974 research outputs found
Light controls motility and phase separation of photosynthetic microbes
Large ensembles of interacting, out-of-equilibrium agents are a paradigm of active matter. Their constituents' intrinsic activity may entail the spontaneous separation into localized phases of high and low densities. Motile microbes, equipped with ATP-fueled engines, are prime examples of such phase-separating active matter, which is fundamental in myriad biological processes. The fact that spontaneous spatial aggregation is not widely recognized as a general feature of microbial communities challenges the generalisation of phase separation beyond artificial active systems. Here, we report on the phase separation of populations of Chlamydomonas reinhardtii that can be controlled by light in a fully reversible manner. We trace this phenomenon back to the light- and density-dependent motility, thus bridging the gap from light perception on the single-cell level to collective spatial self-organization into regions of high and low density. Its spectral sensitivity suggests that microbial motility and phase separation are regulated by the activity of the photosynthetic machinery. Characteristic fingerprints of the stability and dynamics of this active system paint a picture that cannot be reconciled with the current physical understanding of phase separation in artificial active matter, whereby collective behavior can emerge from inherent motility modulation in response to changing stimuli. Our results therefore point towards the existence of a broader class of self-organization phenomena in living systems
A measurement of cosmic ray deuterium from 0.5â2.9 GeV/nucleon
The rare isotopes ^(2)H and ^(3)He in cosmic rays are believed to originate mainly from the interaction of high energy protons and helium with the galactic interstellar medium. The unique propagation history of these rare isotopes provides important constraints on galactic cosmic ray source spectra and on models for their propagation within the Galaxy. Hydrogen and helium isotopes were measured with the balloon-borne experiment, IMAX, which flew from Lynn Lake, Manitoba in 1992. The energy spectrum of deuterium between 0.5 and 3.2 GeV/nucleon measured by the IMAX experiment as well as previously published results of ^(3)He from the same instrument will be compared with predictions of cosmic ray galactic propagation models. The observed composition of the light isotopes is found to be generally consistent with the predictions of the standard Leaky Box Model derived to fit observations of heavier nucle
Cosmic antiprotons as a probe for supersymmetric dark matter?
The flux of cosmic ray antiprotons from neutralino annihilations in the
galactic halo is computed for a large sample of models in the MSSM (the Minimal
Supersymmetric extension of the Standard Model). We also revisit the problem of
estimating the background of low-energy cosmic ray induced secondary
antiprotons, taking into account their subsequent interactions (and energy
loss) and the presence of nuclei in the interstellar matter.
We consider a two-zone diffusion model, with and without a galactic wind. We
find that, given the uncertainties in the background predictions, there is no
need for a primary (exotic) component to explain present data. However,
allowing for a signal by playing with the uncertainties in the background
estimate, we discuss the characteristic features of the supersymmetric models
which give a satisfactory description of the data. We point out that in some
cases the optimal kinetic energy to search for a signal from supersymmetric
dark matter is above several GeV, rather than the traditional sub-GeV region.
The large astrophysical uncertainties involved do not, one the other hand,
allow the exclusion of any of the MSSM models we consider, on the basis of
data.
We present besides numerical results also convenient parameterizations of the
antiproton yields of all `basic' two-body final states. We also give examples
of the yield and differential energy spectrum for a set of supersymmetric
models with high rates.
We also remark that it is difficult to put a limit on the antiproton lifetime
from present measurements, since the injection of antiprotons from neutralino
annihilation can compensate the loss from decay.Comment: 22 pages, 11 figures, uses emulateapj.st
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The Copernicus surface velocity platform drifter with barometer and reference sensor for temperature (SVP-BRST): genesis, design, and initial results
To support calibration and validation of satel- lite sea surface temperature (SST) retrievals, over 60 high- resolution SST (HRSST) drifting buoys were deployed at sea between 2012 and 2017. Their data record is reviewed here. It is confirmed that sea state and immersion depth play an important role in understanding the data collected by such buoys and that the SST sensors need adequate insu- lation. In addition, calibration verification of three recovered drifters suggests that the sensor drift is low, albeit negative at around â0.01 K yearâ1. However, the statistical significance of these results is limited, and the calibration procedure could not be exactly reproduced, introducing additional uncertain- ties into this drift assessment. Based on lessons learnt from these initial buoys, a new sensor package for the Surface Velocity Platform with Barometer (SVP-B) was designed to serve calibration of SST retrievals by European Unionâs Copernicus satellites. The novel sensor package includes an HRSST sensor calibrated by a metrology laboratory. The sensor includes a pressure probe to monitor immersion depth in calm water and acquires SST data at 1 Hz over a 5 min in-
terval every hour. This enables the derivation of mean SST as well as several percentiles of the SST distribution. The HRSST sensor is calibrated with an uncertainty better than 0.01 K. Analysis of the data collected by two prototypes de- ployed in the Mediterranean Sea shows that the buoys are able to capture small-scale SST variations. These variations are found to be smaller when the sea state is well mixed and when the buoys are located within eddy cores. This affects the drifter SST data representativeness, which is an aspect of importance for optimal use of these data
Time dependence of the electron and positron components of the cosmic radiation measured by the PAMELA experiment between July 2006 and December 2015
Cosmic-ray electrons and positrons are a unique probe of the propagation of
cosmic rays as well as of the nature and distribution of particle sources in
our Galaxy. Recent measurements of these particles are challenging our basic
understanding of the mechanisms of production, acceleration and propagation of
cosmic rays. Particularly striking are the differences between the low energy
results collected by the space-borne PAMELA and AMS-02 experiments and older
measurements pointing to sign-charge dependence of the solar modulation of
cosmic-ray spectra. The PAMELA experiment has been measuring the time variation
of the positron and electron intensity at Earth from July 2006 to December 2015
covering the period for the minimum of solar cycle 23 (2006-2009) till the
middle of the maximum of solar cycle 24, through the polarity reversal of the
heliospheric magnetic field which took place between 2013 and 2014. The
positron to electron ratio measured in this time period clearly shows a
sign-charge dependence of the solar modulation introduced by particle drifts.
These results provide the first clear and continuous observation of how drift
effects on solar modulation have unfolded with time from solar minimum to solar
maximum and their dependence on the particle rigidity and the cyclic polarity
of the solar magnetic field.Comment: 11 pages, 2 figure
The Cosmic-Ray Proton and Helium Spectra measured with the CAPRICE98 balloon experiment
A new measurement of the primary cosmic-ray proton and helium fluxes from 3
to 350 GeV was carried out by the balloon-borne CAPRICE experiment in 1998.
This experimental setup combines different detector techniques and has
excellent particle discrimination capabilities allowing clear particle
identification. Our experiment has the capability to determine accurately
detector selection efficiencies and systematic errors associated with them.
Furthermore, it can check for the first time the energy determined by the
magnet spectrometer by using the Cherenkov angle measured by the RICH detector
well above 20 GeV/n. The analysis of the primary proton and helium components
is described here and the results are compared with other recent measurements
using other magnet spectrometers. The observed energy spectra at the top of the
atmosphere can be represented by (1.27+-0.09)x10^4 E^(-2.75+-0.02) particles
(m^2 GeV sr s)^-1, where E is the kinetic energy, for protons between 20 and
350 GeV and (4.8+-0.8)x10^2 E^(-2.67+-0.06) particles (m^2 GeV nucleon^-1 sr
s)^-1, where E is the kinetic energy per nucleon, for helium nuclei between 15
and 150 GeV nucleon^-1.Comment: To be published on Astroparticle Physics (44 pages, 13 figures, 5
tables
Measurement of 0.25-3.2 GeV antiprotons in the cosmic radiation
The balloon-borne Isotope Matter-Antimatter Experiment (IMAX) was flown from Lynn Lake, Manitoba, Canada on 16â17 July 1992. Using velocity and magnetic rigidity to determine mass, we have directly measured the abundances of cosmic ray antiprotons and protons in the energy range from 0.25 to 3.2 GeV. Both the absolute flux of antiprotons and the antiproton/proton ratio are consistent with recent theoretical work in which antiprotons are produced as secondary products of cosmic ray interactions with the interstellar medium. This consistency implies a lower limit to the antiproton lifetime of âŒ10 to the 7th yr
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