Characterizing flows with an instrumented particle measuring Lagrangian accelerations

We present in this article a novel Lagrangian measurement technique: an instrumented particle which continuously transmits the force/acceleration acting on it as it is advected in a flow. We develop signal processing methods to extract information on the flow from the acceleration signal transmitted by the particle. Notably, we are able to characterize the force acting on the particle and to identify the presence of a permanent large-scale vortex structure. Our technique provides a fast, robust and efficient tool to characterize flows, and it is particularly suited to obtain Lagrangian statistics along long trajectories or in cases where optical measurement techniques are not or hardly applicable.Comment: submitted to New Journal of Physic

Weak-strong beam-beam simulations for the Large Hadron Collider

A weak-strong simulation code is used to study the single-particle stability in the presence of triplet field errors, head-on collisions, and long-range beam-beam interactions at the Large Hadron Collider. We present the dependence of the simulated transverse diffusion rate on various parameters, such as starting amplitude, working point in tune diagram, crossing angle, beta function at the interaction points (IPs), beam current, triplet nonlinearities, tune modulation, and a transverse offset at one of two IPs. For several examples, we perform a frequency map analysis a la Laskar, to obtain tune footprints and the tune variation in time. A cursory look at the effect of a Mobius lattice is also reported. (17 refs)

Diffusive Aperture Due to Long-Range Beam-Beam Interaction

Weak-strong tracking simulations for the Large Hadron Collider (LHC) show that long-range beam-beam collisions give rise to a well-defined diffusive aperture beyond which particles are lost quickly. We calculate the tune shift with amplitude and single-resonance driving terms for beam-beam collisions with an arbitrary transverse off-set, and then apply Chirikov resonance overlap criterion to construct an analytical estimate of the diffusive aperture induced by long-range collisions. The analytical results are compared with tracking data for the LHC

Publication Bias in Asset Pricing Research

Researchers are more likely to share notable findings. As a result, published findings tend to overstate the magnitude of real-world phenomena. This bias is a natural concern for asset pricing research, which has found hundreds of return predictors and little consensus on their origins. Empirical evidence on publication bias comes from large scale meta-studies. Meta-studies of cross-sectional return predictability have settled on four stylized facts that demonstrate publication bias is not a dominant factor: (1) almost all findings can be replicated, (2) predictability persists out-of-sample, (3) empirical $t$-statistics are much larger than 2.0, and (4) predictors are weakly correlated. Each of these facts has been demonstrated in at least three meta-studies. Empirical Bayes statistics turn these facts into publication bias corrections. Estimates from three meta-studies find that the average correction (shrinkage) accounts for only 10 to 15 percent of in-sample mean returns and that the risk of inference going in the wrong direction (the false discovery rate) is less than 6%. Meta-studies also find that t-statistic hurdles exceed 3.0 in multiple testing algorithms and that returns are 30 to 50 percent weaker in alternative portfolio tests. These facts are easily misinterpreted as evidence of publication bias effects. We clarify these misinterpretations and others, including the conflating of ``mostly false findings'' with ``many insignificant findings.'' Cross-sectional predictability may not be representative of other fields. Meta-studies of real-time equity premium prediction suggest a much larger effect of publication bias, though publication bias in this and other areas of asset pricing is an important area for future research

Tune Shift Due to Crossing Collision and CRAB Collision

The use of crab cavities in the LHC may not only raise the luminosity, but it could also complicate the beam dynamics, e.g. crab cavities might not only cancel synchrobetatron resonances excited by the crossing angle but they could also excite new ones. In this paper, we use weakstrong beam-beam model to study the incoherent linear tune shift of the weak beam, for the crossing collision case and crab collision case with a nite crossing angle. The tune shift is also compared among the head-on collision, crossing collision and crab collision cases, both analytically and numerically

Effects of sextupole time dependence on the LHC dynamic aperture

A primary concern regarding the LHC dynamic aperture is the time dependence of persistent-current sextupole fields in the superconducting magnets. Decaying slowly during injection, these fields are reinduced rapidly at the start of the acceleration ("snap-back"). If uncompensated, this snap-back would cause a chromaticity change by some 130 units. We investigate how this time dependence and the ramp rate affect the stability of particle motion and we evaluate the efficiency of different correction schemes

CRAB-CAVITY BEAM DYNAMICS ISSUES FOR AN LHC UPGRADE

Modern colliders bring into collision a large number of bunches per pulse or per turn to achieve a high luminosity. The long-range beam-beam effects arising from parasitic encounters at such colliders are mitigated by introducing a crossing angle. Under these conditions, crab cavities can be used to restore effective head-on collisions and thereby to increase the geometric luminosity. The crab cavities have been proposed for both linear and circular colliders. The crab cavities are rf cavities operated in a transverse dipole mode, which imparts on the beam particles a transverse kick that varies with the longitudinal position along the bunch. The use of crab cavities in the LHC may not only raise the luminosity, but it could also complicate the beam dynamics, e.g. crab cavities might not only cancel synchro-betatron resonances excited by the crossing angle but they could also excite new ones, they could reduce the dynamic aperture for off-momentum particles, they could degrade the collimation cleaning efficiency, and so on. In this note, we summarize the results of several preliminary studies of beam-dynamics challenges associated with crab cavities in the LHC

Design Considerations for the CLIC pre-damping rings

The CLIC pre-damping rings (PDR) have to accommodate a large emittance beam, coming in particular from the positron target and reduce its size to low enough values for injection into the main damping rings (DR). The lattice design is based on theoretical minimum emittance (TME) arc cells and long straight sections filled with damping wigglers. Lattice design optimisation considerations are presented with emphasis to the linear optics functions, tunability, chromatic properties and acceptance. A complete phase advance scan of the TME cells is undertaken for reducing the non-linear resonance driving terms and amplitude dependent tunespread and maximizing the ring’s dynamic aperture (DA)

Provenance for SPARQL queries

Determining trust of data available in the Semantic Web is fundamental for applications and users, in particular for linked open data obtained from SPARQL endpoints. There exist several proposals in the literature to annotate SPARQL query results with values from abstract models, adapting the seminal works on provenance for annotated relational databases. We provide an approach capable of providing provenance information for a large and significant fragment of SPARQL 1.1, including for the first time the major non-monotonic constructs under multiset semantics. The approach is based on the translation of SPARQL into relational queries over annotated relations with values of the most general m-semiring, and in this way also refuting a claim in the literature that the OPTIONAL construct of SPARQL cannot be captured appropriately with the known abstract models.Comment: 22 pages, extended version of the ISWC 2012 paper including proof