By using the ATLAS detector, observations have been made of a centrality-dependent dijet asymmetry in the collisions of lead ions at the Large Hadron Collider. In a sample of lead-lead events with a per-nucleon center of mass energy of 2.76 TeV, selected with a minimum bias trigger, jets are reconstructed in fine-grained, longitudinally segmented electromagnetic and hadronic calorimeters. The transverse energies of dijets in opposite hemispheres are observed to become systematically more unbalanced with increasing event centrality leading to a large number of events which contain highly asymmetric dijets. This is the first observation of an enhancement of events with such large dijet asymmetries, not observed in proton-proton collisions, which may point to an interpretation in terms of strong jet energy loss in a hot, dense medium.

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Proton-proton collisions at √ s = 7 TeV and heavy ion collisions at √ s NN = 2.76 TeV were produced by the LHC and recorded using the ATLAS experiment's trigger system in 2010. The LHC is designed with a maximum bunch crossing rate of 40 MHz and the ATLAS trigger system is designed to record approximately 200 of these per second. The trigger system selects events by rapidly identifying signatures of muon, electron, photon, tau lepton, jet, and B meson candidates, as well as using global event signatures, such as missing transverse energy. An overview of the ATLAS trigger system, the evolution of the system during 2010 and the performance of the trigger system components and selections based on the 2010 collision data are shown. A brief outline of plans for the trigger system in 2011 is presented.

Galaxies congregate in clusters and along filaments, and are missing from large regions referred to as voids. These structures are seen in maps derived from spectroscopic surveys 1,2 that reveal networks of structure that are interconnected with no clear boundaries. Extended regions with a high concentration of galaxies are called 'superclusters', although this term is not precise. There is, however, another way to analyse the structure. If the distance to each galaxy from Earth is directly measured, then the peculiar velocity can be derived from the subtraction of the mean cosmic expansion, the product of distance times the Hubble constant, from observed velocity. The peculiar velocity is the line-of-sight departure from the cosmic expansion and arises from gravitational perturbations; a map of peculiar velocities can be translated into a map of the distribution of matter 3 . Here we report a map of structure made using a catalogue of peculiar velocities. We find locations where peculiar velocity flows diverge, as water does at watershed divides, and we trace the surface of divergent points that surrounds us. Within the volume enclosed by this surface, the motions of galaxies are inward after removal of the mean cosmic expansion and long range flows. We define a supercluster to be the volume within such a surface, and so we are defining the extent of our home supercluster, which we call Laniakea.A key component of this paper is an accompanying movie that can be viewed (also in 3D) and downloaded at http://irfu.cea.fr/laniakea or http://vimeo.com/pomarede/laniakeaThe distribution of matter can be alternatively and independently determined from surveys of the distribution of galaxies in projection and redshift or from the motions of galaxies. With the former, using galaxy redshift surveys, the assumption is required that the galaxy lighthouses and mass distribution are strongly correlated, a condition that requires confirmation if, as is suspected, only a minor fraction of matter is baryonic. Moreover with the former there is a stringent demand that the survey be complete, or at least that its incompleteness be well understood. With the latter, studies of galaxy motions, sparse-sampling is acceptable (indeed inevitable) but dealing with errors is a challenge. Except for the very closest galaxies, uncertainties in distance measurements translate into uncertainties in the peculiar velocities of galaxies that are larger in amplitude than the actual peculiar velocities. Many measures are required for suitable averaging and care must be taken to avoid systematics. Overall, the two paths to define the distribution of matter are in good agreement, a circumstance that represents a considerable success for the standard model of structure formation via gravitational instability 4,5,6,7 .The path from velocities to mass distributions benefits from the coherence in velocities on large scales. Multipole components in the velocity field can point to tidal influences beyond the survey region. The current all-sky redshift surve...