Automated optical identification of a large complete northern hemisphere sample of flat spectrum radio sources with S_6cm > 200 mJy

This paper describes the automated optical APM identification of radio sources from the Jodrell Bank - VLA Astrometric Survey (JVAS), as used for the search for distant radio-loud quasars. The sample has been used to investigate possible relations between optical and radio properties of flat spectrum radio sources. From the 915 sources in the sample, 756 have an optical APM identification at a red (e) and/or blue (o) plate,resulting in an identification fraction of 83% with a completeness and reliability of 98% and 99% respectively. About 20% are optically identified with extended APM objects on the red plates, e.g. galaxies. However the distinction between galaxies and quasars can not be done properly near the magnitude limit of the POSS-I plates. The identification fraction appears to decrease from >90% for sources with a 5 GHz flux density of >1 Jy, to <80% for sources at 0.2 Jy. The identification fraction, in particular that for unresolved quasars, is found to be lower for sources with steeper radio spectra. In agreement with previous studies, we find that the quasars at low radio flux density levels also tend to have fainter optical magnitudes, although there is a large spread. In addition, objects with a steep radio-to-optical spectral index are found to be mainly highly polarised quasars, supporting the idea that in these objects the polarised synchrotron component is more prominent. It is shown that the large spread in radio-to-optical spectral index is possibly caused by source to source variations in the Doppler boosting of the synchrotron component [Abridged].Comment: LaTex, 17 pages, 5 gif figures, 4 tables. Accepted for publication in MNRAS. High resolution figures can be found at http://www.roe.ac.uk/~ignas

Learning How to Count: A High Multiplicity Search for the LHC

We introduce a search technique that is sensitive to a broad class of signals with large final state multiplicities. Events are clustered into large radius jets and jet substructure techniques are used to count the number of subjets within each jet. The search consists of a cut on the total number of subjets in the event as well as the summed jet mass and missing energy. Two different techniques for counting subjets are described and expected sensitivities are presented for eight benchmark signals. These signals exhibit diverse phenomenology, including 2-step cascade decays, direct three body decays, and multi-top final states. We find improved sensitivity to these signals as compared to previous high multiplicity searches as well as a reduced reliance on missing energy requirements. One benefit of this approach is that it allows for natural data driven estimates of the QCD background.Comment: 36 pages, 12 Figures, 5 Tables; journal versio