129 research outputs found
Nonlinear material and ionic transport through membrane nanotubes
Membrane nanotubes (NTs) and their networks play an important role in intracellular membrane transport and intercellular communications. The transport characteristics of the NT lumen resemble those of conventional solid-state nanopores. However, unlike the rigid pores, the soft membrane wall of the NT can be deformed by forces driving the transport through the NT lumen. This intrinsic coupling between the NT geometry and transport properties remains poorly explored. Using synchronized fluorescence microscopy and conductance measurements, we revealed that the NT shape was changed by both electric and hydrostatic forces driving the ionic and solute fluxes through the NT lumen. Far from the shape instability, the strength of the force effect is determined by the lateral membrane tension and is scaled with membrane elasticity so that the NT can be operated as a linear elastic sensor. Near shape instabilities, the transport forces triggered large-scale shape transformations, both stochastic and periodic. The periodic oscillations were coupled to a vesicle passage along the NT axis, resembling peristaltic transport. The oscillations were parametrically controlled by the electric field, making NT a highly nonlinear nanofluidic circuitry element with biological and technological implications.This work was partially supported by NIGMS of the National Institutes of Health under award R01GM121725, RYC-2014-01419 to A.V.S.; Spanish Ministry of Science, Innovation and Universities grants PGC2018-099971-B-I00 and EUR2019-103830 to A.V.S.; Basque Government grant IT1270-19; and the Ministry of Science and Higher Education of the Russian Federation to P.I.K. and G.T.R
Spontaneous annihilation of high-density matter in the electroweak theory
In the presence of fermionic matter the topologically distinct vacua of the
standard model are metastable and can decay by tunneling through the sphaleron
barrier. This process annihilates one fermion per doublet due to the anomalous
non-conservation of baryon and lepton currents and is accompanied by a
production of gauge and Higgs bosons. We present a numerical method to obtain
local bounce solutions which minimize the Euclidean action in the space of all
configurations connecting two adjacent topological sectors. These solutions
determine the decay rate and the configuration of the fields after the
tunneling. We also follow the real time evolution of this configuration and
analyze the spectrum of the created bosons. If the matter density exceeds some
critical value, the exponentially suppressed tunneling triggers off an
avalanche producing an enormous amount of bosons.Comment: 38 pages, 6 Postscript figure
Dynamic constriction andfission of endoplasmicreticulum membranes by reticulon
The endoplasmic reticulum (ER) is a continuous cell-wide membrane network. Network formation has been associated with proteins producing membrane curvature and fusion, such as reticulons and atlastin. Regulated network fragmentation, occurring in different physiological contexts, is less understood. Here we find that the ER has an embedded fragmentation mechanism based upon the ability of reticulon to produce fission of elongating network branches. In Drosophila, Rtnl1-facilitated fission is counterbalanced by atlastin-driven fusion, with the prevalence of Rtnl1 leading to ER fragmentation. Ectopic expression of Drosophila reticulon in COS-7 cells reveals individual fission events in dynamic ER tubules. Consistently, in vitro analyses show that reticulon produces velocity-dependent constriction of lipid nanotubes leading to stochastic fission via a hemifission mechanism. Fission occurs at elongation rates and pulling force ranges intrinsic to the ER, thus suggesting a principle whereby the dynamic balance between fusion and fission controlling organelle morphology depends on membrane motility.This work was partially supported by NIH R01GM121725 to V.A.F., a 5x1000 grant from the Italian Ministry of Health and Telethon GGP11189 to A.D., Spanish Ministry of Science, Innovation and Universities grants BFU2015-70552-P to V.A.F. and A.V.S., and BFU2015-63714-R and PGC2018-099341-B-I00 to B.I., Basque Government grant IT1196-19, Russian Science Foundation Grant No. 17-75-30064 and Ministry of Science and Higher Education of the Russian Federation
Fermion sea along the sphaleron barrier
In this revised version we have improved the treatment of the top and bottom
quark mass. This leads to slight changes of the numerical results, especially
of those presented in Fig.4. The discussion of the numerical procedure and
accuracy has been extended.Comment: 39 pages (LaTex) plus 5 figures (uuencoded postscript files);
RUB-TPII-62/93, to appear in Phys.Rev.
Evidence of and search for double-charmonium production in and decays
Using data samples of and
events collected with the Belle detector, a first experimental
search has been made for double-charmonium production in the exclusive decays
, where , , , , and . No significant signal is
observed in the spectra of the mass recoiling against the reconstructed
or except for the evidence of production with a
significance of for . The
measured branching fraction \BR(\Upsilon(1S)\rightarrow J/\psi+\chi_{c1}) is
. The
confidence level upper limits on the branching fractions of the other modes
having a significance of less than are determined. These results are
consistent with theoretical calculations using the nonrelativistic QCD
factorization approach.Comment: 12 pages, 4 figures, 1 table. The fit range was extended to include
X(4160) signal according to referee's suggestions. Other results unchanged.
Paper was accepted for publication as a regular article in Physical Review
Sphaleron transitions in the Minimal Standard Model and the upper bound for the Higgs Mass
We calculate the dissipation of the baryon number after the electroweak phase
transition due to thermal fluctuations above the sphaleron barrier. We consider
not only the classical Boltzmann factor but also fermionic and bosonic one-loop
contributions. We find that both bosonic and especially fermionic fluctuations
can considerably suppress the transition rate. Assuming the Langer--Affleck
formalism for this rate, the condition that an initial baryon asymmetry must
not be washed out by sphaleron transitions leads, in the Minimal Standard Model
(), to an upper bound for the Higgs mass in the range 60 to 75
GeV.Comment: 49 pages, 5 figures (uuencoded PostScript); fixing of the
renormalization scale has been improved, numerics has been extende
A search for charged massive long-lived particles
We report on a search for charged massive long-lived particles (CMLLPs),
based on 5.2 fb of integrated luminosity collected with the D0 detector
at the Fermilab Tevatron collider. We search for events in which one
or more particles are reconstructed as muons but have speed and ionization
energy loss inconsistent with muons produced in beam collisions.
CMLLPs are predicted in several theories of physics beyond the standard model.
We exclude pair-produced long-lived gaugino-like charginos below 267 GeV and
higgsino-like charginos below 217 GeV at 95% C.L., as well as long-lived scalar
top quarks with mass below 285 GeV.Comment: submitted to Phys. Rev. Letter
Winding Transitions at Finite Energy and Temperature: An O(3) Model
Winding number transitions in the two dimensional softly broken O(3)
nonlinear sigma model are studied at finite energy and temperature. New
periodic instanton solutions which dominate the semiclassical transition
amplitudes are found analytically at low energies, and numerically for all
energies up to the sphaleron scale. The Euclidean period beta of these finite
energy instantons increases with energy, contrary to the behavior found in the
abelian Higgs model or simple one dimensional systems. This results in a sharp
crossover from instanton dominated tunneling to sphaleron dominated thermal
activation at a certain critical temperature. Since this behavior is traceable
to the soft breaking of conformal invariance by the mass term in the sigma
model, semiclassical winding number transition amplitudes in the electroweak
theory in 3+1 dimensions should exhibit a similar sharp crossover. We argue
that this is indeed the case in the standard model for M_H < 4 M_W.Comment: 21 pages (14 figures), RevTeX (plus macro), uses eps
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