27,924 research outputs found
Mass Expansions of Screened Perturbation Theory
The thermodynamics of massless phi^4-theory is studied within screened
perturbation theory (SPT). In this method the perturbative expansion is
reorganized by adding and subtracting a mass term in the Lagrangian. We
analytically calculate the pressure and entropy to three-loop order and the
screening mass to two-loop order, expanding in powers of m/T. The truncated
m/T-expansion results are compared with numerical SPT results for the pressure,
entropy and screening mass which are accurate to all orders in m/T. It is shown
that the m/T-expansion converges quickly and provides an accurate description
of the thermodynamic functions for large values of the coupling constant.Comment: 22 pages, 10 figure
Enumeration of chord diagrams on many intervals and their non-orientable analogs
Two types of connected chord diagrams with chord endpoints lying in a
collection of ordered and oriented real segments are considered here: the real
segments may contain additional bivalent vertices in one model but not in the
other. In the former case, we record in a generating function the number of
fatgraph boundary cycles containing a fixed number of bivalent vertices while
in the latter, we instead record the number of boundary cycles of each fixed
length. Second order, non-linear, algebraic partial differential equations are
derived which are satisfied by these generating functions in each case giving
efficient enumerative schemes. Moreover, these generating functions provide
multi-parameter families of solutions to the KP hierarchy. For each model,
there is furthermore a non-orientable analog, and each such model likewise has
its own associated differential equation. The enumerative problems we solve are
interpreted in terms of certain polygon gluings. As specific applications, we
discuss models of several interacting RNA molecules. We also study a matrix
integral which computes numbers of chord diagrams in both orientable and
non-orientable cases in the model with bivalent vertices, and the large-N limit
is computed using techniques of free probability.Comment: 23 pages, 7 figures; revised and extended versio
The Stability Balloon for Two-dimensional Vortex Ripple Patterns
Patterns of vortex ripples form when a sand bed is subjected to an
oscillatory fluid flow. Here we describe experiments on the response of regular
vortex ripple patterns to sudden changes of the driving amplitude a or
frequency f. A sufficient decrease of f leads to a "freezing" of the pattern,
while a sufficient increase of f leads to a supercritical secondary "pearling"
instability. Sufficient changes in the amplitude a lead to subcritical
secondary "doubling" and "bulging" instabilities. Our findings are summarized
in a "stability balloon" for vortex ripple pattern formation.Comment: 4 pages, 5 figure
The stellar content of the infalling molecular clump G286.21+0.17
The early evolution during massive star cluster formation is still uncertain.
Observing embedded clusters at their earliest stages of formation can provide
insight into the spatial and temporal distribution of the stars and thus probe
different star cluster formation models. We present near-infrared imaging of an
8'*13'(5.4pc*8.7pc) region around the massive infalling clump G286.21+0.17(also
known as BYF73). The stellar content across the field is determined and
photometry is derived in order to { obtain} stellar parameters for the cluster
members. We find evidence for some sub-structure (on scales less than a pc
diameter) within the region with apparently at least three different
sub-clusters associated with the molecular clump based on differences in
extinction and disk fractions. At the center of the clump we identify a deeply
embedded sub-cluster. Near-infrared excess is detected for 39-44% in the two
sub-clusters associated with molecular material and 27% for the exposed
cluster. Using the disk excess as a proxy for age this suggests the clusters
are very young. The current total stellar mass is estimated to be at least 200
Msun. The molecular core hosts a rich population of pre-main sequence stars.
There is evidence for multiple events of star formation both in terms of the
spatial distribution within the star forming region and possibly from the disk
frequency.Comment: Submitted to A
Observation of persistent flow of a Bose-Einstein condensate in a toroidal trap
We have observed the persistent flow of Bose-condensed atoms in a toroidal
trap. The flow persists without decay for up to 10 s, limited only by
experimental factors such as drift and trap lifetime. The quantized rotation
was initiated by transferring one unit, , of the orbital angular
momentum from Laguerre-Gaussian photons to each atom. Stable flow was only
possible when the trap was multiply-connected, and was observed with a BEC
fraction as small as 15%. We also created flow with two units of angular
momentum, and observed its splitting into two singly-charged vortices when the
trap geometry was changed from multiply- to simply-connected.Comment: 1 file, 5 figure
Screened Perturbation Theory to Three Loops
The thermal physics of a massless scalar field with a phi^4 interaction is
studied within screened perturbation theory (SPT). In this method the
perturbative expansion is reorganized by adding and subtracting a mass term in
the lagrangian. We consider several different mass prescriptions that
generalize the one-loop gap equation to two-loop order. We calculate the
pressure and entropy to three-loop order and the screening mass to two-loop
order. In contrast to the weak-coupling expansion, the SPT-improved
approximations appear to converge even for rather large values of the coupling
constant.Comment: 30 pages, 10 figure
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