3,382 research outputs found
Relativistic Particle Acceleration in a Folded Current Sheet
Two-dimensional particle simulations of a relativistic Harris current sheet
of pair plasmashave demonstrated that the system is unstable to the
relativistic drift kink instability (RDKI) and that a new kind of acceleration
process takes place in the deformed current sheet. This process contributes to
the generation of non-thermal particles and contributes to the fast magnetic
dissipation in the current sheet structure. The acceleration mechanism and a
brief comparison with relativistic magnetic reconnection are presented.Comment: 11 preprint pages, including 3 .eps figure
Dissipation in Poynting-flux Dominated Flows: the Sigma-Problem of the Crab Pulsar Wind
Flows in which energy is transported predominantly as Poynting flux are
thought to occur in pulsars, gamma-ray bursts and relativistic jets from
compact objects. The fluctuating component of the magnetic field in such a flow
can in principle be dissipated by magnetic reconnection, and used to accelerate
the flow. We investigate how rapidly this transition can take place, by
implementing into a global MHD model, that uses a thermodynamic description of
the plasma, explicit, physically motivated prescriptions for the dissipation
rate: a lower limit on this rate is given by limiting the maximum drift speed
of the current carriers to that of light, an upper limit follows from demanding
that the dissipation zone expand only subsonically in the comoving frame and a
further prescription is obtained by assuming that the expansion speed is
limited by the growth rate of the relativistic tearing mode. In each case,
solutions are presented which give the Lorentz factor of a spherical wind
containing a transverse, oscillating magnetic field component as a function of
radius. In the case of the Crab pulsar, we find that the Poynting flux can be
dissipated before the wind reaches the inner edge of the Nebula if the pulsar
emits electron positron pairs at a rate >1.E40 per second, thus providing a
possible solution to the sigma-problem.Comment: Accepted for publication in Ap
Interactions of UHE cosmic ray nuclei with radiation during acceleration: consequences on the spectrum and composition
In this paper, we study the diffusive shock acceleration of cosmic-ray
protons and nuclei, taking into account all the relevant interaction processes
with photon backgrounds. We investigate how the competition between protons and
nuclei is modified by the acceleration parameters such as the acceleration
rate, its rigidity dependence, the photon density and the confinement
capability of the sources. We find that in the case of interaction-limited
acceleration processes protons are likely to be accelerated to higher energies
than nuclei, whereas for confinement-limited acceleration nuclei are
accelerated to higher energies than protons. Finally, we discuss our results in
the context of possible astrophysical accelerators, and in the light of recent
cosmic-ray data.Comment: 14 pages, 11 figures A few paragraphs and one figure added for
clarity, figures slightly redesigned, no changes in the result
Capture of Solar and Higher-Energy Neutrinos by Iodine 127
We discuss and improve a recent treatment of the absorption of solar
neutrinos by I, in connection with a proposed solar neutrino
detector. With standard-solar-model fluxes and an in-medium value of -1.0 for
the axial-vector coupling constant , we obtain a B-neutrino cross
section of 3.3, about 50\% larger than in our previous work,
and a Be cross section that is less certain but nevertheless also larger
than before. We then apply the improved techniques to higher incoming energies
that obtain at the LAMPF beam dump, where an experiment is underway to finalize
a calibration of the I with electron neutrinos from muon decay. We
find that forbidden operators, which play no role in solar-neutrino absorption,
contribute nonnegligibly to the LAMPF cross section, and that the preliminary
LAMPF mean value is significantly larger than our prediction.Comment: 13 pages + 3 postscript figures (attached), in RevTex 3 , submitted
to Phys. Rev.
Tests of the Equivalence Principle with Neutral Kaons
We test the Principle of Equivalence for particles and antiparticles, using
CPLEAR data on tagged K0 and K0bar decays into pi^+ pi^-. For the first time,
we search for possible annual, monthly and diurnal modulations of the
observables |eta_{+-}| and phi_{+-}, that could be correlated with variations
in astrophysical potentials. Within the accuracy of CPLEAR, the measured values
of |eta_{+-}| and phi_{+-} are found not to be correlated with changes of the
gravitational potential. We analyze data assuming effective scalar, vector and
tensor interactions, and we conclude that the Principle of Equivalence between
particles and antiparticles holds to a level of 6.5, 4.3 and 1.8 x 10^{-9},
respectively, for scalar, vector and tensor potentials originating from the Sun
with a range much greater than the distance Earth-Sun. We also study
energy-dependent effects that might arise from vector or tensor interactions.
Finally, we compile upper limits on the gravitational coupling difference
between K0 and K0bar as a function of the scalar, vector and tensor interaction
range.Comment: 15 pages latex 2e, five figures, one style file (cernart.csl)
incorporate
Test of CPT Symmetry and Quantum Mechanics with Experimental data from CPLEAR
We use fits to recent published CPLEAR data on neutral kaon decays to
and to constrain the CPT--violation parameters
appearing in a formulation of the neutral kaon system as an open
quantum-mechanical system. The obtained upper limits of the CPT--violation
parameters are approaching the range suggested by certain ideas concerning
quantum gravity.Comment: 9 pages of uuencoded postscript (includes 3 figures
News, intelligence and 'little lies' : rumours between the Cherokees and the British 1740-1785
Rumour and information are one of the most fundamental ways in which people engage
with one another. Rumours can change the way that individuals and groups see each other
and the actions that they take. Sociologists and anthropologists have long used rumour as a
way to explore the experiences of their subjects. Historians of early America have, in recent
years, begun to make use of rumour as a way of examining the, often hidden, world of
interactions between American Indians and white Europeans. This thesis will expand upon
this work by exploring the changing role of rumour within an intercultural relationship over
several decades. This thesis will focus on rumour in the relationship between the Cherokee
Nation and the colonists of the British Empire. It will explore the ways that rumour
influenced these interactions and the impact of the rapidly changing backcountry
environment of the latter eighteenth century, both on rumour and on the wider Cherokee-
British relationship. This thesis will argue that rumour shifted in the course of the
eighteenth century from being a diplomatic tool which could be used- either to create
further panic and confusion or to calm and smooth over problems- to an uncontrollable
force which would deepen and exacerbate the divisions between Cherokees and the
British. Rumour played an important role in politics and society in the eighteenth century
backcountry and its changing function offers a way to better understand the shifting
currents of life in early America
Search for CP violation in decays
A model-independent search for direct CP violation in the Cabibbo suppressed
decay in a sample of approximately 370,000 decays is
carried out. The data were collected by the LHCb experiment in 2010 and
correspond to an integrated luminosity of 35 pb. The normalized Dalitz
plot distributions for and are compared using four different
binning schemes that are sensitive to different manifestations of CP violation.
No evidence for CP asymmetry is found.Comment: 13 pages, 8 figures, submitted to Phys. Rev.
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