2,511 research outputs found
Search for evidence of two photon contribution in elastic electron proton data
We reanalyze the most recent data on elastic electron proton scattering. We
look for a deviation from linearity of the Rosenbluth fit to the differential
cross section, which would be the signature of the presence of two photon
exchange. The two photon contribution is parametrized by a one parameter
formula, based on symmetry arguments. The present data do not show evidence for
such deviation.Comment: 15 pages 3 figures More details on the fitting procedure, more
explicit explanation
Could Only Fermions Be Elementary?
In standard Poincare and anti de Sitter SO(2,3) invariant theories,
antiparticles are related to negative energy solutions of covariant equations
while independent positive energy unitary irreducible representations (UIRs) of
the symmetry group are used for describing both a particle and its
antiparticle. Such an approach cannot be applied in de Sitter SO(1,4) invariant
theory. We argue that it would be more natural to require that (*) one UIR
should describe a particle and its antiparticle simultaneously. This would
automatically explain the existence of antiparticles and show that a particle
and its antiparticle are different states of the same object. If (*) is adopted
then among the above groups only the SO(1,4) one can be a candidate for
constructing elementary particle theory. It is shown that UIRs of the SO(1,4)
group can be interpreted in the framework of (*) and cannot be interpreted in
the standard way. By quantizing such UIRs and requiring that the energy should
be positive in the Poincare approximation, we conclude that i) elementary
particles can be only fermions. It is also shown that ii) C invariance is not
exact even in the free massive theory and iii) elementary particles cannot be
neutral. This gives a natural explanation of the fact that all observed neutral
states are bosons.Comment: The paper is considerably revised and the following results are
added: in the SO(1,4) invariant theory i) the C invariance is not exact even
for free massive particles; ii) neutral particles cannot be elementar
Polarization effects in the reaction and determination of the meson form factors in the time--like region
The electron positron annihilation reaction into four pion production has
been studied, through the channel . The
differential (and total) cross sections and various polarization observables
for this reaction have been calculated in terms of the electromagnetic form
factors of the corresponding current. The elements of the
spin--density matrix of the meson were also calculated. Numerical
estimations have been done, with the help of phenomenological form factors
obtained in the space--like region of the momentum transfer squared and
analytically extended to the time-like region.Comment: 19 pages, 2 figures, to appear in Phys Rev
The converging inflow spectrum is an intrinsic signature for a black hole: Monte-Carlo simulations of Comptonization on free-falling electrons
An accreting black hole is, by definition, characterized by the drain.
Namely, the matter falls into a black hole much the same way as water
disappears down a drain - matter goes in and nothing comes out. As this can
only happen in a black hole, it provides an unique way to see it. The accretion
proceeds almost in free fall close to the black hole horizon. In this paper we
calculate (by using Monte -Carlo simulations) the specific features of X-ray
spectra formed as a result of upscattering of the soft (disk) photons in the
converging inflow (CI) within about 3 Schwarzschild radii of the black hole.
The full relativistic treatment has been implemented to reproduce these
spectra. We show that spectra in the soft state of black hole systems can be
described as the sum of a thermal (disk) component and the convolution of some
fraction of this component with the CI upscattering spread function. The latter
boosted photon component is seen as an extended power-law at energies much
higher than the characteristic soft photons energy. We demonstrate the
stability of the power spectral index (alpha= 1.8) over a wide range of the
plasma temperature 0-10 keV and mass accretion rates (higher than 2 in
Eddington units). We also demonstrate that the sharp high energy cutoff occurs
at energies of 200-400 keV which are related to the average rest energy of
electrons impinging upon the horizon. The spectrum is practically identical to
the standard thermal Comptonization spectrum when the CI plasma temperature is
getting of order of 50 keV (hard state of BHS). Also, the change of spectral
shapes from the soft to the hard X-ray state is clearly to be related with the
temperature of the bulk flow. These Monte-Carlo simulated CI spectra are then a
inevitable stamp of the BHS.Comment: 30 pages TeX format, 6 PS figures, accepted for ApJ Main Journa
Disk Diffusion Propagation Model for the Outburst of XTE J1118+480
We present a linear diffusion model for the evolution of the double-peaked
outburst in the transient source XTEJ1118+480. The model treats the two
outbursts as episodic mass deposition at the outer radius of the disk followed
by evolution of disk structure according to a diffusion process. We demonstrate
that light curves with fast-rise, exponential decay profile are a general
consequence of the diffusion process. Deconvolution of the light curve proves
to be feasible and gives an input function specifying mass deposition at the
outer disk edge as well as the total mass of the disk, both as functions of
time. The derived evolution of total disk mass can be correlated with the
observed evolution of the ~0.1 Hz QPO in the source reported in Wood et al.
(2000).Comment: 26 pages, 6 figures, accepted for publication in Ap
Capability in the digital: institutional media management and its dis/contents
This paper explores how social media spaces are occupied, utilized and negotiated by the British Military in relation to the Ministry of Defence’s concerns and conceptualizations of risk. It draws on data from the DUN Project to investigate the content and form of social media about defence through the lens of ‘capability’, a term that captures and describes the meaning behind multiple representations of the military institution. But ‘capability’ is also a term that we hijack and extend here, not only in relation to the dominant presence of ‘capability’ as a representational trope and the extent to which it is revealing of a particular management of social media spaces, but also in relation to what our research reveals for the wider digital media landscape and ‘capable’ digital methods. What emerges from our analysis is the existence of powerful, successful and critically long-standing media and reputation management strategies occurring within the techno-economic online structures where the exercising of ‘control’ over the individual – as opposed to the technology – is highly effective. These findings raise critical questions regarding the extent to which ‘control’ and management of social media – both within and beyond the defence sector – may be determined as much by cultural, social, institutional and political influence and infrastructure as the technological economies. At a key moment in social media analysis, then, when attention is turning to the affordances, criticisms and possibilities of data, our research is a pertinent reminder that we should not forget the active management of content that is being similarly, if not equally, effective
Continuous input nonlocal games
We present a family of nonlocal games in which the inputs the players receive
are continuous. We study three representative members of the family. For the
first two a team sharing quantum correlations (entanglement) has an advantage
over any team restricted to classical correlations. We conjecture that this is
true for the third member of the family as well.Comment: Journal version, slight modification
Quantum Phases of Dipolar Bosons in Bilayer Geometry
We investigate the quantum phases of hard-core dipolar bosons confined to a
square lattice in a bilayer geometry. Using exact theoretical techniques, we
discuss the many-body effects resulting from pairing of particles across layers
at finite density, including a novel pair supersolid phase, superfluid and
solid phases. These results are of direct relevance to experiments with polar
molecules and atoms with large magnetic dipole moments trapped in optical
lattices.Comment: 7 pages, 5 figure
On the problem of interactions in quantum theory
The structure of representations describing systems of free particles in the
theory with the invariance group SO(1,4) is investigated. The property of the
particles to be free means as usual that the representation describing a
many-particle system is the tensor product of the corresponding single-particle
representations (i.e. no interaction is introduced). It is shown that the mass
operator contains only continuous spectrum in the interval
and such representations are unitarily equivalent to ones describing
interactions (gravitational, electromagnetic etc.). This means that there are
no bound states in the theory and the Hilbert space of the many-particle system
contains a subspace of states with the following property: the action of free
representation operators on these states is manifested in the form of different
interactions. Possible consequences of the results are discussed.Comment: 35 pages, Late
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