210 research outputs found
Solar Chameleons
We analyse the creation of chameleons deep inside the sun and their
subsequent conversion to photons near the magnetised surface of the sun. We
find that the spectrum of the regenerated photons lies in the soft X-ray
region, hence addressing the solar corona problem. Moreover, these
back-converted photons originating from chameleons have an intrinsic difference
with regenerated photons from axions: their relative polarisations are mutually
orthogonal before Compton interacting with the surrounding plasma. Depending on
the photon-chameleon coupling and working in the strong coupling regime of the
chameleons to matter, we find that the induced photon flux, when regenerated
resonantly with the surrounding plasma, coincides with the solar flux within
the soft X-ray energy range. Moreover, using the soft X-ray solar flux as a
prior, we find that with a strong enough photon-chameleon coupling the
chameleons emitted by the sun could lead to a regenerated photon flux in the
CAST pipes, which could be within the reach of CAST with upgraded detector
performance. Then, axion helioscopes have thus the potential to detect and
identify particles candidates for the ubiquitous dark energy in the universe.Comment: 16 pages, 6 figures
KWISP: an ultra-sensitive force sensor for the Dark Energy sector
An ultra-sensitive opto-mechanical force sensor has been built and tested in
the optics laboratory at INFN Trieste. Its application to experiments in the
Dark Energy sector, such as those for Chameleon-type WISPs, is particularly
attractive, as it enables a search for their direct coupling to matter. We
present here the main characteristics and the absolute force calibration of the
KWISP (Kinetic WISP detection) sensor. It is based on a thin Si3N4
micro-membrane placed inside a Fabry-Perot optical cavity. By monitoring the
cavity characteristic frequencies it is possible to detect the tiny membrane
displacements caused by an applied force. Far from the mechanical resonant
frequency of the membrane, the measured force sensitivity is 5.0e-14
N/sqrt(Hz), corresponding to a displacement sensitivity of 2.5e-15 m/sqrt(Hz),
while near resonance the sensitivity is 1.5e-14 N/sqrt(Hz), reaching the
estimated thermal limit, or, in terms of displacement, 7.5e-16 N/sqrt(Hz).
These displacement sensitivities are comparable to those that can be achieved
by large interferometric gravitational wave detectors.Comment: 9 pages, 8 figures in colo
Detecting solar chameleons through radiation pressure
Light scalar fields can drive the accelerated expansion of the universe.
Hence, they are obvious dark energy candidates. To make such models compatible
with tests of General Relativity in the solar system and "fifth force" searches
on Earth, one needs to screen them. One possibility is the so-called
"chameleon" mechanism, which renders an effective mass depending on the local
matter density. If chameleon particles exist, they can be produced in the sun
and detected on Earth exploiting the equivalent of a radiation pressure. Since
their effective mass scales with the local matter density, chameleons can be
reflected by a dense medium if their effective mass becomes greater than their
total energy. Thus, under appropriate conditions, a flux of solar chameleons
may be sensed by detecting the total instantaneous momentum transferred to a
suitable opto-mechanical force/pressure sensor. We calculate the solar
chameleon spectrum and the reach in the chameleon parameter space of an
experiment using the preliminary results from a force/pressure sensor,
currently under development at INFN Trieste, to be mounted in the focal plane
of one of the X-Ray telescopes of the CAST experiment at CERN. We show, that
such an experiment signifies a pioneering effort probing uncharted chameleon
parameter space.Comment: revised versio
Background study for the pn-CCD detector of CERN Axion Solar Telescope
The CERN Axion Solar Telescope (CAST) experiment searches for axions from the
Sun converted into photons with energies up to around 10 keV via the inverse
Primakoff effect in the high magnetic field of a superconducting Large Hadron
Collider (LHC) prototype magnet. A backside illuminated pn-CCD detector in
conjunction with an X-ray mirror optics is one of the three detectors used in
CAST to register the expected photon signal. Since this signal is very rare and
different background components (environmental gamma radiation, cosmic rays,
intrinsic radioactive impurities in the set-up, ...) entangle it, a detailed
study of the detector background has been undertaken with the aim to understand
and further reduce the background level of the detector. The analysis is based
on measured data taken during the Phase I of CAST and on Monte Carlo
simulations of different background components. This study will show that the
observed background level (at a rate of (8.00+-0.07)10^-5 counts/cm^2/s/keV
between 1 and 7 keV) seems to be dominated by the external gamma background due
to usual activities at the experimental site, while radioactive impurities in
the detector itself and cosmic neutrons could make just smaller contribution.Comment: Comments: 10 pages, 9 figures and images, submitted to Astroparticle
Physic
A Fast Algorithm for Robust Regression with Penalised Trimmed Squares
The presence of groups containing high leverage outliers makes linear
regression a difficult problem due to the masking effect. The available high
breakdown estimators based on Least Trimmed Squares often do not succeed in
detecting masked high leverage outliers in finite samples.
An alternative to the LTS estimator, called Penalised Trimmed Squares (PTS)
estimator, was introduced by the authors in \cite{ZiouAv:05,ZiAvPi:07} and it
appears to be less sensitive to the masking problem. This estimator is defined
by a Quadratic Mixed Integer Programming (QMIP) problem, where in the objective
function a penalty cost for each observation is included which serves as an
upper bound on the residual error for any feasible regression line. Since the
PTS does not require presetting the number of outliers to delete from the data
set, it has better efficiency with respect to other estimators. However, due to
the high computational complexity of the resulting QMIP problem, exact
solutions for moderately large regression problems is infeasible.
In this paper we further establish the theoretical properties of the PTS
estimator, such as high breakdown and efficiency, and propose an approximate
algorithm called Fast-PTS to compute the PTS estimator for large data sets
efficiently. Extensive computational experiments on sets of benchmark instances
with varying degrees of outlier contamination, indicate that the proposed
algorithm performs well in identifying groups of high leverage outliers in
reasonable computational time.Comment: 27 page
Response-suggestion to The XENON1T excess: an overlooked dark matter signature?
The main alternatives of the recent XENON1T observation are solar axions,
neutrino magnetic moment and tritium. In this short note we suggest to
crosscheck whether the observation is related or not to dark matter (DM)
streams, by searching for planetary dependence of the observed excess. If such
a correlation is derived, this hint (<3.5sigma) can become the overlooked
direct DM discovery. To do this it is necessary to analyze the time
distribution of all the XENON1T data, and in particular the electronic events
with their time stamp and energy. Notably, the velocities of the dark sector
allow for planetary focusing effects towards the earth either by a single
celestial body or combined by the whole solar system. Surprisingly, as yet this
possibility has not been applied in the field of direct dark matter search,
even though DM velocities fit-in well planetary gravitational lensing effects.
The widely used signature of a direct dark matter search needs to be redefined,
while, with luck, such an analysis might confirm or exclude the solar origin of
the observed excess. Therefore, we suggest that XENON1T and DAMA release the
data.Comment: 1 pag
Search for solar Kaluza-Klein axions in theories of low-scale quantum gravity
We explore the physics potential of a terrestrial detector for observing
axionic Kaluza-Klein excitations coming from the Sun within the context of
higher-dimensional theories of low-scale quantum gravity. In these theories,
the heavier Kaluza-Klein axions are relatively short-lived and may be detected
by a coincidental triggering of their two-photon decay mode. Because of the
expected high multiplicity of the solar axionic excitations, we find
experimental sensitivity to a fundamental Peccei-Quinn axion mass up to
eV (corresponding to an effective axion-photon coupling GeV) in theories with 2 extra
dimensions and a fundamental quantum-gravity scale of order 100
TeV, and up to eV (corresponding to GeV) in theories with 3 extra dimensions and
TeV. For comparison, based on recent data obtained from lowest
level underground experiments, we derive the experimental limits: GeV and GeV in the
aforementioned theories with 2 and 3 large compact dimensions, respectively.Comment: 19 pages, extended version, as to appear in Physical Review
Search for axions in streaming dark matter
A new search strategy for the detection of the elusive dark matter (DM) axion
is proposed. The idea is based on streaming DM axions, whose flux might get
temporally enormously enhanced due to gravitational lensing. This can happen if
the Sun or some planet (including the Moon) is found along the direction of a
DM stream propagating towards the Earth location. The experimental requirements
to the axion haloscope are a wide-band performance combined with a fast axion
rest mass scanning mode, which are feasible. Once both conditions have been
implemented in a haloscope, the axion search can continue parasitically almost
as before. Interestingly, some new DM axion detectors are operating wide-band
by default. In order not to miss the actually unpredictable timing of a
potential short duration signal, a network of co-ordinated axion antennae is
required, preferentially distributed world-wide. The reasoning presented here
for the axions applies to some degree also to any other DM candidates like the
WIMPs.Comment: 5 page
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