930 research outputs found
Multifocal Fluorescence Microscope for Fast Optical Recordings of Neuronal Action Potentials
AbstractIn recent years, optical sensors for tracking neural activity have been developed and offer great utility. However, developing microscopy techniques that have several kHz bandwidth necessary to reliably capture optically reported action potentials (APs) at multiple locations in parallel remains a significant challenge. To our knowledge, we describe a novel microscope optimized to measure spatially distributed optical signals with submillisecond and near diffraction-limit resolution. Our design uses a spatial light modulator to generate patterned illumination to simultaneously excite multiple user-defined targets. A galvanometer driven mirror in the emission path streaks the fluorescence emanating from each excitation point during the camera exposure, using unused camera pixels to capture time varying fluorescence at rates that are ∼1000 times faster than the camera’s native frame rate. We demonstrate that this approach is capable of recording Ca2+ transients resulting from APs in neurons labeled with the Ca2+ sensor Oregon Green Bapta-1 (OGB-1), and can localize the timing of these events with millisecond resolution. Furthermore, optically reported APs can be detected with the voltage sensitive dye DiO-DPA in multiple locations within a neuron with a signal/noise ratio up to ∼40, resolving delays in arrival time along dendrites. Thus, the microscope provides a powerful tool for photometric measurements of dynamics requiring submillisecond sampling at multiple locations
Analytic calculations of the spectra of ultra high energy cosmic ray nuclei. II. The general case of background radiation
We discuss the problem of ultra high energy nuclei propagation in
extragalactic background radiations. The present paper is the continuation of
the accompanying paper I where we have presented three new analytic methods to
calculate the fluxes and spectra of ultra high energy cosmic ray nuclei, both
primary and secondary, and secondary protons. The computation scheme in this
paper is based on the analytic solution of coupled kinetic equations, which
takes into account the continuous energy losses due to the expansion of the
universe and pair-production, together with photo-disintegration of nuclei.
This method includes in the most natural way the production of secondary nuclei
in the process of photo-disintegration of the primary nuclei during their
propagation through extragalactic background radiations. In paper I, in order
to present the suggested analytical schemes of calculations, we have considered
only the case of the cosmic microwave background radiation, in the present
paper we generalize this computation to all relevant background radiations,
including infra-red and visible/ultra-violet radiations, collectively referred
to as extragalactic background light. The analytic solutions allow transparent
physical interpretation of the obtained spectra. Extragalactic background light
plays an important role at intermediate energies of ultra high energy cosmic
ray nuclei. The most noticeable effect of the extragalactic background light is
the low-energy tail in the spectrum of secondary nuclei.Comment: The paper is the second part of a two papers series, it is composed
by 25 pages and 16 eps figures, version accepted for publication on
Astroparticle Physic
Global anisotropy of arrival directions of ultra-high-energy cosmic rays: capabilities of space-based detectors
Planned space-based ultra-high-energy cosmic-ray detectors (TUS, JEM-EUSO and
S-EUSO) are best suited for searches of global anisotropies in the distribution
of arrival directions of cosmic-ray particles because they will be able to
observe the full sky with a single instrument. We calculate quantitatively the
strength of anisotropies associated with two models of the origin of the
highest-energy particles: the extragalactic model (sources follow the
distribution of galaxies in the Universe) and the superheavy dark-matter model
(sources follow the distribution of dark matter in the Galactic halo). Based on
the expected exposure of the experiments, we estimate the optimal strategy for
efficient search of these effects.Comment: 19 pages, 7 figures, iopart style. v.2: discussion of the effect of
the cosmic magnetic fields added; other minor changes. Simulated UHECR
skymaps available at http://livni.inr.ac.ru/UHECRskymaps
Lepton Flavor Violation in the Two Higgs Doublet Model type III
We consider the Two Higgs Doublet Model (2HDM) of type III which leads to
Flavour Changing Neutral Currents (FCNC) at tree level in the leptonic sector.
In the framework of this model we can have, in principle, two situations: the
case (a) when both doublets acquire a vacuum expectation value different from
zero and the case (b) when only one of them is not zero. In addition, we show
that we can make two types of rotations for the flavor mixing matrices which
generates four types of lagrangians, with the rotation of type I we recover the
case (b) from the case (a) in the limit , and with the
rotation of type II we obtain the case (b) from (a) in the limit Moreover, two of the four possible lagrangians correspond to the models
of types I and II plus Flavor Changing (FC) interactions. The analitical
expressions of the partial lepton number violating widths and are derived for the cases (a) and (b) and both
types of rotations.In all cases these widths go asymptotically to zero in
the decoupling limit for all Higgses. We present from our analysis upper bounds
for the flavour changing transition and we show that such bounds
are sensitive to the VEV structure and the type of rotation utilized.Comment: 7 pages RevTeX4, 4 figures postscript, new section added and some new
reference
Composition of UHECR and the Pierre Auger Observatory Spectrum
We fit the recently published Pierre Auger ultra-high energy cosmic ray
spectrum assuming that either nucleons or nuclei are emitted at the sources. We
consider the simplified cases of pure proton, or pure oxygen, or pure iron
injection. We perform an exhaustive scan in the source evolution factor, the
spectral index, the maximum energy of the source spectrum Z E_{max}, and the
minimum distance to the sources. We show that the Pierre Auger spectrum agrees
with any of the source compositions we assumed. For iron, in particular, there
are two distinct solutions with high and low E_{max} (e.g. 6.4 10^{20} eV and 2
10^{19} eV) respectively which could be distinguished by either a large
fraction or the near absence of proton primaries at the highest energies. We
raise the possibility that an iron dominated injected flux may be in line with
the latest composition measurement from the Pierre Auger Observatory where a
hint of heavy element dominance is seen.Comment: 19 pages, 6 figures (33 panels)- Uses iopart.cls and iopart12.clo- In
version 2: addition of a few sentences and two reference
Twenty Years of Galactic Observations in Searching for Bursts of Collapse Neutrinos with the Baksan Underground Scintillation Telescope
The results of twenty-year-long Galactic observations in neutrino radiation
are summarized. Except for the recording of a neutrino signal from the
supernova SN 1987A, no Galactic bursts of collapse neutrinos have been
detected. An upper bound on the mean frequency of gravitational collapses in
our Galaxy was obtained, .Comment: latex, 7 pages, 2 eps figure
A celestial gamma-ray foreground due to the albedo of small solar system bodies and a remote probe of the interstellar cosmic ray spectrum
We calculate the gamma-ray albedo flux from cosmic-ray (CR) interactions with
the solid rock and ice in Main Belt asteroids (MBAs), Jovian and Neptunian
Trojan asteroids, and Kuiper Belt objects (KBOs) using the Moon as a template.
We show that the gamma-ray albedo for the Main Belt, Trojans, and Kuiper Belt
strongly depends on the small-body size distribution of each system. Based on
an analysis of the Energetic Gamma Ray Experiment Telescope (EGRET) data we
infer that the diffuse emission from the MBAs, Trojans, and KBOs has an
integrated flux of less than ~6x10^{-6} cm^{-2} s^{-1} (100-500 MeV), which
corresponds to ~12 times the Lunar albedo, and may be detectable by the
forthcoming Gamma Ray Large Area Space Telescope (GLAST). If detected by GLAST,
it can provide unique direct information about the number of small bodies in
each system that is difficult to assess by any other method. Additionally, the
KBO albedo flux can be used to probe the spectrum of CR nuclei at
close-to-interstellar conditions. The orbits of MBAs, Trojans, and KBOs are
distributed near the ecliptic, which passes through the Galactic center and
high Galactic latitudes. Therefore, the asteroid gamma-ray albedo has to be
taken into account when analyzing weak gamma-ray sources close to the ecliptic,
especially near the Galactic center and for signals at high Galactic latitudes,
such as the extragalactic gamma-ray emission. The asteroid albedo spectrum also
exhibits a 511 keV line due to secondary positrons annihilating in the rock.
This may be an important and previously unrecognized celestial foreground for
the INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL) observations of
the Galactic 511 keV line emission including the direction of the Galactic
center.Comment: 10 pages, 5 figures, 1 table, emulateapj.cls; ApJ in press.
Calculations extended to include Jovian and Neptunian Trojan groups, and
Centaurs, in addition to Main Belt Asteroids and Kuiper Belt Objects. Many
other considerable change
Search for Light Gluinos via the Spontaneous Appearance of pi+pi- Pairs with an 800 GeV/c Proton Beam at Fermilab
We searched for the appearance of pi+pi- pairs with invariant mass greater
than 648 MeV in a neutral beam. Such an observation could signify the decay of
a long-lived light neutral particle. We find no evidence for this decay. Our
null result severely constrains the existence of an R0 hadron, which is the
lightest bound state of a gluon and a light gluino, and thereby also the
possibility of a light gluino. Depending on the photino mass, we exclude the R0
in the mass and lifetime ranges of 1.2 -- 4.6 GeV and 2E-10 -- 7E-4 seconds,
respectively. (To Appear in Phys. Rev. Lett.)Comment: Documentstyle aps,epsfig,prl (revtex), 6 pages, 7 figure
Search for the Decay K_L -> pi^0 nu nubar using pi^0 -> e^+ e^- gamma
We report on a search for the decay K_L -> pi^0 nu nubar, carried out as a
part of E799-II, a rare K_L decay experiment at Fermilab. Within the Standard
Model, the K_L -> pi^0 nu nubar decay is dominated by direct CP violating
processes, and thus an observation of the decay implies confirmation of direct
CP violation. Due to theoretically clean calculations, a measurement of B(K_L
-> pi^0 nu nubar) is one of the best ways to determine the CKM parameter eta.
No events were observed, and we set an upper limit B(K_L -> pi^0 nu nubar) <
5.9 times 10^-7 at the 90% confidence level.Comment: 5 pages, 4 figure
Search for the Decay Kl -> pi0 e+ e-
We report on a search for the decay Kl -> pi0 e+ e- carried out by the
KTeV/E799 experiment at Fermilab. This decay is expected to have a significant
CP violating contribution and the measurement of its branching ratio could
support the CKM mechanism for CP violation or could point to new physics. Two
events were observed in the 1997 data with an expected background of 1.06
+-0.41 events, and we set an upper limit Br(Kl -> pi0 e+ e-) < 5.1 x 10^-10 at
the 90% confidence level.Comment: 13 pages, 2 figure
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