38,923 research outputs found
Genisa: A web-based interactive learning environment for teaching simulation modelling
Intelligent Tutoring Systems (ITS) provide students with adaptive instruction and can facilitate the acquisition of problem solving skills in an interactive environment. This paper discusses the role of pedagogical strategies that have been implemented to facilitate the development of simulation modelling knowledge. The learning environment integrates case-based reasoning with interactive tools to guide tutorial remediation. The evaluation of the system shows that the model for pedagogical activities is a useful method for providing efficient simulation modelling instruction
3D printing of gas jet nozzles for laser-plasma accelerators
Recent results on laser wakefield acceleration in tailored plasma channels
have underlined the importance of controlling the density profile of the gas
target. In particular it was reported that appropriate density tailoring can
result in improved injection, acceleration and collimation of laser-accelerated
electron beams. To achieve such profiles innovative target designs are
required. For this purpose we have reviewed the usage of additive layer
manufacturing, commonly known as 3D printing, in order to produce gas jet
nozzles. Notably we have compared the performance of two industry standard
techniques, namely selective laser sintering (SLS) and stereolithography (SLA).
Furthermore we have used the common fused deposition modeling (FDM) to
reproduce basic gas jet designs and used SLA and SLS for more sophisticated
nozzle designs. The nozzles are characterized interferometrically and used for
electron acceleration experiments with the Salle Jaune terawatt laser at
Laboratoire d'Optique Appliqu\'ee
Energy boost in laser wakefield accelerators using sharp density transitions
The energy gain in laser wakefield accelerators is limited by dephasing
between the driving laser pulse and the highly relativistic electrons in its
wake. Since this phase depends on both the driver and the cavity length, the
effects of dephasing can be mitigated with appropriate tailoring of the plasma
density along propagation. Preceding studies have discussed the prospects of
continuous phase-locking in the linear wakefield regime. However, most
experiments are performed in the highly non-linear regime and rely on
self-guiding of the laser pulse. Due to the complexity of the driver evolution
in this regime it is much more difficult to achieve phase locking. As an
alternative we study the scenario of rapid rephasing in sharp density
transitions, as was recently demonstrated experimentally. Starting from a
phenomenological model we deduce expressions for the electron energy gain in
such density profiles. The results are in accordance with particle-in-cell
simulations and we present gain estimations for single and multiple stages of
rephasing
Relativistic corrections to exclusive and inclusive double charm production at B factories
In order to clarify the puzzling problems in double charm production,
relativistic corrections at order to the processes and at B factories are studied
in non-relativistic quantum chromodynamics. The short-distance parts of
production cross sections are calculated perturbatively, while the
long-distance matrix elements are estimated from and decays
up to errors of order . Our results show that the relativistic correction
to the exclusive process is significant, which,
when combined together with the next-to-leading order corrections,
could resolve the large discrepancy between theory and experiment; whereas for
the inclusive process the relativistic
correction is tiny and negligible. The physical reason for the above difference
between exclusive and inclusive processes largely lies in the fact that in the
exclusive process the relative momentum between quarks in charmonium
substantially reduces the virtuality of the gluon that converts into a charm
quark pair, but this is not the case for the inclusive process, in which the
charm quark fragmentation is significant, and QCD radiative
corrections can be more essential.Comment: Version to appear in PRD. In the summary an explicit statement added:
"for the J/\psi eta_c cross section the relativistic correction alone gives
an enhancement factor of 1.7 while the combination of relativistic correction
with QCD radiative correction results in a much larger enhancement factor of
9". One reference added. A few typos correcte
Search for excited charmonium states in Annihilation at GeV
We suggest searching for excited charmonium states in annihilation
via double charmonium production at GeV with factories,
based on a more complete leading order calculation including both QCD and QED
contributions for various processes. In particular, for the C=+ states, the
(n=2,3) and (m=3,4) may have appreciable
potentials to be observed; while for the C=- states, the production
and especially the production might provide opportunities for
observing the with higher statistics in the future. A brief discussion
for the X(3940) observed in the double charmonium production is included.Comment: 13 pages and 8 figures in PRD version; QED contribution added;
experimental and theoretical developments since 2004 summarized; references
adde
Response to tilted magnetic fields in Bi2Sr2CaCu2O8 with columnar defects: Evidence for transverse Meissner effect
The transverse Meissner effect (TME) in the highly layered superconductor
Bi2Sr2CaCu2O(8+y) with columnar defects is investigated by transport
measurements. We present detailed evidence for the persistence of the
Bose-glass phase when H is tilted at an angle theta < theta_c (T) away from the
column direction: (i) the variable-range vortex hopping process for low
currents crosses over to the half-loops regime for high currents; (ii) in both
regimes near theta_c(T) the energy barriers vanish linearly with tan(theta) ;
(iii) the transition temperature is governed by T_{BG}(0) -T_{BG}(theta) sim
|tan(theta)|^{1/\nu_{\perp}} with \nu_{\perp}=1.0 +/- 0.1. Furthermore, above
the transition as theta->\theta_c+, moving kink chains consistent with a
commensurate-incommensurate transition scenario are observed. These results
thereby clearly show the existence of the TME for theta < theta_c(T).Comment: 4 pages, RevTeX, 5 EPS figure
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