5,479 research outputs found
Heat flow of the Earth and resonant capture of solar 57-Fe axions
In a very conservative approach, supposing that total heat flow of the Earth
is exclusively due to resonant capture inside the Earth of axions, emitted by
57-Fe nuclei on Sun, we obtain limit on mass of hadronic axion: m_a<1.8 keV.
Taking into account release of heat from decays of 40-K, 232-Th, 238-U inside
the Earth, this estimation could be improved to the value: m_a<1.6 keV. Both
the values are less restrictive than limits set in devoted experiments to
search for 57-Fe axions (m_a<216-745 eV), but are much better than limits
obtained in experiments with 83-Kr (m_a<5.5 keV) and 7-Li (m_a<13.9-32 keV).Comment: 8 page
Density-dependence of functional development in spiking cortical networks grown in vitro
During development, the mammalian brain differentiates into specialized
regions with distinct functional abilities. While many factors contribute to
functional specialization, we explore the effect of neuronal density on the
development of neuronal interactions in vitro. Two types of cortical networks,
dense and sparse, with 50,000 and 12,000 total cells respectively, are studied.
Activation graphs that represent pairwise neuronal interactions are constructed
using a competitive first response model. These graphs reveal that, during
development in vitro, dense networks form activation connections earlier than
sparse networks. Link entropy analysis of dense net- work activation graphs
suggests that the majority of connections between electrodes are reciprocal in
nature. Information theoretic measures reveal that early functional information
interactions (among 3 cells) are synergetic in both dense and sparse networks.
However, during later stages of development, previously synergetic
relationships become primarily redundant in dense, but not in sparse networks.
Large link entropy values in the activation graph are related to the domination
of redundant ensembles in late stages of development in dense networks. Results
demonstrate differences between dense and sparse networks in terms of
informational groups, pairwise relationships, and activation graphs. These
differences suggest that variations in cell density may result in different
functional specialization of nervous system tissue in vivo.Comment: 10 pages, 7 figure
A geoneutrino experiment at Homestake
A significant fraction of the 44TW of heat dissipation from the Earth's
interior is believed to originate from the decays of terrestrial uranium and
thorium. The only estimates of this radiogenic heat, which is the driving force
for mantle convection, come from Earth models based on meteorites, and have
large systematic errors. The detection of electron antineutrinos produced by
these uranium and thorium decays would allow a more direct measure of the total
uranium and thorium content, and hence radiogenic heat production in the Earth.
We discuss the prospect of building an electron antineutrino detector
approximately 700m^3 in size in the Homestake mine at the 4850' level. This
would allow us to make a measurement of the total uranium and thorium content
with a statistical error less than the systematic error from our current
knowledge of neutrino oscillation parameters. It would also allow us to test
the hypothesis of a naturally occurring nuclear reactor at the center of the
Earth.Comment: proceedings for Neutrino Sciences 2005, submitted to Earth, Moon, and
Planet
Recommended from our members
Observation of Geo-Neutrinos
Geo-neutrinos, electron anti-neutrinos produced in beta decays of naturally
occurring radioactive isotopes in the Earth, are a unique direct probe of our
planet's interior. We report the first observation at more than 3 C.L.
of geo-neutrinos, performed with the Borexino detector at Laboratori Nazionali
del Gran Sasso. Anti-neutrinos are detected through the neutron inverse beta
decay reaction. With a 252.6 ton-yr fiducial exposure after all selection cuts,
we detected 9.9^{+4.1}_{-3.4}(^{+14.6}_{-8.2}) geo-neutrino events, with errors
corresponding to a 68.3%(99.73%) C.L. From the profile, the
statistical significance of the Borexino geo-neutrino observation corresponds
to a 99.997% C.L. Our measurement of the geo-neutrinos rate is
3.9^{+1.6}_{-1.3}(^{+5.8}_{-3.2}) events/(100ton-yr). This measurement rejects
the hypothesis of an active geo-reactor in the Earth's core with a power above
3 TW at 95% C.L. The observed prompt positron spectrum above 2.6 MeV is
compatible with that expected from european nuclear reactors (mean base line of
approximately 1000 km). Our measurement of reactor anti-neutrinos excludes the
non-oscillation hypothesis at 99.60% C.L.Comment: 8 pages, 4 figures, 3 table
Clar's Theory, STM Images, and Geometry of Graphene Nanoribbons
We show that Clar's theory of the aromatic sextet is a simple and powerful
tool to predict the stability, the \pi-electron distribution, the geometry, the
electronic/magnetic structure of graphene nanoribbons with different hydrogen
edge terminations. We use density functional theory to obtain the equilibrium
atomic positions, simulated scanning tunneling microscopy (STM) images, edge
energies, band gaps, and edge-induced strains of graphene ribbons that we
analyze in terms of Clar formulas. Based on their Clar representation, we
propose a classification scheme for graphene ribbons that groups configurations
with similar bond length alternations, STM patterns, and Raman spectra. Our
simulations show how STM images and Raman spectra can be used to identify the
type of edge termination
A search for resonant production of pairs in $4.8\ \rm{fb}^{-1}p\bar{p}\sqrt{s}=1.96\ \rm{TeV}$
We search for resonant production of tt pairs in 4.8 fb^{-1} integrated
luminosity of ppbar collision data at sqrt{s}=1.96 TeV in the lepton+jets decay
channel, where one top quark decays leptonically and the other hadronically. A
matrix element reconstruction technique is used; for each event a probability
density function (pdf) of the ttbar candidate invariant mass is sampled. These
pdfs are used to construct a likelihood function, whereby the cross section for
resonant ttbar production is estimated, given a hypothetical resonance mass and
width. The data indicate no evidence of resonant production of ttbar pairs. A
benchmark model of leptophobic Z \rightarrow ttbar is excluded with m_{Z'} <
900 GeV at 95% confidence level.Comment: accepted for publication in Physical Review D Sep 21, 201
Observation of Exclusive Gamma Gamma Production in p pbar Collisions at sqrt{s}=1.96 TeV
We have observed exclusive \gamma\gamma production in proton-antiproton
collisions at \sqrt{s}=1.96 TeV, using data from 1.11 \pm 0.07 fb^{-1}
integrated luminosity taken by the Run II Collider Detector at Fermilab. We
selected events with two electromagnetic showers, each with transverse energy
E_T > 2.5 GeV and pseudorapidity |\eta| < 1.0, with no other particles detected
in -7.4 < \eta < +7.4. The two showers have similar E_T and azimuthal angle
separation \Delta\phi \sim \pi; 34 events have two charged particle tracks,
consistent with the QED process p \bar{p} to p + e^+e^- + \bar{p} by two-photon
exchange, while 43 events have no charged tracks. The number of these events
that are exclusive \pi^0\pi^0 is consistent with zero and is < 15 at 95% C.L.
The cross section for p\bar{p} to p+\gamma\gamma+\bar{p} with |\eta(\gamma)| <
1.0 and E_T(\gamma) > 2.5$ GeV is
2.48^{+0.40}_{-0.35}(stat)^{+0.40}_{-0.51}(syst) pb.Comment: 7 pages, 4 figure
Measurement of branching ratio and Bs0 lifetime in the decay Bs0 -> J/psi f0(980) at CDF
We present a study of Bs0 decays to the CP-odd final state J/psi f0(980) with
J/psi -> mu+ mu- and f0(980) -> pi+ pi-. Using ppbar collision data with an
integrated luminosity of 3.8/fb collected by the CDF II detector at the
Tevatron we measure a Bs0 lifetime of tau(Bs0 -> J/psi f0(980)) = 1.70
-0.11+0.12(stat) +-0.03(syst) ps. This is the first measurement of the Bs0
lifetime in a decay to a CP eigenstate and corresponds in the standard model to
the lifetime of the heavy Bs0 eigenstate. We also measure the product of
branching fractions of Bs0 -> J/psi f0(980) and f0(980) -> pi+ pi- relative to
the product of branching fractions of Bs0 -> J/psi phi and phi -> K+ K- to be
R_f0/phi = 0.257 +_0.020(stat) +-0.014(syst), which is the most precise
determination of this quantity to date.Comment: accepted by Phys. Rev.
Evidence for t\bar{t}\gamma Production and Measurement of \sigma_t\bar{t}\gamma / \sigma_t\bar{t}
Using data corresponding to 6.0/fb of ppbar collisions at sqrt(s) = 1.96 TeV
collected by the CDF II detector, we present a cross section measurement of
top-quark pair production with an additional radiated photon. The events are
selected by looking for a lepton, a photon, significant transverse momentum
imbalance, large total transverse energy, and three or more jets, with at least
one identified as containing a b quark. The ttbar+photon sample requires the
photon to have 10 GeV or more of transverse energy, and to be in the central
region. Using an event selection optimized for the ttbar+photon candidate
sample we measure the production cross section of, and the ratio of cross
sections of the two samples. Control samples in the dilepton+photon and
lepton+photon+\met, channels are constructed to aid in decay product
identification and background measurements. We observe 30 ttbar+photon
candidate events compared to the standard model expectation of 26.9 +/- 3.4
events. We measure the ttbar+photon cross section to be 0.18+0.08 pb, and the
ratio of the cross section of ttbar+photon to ttbar to be 0.024 +/- 0.009.
Assuming no ttbar+photon production, we observe a probability of 0.0015 of the
background events alone producing 30 events or more, corresponding to 3.0
standard deviations.Comment: 9 pages, 3 figure
- …