13,001 research outputs found
nuSTORM: Neutrinos from Stored Muons
nuSTORM (Neutrinos from STORed Muons) is a proposed storage ring facility to
deliver beams of muon antineutrinos and electron neutrinos from positive muon
decays (muon neutrinos and electron antineutrinos from negative muon decays),
with a central muon momentum of 3.8 GeV/c and a momentum acceptance of 10%. The
facility will allow searches for eV-scale sterile neutrinos at better than 10
sigma sensitivity, it will be able to provide measurements of neutrino and
antineutrino-nucleus scattering cross sections with percent-level precision and
will serve as a first step towards developing muon accelerators for particle
physics. We report on the physics capabilities of the nuSTORM facility and we
specify the main features of its design, which does not require any new
technology. The flux of the neutrino beam can be determined with percent-level
accuracy to perform cross-section measurements for future neutrino oscillation
experiments and to resolve the hints for eV-scale sterile neutrinos. nuSTORM
may be considered as a first step towards a Neutrino Factory and a Muon
Collider.Comment: 10 pages, 5 figures, Prospects in Neutrino Physics Conference
(NuPhys). eConf (CNUM: C14-12-15
Electron-phonon interaction in Fe-based superconductors: Coupling of magnetic moments with phonons in LaFeAsOF
The coupling of Fe magnetic moments in LaFeAsOF with the As
phonon is calculated. We present first principles calculations of the
atomic and electronic structure of LaFeAsO as a function of electron doping. We
perform calculations using the virtual crystal approximation as well as
supercell calculations with F substitutional impurity atoms. The results
validate the virtual crystal approximation for the electronic structure near
the Fermi level. Its is found that the electronic density of states at the
Fermi level is maximum for x=0.125, enhancing the electron-phonon interaction.
An additional increase of the electron-phonon parameter is obtained
if the coupling between the phonon and the Fe magnetic moment is
included. It is found that the electron-phonon interaction can be one order of
magnitude larger than its value if no spin resolution is included in the
calculation. The implications of these results on the superconducting
transition are discusse
Kelvin-Helmholtz instability in partially ionized compressible plasmas
The Kelvin-Helmholtz Instability (KHI) has been observed in the solar
atmosphere. Ion-neutral collisions may play a relevant role for the growth rate
and evolution of the KHI in solar partially ionized plasmas as in, e.g., solar
prominences. Here, we investigate the linear phase of the KHI at an interface
between two partially ionized magnetized plasmas in the presence of a shear
flow. The effects of ion-neutral collisions and compressibility are included in
the analysis. We obtain the dispersion relation of the linear modes and perform
parametric studies of the unstable solutions. We find that in the
incompressible case the KHI is present for any velocity shear regardless the
value of the collision frequency. In the compressible case, the domain of
instability depends strongly on the plasma parameters, specially the collision
frequency and the density contrast. For high collision frequencies and low
density contrasts the KHI is present for super-Alfvenic velocity shear only.
For high density contrasts the threshold velocity shear can be reduced to
sub-Alfvenic values. For the particular case of turbulent plumes in
prominences, we conclude that sub-Alfvenic flow velocities can trigger the KHI
thanks to the ion-neutral coupling.Comment: Accepted for publication in Ap
The Thermal Instability of Solar Prominence Threads
The fine structure of solar prominences and filaments appears as thin and
long threads in high-resolution images. In H-alpha observations of filaments,
some threads can be observed for only 5 - 20 minutes before they seem to fade
and eventually disappear, suggesting that these threads may have very short
lifetimes. The presence of an instability might be the cause of this quick
disappearance. Here, we study the thermal instability of prominence threads as
an explanation of their sudden disappearance from H-alpha observations. We
model a prominence thread as a magnetic tube with prominence conditions
embedded in a coronal environment. We assume a variation of the physical
properties in the transverse direction, so that the temperature and density
continuously change from internal to external values in an inhomogeneous
transitional layer representing the particular prominence-corona transition
region (PCTR) of the thread. We use the nonadiabatic and resistive
magnetohydrodynamic equations, which include terms due to thermal conduction
parallel and perpendicular to the magnetic field, radiative losses, heating,
and magnetic diffusion. We combine both analytical and numerical methods to
study linear perturbations from the equilibrium state, focusing on unstable
thermal solutions. We find that thermal modes are unstable in the PCTR for
temperatures higher than 80,000 K, approximately. These modes are related to
temperature disturbances that can lead to changes in the equilibrium due to
rapid plasma heating or cooling. For typical prominence parameters, the
instability time scale is of the order of a few minutes and is independent of
the form of the temperature profile within the PCTR of the thread. This result
indicates that thermal instability may play an important role for the short
lifetimes of threads in the observations.Comment: Accepted for publication in Ap
Seismology of Standing Kink Oscillations of Solar Prominence Fine Structures
We investigate standing kink magnetohydrodynamic (MHD) oscillations in a
prominence fine structure modeled as a straight and cylindrical magnetic tube
only partially filled with the prominence material, and with its ends fixed at
two rigid walls representing the solar photosphere. The prominence plasma is
partially ionized and a transverse inhomogeneous transitional layer is included
between the prominence thread and the coronal medium. Thus, ion-neutral
collisions and resonant absorption are the considered damping mechanisms.
Approximate analytical expressions of the period, the damping time, and their
ratio are derived for the fundamental mode in the thin tube and thin boundary
approximations. We find that the dominant damping mechanism is resonant
absorption, which provides damping ratios in agreement with the observations,
whereas ion-neutral collisions are irrelevant for the damping. The values of
the damping ratio are independent of both the prominence thread length and its
position within the magnetic tube, and coincide with the values for a tube
fully filled with the prominence plasma. The implications of our results in the
context of the MHD seismology technique are discussed, pointing out that the
reported short-period (2 - 10 min) and short-wavelength (700 - 8,000 km) thread
oscillations may not be consistent with a standing mode interpretation and
could be related to propagating waves. Finally, we show that the inversion of
some prominence physical parameters, e.g., Alfv\'en speed, magnetic field
strength, transverse inhomogeneity length-scale, etc., is possible using
observationally determined values of the period and damping time of the
oscillations along with the analytical approximations of these quantities.Comment: Accepted for publication in Ap
Prominence seismology using the period ratio of transverse thread oscillations
The ratio of the period of the fundamental mode to that of the first overtone
of kink oscillations, from here on the "period ratio", is a seismology tool
that can be used to infer information about the spatial variation of density
along solar magnetic flux tubes. The period ratio is 2 in longitudinally
homogeneous thin tubes, but it differs from 2 due to longitudinal
inhomogeneity. In this paper we investigate the period ratio in longitudinally
inhomogeneous prominence threads and explore its implications for prominence
seismology. We numerically solve the two-dimensional eigenvalue problem of kink
oscillations in a model of a prominence thread. We take into account three
nonuniform density profiles along the thread. In agreement with previous works
that used simple piecewise constant density profiles, we find that the period
ratio is larger than 2 in prominence threads. When the ratio of the central
density to that at the footpoints is fixed, the period ratio depends strongly
on the form of the density profile along the thread. The more concentrated the
dense prominence plasma near the center of the tube, the larger the period
ratio. However, the period ratio is found to be independent of the specific
density profile when the spatially averaged density in the thread is the same
for all the profiles. An empirical fit of the dependence of the period ratio on
the average density is given and its use for prominence seismology is
discussed.Comment: Accepted for publication in A&
: Confronting theory and lattice simulations
We consider a recent -matrix analysis by Albaladejo {\it et al.}, [Phys.\
Lett.\ B {\bf 755}, 337 (2016)] which accounts for the and
coupled--channels dynamics, and that successfully describes the
experimental information concerning the recently discovered .
Within such scheme, the data can be similarly well described in two different
scenarios, where the is either a resonance or a virtual state. To
shed light into the nature of this state, we apply this formalism in a finite
box with the aim of comparing with recent Lattice QCD (LQCD) simulations. We
see that the energy levels obtained for both scenarios agree well with those
obtained in the single-volume LQCD simulation reported in Prelovsek {\it et
al.} [Phys.\ Rev.\ D {\bf 91}, 014504 (2015)], making thus difficult to
disentangle between both possibilities. We also study the volume dependence of
the energy levels obtained with our formalism, and suggest that LQCD
simulations performed at several volumes could help in discerning the actual
nature of the intriguing state
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