394 research outputs found
Disorder-induced microscopic magnetic memory
Using coherent x-ray speckle metrology, we have measured the influence of
disorder on major loop return point memory (RPM) and complementary point memory
(CPM) for a series of perpendicular anisotropy Co/Pt multilayer films. In the
low disorder limit, the domain structures show no memory with field cycling--no
RPM and no CPM. With increasing disorder, we observe the onset and the
saturation of both the RPM and the CPM. These results provide the first direct
ensemble-sensitive experimental study of the effects of varying disorder on
microscopic magnetic memory and are compared against the predictions of
existing theories.Comment: 4 pages, 4 figures. Accepted for publication in Physical Review
Letters in Nov. 200
Disorder-induced magnetic memory: Experiments and theories
Beautiful theories of magnetic hysteresis based on random microscopic
disorder have been developed over the past ten years. Our goal was to directly
compare these theories with precise experiments. We first developed and then
applied coherent x-ray speckle metrology to a series of thin multilayer
perpendicular magnetic materials. To directly observe the effects of disorder,
we deliberately introduced increasing degrees of disorder into our films. We
used coherent x-rays to generate highly speckled magnetic scattering patterns.
The apparently random arrangement of the speckles is due to the exact
configuration of the magnetic domains in the sample. In effect, each speckle
pattern acts as a unique fingerprint for the magnetic domain configuration.
Small changes in the domain structure change the speckles, and comparison of
the different speckle patterns provides a quantitative determination of how
much the domain structure has changed. How is the magnetic domain configuration
at one point on the major hysteresis loop related to the configurations at the
same point on the loop during subsequent cycles? The microscopic return-point
memory(RPM) is partial and imperfect in the disordered samples, and completely
absent when the disorder was not present. We found the complementary-point
memory(CPM) is also partial and imperfect in the disordered samples and
completely absent when the disorder was not present. We found that the RPM is
always a little larger than the CPM. We also studied the correlations between
the domains within a single ascending or descending loop. We developed new
theoretical models that do fit our experiments.Comment: 26 pages, 25 figures, Accepted by Physical Review B 01/25/0
Dynamical spin-flip susceptibility for a strongly interacting ultracold Fermi gas
The Stoner model predicts that a two-component Fermi gas at increasing
repulsive interactions undergoes a ferromagnetic transition. Using the
random-phase approximation we study the dynamical properties of the interacting
Fermi gas. For an atomic Fermi gas under harmonic confinement we show that the
transverse (spin-flip) dynamical susceptibility displays a clear signature of
the ferromagnetic phase in a magnon peak emerging from the Stoner particle-hole
continuum. The dynamical spin susceptibilities could be experimentally explored
via spin-dependent Bragg spectroscopy.Comment: 4 pages, 3 figure
Seroprevalence of Zika virus in wild African green monkeys and baboons
ABSTRACT Zika virus (ZIKV) has recently spread through the Americas and has been associated with a range of health effects, including birth defects in children born to women infected during pregnancy. Although the natural reservoir of ZIKV remains poorly defined, the virus was first identified in a captive “sentinel” macaque monkey in Africa in 1947. However, the virus has not been reported in humans or nonhuman primates (NHPs) in Africa outside Gabon in over a decade. Here, we examine ZIKV infection in 239 wild baboons and African green monkeys from South Africa, the Gambia, Tanzania, and Zambia using combinations of unbiased deep sequencing, quantitative reverse transcription-PCR (qRT-PCR), and an antibody capture assay that we optimized using serum collected from captive macaque monkeys exposed to ZIKV, dengue virus, and yellow fever virus. While we did not find evidence of active ZIKV infection in wild NHPs in Africa, we found variable ZIKV seropositivity of up to 16% in some of the NHP populations sampled. We anticipate that these results and the methodology described within will help in continued efforts to determine the prevalence, natural reservoir, and transmission dynamics of ZIKV in Africa and elsewhere. IMPORTANCE Zika virus (ZIKV) is a mosquito-borne virus originally discovered in a captive monkey living in the Zika Forest of Uganda, Africa, in 1947. Recently, an outbreak in South America has shown that ZIKV infection can cause myriad health effects, including birth defects in the children of women infected during pregnancy. Here, we sought to investigate ZIKV infection in wild African primates to better understand its emergence and spread, looking for evidence of active or prior infection. Our results suggest that up to 16% of some populations of nonhuman primate were, at some point, exposed to ZIKV. We anticipate that this study will be useful for future studies that examine the spread of infections from wild animals to humans in general and those studying ZIKV in primates in particular. Podcast: A podcast concerning this article is available
Lattice Matrix Elements and CP Violation in B and K Physics: Status and Outlook
Status of lattice calculations of hadron matrix elements along with CP
violation in B and in K systems is reviewed. Lattice has provided useful input
which, in conjunction with experimenatl data, leads to the conclusion that
CP-odd phase in the CKM matrix plays the dominant role in the observed
asymmetry in . It is now quite likely that any beyond the SM,
CP-odd, phase will cause only small deviations in B-physics. Search for the
effects of the new phase(s) will consequently require very large data samples
as well as very precise theoretical predictions. Clean determination of {\it
all} the angles of the unitarity triangle therefore becomes essential. In this
regard processes play a unique role. Regarding K-decays,
remarkable progress made by theory with regard to maintenance of chiral
symmetry on the lattice is briefly discussed. First application already provide
quantitaive information on and the rule. The enhancement
in appears to arise solely from tree operators, esp. ; penguin
contribution to appears to be very small. However, improved
calculations are necessary for \epsilon^'/epsilon as there the contributions
of QCD penguins and electroweak penguins largely seem to cancel. There are good
reasons, though, to believe that these cancellations will not survive
improvements that are now underway. Importance of determining the unitarity
triangle purely from K-decays is also emphasized.Comment: Invited talk at the 9th International Symposium on Particles, Strings
and Cosmology (PASCOS 03), Mumbai (Bombay) India,3-8 Jan 200
Prefigurative politics between ethical practice and absent promise
'Prefigurative politics' has become a popular term for social movements' ethos of unity between means and ends, but its conceptual genealogy has escaped attention. This article disentangles two components: an ethical revolutionary practice, chiefly indebted to the anarchist tradition, which fights domination while directly constructing alternatives; and prefiguration as a recursive temporal framing, unknowingly drawn from Christianity, in which a future radiates backwards on its past. Tracing prefiguration from the Church Fathers to politicised re-surfacings in the Diggers and the New Left, I associate it with Koselleck's 'process of reassurance' in a pre-ordained historical path. Contrasted to recursive prefiguration are the generative temporal framings couching defences of means-ends unity in the anarchist tradition. These emphasised the path dependency of revolutionary social transformation and the ethical underpinnings of anti-authoritarian politics. Misplaced recursive terminology, I argue, today conveniently distracts from the generative framing of means-ends unity, as the promise of revolution is replaced by that of environmental and industrial collapse. Instead of prefiguration, I suggest conceiving of means-ends unity in terms of Bloch's 'concrete utopia', and associating it with 'anxious' and 'catastrophic' forms of hope
Measurement of the ratio of branching fractions BR(B0 -> K*0 gamma)/BR(Bs0 -> phi gamma)
The ratio of branching fractions of the radiative B decays B0 -> K*0 gamma
and Bs0 -> phi gamma has been measured using 0.37 fb-1 of pp collisions at a
centre of mass energy of sqrt(s) = 7 TeV, collected by the LHCb experiment. The
value obtained is BR(B0 -> K*0 gamma)/BR(Bs0 -> phi gamma) = 1.12 +/- 0.08
^{+0.06}_{-0.04} ^{+0.09}_{-0.08}, where the first uncertainty is statistical,
the second systematic and the third is associated to the ratio of fragmentation
fractions fs/fd. Using the world average for BR(B0 -> K*0 gamma) = (4.33 +/-
0.15) x 10^{-5}, the branching fraction BR(Bs0 -> phi gamma) is measured to be
(3.9 +/- 0.5) x 10^{-5}, which is the most precise measurement to date.Comment: 15 pages, 1 figure, 2 table
Photoproduction of pions and properties of baryon resonances from a Bonn-Gatchina partial wave analysis
Masses, widths and photocouplings of baryon resonances are determined in a
coupled-channel partial wave analysis of a large variety of data. The
Bonn-Gatchina partial wave formalism is extended to include a decomposition of
t- and u-exchange amplitudes into individual partial waves. The multipole
transition amplitudes for and are
given and compared to results from other analyses.Comment: 18 pages, 14 figure
P-wave excited baryons from pion- and photo-induced hyperon production
We report evidence for , , ,
, , and , and find
indications that might have a companion state at 1970\,MeV. The
controversial is not seen. The evidence is derived from a
study of data on pion- and photo-induced hyperon production, but other data are
included as well. Most of the resonances reported here were found in the
Karlsruhe-Helsinki (KH84) and the Carnegie-Mellon (CM) analyses but were
challenged recently by the Data Analysis Center at GWU. Our analysis is
constrained by the energy independent scattering amplitudes from either
KH84 or GWU. The two amplitudes from KH84 or GWU, respectively, lead to
slightly different branching ratios of contributing resonances but the
debated resonances are required in both series of fits.Comment: 22 pages, 28 figures. Some additional sets of data are adde
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