattering VIII,
Marciana Marina, Isola d'Elba, June 21-25, 200
Analysis Tools for Discovering Strong Parity Violation at Hadron Colliders
Several arguments suggest parity violation may be observable in high energy
strong interactions. We introduce new analysis tools for describing the
azimuthal dependence of multi-particle distributions, or "azimuthal flow."
Analysis uses the representations of the orthogonal group O(2) and dihedral
groups necessary to define parity correctly in two dimensions.
Classification finds that collective angles used in event-by-event statistics
represent inequivalent tensor observables that cannot generally be represented
by a single "reaction plane". Many new parity-violating observables exist that
have never been measured, while many new parity-conserving observables formerly
lumped together are now distinguished. We use the concept of "event shape
sorting" to suggest separating right- and left-handed events, and we discuss
the effects of transverse and longitudinal spin. The analysis tools are
statistically robust, and can be applied equally to low or high multiplicity
events at the Tevatron, or , and the .Comment: 18 pages, 2 figures. Final version, accepted for publication in PRD.
Updated references. Modified presentation and discussion of previous wor
Nonlinear Boltzmann equation for the homogeneous isotropic case: Minimal deterministic Matlab program
The homogeneous isotropic Boltzmann equation (HIBE) is a fundamental dynamic
model for many applications in thermodynamics, econophysics and sociodynamics.
Despite recent hardware improvements, the solution of the Boltzmann equation
remains extremely challenging from the computational point of view, in
particular by deterministic methods (free of stochastic noise). This work aims
to improve a deterministic direct method recently proposed [V.V. Aristov,
Kluwer Academic Publishers, 2001] for solving the HIBE with a generic
collisional kernel and, in particular, for taking care of the late dynamics of
the relaxation towards the equilibrium. Essentially (a) the original problem is
reformulated in terms of particle kinetic energy (exact particle number and
energy conservation during microscopic collisions) and (b) the computation of
the relaxation rates is improved by the DVM-like correction, where DVM stands
for Discrete Velocity Model (ensuring that the macroscopic conservation laws
are exactly satisfied). Both these corrections make possible to derive very
accurate reference solutions for this test case. Moreover this work aims to
distribute an open-source program (called HOMISBOLTZ), which can be
redistributed and/or modified for dealing with different applications, under
the terms of the GNU General Public License. The program has been purposely
designed in order to be minimal, not only with regards to the reduced number of
lines (less than 1,000), but also with regards to the coding style (as simple
as possible).Comment: 35 pages, 4 figures, it describes the code HOMISBOLTZ to be
distributed with the pape
A prescription for projectors to compute helicity amplitudes in D dimensions
This article discusses a prescription to compute polarized dimensionally
regularized amplitudes, providing a recipe for constructing simple and general
polarized amplitude projectors in D dimensions that avoids conventional Lorentz
tensor decomposition and avoids also dimensional splitting. Because of the
latter, commutation between Lorentz index contraction and loop integration is
preserved within this prescription, which entails certain technical advantages.
The usage of these D-dimensional polarized amplitude projectors results in
helicity amplitudes that can be expressed solely in terms of external momenta,
but different from those defined in the existing dimensional regularization
schemes. Furthermore, we argue that despite being different from the
conventional dimensional regularization scheme (CDR), owing to the
amplitude-level factorization of ultraviolet and infrared singularities, our
prescription can be used, within an infrared subtraction framework, in a hybrid
way without re-calculating the (process-independent) integrated subtraction
coefficients, many of which are available in CDR. This hybrid CDR-compatible
prescription is shown to be unitary. We include two examples to demonstrate
this explicitly and also to illustrate its usage in practice.Comment: Matched to the version to be published in Eur. Phys. J.
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