The ATLAS Experiment

at the Physikalisches Institut Heidelberg

ATLAS is one out of four large experiments at the Large-Hadron-Collider (LHC) at CERN in Geneva. It is dedicated to a comprehensive study of very high-energy proton-proton collisions at the terascale. Prime goal is the search for new phenomena beyond the horizons of the known standard model of particle physics. Such phenomena will possibly be crucial for the understanding of the development of the early universe at times as small as 10^-12 seconds after the Big Bang.

Our group at the Physikalisches Institut is one of three ATLAS groups in Heidelberg. The group at the Kirchhoff Insitute has been one of the ATLAS founding members in 1992 and has built together with other internation groups the ATLAS level-1 calorimeter trigger. The Lehrstuhl für Informatik has made several contributions to the ATLAS trigger.

The Physikalisches Institut contributes to the commissioning and operation of the Detector and engages in Super-LHC upgrade projects related to tracking. A special focus is in designing and constructing a level-1 track trigger.

In March 2010 first proton-proton collisions at a center of mass energy were establiched. The analysis of the data keeps scientists busy all over in the world. The physics topics addressed by our group is the search for exotics (e.g. search for fourth generation quarks), Standard Model physics (Z production) and tests of QCD.

Physics Analysis

Search for Heavy Quarks in Same-Sign Dilepton Events

We are searching for pair-production of heavy quarks in events with four W bosons: .

Our search channel signature is two leptons (e or μ) in the event with equal electric charge and high momentum, and several jets. While the standard-model background for like-sign dileptons is small, jets can be reconstructed as leptons. To suppress this fake lepton background we are studying the application of a top finder.

Heavy quarks are predicted by Little Higgs models and/or could appear as part of a heavy fourth generation of fermions.

Super-LHC

Data taking at Atlas has just begun, however, the next big step is already planned. At the end of this decade a major upgrade to the LHC, the so called Super-LHC, is foreseen. The upgrade will tenfold the luminosity to allow precision measurements of particles maybe discovered at the LHC and extend the sensitivity to see even the rarest phenomena. The luminosity gives, together with the cross section, the number of events that are expected per unit of time. A higher luminosity leads to more events per bunch crossing, up to 400, that are on top of each other and have to be disentangled. The system that decides if the result of a bunch crossing is stored for later analysis is called the trigger. To take the decision on a firm basis it is planned to use information from the tracker system on an early (level-1) trigger level. The tracker system records which path a particle took through the inner part of the detector from which properties like the momentum and charge can be deduced. Because the early trigger level has to give its decision on a very short time scale (order micro seconds) specialized hardware has to be used. Our group works on the design and implementation of a level-1 track trigger which involves simulation of events, software engineering and hardware concepts.

The ATLAS project has been a strong driver for the foundation of the Heidelberg ASIC laboratory for microelectronics. At the department level the group contributes to the Graduate School "Fundamental Physics" and to the International DFG Graduate School "Intelligent Detectors". On the national scale the Heidelberg group is a member of the BMBF Research Unit FSP101 "ATLAS" and of the Helmholtz-Alliance "Physics at the Terascale".

Acknowledgements