2,140 research outputs found
Anderson localization on the Cayley tree : multifractal statistics of the transmission at criticality and off criticality
In contrast to finite dimensions where disordered systems display
multifractal statistics only at criticality, the tree geometry induces
multifractal statistics for disordered systems also off criticality. For the
Anderson tight-binding localization model defined on a tree of branching ratio
K=2 with generations, we consider the Miller-Derrida scattering geometry
[J. Stat. Phys. 75, 357 (1994)], where an incoming wire is attached to the root
of the tree, and where outcoming wires are attached to the leaves of
the tree. In terms of the transmission amplitudes , the total
Landauer transmission is , so that each channel
is characterized by the weight . We numerically measure the
typical multifractal singularity spectrum of these weights as a
function of the disorder strength and we obtain the following conclusions
for its left-termination point . In the delocalized phase ,
is strictly positive and is associated with a
moment index . At criticality, it vanishes and is
associated with the moment index . In the localized phase ,
is associated with some moment index . We discuss the
similarities with the exact results concerning the multifractal properties of
the Directed Polymer on the Cayley tree.Comment: v2=final version (16 pages
Molecular line intensities as measures of cloud masses - II. Conversion factors for specific galaxy types
We present theoretically-established values of the CO-to-H2 and C-to-H2
conversion factors that may be used to estimate the gas masses of external
galaxies. We consider four distinct galaxy types, represented by M51, NGC 6946,
M82 and SMC N27. The physical parameters that best represent the conditions
within the molecular clouds in each of the galaxy types are estimated using a
chi^2 analysis of several observed atomic fine structure and CO rotational
lines. This analysis is explored over a wide range of density, radiation field,
extinction, and other relevant parameters. Using these estimated physical
conditions in methods that we have previously established, CO-to-H2 conversion
factors are then computed for CO transitions up to J=9-8. For the conventional
CO(1-0) transition, the computed conversion factor varies significantly below
and above the canonical value for the Milky Way in the four galaxy types
considered. Since atomic carbon emission is now frequently used as a probe of
external galaxies, we also present, for the first time, the C-to-H2 conversion
factor for this emission in the four galaxy types considered.Comment: 14 pages, 11 figures, accepted for publication in MNRA
Colour reconnection in e+e- -> W+W- at sqrt(s) = 189 - 209 GeV
The effects of the final state interaction phenomenon known as colour
reconnection are investigated at centre-of-mass energies in the range sqrt(s) ~
189-209 GeV using the OPAL detector at LEP. Colour reconnection is expected to
affect observables based on charged particles in hadronic decays of W+W-.
Measurements of inclusive charged particle multiplicities, and of their angular
distribution with respect to the four jet axes of the events, are used to test
models of colour reconnection. The data are found to exclude extreme scenarios
of the Sjostrand-Khoze Type I (SK-I) model and are compatible with other
models, both with and without colour reconnection effects. In the context of
the SK-I model, the best agreement with data is obtained for a reconnection
probability of 37%. Assuming no colour reconnection, the charged particle
multiplicity in hadronically decaying W bosons is measured to be (nqqch) =
19.38+-0.05(stat.)+-0.08 (syst.).Comment: 30 pages, 9 figures, Submitted to Euro. Phys. J.
Scaling violations of quark and gluon jet fragmentation functions in e+e- annihilations at sqrt(s) = 91.2 and 183-209 GeV
Flavour inclusive, udsc and b fragmentation functions in unbiased jets, and
flavour inclusive, udsc, b and gluon fragmentation functions in biased jets are
measured in e+e- annihilations from data collected at centre-of-mass energies
of 91.2, and 183-209 GeV with the OPAL detector at LEP. The unbiased jets are
defined by hemispheres of inclusive hadronic events, while the biased jet
measurements are based on three-jet events selected with jet algorithms.
Several methods are employed to extract the fragmentation functions over a wide
range of scales. Possible biases are studied in the results are obtained. The
fragmentation functions are compared to results from lower energy e+e-
experiments and with earlier LEP measurements and are found to be consistent.
Scaling violations are observed and are found to be stronger for the
fragmentation functions of gluon jets than for those of quarks. The measured
fragmentation functions are compared to three recent theoretical
next-to-leading order calculations and to the predictions of three Monte Carlo
event generators. While the Monte Carlo models are in good agreement with the
data, the theoretical predictions fail to describe the full set of results, in
particular the b and gluon jet measurements.Comment: 46 pages, 17 figures, Submitted to Eur. Phys J.
W Boson Polarisation at LEP2
Elements of the spin density matrix for W bosons in e+e- -> W+W- -> qqln
events are measured from data recorded by the OPAL detector at LEP. This
information is used calculate polarised differential cross-sections and to
search for CP-violating effects. Results are presented for W bosons produced in
e+e- collisions with centre-of-mass energies between 183 GeV and 209 GeV. The
average fraction of W bosons that are longitudinally polarised is found to be
(23.9 +- 2.1 +- 1.1)% compared to a Standard Model prediction of (23.9 +-
0.1)%. All results are consistent with CP conservation.Comment: 20 pages, 3 figures, Submitted to Phys. Letts.
Search for the Standard Model Higgs Boson with the OPAL Detector at LEP
This paper summarises the search for the Standard Model Higgs boson in e+e-
collisions at centre-of-mass energies up to 209 GeV performed by the OPAL
Collaboration at LEP. The consistency of the data with the background
hypothesis and various Higgs boson mass hypotheses is examined. No indication
of a signal is found in the data and a lower bound of 112.7GeV/C^2 is obtained
on the mass of the Standard Model Higgs boson at the 95% CL.Comment: 51 pages, 21 figure
Tests of model of color reconnection and a search for glueballs using gluon jets with a rapidity gap
Gluon jets with a mean energy of 22 GeV and purity of 95% are selected from
hadronic Z0 decay events produced in e+e- annihilations. A subsample of these
jets is identified which exhibits a large gap in the rapidity distribution of
particles within the jet. After imposing the requirement of a rapidity gap, the
gluon jet purity is 86%. These jets are observed to demonstrate a high degree
of sensitivity to the presence of color reconnection, i.e. higher order QCD
processes affecting the underlying color structure. We use our data to test
three QCD models which include a simulation of color reconnection: one in the
Ariadne Monte Carlo, one in the Herwig Monte Carlo, and the other by Rathsman
in the Pythia Monte Carlo. We find the Rathsman and Ariadne color reconnection
models can describe our gluon jet measurements only if very large values are
used for the cutoff parameters which serve to terminate the parton showers, and
that the description of inclusive Z0 data is significantly degraded in this
case. We conclude that color reconnection as implemented by these two models is
disfavored. The signal from the Herwig color reconnection model is less clear
and we do not obtain a definite conclusion concerning this model. In a separate
study, we follow recent theoretical suggestions and search for glueball-like
objects in the leading part of the gluon jets. No clear evidence is observed
for these objects.Comment: 42 pages, 18 figure
Gravitational Waves From Known Pulsars: Results From The Initial Detector Era
We present the results of searches for gravitational waves from a large selection of pulsars using data from the most recent science runs (S6, VSR2 and VSR4) of the initial generation of interferometric gravitational wave detectors LIGO (Laser Interferometric Gravitational-wave Observatory) and Virgo. We do not see evidence for gravitational wave emission from any of the targeted sources but produce upper limits on the emission amplitude. We highlight the results from seven young pulsars with large spin-down luminosities. We reach within a factor of five of the canonical spin-down limit for all seven of these, whilst for the Crab and Vela pulsars we further surpass their spin-down limits. We present new or updated limits for 172 other pulsars (including both young and millisecond pulsars). Now that the detectors are undergoing major upgrades, and, for completeness, we bring together all of the most up-to-date results from all pulsars searched for during the operations of the first-generation LIGO, Virgo and GEO600 detectors. This gives a total of 195 pulsars including the most recent results described in this paper.United States National Science FoundationScience and Technology Facilities Council of the United KingdomMax-Planck-SocietyState of Niedersachsen/GermanyAustralian Research CouncilInternational Science Linkages program of the Commonwealth of AustraliaCouncil of Scientific and Industrial Research of IndiaIstituto Nazionale di Fisica Nucleare of ItalySpanish Ministerio de Economia y CompetitividadConselleria d'Economia Hisenda i Innovacio of the Govern de les Illes BalearsNetherlands Organisation for Scientific ResearchPolish Ministry of Science and Higher EducationFOCUS Programme of Foundation for Polish ScienceRoyal SocietyScottish Funding CouncilScottish Universities Physics AllianceNational Aeronautics and Space AdministrationOTKA of HungaryLyon Institute of Origins (LIO)National Research Foundation of KoreaIndustry CanadaProvince of Ontario through the Ministry of Economic Development and InnovationNational Science and Engineering Research Council CanadaCarnegie TrustLeverhulme TrustDavid and Lucile Packard FoundationResearch CorporationAlfred P. Sloan FoundationAstronom
Search for Yukawa Production of a Light Neutral Higgs Boson at LEP
Within a Two-Higgs-Doublet Model (2HDM) a search for a light Higgs boson in
the mass range of 4-12 GeV has been performed in the Yukawa process e+e- -> b
bbar A/h -> b bbar tau+tau-, using the data collected by the OPAL detector at
LEP between 1992 and 1995 in e+e- collisions at about 91 GeV centre-of-mass
energy. A likelihood selection is applied to separate background and signal.
The number of observed events is in good agreement with the expected
background. Within a CP-conserving 2HDM type II model the cross-section for
Yukawa production depends on xiAd = |tan beta| and xihd = |sin alpha/cos beta|
for the production of the CP-odd A and the CP-even h, respectively, where tan
beta is the ratio of the vacuum expectation values of the Higgs doublets and
alpha is the mixing angle between the neutral CP-even Higgs bosons. From our
data 95% C.L. upper limits are derived for xiAd within the range of 8.5 to 13.6
and for xihd between 8.2 to 13.7, depending on the mass of the Higgs boson,
assuming a branching fraction into tau+tau- of 100%. An interpretation of the
limits within a 2HDM type II model with Standard Model particle content is
given. These results impose constraints on several models that have been
proposed to explain the recent BNL measurement of the muon anomalous magnetic
moment.Comment: 24 pages, 9 figures, Submitted to Euro. Phys. J.
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