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The ATLAS detector 

The Large Hadron Collider (LHC) located at CERN (Conseil Européen pour la Recherche Nucléaire) is the most powerful particle accelerator built to date. This hadron machine is extending the frontiersof the particle physics due to its unprecedentedly high energy. It is designed to collide proton beams 40 million times per second at a centre-of-mass energy of 14 TeV. Since the start of the LHC operation in 2009, the accelerator has been improving its performance: increasing the luminosity and doubling the beam energy, until reaching 13 TeV in p-p collisions. 

The ATLAS (A Toroidal LHC ApparatuS) Detector is a general purpose experiment built to fully exploit the physics produced by the LHC, from precision physics within the Standard Model all the way to new physics phenomena beyond it. Thanks to the good operation of the machine and detectors in the first data-taking run, some of the major goals of the ATLAS detector, such as the discovery of the Higgs boson in 2012, have already been achieved. The second run of data-taking, started in spring 2015. It will allow ATLAS to continue investigating a wide range of physics, from the properties of the fundamental particles and their interactions to topics such as extra dimensions and the search for dark matter candidates.

At the LHC design luminosity, a large number of particles emerge from the interaction point every collision creating a high charged-track multiplicity in the detector. In order to operate under these conditions, the ATLAS detector is equipped with fast and radiation-hard electronics and sensors. In addition, the high granularity of the detector allows handling the huge particle flux and reducing 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. This detector weights 7000 tons and it is 45 m long and 22 m tall. Its large size together with a powerful magnet system allow for a good momentum resolution of the charged particles. The magnet system is composed of a thin superconducting solenoid in the inner part and three large superconducting toroids in the outer region of the detector.

The Inner Detector (ID), immersed in a 2 T solenoidal field, is responsible for the pattern recognition, the momentum measurement of the charged particles and the reconstruction of the primary and secondary vertices. This is achieved using a combination of discrete, highly granular semiconductor pixel and strip detectors in the inner part of the tracking volume, surrounded by a straw-tube gaseous detector (TRT) with the capability to discriminate electrons from charged pions.The ID is then followed by the calorimeters, which are the responsible for measuring the energy of the particles. ATLAS uses a high granularity liquid-argon (LAr) electromagnetic sampling calorimeter, with excellent performance in terms of energy and position resolution. The hadronic calorimetry consists of iron and scintillator-tiles in the central part of the detector, and LAr technology in the forward region, matching the outer limits of electromagnetic calorimeter.The outermost detector is the Muon Spectrometer (MS) which includes three large superconducting air-core toroids, generating a strong bending power in a large volume within a light and open structure. An excellent muon momentum resolution is achieved by means of three layers of high precision tracking chambers. The good timing resolution of the muon detector also plays a crucial role in the trigger system.


             Figure 1: Schematic layout of the ATLAS detector

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