The ATLAS detector
The Large Hadron Collider (LHC) at CERN will extend the frontiers of particle physics with the unprecedented high energy that will be achived. The LHC is designed to collide bunches of up to 1011 protons at a rate of 40 million times per second to provide 14 TeV proton-proton collisions. The LHC is also designed to collide heavy ions (A), in particular Lead nuclei, at 5.5 TeV per nucleon pair.
The ATLAS (A Toroidal LHC ApparatuS) Detector is a general purpose detector which has been built for probing p-p and A-A collisions at LHC.
The ATLAS experiment will explore the fundamental nature of matter and the forces that shape our Universe. Looking for answers to unknowns like the origin of mass, the existence of extra dimensions of space or microscopic black holes, as well as searching for evidence of dark matter.
Due to the experimental conditions at the LHC, the ATLAS detector was equiped with fast, radiation-hard electronics and sensor elements. In addition, a high detector granularity was nedded to handle the particle fluxes and to reduce the influence of overlapping events. The overall ATLAS detector layout is shown in the Figure on the right.
The ATLAS detector is nominally symmetric with respect to the interaction point. The magnet configuration comprises a thin superconducting solenoid surronding the inner detector cavity, and three large superconducting toroids (one barrel and two end-caps) arranged with an eight-fold azimuthal symmetry around the calorimeters. This fundamental choice has driven the design of the rest of the detector.
The inner detector is immersed in a 2 T solenoidal field. Pattern recognition, momentum and vertex measurements, and electron identification are achieved with a combination of discrete, high-resolution semiconductor pixel and strip detectors in the inner part of the tracking volume, and straw-tube tracking detectors with the capability to generate and detect transition radiation in its outher part.
ATLAS uses high granularity liquid-argon (LAr) electromagnetic sampling calorimeters, with excellent performance in terms of energy and position resolution. The hadronic calorimetry is provided by a scintillator-tile calorimeter, which is separated into a large barrel and two smaller extended barrel cylinders, one on either side of the central barrel. In the end-caps, LAr technology is also used for the hadronic calorimeters, matching the outer limits of end-cap electromagnetic calorimeters. The LAr forward calorimeters provide both electromagnetic and hadronic energy measurements.
The calorimeter is surrounded by the muon spectrometer. The toroid system generates strong bending power in a large volume within a light and open structure. Multiple-scaterring effects are thereby minimased, and excellent muon momentum resolution is achieved with three layers of high precision tracking chambers. The muon instrumentation includes, as a key component, trigger chambers with timming resolution of order of 1.5 - 4 ns. The muon spectrometer defines the overall dimensions of the ATLAS detector.