ATLAS Images: ATLAS Experiment © 2014 CERN
ATLAS is one of the two multi-purpose detectors on the Large Hadron Collider (LHC) at CERN. The LHC is a circular accelerator, 27 km long, located about 100 meters underground across the French-Swiss border. It will collide protons onto protons at unprecedented high energies, recreating conditions thought to have existed in our Universe shortly after the Big Bang.
The gigantic ATLAS experiment, the size of a five-storey building, will allow scientists from all over the world to probe deeper into the heart of matter and further back in time than it has ever been possible before. By pushing the energy frontier into unexplored territory, ATLAS hopes to shed light on fundamental questions such as the origin of mass, the nature of Dark Matter in our Universe, or even to explore the existence of yet undiscovered additional dimensions of space.
ATLAS at Sussex
Members of the Sussex ATLAS team are making major contributions to the ATLAS physics programme, searching for Supersymmetry and other manifestations of "new physics" beyond the so-called Standard Model of particle physics, which is known to break down at LHC energies. They are also making significant contributions to the ATLAS trigger system, the experiment's "brain" that must decide almost instantaneously which proton-proton collisions to keep for further analysis, and which ones to reject.
The Sussex ATLAS group have established themselves in prominent physics analysis roles within the Collaboration, leading two strands of physics analysis at ATLAS – the multilepton SUSY effort (with Antonella De Santo, group Leader, Tina Potter and PhD students Zara Grout, Stewart Martin-Haugh and Yusufu Shehu) and the SUSY searches in tau channels (Fabrizio Salvatore and PhD students Anthony Rose and Nicky Santoyo-Castillo). On the technical front, Sussex ATLAS members also lead in their own area of expertise, with in particular Fabrizio Salvatore, Valeria Bartsch and Mark Sutton making key contributions to the core software for the ATLAS High-Level Trigger (HLT) system and to trigger operations. The group has also joined the ATLAS upgrades programme, at present through participation in the L1Track project (Mark Sutton).
Supersymmetry (or SUSY) is one of the favoured models of "new physics" that could be realized at LHC energies, where the Standard Model of particle physics is known to break down. In SUSY theories, for each Standard Model particle there exist a new supersymmetric particle (or "sparticle") with a spin that differs by half-unit from that of its Standard Model partner. In many SUSY models, the lightest supersymmetric particle, or LSP, is the lightest neutralino. This is a massive, stable, electrically neutral and weakly interacting particle which, if it existed, would represent an excellent candidate constituent of the elusive Dark Matter in our Universe, therefore providing a clear link between Particle Physics and Cosmology. The ATLAS group at Sussex has a leading role in the search for supersymmetry in leptonic final states, including electrons, muons and tau leptons.
ATLAS Publications with leading Sussex involvement
The ATLAS Trigger
The ATLAS trigger is the experiment's "brain", which must make split-second decision as to which proton-proton interactions to save onto permanent storage, and which ones to reject because non-interesting. The ATLAS trigger consists of three stages: "Level-1" (L1), which is hardware-based, and "Level-2" (L2) and "Event Filter" (EF) (collectively known as the "High-Level Trigger", or HLT), which are software-based. The LHC 25 ns bunch crossing determines the L1 input rate of 40 MHz. With an average event size of 1-2 MBytes, this must be reduced to a more manageable overall output rate of 100-200 Hz of interesting events to be saved to permanent storage. The HLT uses a "seeded" and "stepwise" strategy to accomplish early event rejection of uninteresting events. To achieve the necessary event suppression within the required bandwidth, the HLT implements the concept of "Regions of Interest" (RoIs). These are constructed at L1 based on coarse information from the calorimeters and the muon system. Based on specific "trigger hypotheses" (e.g. electromagnetic or hadronic activity, etc), only a small fraction (< 10%) of the detector is analyzed and possibly passed to L2 and EF for further analysis. The Sussex ATLAS group is involved in core software development and validation for the ATLAS trigger and in the coordination of the Event Data Model activities for the trigger.
The ATLAS Upgrade
The Sussex ATLAS group is involved in preparations for a possible luminosity upgrade of the LHC, projected to deliver peak luminosities of up to 1035cm-2s-1. Besides investigating the physics potential of a luminosity upgrade for SUSY, members of the group are also involved in the so-called "Level-1 track-trigger" project, exploring the potential trigger improvements that can be achieved at high luminosity when also tracking information is used at Level-1.