7,513 research outputs found
Spin density matrix elements in vector meson leptoproduction
It is reported on an analysis of vector meson leptoproduction at small
Bjorken- and large photon virtuality within an approach that bases on
generalized parton distributions (GPDs). This approach leads to results for
cross sections and spin density matrix elements for and
leptoproduction in fair agreement with experiment.Comment: Report at Spin 2004 Symposium, October 10-16, Trieste, Ital
Transversity in exclusive vector-meson leptoproduction
The role of transversity or helicity-flip generalized parton distributions
(GPDs) in leptoproduction of vector mesons is investigated within the framework
of the handbag approach. The transversity GPDs in combination with twist-3
meson wave functions, occur in the amplitudes for transitions from a
transversely polarized virtual photon to a longitudinal polarized vector meson.
The importance of the transversity GPDs can be examined in some of the spin
density matrix elements and in transverse target spin asymmetries. Using
suitable parametrizations of both helicity-flip and non-flip GPDs, which are
essentially taken from our previous papers, we estimate these observables and
compare the results with available data.Comment: 30 pages, 19 figure
The pion pole in hard exclusive vector-meson leptoproduction
Exploiting a set of generalized parton distributions (GPDs) derived from
analyses of hard exclusive leptoproduction of rho0, phi and pi+ mesons, we
investigate the omega spin density matrix elements (SDMEs) recently measured by
the HERMES collaboration. It turns out from our study that the pion pole is an
important contribution to omega production. It will be treated as a
one-particle exchange since its evaluation from the GPD E-tilde considerably
underestimates its contribution. As an intermediate step of our analysis we
extract the pi-omega transition form factor for photon virtualities less than 4
GeV^2. From our approach we achieve results for the omega SDMEs in good
agreement with the HERMES data. The role of the pion pole in exclusive rho0 and
phi leptoproduction is discussed too.Comment: 27 pages, 33 figure
The exclusive limit of the pion-induced Drell-Yan process
Based on previous studies of hard exclusive leptoproduction of pions in which
the essential role of the pion pole and the transversity generalized parton
distributions has been pointed out, we present predictions for the four partial
cross sections of the exclusive Drell-Yan process, pi^-p -> l^-l^+n.Comment: 14 pages, 6 figure
Ising magnets with mobile defects
Motivated by recent experiments on cuprates with low-dimensional magnetic
interactions, a new class of two-dimensional Ising models with short-range
interactions and mobile defects is introduced and studied. The non-magnetic
defects form lines, which, as temperature increases, first meander and then
become unstable. Using Monte Carlo simulations and analytical low- and
high-temperature considerations, the instability of the defect stripes is
monitored for various microscopic and thermodynamic quantities in detail for a
minimal model, assuming some of the couplings to be indefinitely strong. The
robustness of the findings against weakening the interactions is discussed as
well
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Aperture judgement in fire-appliance drivers
Low-speed collisions are a common occurrence for UK Fire and Rescue Services, with frequent bumps and scrapes adding up to create a significant drain on emergency service budgets, and damaging public faith in the service. We hypothesised that the decision to drive through a gap, or aperture, created by other vehicles and/or road furniture, may be related to driver experience and risk-level. To create a test to assess this skill, video clips were recorded from a fire appliance on bluelight training runs. Eighteen clips were selected on the basis that they culminated in a narrow aperture that the film-driver either navigated through, or stopped and waited for other vehicles to move and the gap to widen. Drivers were required to watch these clips, which paused at the point that the film-driver made the decision to either go though, or to wait. The participants rated their belief that the appliance would fit through the gap, and how confident they would be to drive through that aperture themselves (on a 1-8 scale). Novice fire-appliance drivers were less accurate in deciding whether to proceed or not, though their criterion threshold did not differ to that of more experienced drivers. All drivers favoured looking at the right edge of the aperture (either a vehicle or road furniture), but low-risk, experienced fire appliance drivers had the shortest fixations on this area. The results suggest that a video-based test of aperture judgement can differentiate between fire-appliance driver groups based on experience, opening the way for future testing and training tools using this methodology
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