26,004 research outputs found
Determining the inelastic proton-proton cross section at the Large Hadron Collider using minimum bias events
Described in this paper is a new method for determining the non-diffractive
part of the inelastic proton-proton cross section, at the LHC centre of mass
energy of 14TeV. The method is based on counting the number of inelastic
proton-proton interactions in the collision regions. According to preliminary
investigation, this measurement will be best suited for the initial low
luminosity phase of the LHC. The dominant uncertainty on this measurement comes
from knowledge of the proton-proton luminosity
Seeing the trees as well as the forest: the importance of managing forest genetic resources
Reliable data on the status and trends of forest genetic resources are essential for their sustainable management. The reviews presented in this special edition of Forest Ecology and Management on forest genetic resources complement the first ever synthesis of the State of the World’s Forest Genetic Resources (SOW-FGR) that has just been published by the Food and Agriculture Organization. In this editorial, we present some of the key findings of the SOW-FGR and introduce the seven reviews presented in this special edition on: (1) tree genetic resources and livelihoods; (2) the benefits and dangers of international germplasm transfers; (3) genetic indicators for monitoring threats to populations and the effectiveness of ameliorative actions; (4) the genetic impacts of timber management practices; (5) genetic considerations in forest ecosystem restoration projects using native trees; (6) genetic-level responses to climate change; and (7) ex situ conservation approaches and their integration with in situ methods. Recommendations for action arising from the SOW-FGR, which are captured in the first Global Plan of Action for the Conservation, Sustainable Use and Development of Forest Genetic Resources, and the above articles are discussed. These include: increasing the awareness of the importance of and threats to forest genetic resources and the mainstreaming of genetic considerations into forest management and restoration; establishing common garden provenance trials to support restoration and climate change initiatives that extend to currently little-researched tree species; streamlining processes for germplasm exchange internationally for research and development; and the intelligent use of modern molecular marker methods as genetic indicators in management and for improvement purposes
Random copolymer: Gaussian variational approach
We study the phase transitions of a random copolymer chain with quenched
disorder. We apply a replica variational approach based on a Gaussian trial
Hamiltonian in terms of the correlation functions of monomer Fourier
coordinates. This allows us to study collapse, phase separation and freezing
transitions within the same mean field theory. The effective free energy of the
system is derived analytically and analysed numerically. Such quantities as the
radius of gyration or the average value of the overlap between different
replicas are treated as observables and evaluated by introducing appropriate
external fields to the Hamiltonian. We obtain the phase diagram and show that
this system exhibits a scale dependent freezing transition. The correlations
between replicas appear at different length scales as the temperature
decreases. This indicates the existence of the topological frustration.Comment: 15 pages, 4 Postscript figure
Exact Solution of a Jamming Transition: Closed Equations for a Bootstrap Percolation Problem
Jamming, or dynamical arrest, is a transition at which many particles stop
moving in a collective manner. In nature it is brought about by, for example,
increasing the packing density, changing the interactions between particles, or
otherwise restricting the local motion of the elements of the system. The onset
of collectivity occurs because, when one particle is blocked, it may lead to
the blocking of a neighbor. That particle may then block one of its neighbors,
these effects propagating across some typical domain of size named the
dynamical correlation length. When this length diverges, the system becomes
immobile. Even where it is finite but large the dynamics is dramatically
slowed. Such phenomena lead to glasses, gels, and other very long-lived
nonequilibrium solids. The bootstrap percolation models are the simplest
examples describing these spatio-temporal correlations. We have been able to
solve one such model in two dimensions exactly, exhibiting the precise
evolution of the jamming correlations on approach to arrest. We believe that
the nature of these correlations and the method we devise to solve the problem
are quite general. Both should be of considerable help in further developing
this field.Comment: 17 pages, 4 figure
Geometry of Empty Space is the Key to Near-Arrest Dynamics
We study several examples of kinetically constrained lattice models using
dynamically accessible volume as an order parameter. Thereby we identify two
distinct regimes exhibiting dynamical slowing, with a sharp threshold between
them. These regimes are identified both by a new response function in
dynamically available volume, as well as directly in the dynamics. Results for
the selfdiffusion constant in terms of the connected hole density are
presented, and some evidence is given for scaling in the limit of dynamical
arrest.Comment: 11 page
Mode-Coupling Theory of Colloids with Short-range Attractions
Within the framework of the mode-coupling theory of super-cooled liquids, we
investigate new phenomena in colloidal systems on approach to their glass
transitions. When the inter-particle potential contains an attractive part,
besides the usual repulsive hard core, two intersecting liquid-glass transition
lines appear, one of which extends to low densities, while the other one, at
high densities, shows a re-entrant behaviour. In the glassy region a new type
of transition appears between two different types of glasses. The complex
phenomenology can be described in terms of higher order glass transition
singularities. The various glass phases are characterised by means of their
viscoelastic properties. The glass driven by attractions has been associated to
particle gels, and the other glass is the well known repulsive colloidal glass.
These correspondences, in associations with the new predictions of glassy
behaviour mean that such phenomena may be expected in colloidal systems with,
for example, strong depletion or other short-ranged attractive potentials.Comment: 17 pages, 8 figure
Dead Time Compensation for High-Flux Ranging
Dead time effects have been considered a major limitation for fast data
acquisition in various time-correlated single photon counting applications,
since a commonly adopted approach for dead time mitigation is to operate in the
low-flux regime where dead time effects can be ignored. Through the application
of lidar ranging, this work explores the empirical distribution of detection
times in the presence of dead time and demonstrates that an accurate
statistical model can result in reduced ranging error with shorter data
acquisition time when operating in the high-flux regime. Specifically, we show
that the empirical distribution of detection times converges to the stationary
distribution of a Markov chain. Depth estimation can then be performed by
passing the empirical distribution through a filter matched to the stationary
distribution. Moreover, based on the Markov chain model, we formulate the
recovery of arrival distribution from detection distribution as a nonlinear
inverse problem and solve it via provably convergent mathematical optimization.
By comparing per-detection Fisher information for depth estimation from high-
and low-flux detection time distributions, we provide an analytical basis for
possible improvement of ranging performance resulting from the presence of dead
time. Finally, we demonstrate the effectiveness of our formulation and
algorithm via simulations of lidar ranging.Comment: Revision with added estimation results, references, and figures, and
modified appendice
Reliability considerations in the design, assembly, and testing of the mariner iv power system
Reliability considerations in design, assembly, and testing of Mariner IV power syste
Resummation Effects in Vector-Boson and Higgs Associated Production
Fixed-order QCD radiative corrections to the vector-boson and Higgs
associated production channels, pp -> VH (V=W, Z), at hadron colliders are well
understood. We combine higher order perturbative QCD calculations with
soft-gluon resummation of both threshold logarithms and logarithms which are
important at low transverse momentum of the VH pair. We study the effects of
both types of logarithms on the scale dependence of the total cross section and
on various kinematic distributions. The next-to-next-to-next-to-leading
logarithmic (NNNLL) resummed total cross sections at the LHC are almost
identical to the fixed-order perturbative next-to-next-to-leading order (NNLO)
rates, indicating the excellent convergence of the perturbative QCD series.
Resummation of the VH transverse momentum (p_T) spectrum provides reliable
results for small values of p_T and suggests that implementing a jet-veto will
significantly decrease the cross sections.Comment: 25 pages, references update
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