1,914 research outputs found
Constraints on string networks with junctions
We consider the constraints on string networks with junctions in which the
strings may all be different, as may be found for example in a network of
cosmic superstrings. We concentrate on three aspects of junction
dynamics. First we consider the propagation of small amplitude waves across a
static three-string junction. Then, generalizing our earlier work, we determine
the kinematic constraints on two colliding strings with different tensions. As
before, the important conclusion is that strings do not always reconnect with a
third string; they can pass straight through one another (or in the case of
non-abelian strings become stuck in an X configuration), the constraint
depending on the angle at which the strings meet, on their relative velocity,
and on the ratios of the string tensions. For example, if the two colliding
strings have equal tensions, then for ultra-relativistic initial velocities
they pass through one another. However, if their tensions are sufficiently
different they can reconnect. Finally, we consider the global properties of
junctions and strings in a network. Assuming that, in a network, the incoming
waves at a junction are independently randomly distributed, we determine the
r.m.s. velocities of strings and calculate the average speed at which a
junction moves along each of the three strings from which it is formed. Our
findings suggest that junction dynamics may be such as to preferentially remove
the heavy strings from the network leaving a network of predominantly light
strings. Furthermore the r.m.s. velocity of strings in a network with junctions
is smaller than 1/\sqrt{2}, the result for conventional Nambu-Goto strings
without junctions in Minkowski spacetime.Comment: 12 pages, 6 figures. Version to appear in PRD. (2 new references and
slightly extended discussion in section VII
Wick's Theorem for non-symmetric normal ordered products and contractions
We consider arbitrary splits of field operators into two parts, and use the
corresponding definition of normal ordering introduced by Evans and Steer. In
this case the normal ordered products and contractions have none of the special
symmetry properties assumed in existing proofs of Wick's theorem. Despite this,
we prove that Wick's theorem still holds in its usual form as long as the
contraction is a c-number. Wick's theorem is thus shown to be much more general
than existing derivations suggest, and we discuss possible simplifying
applications of this result.Comment: 17 page
The Role Of Sentencing Guideline Amendments In Reducing Unwarranted Sentencing Disparity
Collisions of strings with Y junctions
We study the dynamics of Nambu--Goto strings with junctions at which three
strings meet. In particular, we exhibit one simple exact solution and examine
the process of intercommuting of two straight strings, in which they exchange
partners but become joined by a third string. We show that there are important
kinematical constraints on this process. The exchange cannot occur if the
strings meet with very large relative velocity. This may have important
implications for the evolution of cosmic superstring networks and non-abelian
string networks.Comment: 4 pages, 1 figure, uses revtex 4. Clarifying comments added to
correct a conceptual error, reference updated. Version accepted by Phys Rev
Letters, with additional references and minor change
Kinematic Constraints on Formation of Bound States of Cosmic Strings - Field Theoretical Approach
Superstring theory predicts the potential formation of string networks with
bound states ending in junctions. Kinematic constraints for junction formation
have been derived within the Nambu-Goto thin string approximation. Here we test
these constraints numerically in the framework of the Abelian-Higgs model in
the Type-I regime and report on good agreement with the analytical predictions.
We also demonstrate that strings can effectively pass through each other when
they meet at speeds slightly above the critical velocity permitting bound state
formation. This is due to reconnection effects that are beyond the scope of the
Nambu-Goto approximation.Comment: 6 pages, 12 eps figures - matches the published versio
MERLIN/VLA imaging of the gravitational lens system B0218+357
Gravitational lenses offer the possibility of accurately determining the
Hubble parameter (H_0) over cosmological distances, and B0218+357 is one of the
most promising systems for an application of this technique. In particular this
system has an accurately measured time delay (10.5+/-0.4 d; Biggs et al. 1999)
and preliminary mass modelling has given a value for H_0 of 69 +13/-19
km/s/Mpc. The error on this estimate is now dominated by the uncertainty in the
mass modelling. As this system contains an Einstein ring it should be possible
to constrain the model better by imaging the ring at high resolution. To
achieve this we have combined data from MERLIN and the VLA at a frequency of 5
GHz. In particular MERLIN has been used in multi-frequency mode in order to
improve substantially the aperture coverage of the combined data set. The
resulting map is the best that has been made of the ring and contains many new
and interesting features. Efforts are currently underway to exploit the new
data for lensing constraints using the LensClean algorithm (Kochanek & Narayan
1992).Comment: Accepted for publication in MNRAS. 6 pages, 4 included PostScript
figure
High-Resolution, Wide-Field Imaging of the Galactic Center Region at 330 MHz
We present a wide field, sub-arcminute resolution VLA image of the Galactic
Center region at 330 MHz. With a resolution of ~ 7" X 12" and an RMS noise of
1.6 mJy/beam, this image represents a significant increase in resolution and
sensitivity over the previously published VLA image at this frequency. The
improved sensitivity has more than tripled the census of small diameter sources
in the region, has resulted in the detection of two new Non Thermal Filaments
(NTFs), 18 NTF candidates, 30 pulsar candidates, reveals previously known
extended sources in greater detail, and has resulted in the first detection of
Sagittarius A* in this frequency range.
A version of this paper containing full resolution images may be found at
http://lwa.nrl.navy.mil/nord/AAAB.pdf.Comment: Astronomical Journal, Accepted 62 Pages, 21 Figure
Measurement of the orientation of buffer-gas-cooled, electrostatically-guided ammonia molecules
AbstractThe extent to which the spatial orientation of internally and translationally cold ammonia molecules can be controlled as molecules pass out of a quadrupole guide and through different electric field regions is examined. Ammonia molecules are collisionally cooled in a buffer gas cell, and are subsequently guided by a three-bend electrostatic quadrupole into a detection chamber. The orientation of ammonia molecules is probed using (2+1) resonance-enhanced multiphoton ionisation (REMPI), with the laser polarisation axis aligned both parallel and perpendicular to the time-of-flight axis. Even with the presence of a near-zero field region, the ammonia REMPI spectra indicate some retention of orientation. Monte Carlo simulations propagating the time-dependent Schrödinger equation in a full basis set including the hyperfine interaction enable the orientation of ammonia molecules to be calculated â with respect to both the local field direction and a space-fixed axis â as the molecules pass through different electric field regions. The simulations indicate that the orientation of âŒ95% of ammonia molecules in JK=11 could be achieved with the application of a small bias voltage (17V) to the mesh separating the quadrupole and detection regions. Following the recent combination of the buffer gas cell and quadrupole guide apparatus with a linear Paul ion trap, this result could enable one to examine the influence of molecular orientation on ion-molecule reaction dynamics and kinetics
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