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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The silicon pixel tracking system for the ATLAS experiment at the Large Hadron Collider is described and the performance requirements are summarized. Detailed descriptions of the pixel detector electronics and the silicon sensors are given. The design, fabrication, assembly and performance of the pixel detector modules are presented. Data obtained from test beams as well as studies using cosmic rays are also discussed.

This paper introduces an experimental probe of the sterile neutrino with a novel, high-intensity source of electron antineutrinos from the production and subsequent decay of 8 Li. When paired with an existing ∼1 kton scintillator-based detector, this Eν =6.4 MeV source opens a wide range of possible searches for beyond standard model physics via studies of the inverse beta decay interaction νe + p → e + + n. In particular, the experimental design described here has unprecedented sensitivity toνe disappearance at ∆m 2 ∼ 1 eV 2 and features the ability to distinguish between the existence of zero, one, and two sterile neutrinos.PACS numbers: 14.60.Pq, 14.60.StThe beta decay-at-rest of 8 Li produces an isotropic electron antineutrino flux with an average energy of 6.4 MeV. An underground liquid scintillator based detector can be used to detect these antineutrinos via the inverse beta decay (IBD) processν e + p → e + + n. The antineutrino rate and energy, peaking at 9 MeV, can be fully reconstructed by the detector. Precise energy and vertex reconstruction opens the possibility of searching for antineutrino disappearance due to oscillations, which, in the simplest two-neutrino form, has the probabilitywhere θ is the disappearance mixing angle; ∆m 2 (eV 2 ) is the squared mass splitting; L is the distance (in meters) from the antineutrino source to the detector; and E (MeV) is the antineutrino energy. This probability is maximized in the range of ∆m 2 ∼ E/L. An existing large scintillator-based antineutrino detector with a diameter of O(10 m), when combined with an 8 Li isotope decay-at-rest source, is sensitive to oscillations at ∆m 2 ∼ 1 eV 2 . This is an oscillation region of high interest due to anomalies that have been observed in the data from LSND [1], MiniBooNE [2], short-baseline reactor studies [3], and gallium source calibration runs [4]. These anomalies are often interpreted as being due to sterile neutrinos [5][6][7][8] and have motivated the development of the IsoDAR (Isotope Decay-At-Rest) concept.IsoDAR-style sources have been considered before [9][10][11]. The design presented here, consisting of an ion source, cyclotron, and target, is the first with a sufficiently high antineutrino flux to address the existence of one or more sterile neutrinos. 5 IBD interactions in a five year run. Such events allow a definitive search for antineutrino oscillations with the added ability to distinguish between models with one and two sterile neutrinos. A sample of more than 7200ν e -electron scatters is also accumulated during this time and can be used as a sensitive electroweak probe.The charged particle beam, used for electron antineutrino production, originates with a 60 MeV/amu cyclotron accelerating 5 mA of H + 2 ions. The design of this compact cyclotron [15] is ongoing and is envisaged as the injector for the accelerator system of the DAEδALUS physics program [16,17]. The IsoDAR design calls for about a factor of six increase in intensity compared to compact cyclotrons used in the medical isotope industry. ...

This paper describes the silicon microstrip modules in the barrel section of the SemiConductor Tracker (SCT) of the ATLAS experiment at the CERN Large Hadron Collider (LHC). The module requirements, components and assembly techniques are given, as well as first results of the module performance on the fully-assembled barrels that make up the detector being installed in the ATLAS experiment.