7,730 research outputs found
Collision of Polymers in a Vacuum
In a number of experimental situations, single polymer molecules can be
suspended in a vacuum. Here collisions between such molecules are considered.
The limit of high collision velocity is investigated numerically for a variety
of conditions. The distribution of contact times, scattering angles, and final
velocities are analyzed. In this limit, self avoiding chains are found to
become highly stretched as they collide with each other, and have a
distribution of scattering times that depends on the scattering angle. The
velocity of the molecules after the collisions is similar to predictions of a
model assuming thermal equilibration of molecules during the collision. The
most important difference is a significant subset of molecules that
inelastically scatter but do not substantially change direction.Comment: 7 pages, 6 figure
Time-Resolved Ultraviolet Observations of the Globular Cluster X-ray Source in NGC 6624: The Shortest Known Period Binary System
Using the Faint Object Spectrograph (FOS) aboard the Hubble Space Telescope,
we have obtained the first time-resolved spectra of the King et al.
ultraviolet-bright counterpart to the 11-minute binary X-ray source in the core
of the globular cluster NGC 6624. This object cannot be readily observed in the
visible, even from HST, due to a much brighter star superposed <0.1'' distant.
Our FOS data show a highly statistically significant UV flux modulation with a
period of 11.46+-0.04 min, very similar to the 685 sec period of the known
X-ray modulation, definitively confirming the association between the King et
al. UV counterpart and the intense X-ray source. The UV amplitude is very large
compared with the observed X-ray oscillations: X-ray variations are generally
reported as 2-3% peak-to-peak, whereas our data show an amplitude of about 16%
in the 126-251 nm range. A model for the system by Arons & King predicts
periodic UV fluctuations in this shortest-known period binary system, due to
the cyclically changing aspect of the X-ray heated face of the secondary star
(perhaps a very low mass helium degenerate). However, prior to our
observations, this predicted modulation has not been detected. Employing the
Arons & King formalism, which invokes a number of different physical
assumptions, we infer a system orbital inclination 35deg<i<50 deg. Amongst the
three best-studied UV/optical counterparts to the intense globular cluster
X-ray sources, two are now thought to consist of exotic double-degenerate
ultrashort period binary systems.Comment: 10 pages including 2 figures in Latex (AASTeX 4.0). Accepted for
publication in vol. 482 (1997 June 10 issue) of The Astrophysical Journal
(Letters
Supersymmetric Contributions to Weak Decay Correlation Coefficients
We study supersymmetric contributions to correlation coefficients that
characterize the spectral shape and angular distribution for polarized muon-
and beta-decays. In the minimal supersymmetric Standard Model (MSSM), one-loop
box graphs containing superpartners can give rise to non-(V-A)x(V-A) four
fermion operators in the presence of left-right or flavor mixing between
sfermions. We analyze the present phenomenological constraints on such mixing
and determine the range of allowed contributions to the weak decay correlation
coefficients. We discuss the prospective implications for future muon- and
beta-decay experiments, and argue that they may provide unique probes of
left-right mixing in the first generation scalar fermion sector.Comment: Revised version - to appear in Phys.Rev.
A Search for the Optical Counterpart of the Luminous X-ray Source in NGC 6652
We examine images of the field of X1832-330, the luminous (Lx ~ 10^36 erg/s)
X-ray burst source near the center of the globular cluster NGC 6652, in order
to identify the optical counterpart for further study. U and B ground-based
images allow us to set a limit M_B > 3.5 for the counterpart at the time of
those observations, provided that the color is (U-B)_0 ~ -1, similar to the
sources known in other clusters. Archival Hubble Space Telescope observations
survey most but not all of the 1 sigma X-ray error circle, and allow us to set
limits M_B > 5.9 and M_B > 5.2 in the WF/PC and WFPC2 regions, respectively. In
the WF/PC images we do weakly detect a faint object with UV-excess, but it is
located 11.7'' from the ROSAT X-ray position. This considerable (2.3 sigma)
discrepancy in position suggests that this candidate be treated with caution,
but it remains the only reasonable one advanced thus far. We measure for this
star m_439 = 20.2 +- 0.2, (m_336 - m_439) = -0.5 +- 0.2, and estimate M_B =
5.5, (U-B)_0 = -0.9, similar to other known optical counterparts. If this
candidate is not the identification, our limits imply that the true
counterpart, not yet identified, is probably the optically-faintest cluster
source yet known, or alternatively that it did not show significant UV excess
at the time of these observations. Finally, we assess the outlook for the
identification of the remaining luminous globular cluster X-ray sources.Comment: 15 pages including 5 figures and no tables. Accepted for publication
in The Astronomical Journal; to appear in Volume 116, September 1998. A
preprint with full resolution figures may be downloaded from
http://www.astro.washington.edu/deutsch/pubs
A mean-field theory for self-propelled particles interacting by velocity alignment mechanisms
Holonomic quantum computation in the presence of decoherence
We present a scheme to study non-abelian adiabatic holonomies for open
Markovian systems. As an application of our framework, we analyze the
robustness of holonomic quantum computation against decoherence. We pinpoint
the sources of error that must be corrected to achieve a geometric
implementation of quantum computation completely resilient to Markovian
decoherence.Comment: I. F-G. Now publishes under name I. Fuentes-Schuller Published
versio
Quantum ergodicity and entanglement in kicked coupled-tops
We study the dynamical generation of entanglement as a signature of chaos in
a system of periodically kicked coupled-tops, where chaos and entanglement
arise from the same physical mechanism. The long-time averaged entanglement as
a function of the position of an initially localized wave packet very closely
correlates with the classical phase space surface of section -- it is nearly
uniform in the chaotic sea, and reproduces the detailed structure of the
regular islands. The uniform value in the chaotic sea is explained by the
random state conjecture. As classically chaotic dynamics take localized
distributions in phase space to random distributions, quantized versions take
localized coherent states to pseudo-random states in Hilbert space. Such random
states are highly entangled, with an average value near that of the maximally
entangled state. For a map with global chaos, we derive that value based on new
analytic results for the typical entanglement in a subspace defined by the
symmetries of the system.
For a mixed phase space, we use the Percival conjecture to identify a
"chaotic subspace" of the Hilbert space. The typical entanglement, averaged
over the unitarily invariant Haar measure in this subspace, agrees with the
long-time averaged entanglement for initial states in the chaotic sea. In all
cases the dynamically generated entanglement is predicted by a unitary ensemble
of random states, even though the system is time-reversal invariant, and the
Floquet operator is a member of the circular orthogonal ensemble.Comment: 12 pages with 8 figure
Early out-of-equilibrium beam-plasma evolution
We solve analytically the out-of-equilibrium initial stage that follows the
injection of a radially finite electron beam into a plasma at rest and test it
against particle-in-cell simulations. For initial large beam edge gradients and
not too large beam radius, compared to the electron skin depth, the electron
beam is shown to evolve into a ring structure. For low enough transverse
temperatures, the filamentation instability eventually proceeds and saturates
when transverse isotropy is reached. The analysis accounts for the variety of
very recent experimental beam transverse observations.Comment: to appear in Phys. Rev. Letter
Scaling of polymers in aligned rods
We study the behavior of self avoiding polymers in a background of vertically
aligned rods that are either frozen into random positions or free to move
horizontally. We find that in both cases the polymer chains are highly
elongated, with vertical and horizontal size exponents that differ by a factor
of 3. Though these results are different than previous predictions, our results
are confirmed by detailed computer simulations.Comment: 4 pages, 4 figure
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