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Abstract:

We have developed a semi-empirical simulation of the extragalactic radio continuum sky suitable for aiding the design of next generation radio interferometers such as the Square Kilometre Array (SKA). The emphasis is on modelling the large-scale cosmological distribution of radio sources rather than the internal details of individual galaxies. Here we provide a description of the simulation to accompany the online release of a catalogue of 320 million simulated radio sources. The simulation covers 20x20 deg^2 - a plausible upper limit to the instantaneous field of view attainable with future (e.g. SKA) aperture array technologies - out to redshift z=20, and down to flux density limits of 10 nJy at 151, 610 MHz, 1.4, 4.86 and 18 GHz. Five distinct source types are included: radio-quiet AGN, radio-loud AGN of the FRI and FRII structural classes, and star-forming galaxies, the latter split into populations of quiescent and starbursting galaxies. In our semi-empirical approach, the simulated sources are drawn from observed (or extrapolated) luminosity functions and grafted onto an underlying dark matter density field with biases which reflect their measured large-scale clustering. A numerical Press-Schechter-style filtering of the density field is used to identify and populate clusters of galaxies. Radio source structures are built from point source and elliptical sub-components, and for FRI and FRII sources an orientation-based unification and beaming model is used to partition flux between the core and extended lobes and hotspots. The simulation output can be post-processed to achieve more complete agreement with observational data in the years ahead, with the aim of using these 'idealised skies' in telescope simulators to optimise the design of the SKA itself (abridged).

Abstract:

We report the first evidence of Z boson pair production at a hadron collider with a significance exceeding 4 standard deviations. This result is based on a data sample corresponding to 1.9fb-1 of integrated luminosity from pp̄ collisions at s=1.96TeV collected with the Collider Detector at Fermilab II detector. In the ′ ′ channel, we observe three ZZ candidates with an expected background of 0.096-0.063+0.092 events. In the νν channel, we use a leading-order calculation of the relative ZZ and WW event probabilities to discriminate between signal and background. In the combination of ′ ′ and νν channels, we observe an excess of events with a probability of 5.1×10-6 to be due to the expected background. This corresponds to a significance of 4.4 standard deviations. The measured cross section is σ(pp̄→ZZ)=1.4-0.6+0.7(stat+syst)pb, consistent with the standard model expectation. © 2008 The American Physical Society.

Abstract:

We present the first measurement of two-particle momentum correlations in jets produced in pp̄ collisions at s=1.96TeV. Results are obtained for charged particles within a restricted cone with an opening angle of 0.5 radians around the jet axis and for events with dijet masses between 66 and 563GeV/c2. A comparison of the experimental data to theoretical predictions obtained for partons within the framework of resummed perturbative QCD in the next-to-leading log approximation shows that the parton momentum correlations survive the hadronization stage of jet fragmentation, giving further support to the hypothesis of local parton-hadron duality. The extracted value of the next-to-leading-log-approximation parton shower cutoff scale Qeff set equal to ΛQCD is found to be (1.4-0.7+0.9)×100MeV. © 2008 The American Physical Society.