9,775 research outputs found
Semi-inclusive structure functions in the spectator model
We establish the relationship between distribution and fragmentation
functions and the structure functions appearing in the cross section of
polarized 1-particle inclusive deep-inelastic scattering. We present spectator
model evaluations of these structure functions focusing on the case of an
outgoing spin-1/2 baryon. Distribution functions obtained in the spectator
model are known to fairly agree at low energy scales with global
parameterizations extracted, for instance, from totally inclusive DIS data.
Therefore, we expect it to give good hints on the functional dependence of the
structure functions on the scaling variables x(Bjorken), z and on the
transverse momentum of the observed outgoing hadron, P_{h\perp}. Presently,
this dependence is not very well known, but experiments are planned in the near
future.Comment: 19 pages, 16 figures, submitted to Eur. Phys. J.
Role of anisotropy for protein-protein encounter
Protein-protein interactions comprise both transport and reaction steps.
During the transport step, anisotropy of proteins and their complexes is
important both for hydrodynamic diffusion and accessibility of the binding
site. Using a Brownian dynamics approach and extensive computer simulations, we
quantify the effect of anisotropy on the encounter rate of ellipsoidal
particles covered with spherical encounter patches. We show that the encounter
rate depends on the aspect ratios mainly through steric effects,
while anisotropic diffusion has only a little effect. Calculating analytically
the crossover times from anisotropic to isotropic diffusion in three
dimensions, we find that they are much smaller than typical protein encounter
times, in agreement with our numerical results.Comment: 4 pages, Revtex with 3 figures, to appear as a Rapid Communication in
Physical Review
The size of the nucleosome
The structural origin of the size of the 11 nm nucleosomal disc is addressed.
On the nanometer length-scale the organization of DNA as chromatin in the
chromosomes involves a coiling of DNA around the histone core of the
nucleosome. We suggest that the size of the nucleosome core particle is
dictated by the fulfillment of two criteria: One is optimizing the volume
fraction of the DNA double helix; this requirement for close-packing has its
root in optimizing atomic and molecular interactions. The other criterion being
that of having a zero strain-twist coupling; being a zero-twist structure is a
necessity when allowing for transient tensile stresses during the
reorganization of DNA, e.g., during the reposition, or sliding, of a nucleosome
along the DNA double helix. The mathematical model we apply is based on a
tubular description of double helices assuming hard walls. When the base-pairs
of the linker-DNA is included the estimate of the size of an ideal nucleosome
is in close agreement with the experimental numbers. Interestingly, the size of
the nucleosome is shown to be a consequence of intrinsic properties of the DNA
double helix.Comment: 11 pages, 5 figures; v2: minor modification
Electrochemical control of quantum interference in anthraquinone-based molecular switches
Using first-principles calculations we analyze the electronic transport
properties of a recently proposed anthraquinone based electrochemical switch.
Robust conductance on/off ratios of several orders of magnitude are observed
due to destructive quantum interference present in the anthraquinone, but
absent in the hydroquinone molecular bridge. A simple explanation of the
interference effect is achieved by transforming the frontier molecular orbitals
into localized molecular orbitals thereby obtaining a minimal tight-binding
model describing the transport in the relevant energy range in terms of hopping
via the localized orbitals. The topology of the tight-binding model, which is
dictated by the symmetries of the molecular orbitals, determines the amount of
quantum interference.Comment: 6 pages, 6 figure
Current dependence of grain boundary magnetoresistance in La_0.67Ca_0.33MnO_3 films
We prepared epitaxial ferromagnetic manganite films on bicrystal substrates
by pulsed laser ablation. Their low- and high-field magnetoresistance (MR) was
measured as a function of magnetic field, temperature and current. At low
temperatures hysteretic changes in resistivity up to 70% due to switching of
magnetic domains at the coercitive field are observed. The strongly non-ohmic
behavior of the current-voltage leads to a complete suppression of the MR
effect at high bias currents with the identical current dependence at low and
high magnetic fields. We discuss the data in view of tunneling and mesoscale
magnetic transport models and propose an explicit dependence of the spin
polarization on the applied current in the grain boundary region.Comment: 5 pages, to appear in J. Appl. Phy
Calculation of fragmentation functions in two-hadron semi-inclusive processes
We investigate the properties of interference fragmentation functions arising
from the emission of two leading hadrons inside the same jet for inclusive
lepton-nucleon deep-inelastic scattering. Using an extended spectator model for
the mechanism of the hadronization, we give a complete calculation and
numerical estimates for the examples of a proton-pion pair produced with
invariant mass on the Roper resonance, and of two pions produced with invariant
mass close to the mass. We discuss azimuthal angular dependence of the
leading order cross section to point up favourable conditions for extracting
transversity from experimental data.Comment: 5 pages, 3 figures in .eps format, AIP and epsfig styles included, to
appear in proceedings of "Second Workshop on Physics with an Electron
Polarized Light Ion Collider", MIT, Sept. 14-16, 200
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