3,305,934 research outputs found
Medium-modified fragmentation of b-jets tagged by a leading muon in ultrarelativistic heavy ion collisions
The possibility to observe the medium-modified fragmentation of hard b-quarks
tagged by a leading muon in ultrarelativistic heavy ion collisions is analyzed.
We have found that reasonable statistics, ~20000 events per 1 month of LHC run
with lead beams, can be expected for the realistic geometrical acceptance and
kinematic cuts. The numerical estimates on the effect of the medium-induced
softening b-jet fragmentation function are given.Comment: 12 pages in LaTeX (including 3 figures in EPS-format
Irreversible Adsorption from Dilute Polymer Solutions
We study irreversible polymer adsorption from dilute solutions theoretically.
Universal features of the resultant non-equilibrium layers are predicted. Two
cases are considered, distinguished by the value of the local monomer-surface
sticking rate Q: chemisorption (very small Q) and physisorption (large Q).
Early stages of layer formation entail single chain adsorption. While single
chain physisorption times tau_ads are typically microsecs, for chemisorbing
chains of N units we find experimentally accessible times tau_ads = Q^{-1}
N^{3/5}, ranging from secs to hrs. We establish 3 chemisorption universality
classes, determined by a critical contact exponent: zipping, accelerated
zipping and homogeneous collapse. For dilute solutions, the mechanism is
accelerated zipping: zipping propagates outwards from the first attachment,
accelerated by occasional formation of large loops which nucleate further
zipping. This leads to a transient distribution omega(s) \sim s^{-7/5} of loop
lengths s up to a size s_max \approx (Q t)^{5/3} after time t. By tau_ads the
entire chain is adsorbed. The outcome of the single chain adsorption episode is
a monolayer of fully collapsed chains. Having only a few vacant sites to adsorb
onto, late arriving chains form a diffuse outer layer. In a simple picture we
find for both chemisorption and physisorption a final loop distribution
Omega(s) \sim s^{-11/5} and density profile c(z) \sim z^{-4/3} whose forms are
the same as for equilibrium layers. In contrast to equilibrium layers, however,
the statistical properties of a given chain depend on its adsorption time; the
outer layer contains many classes of chain, each characterized by different
fraction of adsorbed monomers f. Consistent with strong physisorption
experiments, we find the f values follow a distribution P(f) \sim f^{-4/5}.Comment: 18 pages, submitted to Eur. Phys. J. E, expanded discussion sectio
Finally, results from Gravity Probe-B
Nearly fifty years after its inception, the Gravity Probe B satellite mission
delivers the first measurements of how a spinning gyroscope precesses in the
gravitational warping of spacetime.Comment: A Viewpoint article, published in Physics 4, 43 (2011), available at
http://physics.aps.org/articles/v4/43 Submitted to the arXiv by permission of
the American Physical Societ
Experimental Observation Of Energetic Ions Accelerated By Three-Dimensional Magnetic Reconnection In A Laboratory Plasma
Magnetic reconnection is widely believed responsible for heating the solar corona as well as for generating X-rays and energetic particles in solar flares. On astrophysical scales, reconnection in the intergalactic plasma is a prime candidate for a local source (Mpc) of cosmic rays exceeding the Greisen-Zatsepin-Kuzmin cutoff (∼10(19) eV). In a laboratory astrophysics experiment, we have made the first observation of particles accelerated by magnetic reconnection events to energies significantly above both the thermal and the characteristic magnetohydrodynamic energies. These particles are correlated temporally and spatially with the formation of three-dimensional magnetic structures in the reconnection region
Optimization Of Detergent-Mediated Reconstitution Of Influenza A M2 Protein Into Proteoliposomes
We report the optimization of detergent-mediated reconstitution of an integral membrane-bound protein, full-length influenza M2 protein, by direct insertion into detergent-saturated liposomes. Detergent-mediated reconstitution is an important method for preparing proteoliposomes for studying membrane proteins, and must be optimized for each combination of protein and membrane constituents used. The purpose of the reconstitution was to prepare samples for site-directed spin-labeling electron paramagnetic resonance (SDSL-EPR) studies. Our goals in optimizing the protocol were to minimize the amount of detergent used, reduce overall proteoliposome preparation time, and confirm the removal of all detergent. The liposomes were comprised of (1-palmitoyl-2-oleyl-sn-glycero-phosphocholine (POPC) and 1-palmitoyl-2-oleyl-sn-glycero-3-[phospho-rac-(1-glycerol)] (POPG), and the detergent octylglucoside (OG) was used for reconstitution. Rigorous physical characterization was applied to optimize each step of the reconstitution process. We used dynamic light scattering (DLS) to determine the amount of OG needed to saturate the preformed liposomes. During detergent removal by absorption with Bio-Beads, we quantified the detergent concentration by means of a colorimetric assay, thereby determining the number of Bio-Bead additions needed to remove all detergent from the final proteoliposomes. We found that the overnight Bio-Bead incubation used in previously published protocols can be omitted, reducing the time needed for reconstitution. We also monitored the size distribution of the proteoliposomes with DLS, confirming that the size distribution remains essentially constant throughout the reconstitution process
A Test Of Pre-Main-Sequence Lithium Depletion Models
Despite the extensive study of lithium depletion during pre-main-sequence (PMS) contraction, studies of individual stars show discrepancies between ages determined from the Hertzsprung-Russell (H-R) diagram and ages determined from lithium depletion, indicating open questions in the PMS evolutionary models. To further test these models, we present high-resolution spectra for members of the beta Pictoris Moving Group (BPMG), which is young and nearby. We measure equivalent widths of the 6707.8 angstrom Li I line in these stars and use them to determine lithium abundances. We combine the lithium abundance with the predictions of PMS evolutionary models in order to calculate a lithium depletion age for each star. We compare this age to the age predicted by the H-R diagram of the same model. We find that the evolutionary models underpredict the amount of lithium depletion for the BPMG given its nominal H-R diagram age of similar to 12 Myr, particularly for the mid-M stars, which have no observable Li I line. This results in systematically older ages calculated from lithium depletion isochrones than from the H-R diagram. We suggest that this discrepancy may be related to the discrepancy between measured M-dwarf radii and the smaller radii predicted by evolutionary models
Generalized Ohm\u27s Law In A 3-D Reconnection Experiment
We report the measurement of non-ideal terms of the generalized Ohm\u27s law at a reconnection site of a weakly collisional laboratory magnetohydrodynamic plasma. Results show that the Hall term dominates the measured terms; resistive and electron inertia terms are small. We suggest that electron pressure (not measured) supports the observed quasistatic reconnection rate, and that anomalous resistivity, while not ruled out, is not required to account for the results
Cosmological Implications Of Ultralight Axionlike Fields
Cosmological observations are used to test for imprints of an ultralight axionlike field (ULA), with a range of potentials V(ϕ)∝[1−cos(ϕ/f)]ⁿ set by the axion-field value ϕ and decay constant f. Scalar field dynamics dictate that the field is initially frozen and then begins to oscillate around its minimum when the Hubble parameter drops below some critical value. For n=1, once dynamical, the axion energy density dilutes as matter; for n=2 it dilutes as radiation and for n=3 it dilutes faster than radiation. Both the homogeneous evolution of the ULA and the dynamics of its linear perturbations are included, using an effective fluid approximation generalized from the usual n=1 case. ULA models are parametrized by the redshift z(c) when the field becomes dynamical, the fractional energy density f(z(c))≡Ωₐ(z(c))/Ωₜₒₜ(z(c)) in the axion field at zc, and the effective sound speed c²ₛ. Using Planck, BAO and JLA data, constraints on fzc are obtained. ULAs are degenerate with dark energy for all three potentials if 1+z(c)≲10. When 3×10⁴≳1+z(c)≳10, f(z(c)) is constrained to be ≲0.004 for n=1 and f(z(c))≲0.02 for the other two potentials. The constraints then relax with increasing zc. These results have implications for ULAs as a resolution to cosmological tensions, such as discrepant measurements of the Hubble constant, or the EDGES measurement of the global 21 cm signal
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