1,721 research outputs found
Precise mass-dependent QED contributions to leptonic g-2 at order alpha^2 and alpha^3
Improved values for the two- and three-loop mass-dependent QED contributions
to the anomalous magnetic moments of the electron, muon, and tau lepton are
presented. The Standard Model prediction for the electron (g-2) is compared
with its most precise recent measurement, providing a value of the
fine-structure constant in agreement with a recently published determination.
For the tau lepton, differences with previously published results are found and
discussed. An updated value of the fine-structure constant is presented in
"Note added after publication."Comment: 6 pages, 1 figure. v2: New determination of alpha presented (based on
the recent electron g-2 measurement). v3: New formulae added in Sec.IIB. v4:
Updated value of alpha presente
Explaining Jupiter's magnetic field and equatorial jet dynamics
Spacecraft data reveal a very Earth-like Jovian magnetic field. This is
surprising since numerical simulations have shown that the vastly different
interiors of terrestrial and gas planets can strongly affect the internal
dynamo process. Here we present the first numerical dynamo that manages to
match the structure and strength of the observed magnetic field by embracing
the newest models for Jupiter's interior. Simulated dynamo action primarily
occurs in the deep high electrical conductivity region while zonal flows are
dynamically constrained to a strong equatorial jet in the outer envelope of low
conductivity. Our model reproduces the structure and strength of the observed
global magnetic field and predicts that secondary dynamo action associated to
the equatorial jet produces banded magnetic features likely observable by the
Juno mission. Secular variation in our model scales to about 2000 nT per year
and should also be observable during the one year nominal mission duration.Comment: 7 pages, 4 figures, accepted for publication in Geophysical Research
Letter
From solar-like to anti-solar differential rotation in cool stars
Stellar differential rotation can be separated into two main regimes:
solar-like when the equator rotates faster than the poles and anti-solar when
the polar regions rotate faster than the equator. We investigate the transition
between these two regimes with 3-D numerical simulations of rotating spherical
shells. We conduct a systematic parameter study which also includes models from
different research groups. We find that the direction of the differential
rotation is governed by the contribution of the Coriolis force in the force
balance, independently of the model setup (presence of a magnetic field,
thickness of the convective layer, density stratification). Rapidly-rotating
cases with a small Rossby number yield solar-like differential rotation, while
weakly-rotating models sustain anti-solar differential rotation. Close to the
transition, the two kinds of differential rotation are two possible bistable
states. This study provides theoretical support for the existence of anti-solar
differential rotation in cool stars with large Rossby numbers.Comment: 5 pages, 6 figures, accepted for publication in MNRA
What controls the large-scale magnetic fields of M dwarfs?
Observations of active M dwarfs show a broad variety of large-scale magnetic
fields encompassing dipole-dominated and multipolar geometries. We detail the
analogy between some anelastic dynamo simulations and spectropolarimetric
observations of 23 M stars. In numerical models, the relative contribution of
inertia and Coriolis force in the global force balance -estimated by the
so-called local Rossby number- is known to have a strong impact on the magnetic
field geometry. We discuss the relevance of this parameter in setting the
large-scale magnetic field of M dwarfs.Comment: 4 pages, 3 figures, conference proceeding, IAUS 302 'Magnetic Fields
Throughout the Stellar Evolution', (26-30 Aug 2013, Biarritz, France
What controls the magnetic geometry of M dwarfs?
Context: observations of rapidly rotating M dwarfs show a broad variety of
large-scale magnetic fields encompassing dipole-dominated and multipolar
geometries. In dynamo models, the relative importance of inertia in the force
balance -- quantified by the local Rossby number -- is known to have a strong
impact on the magnetic field geometry. Aims: we aim to assess the relevance of
the local Rossby number in controlling the large-scale magnetic field geometry
of M dwarfs. Methods: we explore the similarities between anelastic dynamo
models in spherical shells and observations of active M-dwarfs, focusing on
field geometries derived from spectropolarimetric studies. To do so, we
construct observation-based quantities aimed to reflect the diagnostic
parameters employed in numerical models. Results: the transition between
dipole-dominated and multipolar large-scale fields in early to mid M dwarfs is
tentatively attributed to a Rossby number threshold. We interpret late M dwarfs
magnetism to result from a dynamo bistability occurring at low Rossby number.
By analogy with numerical models, we expect different amplitudes of
differential rotation on the two dynamo branches.Comment: 4 pages, 4 figures, accepted for publication in A&
A Bose-Einstein condensate interferometer with macroscopic arm separation
A Michelson interferometer using Bose-Einstein condensates is demonstrated
with coherence times of up to 44 ms and arm separations up to 0.18 mm. This arm
separation is larger than that observed for any previous atom interferometer.
The device uses atoms weakly confined in a magnetic guide and the atomic motion
is controlled using Bragg interactions with an off-resonant standing wave laser
beam.Comment: 4 pages, 3 figure
A compact and robust diode laser system for atom interferometry on a sounding rocket
We present a diode laser system optimized for laser cooling and atom
interferometry with ultra-cold rubidium atoms aboard sounding rockets as an
important milestone towards space-borne quantum sensors. Design, assembly and
qualification of the system, combing micro-integrated distributed feedback
(DFB) diode laser modules and free space optical bench technology is presented
in the context of the MAIUS (Matter-wave Interferometry in Microgravity)
mission.
This laser system, with a volume of 21 liters and total mass of 27 kg, passed
all qualification tests for operation on sounding rockets and is currently used
in the integrated MAIUS flight system producing Bose-Einstein condensates and
performing atom interferometry based on Bragg diffraction. The MAIUS payload is
being prepared for launch in fall 2016.
We further report on a reference laser system, comprising a rubidium
stabilized DFB laser, which was operated successfully on the TEXUS 51 mission
in April 2015. The system demonstrated a high level of technological maturity
by remaining frequency stabilized throughout the mission including the rocket's
boost phase
Semiclassical limits to the linewidth of an atom laser
We investigate the linewidth of a quasi-continuous atom laser within a
semiclassical framework. In the high flux regime, the lasing mode can exhibit a
number of undesirable features such as density fluctuations. We show that the
output therefore has a complicated structure that can be somewhat simplified
using Raman outcoupling methods and energy-momentum selection rules. In the
weak outcoupling limit, we find that the linewidth of an atom laser is
instantaneously Fourier limited, but, due to the energy `chirp' associated with
the draining of a condensate, the long-term linewidth of an atom laser is
equivalent to the chemical potential of the condensate source. We show that
correctly sweeping the outcoupling frequency can recover the Fourier-limited
linewidth.Comment: 9 Figure
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