# The ATLAS experiment

 Monica Lynn Dunford and Peter Jenni (2014), Scholarpedia, 9(10):32147. doi:10.4249/scholarpedia.32147 revision #146229 [link to/cite this article]
Post-publication activity

Curator: Peter Jenni

# Introduction

In particle physics experiments, the discovery of increasingly more massive particles has brought deep understanding of the basic constituents of matter and of the fundamental forces among them. In order to explore Nature in its deepest elementary secrets, the Large Hadron Collider (LHC) was built at CERN, Geneva. The LHC provides the highest energy collisions in a laboratory, at very high rates to allow one to study very rare reactions. Two independent sophisticated huge instruments, called ATLAS and CMS detectors, are operated to explore in a most broad way the physics of these collisions. In addition to these two general-purpose detectors, smaller specialized experiments (LHCb, ALICE and some others) are collecting collision data as well.

# Project History

The initial ideas for LHC detectors and their physics potential were studied in 1984 at a workshop in Lausanne bringing together experimental and theoretical physicists and accelerator experts. These studies evolved in the late 1980s and early 1990s to informal detector collaborations of many dozens of Institutes developing technologies to be used in a future LHC experiment. The ATLAS Collaboration was born in summer 1992 from the merging of two such working groups which both developed detector concepts based on a toroidal muon magnet configuration. ATLAS submitted in October 1992 a Letter of Intent (LoI, a 100-page document) to the new CERN LHC Experiments Committee (LHCC) proposing a general-purpose experiment for the LHC. The LHCC is an international peer reviewing committee examining closely all scientific, technical and financial aspects of the LHC experiments.

The LoI contained a number of conceptual and technical design options that needed to be narrowed down over the course of the following years, including the critical choice of the superconducting toroid magnet system. The detector concept was basically settled by the time of the submission of the Technical Proposal (TP) to the LHCC in December 1994, which also reduced cost wherever this was possible. Nearly 20 detailed Technical Design Reports were reviewed by the LHCC for the various detector components over the years 1995 to 2005. After the TP the design underwent several changes in regards to the detailed implementation of the selected technologies, based on prototype studies in particle beam tests as well as important further cost optimizations.

The project was formally approved in January 1996, and the budget for the full construction was established with an expenditure ceiling set at 475 MCHF (in 1995 currency rate) in 1998. Only a small fraction of the funding, shared among all partners of the project, was centrally available. Detector components were built all over the world in the collaboration Institutes and local industries, under their responsibility, and then delivered to CERN as ‘in-kind’ contributions to the ATLAS detector.

ATLAS is an international, worldwide Collaboration, which grew from about 850 scientists from 88 institutions at the time of the LoI to today’s size of 3000 scientists from 178 institutions located in 38 countries from all inhabited continents. A good third of these participants are PhD students.

# Description of the ATLAS Detector and its Main Components

## Overall Layout

A broad spectrum of detailed studies addressing the great physics potential offered by the LHC led to the final adopted overall detector design. The primary goal of the experiment is to operate at the highest proton-proton collision rates (a luminosity of 1034 cm-2 s-1, for example, means almost 109 collisions per second) with a detector that provides as many signatures (signals indicating that something interesting happened) as possible. Sensitivity to a variety of signatures is important in the harsh environment of the LHC; this is required to achieve robust and redundant physics measurements with the ability of internal cross-check, while being sensitive to as many physics scenarios as possible.

Figure 1: Longitudinal cut-away view of the ATLAS detector, showing the different layers around the LHC beam axis. The collisions occur in the centre of detector. The main detector components are indicated.

The search for the Higgs boson strongly guided the desired capabilities needed for a general-purpose LHC detector. It has to select collisions containing signatures of particles that could occur in Higgs boson decays, and measure kinematical and geometrical (flight direction) properties of these particles as accurately as possible. This information allows one then to reconstruct what happened in the collision, for example the production of a massive new particle decaying into known measurable final state particles, which are either stable or decaying further in a known way into stable particles. Examples of such signatures are photons, electrons, muons, W and Z bosons, or quarks and gluons which manifest themselves as narrow sprays of particles, known as ‘hadronic jets’, resulting from their fragmentation into mesons and baryons. Finally, neutrinos can be produced, which, as neutral weakly interacting leptons, leave the detector without direct trace. Momentum balance appears to be violated, leading to the signature of missing transverse momentum (loosely referred to as missing transverse energy, ETmiss), where ‘transverse’ means perpendicular to the beam axis.

The basic design criteria of the detector included the following points:

• Excellent electromagnetic calorimetry for electron and photon identification and measurements, complemented by full-coverage hadronic calorimetry for accurate jet and ETmiss measurements;
• High-precision muon momentum measurements, with the capability of accurate measurements at the highest collision rates using the external muon spectrometer alone;
• Efficient charged particle tracking at high luminosity for high transverse momentum (pT) lepton-momentum measurements, electron and photon identification, τ-lepton and heavy-flavour identification, and full event reconstruction capability at lower luminosity;
• Large acceptance in pseudorapidity (η) with almost full azimuthal angle (φ) coverage everywhere. The azimuthal angle is measured around the beam axis z, whereas pseudorapidity relates to the polar angle (θ) where θ is the angle from the z direction, $$\eta = - \ln \left(\tan \frac{\theta}{2} \right)$$.
• Triggering and measurements of particles at low-pT thresholds, providing high efficiencies for most physics processes of interest at the LHC.

The overall detector layout is shown in Figure 1. The beams collide in the centre of the detector, which is the origin of the coordinate system. A cross-section through the barrel part, perpendicular to the beam axis, illustrating schematically the functions of the different detector layers, is given in Figure 2.

The main active detector components of the ATLAS detector, from the beam line towards the outside. The total readout channels for each component is given, as well as its pseudorapidity coverage.
Detector component Position Channels (total) η - coverage
Tracking
Pixel 3 cylindrical barrel layers 80.4 million $$\pm 2.5$$
3 end-cap disks on each side
Radial envelope 45.5 - 242 mm
SCT strips 4 cylindrical barrel layers 6.3 million $$\pm 2.5$$
9 end-cap disks on each side
Radial envelope 251 - 610 mm
TRT 73 barrel straw planes 351,000 $$\pm 2.0$$
80 end-cap straw planes
Radial envelope 554 - 1106 mm
Calorimetry
EM presampler Barrel 7,808 $$\pm 1.52$$
End-caps 1,536 $$1.5 < | \eta | < 1.8$$
EM calorimeter 3 depth samples barrel 101,760 $$\pm 1.48$$
3 depth layers end-caps 62,208 $$1.375 < | \eta | < 3.2$$
Hadronic tile calorimeter 3 depth samples barrel 5,760 $$\pm 1.0$$
3 depth samples extended barrel 4,092 $$0.8 < | \eta | < 1.7$$
LAr hadronic end-caps 4 depth layers 5,632 $$1.5 < | \eta | < 3.2$$
LAr forward hadronic calorimeter 3 depth layers 3,524 $$3.1 < | \eta | < 4.9$$
Muon spectrometer
MDT precision tracking 3 multi-layer stations 354,000 $$\pm 2.7$$
CSC precision tracking 1 innermost station end-caps 31,000 $$2.0 < | \eta | < 2.7$$
RPC trigger chambers 2 multi-layer stations barrel 373,000 $$\pm 1.05$$
TGC trigger chambers 2 multi-layer stations end-cap 318,000 $$1.05 < | \eta | < 2.4$$

The magnet configuration is based on an inner thin superconducting solenoid surrounding the inner detector cavity, and large superconducting air-core toroids consisting of independent coils arranged with an eight-fold symmetry outside the calorimeters. Details for the various detection layers including a breakdown of the total electronics channel count of almost 100 million signals are given in Table 1. It is remarkable that during operation almost all of them, typically 99% per detection layer, are fully operational.

Figure 2: Schematic representation of the detector components in a plane perpendicular to the LHC beam line (transverse plane) in the barrel region. Only a small sector of the azimuthally symmetric detector is shown, starting outwards from the LHC beam vacuum pipe. Typical signatures for various particles as measured in the different detector layers are illustrated, where solid lines represent directly measurable trajectories of charged particles, and dashed lines the straight-line trajectories of neutral particles leaving no direct signals in tracking detectors.

The inner tracking detector around the collision point is contained within a cylinder of length 7 m and a radius of 1.15 m, in an axial magnetic field of 2 Teslas (T). Pattern recognition, momentum and vertex measurements, as well as additional electron identification, are achieved with a combination of a few discrete high-resolution semiconductor pixel and strip detector layers in the inner part of the tracking volume, and ‘continuous’ straw-tube tracking detectors giving 30-40 points along the tracks with transition radiation capability in its outer part.

Highly granular liquid-argon (LAr) electromagnetic (EM) sampling calorimetry, with excellent performance in terms of energy and position resolution, covers the pseudorapidity range |η|< 3.2. In the end-caps, the LAr technology is also used for the hadronic calorimeters, which share the cryostats with the EM end-caps. The same cryostats also house the special LAr forward calorimeters which extend the pseudorapidity coverage to |η|= 4.9 (which corresponds to only 0.85 degrees from the beam axis). The bulk of the hadronic calorimetry is provided by a novel scintillator-tile calorimeter, which is separated into a large barrel and two smaller extended barrel cylinders, one on each side of the barrel. The overall calorimeter system provides the very good jet and ETmiss performance of the detector. The LAr calorimetry is contained in a cylinder with an outer radius of 2.25 m and extends longitudinally to ±6.65 m along the beam axis. The outer radius of the scintillator-tile calorimeter is 4.25 m and its half-length is 6.10 m. The total weight of the calorimeter system, including the solenoid flux-return iron yoke which is integrated into the tile calorimeter support structure, is about 4,000 tons.

Figure 3: Photograph of one end of the ATLAS detector barrel with the calorimeter end-cap still retracted before its insertion into the barrel toroid magnet structure (February 2007 during the installation phase).

The calorimeter is surrounded by the muon spectrometer. The air-core toroid system, with a long barrel and two inserted end-cap magnets, generates a large magnetic field volume with strong bending power within a light and open structure. Multiple-scattering effects are thereby minimised, and excellent muon momentum resolution is achieved with three stations of high-precision tracking chambers. The muon instrumentation also includes as a key component trigger chambers with very fast time response. The muon spectrometer defines the overall dimensions of the ATLAS detector. The outer chambers of the barrel are at a radius of about 11 m. The half-length of the barrel toroid coils is 12.5 m, and the third layer of the forward muon chambers, mounted on the cavern wall, is located about 23 m from the interaction point.

The overall weight of the ATLAS detector is about 7,000 tons. It is installed in a large underground cavern around the LHC beam line which is about 85 m below the surface at this place. A photograph of one end of the cylindrical barrel detector, taken in February 2007, is shown in Figure 3, after about 4 years of installation work and 1.5 years before its completion.

## Magnet System

The ATLAS superconducting magnet system is an arrangement of a central solenoid (CS) providing the inner tracking with magnetic field, surrounded by a system of three large air-core toroids generating the magnetic field for the muon spectrometer. This magnet system configuration is unique in the history of large particle physics experiments. The overall dimensions of the magnet system are 26 m in length and 20 m in diameter.

The two end-cap toroids (ECT) are inserted in the barrel toroid (BT) at each end and line up with the CS. They have a length of 5 m, an outer diameter of 10.7 m and an inner bore of 1.65 m. The CS extends over a length of 5.3 m and has a bore of 2.4 m. The unusual configuration and large size made the magnet system a considerable technical challenge requiring careful engineering. The CS provides a central field of 2 T with a peak magnetic field of 2.6 T at the superconductor itself. The peak magnetic fields on the superconductors in the BT and ECT are 3.9 and 4.1 T respectively.

The performance in terms of bending power is characterised by the field integral $$\int B dl$$, where B is the azimuthal field component and the integral is taken on a straight line trajectory between the inner and outer radius of the toroids. The BT provides 2 to 6 Tm and the ECT contributes with 4 to 8 Tm in the 0.0-1.3 and 1.6-2.7 pseudorapidity ranges respectively. The position of the CS in front of the EM calorimeter demanded a careful minimisation of the material in order to degrade as little as possible the calorimeter performance. Therefore the CS and the LAr calorimeter share one common vacuum vessel, thereby eliminating two vacuum walls.

Each one of the three toroids consists of eight coils assembled radially and symmetrically around the beam axis. The ECT coil system is rotated by 22.5° with respect to the BT coil system in order to provide radial overlap (see Figure 1). The coils are of a flat ‘racetrack’ type with two so-called double-pancake windings made of 20.5 kA aluminium-stabilised NbTi superconductors. Each ECT consists of eight coils cold-linked and assembled as a single cold mass, housed in one large cryostat. Due to the magnetic forces, the ECT magnets are pulled into the BT and the corresponding axial forces are transferred to the BT cryostats via axial transfer points linking both magnet systems.

A central refrigeration plant located in the side cavern supplies the cooling power. Electrically all toroid coils are connected in series; they have a 21 kA power supply and are equipped with control systems for fast and slow energy dumps. The CS is energised by an 8 kA power supply. An adequate and proven quench protection system has been designed to safely dissipate the stored energy (1.6 GJ total energy) without overheating the coil windings.

## Tracking Detectors

Figure 4: Cut-away view of the ATLAS tracking detectors. In this longitudinal view the different tracking layers around the LHC beam pipe are shown. The interaction point is in the centre of tracking detector.

The layout of the tracking detector is shown in Figure 4. Given the very large track density at the LHC, the momentum and vertex resolution requirements call for high-precision measurements to be made with fine-granularity detectors. Semiconductor tracking detectors, using silicon microstrip (SCT) and pixel technologies offer these features. The highest granularity is achieved around the vertex region using semi-conductor pixel detectors. The total number of precision layers was limited because of the material they introduce, and because of their high cost. Typically, three pixel layers and eight strip layers (four space points) are crossed by each charged particle. A large number of tracking points (typically 36 per track) is obtained by the straw-tube tracker which provides continuous track-following with much less material per point and a lower cost.

The combination of the two techniques gives very robust pattern recognition and high precision in both φ and z coordinates. In addition, the electron identification capabilities of the whole experiment are enhanced by the detection of transition-radiation photons in the xenon-based gas mixture of the straw tubes. Mechanically, the tracking consists of three units: a barrel part extending over ± 80 cm, and two identical end-caps covering the rest of the cylindrical detector of 6.2 m length and 2.1 m diameter. In the barrel region, the high-precision detector layers are arranged on concentric cylinders around the beam axis, while the end-cap detectors are mounted on disks perpendicular to the beam axis. The secondary vertex measurement performance is enhanced by the innermost layer of pixels, at a radius of about 4 cm, as close as is practical to the beam pipe.

## Calorimeters

The ATLAS calorimeters, Figure 5, consist of an electromagnetic (EM) calorimeter covering the region |η|< 3.2, a hadronic barrel calorimeter covering |η|< 1.7, hadronic end-cap calorimeters covering 1.5 <|η|< 3.2, and forward calorimeters covering 3.1 <|η|< 4.9.

Figure 5: Cut-away view of the ATLAS calorimetry. The three distinct cylinders, barrel and end-caps, are visible. The smaller radial regions use the LAr technology requiring cryostats, whereas the outer cylinders use scintillator tiles embedded in an iron absorber structure. The end-caps can be moved longitudinally along the LHC beam line for creating access space to maintain the barrel region.

The EM calorimeter is a lead/liquid-argon detector with alternating sampling layers of 2.1 mm gaps filled by LAr with readout electrodes in the middle, and lead absorber plates (typically 2 mm thick), all shaped in a novel ‘accordion geometry’. This geometry provides fast response and full azimuthal coverage without dead regions. The EM calorimeter is preceded by a LAr presampler detector, installed immediately behind the cryostat cold wall, and used to correct for the energy lost in the material (tracker, cryostats, and solenoid coil in the barrel region) upstream of the calorimeter. The LAr technology requires cryogenic installations, as the operation temperature is typically 87 degrees Kelvin.

The hadronic barrel calorimeter is a cylinder divided into three sections: the central barrel and two identical extended barrels. It is based on a sampling technique with plastic scintillator plates (tiles) embedded in an iron absorber. The tiles are placed radially and staggered in depth. The structure is periodic along the beam axis. The tiles are 3 mm thick and the total thickness of the iron plates in one period is 14 mm. Two sides of the scintillating tiles are read out by wavelength shifting fibres into two separate photomultipliers.

At larger pseudorapidities, closer to the beams, where higher radiation resistance is needed, the intrinsically radiation-hard LAr technology is used for all the calorimeters: the hadronic end-cap calorimeter, a copper/LAr detector with parallel-plate geometry, and the forward calorimeter, a dense LAr calorimeter with rod-shaped electrodes in a tungsten matrix.

The barrel EM calorimeter is contained in a barrel cryostat, which surrounds the tracking detectors. The solenoid which supplies the 2 T magnetic tracker field is integrated into the vacuum of the barrel cryostat and is placed in front of the EM calorimeter. Two end-cap cryostats house the end-cap EM and hadronic calorimeters, as well as the integrated forward calorimeter. The barrel and extended barrel tile calorimeters support the LAr cryostats, and also act as the main solenoid flux return.

The approximately 200,000 signals from the LAr calorimeters leave the cryostats through cold-to-warm feedthroughs located between the barrel and the extended barrel tile calorimeters, and at the back of each end-cap. The electronics up to the digitisation stage is contained in radial boxes attached to each feedthrough and located in the vertical gaps between the barrel and extended barrel tile calorimeters.

## Muon Spectrometer

The layout of the muon spectrometer is visible in Figure 1. It is instrumented with separate trigger and high-precision tracking chambers. Measurements are based on the magnetic deflection of muon tracks in the large superconducting air-core toroid magnets. The toroidal magnet configuration provides a field that is mostly orthogonal to the muon trajectories, while minimising the degradation of resolution due to multiple scattering. The anticipated high level of particle fluxes has had a major impact on the choice and design of the spectrometer instrumentation, affecting required performance parameters such as rate capability, granularity, ageing properties and radiation hardness. Trigger and reconstruction algorithms have been optimised to cope with the difficult background conditions resulting from hadrons penetrating the calorimeters and from radiation backgrounds, mostly neutrons and photons in the 1 MeV range, produced from secondary interactions in the calorimeters, shielding material, beam pipe and LHC machine elements.

In the barrel region, tracks are measured in chambers arranged in three cylindrical layers (‘stations’) around the beam axis; in the transition and end-cap regions, the chambers are installed vertically, also in three stations. Over most of the η-range, a precision measurement of the track coordinates in the principal bending direction of the magnetic field is provided by Monitored Drift Tubes (MDTs). Close to the beam axis and near to the interaction point, Cathode Strip Chambers (CSCs) with higher granularity are used in the innermost plane, to withstand the demanding rate and background conditions. Optical alignment systems have been designed to meet the stringent requirements on the position accuracy and the survey of the precision chambers. The precision measurement of the muon tracks is made in the R–z projection, the direction parallel to the bending direction of the magnetic field; the axial coordinate (z) is measured in the barrel and the radial coordinate (R) in the transition and end-cap regions. The MDTs provide a single-wire resolution of ~80 microns when operated at high gas pressure (3 bars). The fast trigger system covers the range |η|< 2.4. Resistive Plate Chambers (RPCs) are used in the barrel and Thin Gap Chambers (TGCs) in the end-cap regions.

# How the data flows (Trigger, Computing, and Data Analysis)

In order to process large volumes of data within nanosecond timescales, the ‘trigger’ system is designed to select interesting events quickly and efficiently. At the LHC design intensities, one billion events per second occur within the ATLAS detector but only one Higgs boson is produced in 10 seconds. It is not technically possible to store the data for all events, nor is it wanted; therefore the trigger is used to reject large numbers of events and retain only the interesting events. This is done in three successive stages by the trigger system, called the Level-1, Level-2 and the Event Filter triggers. Figure 6 shows a simplified drawing of the data flow, which is discussed in more detail below.

Figure 6: An illustration of the data flow path from the ATLAS detector to permanent storage on disk and tapes as described in the text.

The goal of the Level-1 trigger is to reduce the event rate from 40 MHz to 75 kHz, within a time frame of 2.5 microseconds. Some of the most interesting events, such as Higgs decays, contain energetic leptons, photons and jets and the Level-1 trigger uses specialized hardware to find these objects. This is done with two dedicated systems: the Level-1 calorimeter trigger and the Level-1 muon trigger. The Level-1 calorimeter trigger receives 7,200 analog signals from the electromagnetic and hadronic calorimeters, which are a sum of several calorimeter cells and therefore have a reduced granularity, with respect to that shown in Table 1. Using these signals, specialized hardware is used to search for patterns as expected for electrons, photons, τ-leptons and jets. The Level-1 calorimeter system can also calculate the amount of missing transverse energy in an event. Muon tracks are found by measuring hits in one plane and then searching for additional hits in nearby planes along pre-determined patterns. In parallel during this time, the analog signals from the interacting particles have been processed by the front-end electronics for all the sub-detectors. These signals are digitized and stored (either as analog or digital signals) in the so-called Level-1 buffer. The data waits in these buffers until the decision from the Level-1 trigger system is made. For each object found by the Level-1 systems, a ‘region-of-interest’ is identified. The number of identified objects passing different energy thresholds is then sent to the central trigger processor, which determines, for example, if a sufficient number of energetic objects has been found in this event. The central trigger has up to 128 different criteria to which to compare the event. If the event passes one of these criteria, a so-called Level-1 accept is sent to the detector, which signals to the front-end hardware to send the data to the Readout Drivers (RODs) for further processing. Otherwise, the data is removed from the Level-1 buffer.

Unlike the front-end hardware, the RODs are not located in the detector hall but in a room several dozen meters away, which is shielded from radiation. Here the data is stored and can be requested by the Level-2 and event filter trigger systems. To minimize the data transfer volume, the Level-2 system only requests detector information within the region-of-interests as specified by the Level-1 systems. Using commercial computers, the Level-2 systems runs more complex lepton, photon and jet identification algorithms using information from the tracking detectors, the muon spectrometer and in addition the full granularity information from the calorimeters. This system reduces the event rate to 3.5 kHz within 10 milliseconds. If the event passes one of 256 different Level-2 event criteria, the full detector information is transferred to the event filter system, where a final selection is run using commercial computers. The event rate is reduced at this stage to a few hundreds of Hz within one second. Over 17,000 computing cores are used in the Level-2 and event filter systems. If the event is selected, the data is transferred to permanent storage at the CERN computing centre. If it is not, the event is deleted from the RODs.

Once the event is saved, it still needs to be processed so that it can be analysed by ATLAS members. Here the computing system faces many challenges; it must process high volumes of data quickly and distribute that data to ATLAS collaborators around the world. For this a pyramid-structured computing model is used, called the Worldwide LHC Computing Grid (WLCG).

The CERN computing centre, called the Tier-0, processes all the data for the first time. This first processing plays an important role in understanding the detector. The conditions of the detector components and therefore the response can change over time or even malfunction. Some detector changes, such as the failure of a single channel can be corrected for but other problems, such as a malfunctioning power supply affecting many channels must be addressed immediately. An ATLAS member therefore monitors the data from each stage of the trigger live in the control room. At the Tier-0 a small set of data is processed within one hour, so that analysers can study the data in more detail within the next 24 hours. Data, which are unusable, are flagged so that analysers can reject these events. But a large fraction, approximately 95% of the data, is usable for analysis.

After processing at the Tier-0, the data is then copied to one or more of the Tier-1s, which are approximately a dozen large computing centres located worldwide. The Tier-1s re-process the data when needed. Small sub-sets of the data are copied to one or more of the roughly 150 Tier-2s, which are located mainly at universities and are the most convenient sites for ATLAS members to access the data. The Tier-2s are also the main production sites for generating the large samples of simulated events. For analysis, the data is stored in user-friendly formats, which contain information about the reconstructed objects for each event. Analysers then process this information on local computers and produce histograms of the data. This model enables researchers including students to analyse the data from their home institute.

# Commissioning and Performance

The calibration of the ATLAS detector is a complex task and has involved the work of thousands of physicists. Two examples of the calibration are highlighted here: the determination of the detector’s response to electrons, photons, muons and jets and the alignment of the tracking and muon systems.

The detector response refers to the determination of both the energy or momentum scale, which is an estimate of the amount of energy/momentum a particle had and the resolution, which is an estimate of how precisely that energy/momentum is measured. Before the detector was fully installed in the cavern, one important first study of the response was to place a small but complete slice of the ATLAS detector in a particle beam test at CERN. In this test in 2004, the beams of electrons, photons, pions and muon with energies between 1 and 350 GeV, were targeted on the detector components. The response of the detector is measured and then using the known energy of the beam, this response is calibrated into an amount of deposited energy.

Figure 7: Invariant mass in GeV of events with two muons. The resonance peaks for the various mesons and the Z boson are labelled. EF_mu15 refers to the name of the trigger used to select the events.

To verify the detector response using particle beam test data as well as to monitor that response over time, well-known Standard Model particles are used. The Z boson, whose mass is well measured from the LEP collider experiments, can decay to two electrons or two muons with opposite charge. The invariant mass distribution, which is a reconstruction of the Z mass using its decay products, is then used to determine the energy scale and resolution of electrons and muons. Figure 7 shows the invariant mass distribution for muon pairs. The Z boson mass peak, as well as the mass peaks from the decays of mesons such as the J/ψ, are clearly visible. Studies of the decay products of the Z boson are also used to estimate the efficiency with which the detector finds electrons and muons.

Another important calibration for the tracking and muon systems is their alignment, which refers to the determination of the exact location of each tracking module or muon chamber. In order to precisely measure the bending of tracks from charged particles in the magnetic field, the location of each pixel or silicon module, for example, must be known to better than 15 microns. To test the alignment before the start of the LHC, the only available source of energetic particles were muons from cosmic rays. Between 2008 and 2009, the ATLAS detector collected signals from over 300 million cosmic ray muons. These events were used to establish a first, rough alignment and they were also a critical test of the full ATLAS readout system from the Level-1 trigger to the processing of the data on the Grid. To determine the final alignment, charged particles from the beam interactions are used. To achieve this, the trajectory of single, isolated tracks (i.e. a track which has no other tracks nearby) is measured and any misalignment of the components can be determined. Several million tracks are needed in order to determine the alignment of the pixel and silicon detectors.

The detector performance has immediate impact on physics measurements; a worsening of the photon energy resolution, for example would lead to a worsening of the two photon invariant mass distribution used for the Higgs discovery. Overall the performance of the ATLAS detector has been spectacular. Within only three years of LHC running, the calibration and performance of the detector has reached its design goals. The relative resolution for electrons, photons and muons is at the percent level over large momentum and energy ranges. These resolutions are summarised in Table 2.

Indicative resolutions of the ATLAS detector components. The units for energy E and transverse momentum $$p_T$$ are in GeV. The symbol $$\oplus$$ means adding both parts in quadrature.
Detector component Resolution
Tracking $$\sigma_{p_T} / p_T = 0.05\%\, p_T \oplus 1\%$$
EM calorimetry $$\sigma_E / E = 10\% / \sqrt{E} \oplus 0.7\%$$
barrel and end-caps $$\sigma_E / E = 50\% / \sqrt{E} \oplus 3\%$$
forward $$\sigma_E / E = 100\% / \sqrt{E} \oplus 10\%$$
Muon spectrometer $$\sigma_{p_T} / p_T = 10\%\ {\rm at }\ p_T = 1\, {\rm TeV}$$

# Physics Highlight Examples

Figure 8: The invariant mass distribution in GeV for events with two photons. The black points are the data. The blue dashed line is an estimate of the background contribution, while the black line is an estimate of the signal contribution from a Higgs particle. The red curve is the signal plus background fit to the data. The lower figure shows the difference between the data and the background only model.

Before the turn-on of the LHC, the biggest question of the day was: Does the Higgs boson, the last of the undiscovered particles in the Standard Model, exist? For the last several decades, the Standard Model theory of particle physics has withstood rigorous experimental verification. One critical aspect of that theory is that elementary particles in the Standard Model obtain their mass through the so-called Brout–Englert–Higgs mechanism, and therefore the Higgs boson should also exist. The beams at the LHC have a high enough energy to produce Higgs particles. This particle, though, is not stable and will decay immediately into other Standard Model particles, such as two photons or two Z bosons. By detecting and measuring the two photons, the mass of the Higgs can then be reconstructed as shown in Figure 8. The figure shows a smooth falling background, which comes from, for example, quark/gluon interactions producing two photons. At around an invariant mass of 125 GeV, a bump above the expected background is seen, which comes from Higgs decays into two photons. In Figure 9 the corresponding invariant mass can be seen for decays to four charged leptons via two Z bosons. Studying the difference in the decay rates allows one to understand the coupling of the Higgs boson to Standard Model particles. Since the July 2012 discovery, all new studies indicate that the new particle is the Higgs boson as predicted by the Standard Model; however, many more detailed studies are needed to confirm this.

Figure 9: The invariant mass distribution in GeV for events with four charged leptons. The data are shown by the black points. The Higgs signal is shown by the blue histogram while the expected backgrounds are shown by the red and purple histograms.
Figure 10: The missing transverse momentum (ETmiss) in the event in GeV. The data are shown by the black points and the Standard Model backgrounds are shown by the coloured histograms. The hashed red line shows the sum of the Standard Model backgrounds with uncertainties. The predictions from two additional theories are also shown; “ADD” is a theory with additional large extra spatial dimensions. “WIMP, D5” is a theory with pair production of dark matter particles.

Although the Standard Model has been experimentally well tested, many physicists believe that it is not a complete description of Nature. For example the Standard Model has no explanation for Dark Matter, which has been inferred from observations by astronomers and is expected to make up 25% of the energy density of the universe. Also the Standard Model does not include gravity in its description. New theories which extend the Standard Model have been proposed to explain these and other open questions. Many of these new theories postulate additional particles, which can be produced at the LHC. Figure 10 illustrates the results of such a search, where events are required to have large amounts of missing transverse momentum and an energetic photon. In this search, the ETmiss in the data is compared to the predictions from the Standard Model theory. Two example theories, which include heavy dark matter particles or additional space dimensions, are also shown on the figure. These theories predict massive new particles, which do not interact in the detector and are therefore measured as ETmiss. No difference between the data and the Standard Model predictions are seen, therefore limits can be set on the masses for any new particles for these theories. The LHC experiments have searched extensively for new particles in the data and as of yet, found none.

The LHC programme includes every year also a running period of a few weeks where not proton beams collide, but beams of lead nuclei (producing either Pb-Pb or Pb-p collisions). These so-called Heavy Ion (HI) collisions allow one to study the behaviour of hadronic matter under extreme conditions, known as quark-gluon plasma. To make measurements in these extreme conditions, the versatility of the ATLAS detector plays a strong role; for example its ability to measure high transverse momenta jets and bosons over a large rapidity range. The many interesting results from ATLAS complement those from the dedicated LHC HI experiment ALICE.

# Outlook

The LHC operation at 7 and 8 TeV during the first three years has been successful beyond expectations. The collider performed beyond its initial designs, reaching peak luminosities of 7 x 10 33 cm-2 s-1. The centre-of-mass energy will nearly double to 13-14 TeV in 2015 and the luminosity will continue to be increased in two major steps: the first in 2020 and a second in 2025. The ATLAS detector will also be improved to keep up with the higher collision rates and maintain its good performance of measurements of objects like leptons, photons and jets. Already for the operation starting in 2015 a fourth cylindrical barrel layer of pixel detectors has been added very close to a new beam pipe. This ‘Insertable B-Layer’ at an average radial distance of 33 mm will improve the secondary vertex measurements at high luminosity. Furthermore, the trigger system has been improved to accept a rate of up to 100 kHz after the Level-1 trigger.

After a shutdown to upgrade parts of the accelerator, the LHC will resume in 2020 at luminosity of up to 2-3 x 10 34 cm-2 s-1 and the number of simultaneous interactions per crossing will increase by a factor of two. Additional collisions result in unwanted particle interactions in the detector, which can lead to a worsening of the energy resolution in the calorimeters and complicate the reconstruction of charged particle tracks. To run under these conditions, one of the main improvements to ATLAS will be the trigger system. The Level-1 trigger will be enhanced, so that it can continue to reject events quickly in this new hostile environment. To achieve this, new tracking detectors will be added to the muon trigger system. This will reduce the amount of spurious tracks that are detected and misidentified as muons in some critical regions. The front-end electronics of the electromagnetic calorimeter will be improved to send the full calorimeter granularity to the Level-1 calorimeter trigger. This will help in distinguishing electrons from jets and therefore improve the trigger acceptance.

In 2025 after a second upgrade phase, the LHC will reach the luminosity of 5 x 1034 cm-2 s-1. At this point, the components of the ATLAS detector will be 15-20 years old and therefore major upgrades of many sub-detectors will be necessary, most notably the tracking detector, which in its current form will not survive the harsher radiation environment. To meet this demand, the tracking detectors will be completely replaced with a new all-silicon detector design. The Level-1 trigger will also have to undergo major renovations to cope with the increased event rates. To achieve this, an additional decision layer will be added to the trigger system. The current Level-1 system will be upgraded to reduce the overall event rate to 500 kHz within six microseconds. Next, a new trigger system will be added which uses information from the tracking detector to reduce the rate to 100 kHz within 14 microseconds. Major changes are also planned for the forward calorimeters to be more resilient to radiation damage. In addition the computing model will be overhauled to handle the even larger data samples.