13,980 research outputs found
Parabolic and Quasiparabolic Subgroups of Free Partially Commutative Groups
Let S be a finite graph and G be the corresponding free partially commutative
group. In this paper we study subgroups generated by vertices of the graph S,
which we call canonical parabolic subgroups. A natural extension of the
definition leads to canonical quasiparabolic subgroups. It is shown that the
centralisers of subsets of G are the conjugates of canonical quasiparabolic
centralisers satisfying certain graph theoretic conditions.Comment: 18 pages, 1 figur
Automorphisms of Partially Commutative Groups II: Combinatorial Subgroups
We define several "standard" subgroups of the automorphism group Aut(G) of a
partially commutative (right-angled Artin) group and use these standard
subgroups to describe decompositions of Aut(G). If C is the commutation graph
of G, we show how Aut(G) decomposes in terms of the connected components of C:
obtaining a particularly clear decomposition theorem in the special case where
C has no isolated vertices.
If C has no vertices of a type we call dominated then we give a semi-direct
decompostion of Aut(G) into a subgroup of locally conjugating automorphisms by
the subgroup stabilising a certain lattice of "admissible subsets" of the
vertices of C. We then characterise those graphs for which Aut(G) is a product
(not necessarily semi-direct) of two such subgroups.Comment: 7 figures, 63 pages. Notation and definitions clarified and typos
corrected. 2 new figures added. Appendix containing details of presentation
and proof of a theorem adde
Prospective Analysis Spin- and CP-sensitive Variables in H -> ZZ -> l_1 l_1 l_2 l_2 with Atlas
A possibility to prove spin and CP-eigenvalue of a Standard Model (SM) Higgs
boson is presented. We exploit angular correlations in the subsequent decay H
-> ZZ -> 4l (muons or electrons) for Higgs masses above 200 GeV. We compare the
angular distributions of the leptons originating from the SM Higgs with those
resulting from decays of hypothetical particles with differing quantum numbers.
We restrict our analysis to the use of the Atlas-detector which is one of two
multi-purpose detectors at the upcoming 14 TeV proton-proton-collider (LHC) at
CERN. By applying a fast simulation of the Atlas detector it can be shown that
these correlations will be measured sufficiently well that consistency with the
spin-CP hypothesis 0+ of the Standard Model can be verified and the 0- and 1+-
can be ruled out with an integrated luminosity of 100 fb^-1.Comment: 25 pages, 9 figures Version 2: Minor changes made as requested by
Atlas referee and Springer editor. Added a chapter where background
subtraction is detaile
Convergence of the Optimized Delta Expansion for the Connected Vacuum Amplitude: Zero Dimensions
Recent proofs of the convergence of the linear delta expansion in zero and in
one dimensions have been limited to the analogue of the vacuum generating
functional in field theory. In zero dimensions it was shown that with an
appropriate, -dependent, choice of an optimizing parameter \l, which is an
important feature of the method, the sequence of approximants tends to
with an error proportional to . In the present paper we
establish the convergence of the linear delta expansion for the connected
vacuum function . We show that with the same choice of \l the
corresponding sequence tends to with an error proportional to . The rate of convergence of the latter sequence is governed by
the positions of the zeros of .Comment: 20 pages, LaTeX, Imperial/TP/92-93/5
Activities of \gamma-ray emitting isotopes in rainwater from Greater Sudbury, Canada following the Fukushima incident
We report the activity measured in rainwater samples collected in the Greater
Sudbury area of eastern Canada on 3, 16, 20, and 26 April 2011. The samples
were gamma-ray counted in a germanium detector and the isotopes 131I and 137Cs,
produced by the fission of 235U, and 134Cs, produced by neutron capture on
133Cs, were observed at elevated levels compared to a reference sample of
ice-water. These elevated activities are ascribed to the accident at the
Fukushima Dai-ichi nuclear reactor complex in Japan that followed the 11 March
earthquake and tsunami. The activity levels observed at no time presented
health concerns.Comment: 4 pages, 8 figure
Dependence of the Fundamental Plane Scatter on Galaxy Age
The fundamental plane (FP) has an intrinsic scatter that can not be explained
purely by observational errors. Using recently available age estimates for
nearby early type galaxies, we show that a galaxy's position relative to the FP
depends on its age. In particular, the mean FP corresponds to ellipticals with
an age of ~10 Gyr. Younger galaxies are systematically brighter with higher
surface brightness relative to the mean relation. Old ellipticals form an
`upper envelope' to the FP. For our sample of mostly non-cluster galaxies, age
can account for almost half of the scatter in the B band FP. Distance
determinations based on the FP may have a systematic bias, if the mean age of
the sample varies with redshift.
We also show that fundamental plane residuals, B-V colors and Mg_2 line
strength are consistent with an ageing central burst superposed on an old
stellar population. This reinforces the view that these age estimates are
tracing the last major episode of star formation induced by a gaseous merger
event. We briefly discuss the empirical `evolutionary tracks' of
merger-remnants and young ellipticals in terms of their key observational
parameters.Comment: 14 pages, Latex, 2 figures, accepted by ApJ Letter
Periodic Thermonuclear X-ray Bursts from GS 1826-24 and the Fuel Composition as a Function of Accretion Rate
We analyze 24 type I X-ray bursts from GS 1826-24 observed by the Rossi X-ray
Timing Explorer between 1997 November and 2002 July. The bursts observed
between 1997-98 were consistent with a stable recurrence time of 5.74 +/- 0.13
hr. The persistent intensity of GS 1826-24 increased by 36% between 1997-2000,
by which time the burst interval had decreased to 4.10 +/- 0.08 hr. In 2002
July the recurrence time was shorter again, at 3.56 +/- 0.03 hr. The bursts
within each epoch had remarkably identical lightcurves over the full approx.
150 s burst duration; both the initial decay timescale from the peak, and the
burst fluence, increased slightly with the rise in persistent flux. The
decrease in the burst recurrence time was proportional to Mdot^(-1.05+/-0.02)
(where Mdot is assumed to be linearly proportional to the X-ray flux), so that
the ratio alpha between the integrated persistent and burst fluxes was
inversely correlated with Mdot. The average value of alpha was 41.7 +/- 1.6.
Both the alpha value, and the long burst durations indicate that the hydrogen
is burning during the burst via the rapid-proton (rp) process. The variation in
alpha with Mdot implies that hydrogen is burning stably between bursts,
requiring solar metallicity (Z ~ 0.02) in the accreted layer. We show that
solar metallicity ignition models naturally reproduce the observed burst
energies, but do not match the observed variations in recurrence time and burst
fluence. Low metallicity models (Z ~ 0.001) reproduce the observed trends in
recurrence time and fluence, but are ruled out by the variation in alpha. We
discuss possible explanations, including extra heating between bursts, or that
the fraction of the neutron star covered by the accreted fuel increases with
Mdot.Comment: 9 pages, 6 figures, accepted by ApJ. Minor revisions following the
referee's repor
Unquenched QCD with Light Quarks
We present recent results in unquenched lattice QCD with two degenerate light
sea quarks using the truncated determinant approximation (TDA). In the TDA the
infrared modes contributing to the quark determinant are computed exactly up to
some cutoff in quark off-shellness (typically 2). This approach
allows simulations to be performed at much lighter quark masses than possible
with conventional hybrid MonteCarlo techniques. Results for the static energy
and topological charge distributions are presented using a large ensemble
generated on very coarse (6) but physically large lattices. Preliminary
results are also reported for the static energy and meson spectrum on 10x20
lattices (lattice scale =1.15 GeV) at quark masses corresponding to
pions of mass 200 MeV. Using multiboson simulation to compute the
ultraviolet part of the quark determinant the TDA approach becomes an exact
with essentially no increase in computational effort. Some preliminary results
using this fully unquenched algorithm are presented.Comment: LateX, 39 pages, 16 eps figures, 1 ps figur
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