4,364 research outputs found
Heavy boson production through the collision of an ultrahigh-energy neutrino on a target nucleon
We discuss W and Z production through the deep inelastic neutrino-nucleon
scattering in the context of the standard model SU(3)x SU(2)x U(1) of the
strong and electroweak interactions. We find the cross section rates for the
process neutrino + nucleon --> lepton(-) + W(+) + X for the case of
ultrahigh-energy neutrinos colliding on a target nucleon.Comment: 20 pages, 6 figure
Hopf Categories
We introduce Hopf categories enriched over braided monoidal categories. The
notion is linked to several recently developed notions in Hopf algebra theory,
such as Hopf group (co)algebras, weak Hopf algebras and duoidal categories. We
generalize the fundamental theorem for Hopf modules and some of its
applications to Hopf categories.Comment: 47 pages; final version to appear in Algebras and Representation
Theor
Long-term magnetic field stability of Vega
We present new spectropolarimetric observations of the normal A-type star
Vega, obtained during the summer of 2010 with NARVAL at T\'elescope Bernard
Lyot (Pic du Midi Observatory). This new time-series is constituted of 615
spectra collected over 6 different nights. We use the
Least-Square-Deconvolution technique to compute, from each spectrum, a mean
line profile with a signal-to-noise ratio close to 20,000. After averaging all
615 polarized observations, we detect a circularly polarized Zeeman signature
consistent in shape and amplitude with the signatures previously reported from
our observations of 2008 and 2009. The surface magnetic geometry of the star,
reconstructed using the technique of Zeeman-Doppler Imaging, agrees with the
maps obtained in 2008 and 2009, showing that most recognizable features of the
photospheric field of Vega are only weakly distorted by large-scale surface
flows (differential rotation or meridional circulation).Comment: Proceedings of the conference "Stellar polarimetry: from birth to
death", 2011 Jun 27-30, Madiso
Vortices, shocks, and heating in the solar photosphere: effect of a magnetic field
Aims: We study the differences between non-magnetic and magnetic regions in
the flow and thermal structure of the upper solar photosphere. Methods:
Radiative MHD simulations representing a quiet region and a plage region,
respectively, which extend into the layers around the temperature minimum, are
analyzed. Results: The flow structure in the upper photospheric layers of the
two simulations is considerably different: the non-magnetic simulation is
dominated by a pattern of moving shock fronts while the magnetic simulation
shows vertically extended vortices associated with magnetic flux
concentrations. Both kinds of structures induce substantial local heating. The
resulting average temperature profiles are characterized by a steep rise above
the temperature minimum due to shock heating in the non-magnetic case and by a
flat photospheric temperature gradient mainly caused by Ohmic dissipation in
the magnetic run. Conclusions: Shocks in the quiet Sun and vortices in the
strongly magnetized regions represent the dominant flow structures in the
layers around the temperature minimum. They are closely connected with
dissipation processes providing localized heating.Comment: Accepted for publicaton in A&
Exchange-correlation potential for Current Density Functional Theory of frequency dependent linear response
The dynamical, long-wavelength longitudinal and transverse
exchange-correlation potentials for a homogeneous electron gas are evaluated in
a microscopic model based on an approximate decoupling of the equation of
motion for the current-current response function. The transverse spectrum turns
out to be very similar to the longitudinal one. We obtain evidence for a strong
spectral structure near twice the plasma frequency due to a two-plasmon
threshold for two-pair excitations, which may be observable in inelastic
scattering experiments. Our results give the entire input needed to implement
the Time-Dependent Current Density Functional Theory scheme recently developed
by G. Vignale and W. Kohn [Phys. Rev. Lett. 77, 2037 (1996)] and are fitted to
analytic functions to facilitate such applications.Comment: 6 pages, 3 figure
Discovery of starspots on Vega - First spectroscopic detection of surface structures on a normal A-type star
The theoretically studied impact of rapid rotation on stellar evolution needs
to be confronted with the results of high resolution spectroscopy-velocimetry
observations. A weak surface magnetic field had recently been detected in the
A0 prototype star Vega, potentially leading to a (yet undetected) structured
surface. The goal of this article is to present a thorough analysis of the line
profile variations and associated estimators in the early-type standard star
Vega (A0) in order reveal potential activity tracers, exoplanet companions and
stellar oscillations. Vega was monitored in high-resolution spectroscopy with
the velocimeter Sophie/OHP. A total of 2588 high S/N spectra was obtained
during 5 nights (August 2012) at R = 75000 and covering the visible domain. For
each reduced spectrum, Least Square Deconvolved (LSD) equivalent photospheric
profiles were calculated with a Teff = 9500 and logg = 4.0 spectral line mask.
Several methods were applied to study the dynamic behavior of the profile
variations (evolution of radial velocity, bisectors, vspan, 2D profiles,
amongst others). We present the discovery of a starspotted stellar surface in
an A-type standard star with faint spot amplitudes Delta F/Fc ~5 10^{-4}. A
rotational modulation of spectral lines with a period of rotation P = 0.68 d
has clearly been exhibited, confirming the results of previous
spectropolarimetric studies. Either a very thin convective layer can be
responsible for magnetic field generation at small amplitudes, or a new
mechanism has to be invoked in order to explain the existence of activity
tracing starspots. This first strong evidence that standard A-type stars can
show surface structures opens a new field of research and asks the question
about a potential link with the recently discovered weak magnetic field
discoveries in this category of stars.Comment: accepted for publication by Astronomy & Astrophysics (23rd of March
2015
The Enigmatic HH 255
To gain insight into the nature of the peculiar Herbig-Haro object HH 255
(also called Burnham's nebula), we use previously published observations to
derive information about the emission line fluxes as a function of position
within HH 255 and compare them with the well-studied, and relatively
well-behaved bow shock HH 1. There are some qualitative similarities in the
H and [O III] 5007 lines in both objects. However, in contrast to the
expectation of the standard bow shock model, the fluxes of the [O I] 6300, [S
II] 6731, and [N II] 6583 lines are essentially constant along the axis of the
flow, while the electron density decreases, over a large distance within HH
255.
We also explore the possibility that HH 255 represents the emission behind a
standing or quasi-stationary shock. The shock faces upwind, and we suggest,
using theoretical arguments, that it may be associated with the collimation of
the southern outflow from T Tauri. Using a simplified magnetohydrodynamic
simulation to illustrate the basic concept, we demonstrate that the existence
of such a shock at the north edge of HH 255 could indeed explain its unusual
kinematic and ionization properties. Whether or not such a shock can explain
the detailed emission line stratification remains an open question.Comment: Accepted by PASP, 12 pages including 8 figure
Full O(alpha) corrections to e+e- -> sf_i sf_j
We present a complete precision analysis of the sfermion pair production
process e+e- -> sf_i sf_j (f = t, b, tau, nu_tau) in the Minimal Supersymmetric
Standard Model. Our results extend the previously calculated weak corrections
by including all one-loop corrections together with higher order QED
corrections. We present the details of the analytical calculation and discuss
the renormalization scheme. The numerical analysis shows the results for total
cross-sections, forward-backward and left-right asymmetries. It is based on the
SPS1a' point from the SPA project. The complete corrections are about 10% and
have to be taken into account in a high precision analysis.Comment: 32 pages, 24 figures, RevTeX
Path Integrals in Polar Field Variables in QFT
We show how to transform a -dimensional Euclidean path integral in terms
of two (Cartesian) fields to a path integral in terms of polar field variables.
First we present a conjecture that states how this transformation should be
done. Then we show that this conjecture is correct in the case of two toy
models. Finally the conjecture will be proven for a general QFT model with two
fields
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