By studying in detail the collision products and their characteristics, information can be obtained on the underlying physics processes giving rise to the production of these particles.
When particles traverse any material, they interact with it, resulting in a loss of energy of the traversing particle: either because the traversed material gets ionised or excited or because the particle emits radiation. Particle detectors function such that they convert this lost energy into an electronic signal that is recorded and can be analysed. Tracking detectors constrain the point of energy loss spatially in order to reconstruct the trajectory of the particle (track). If a magnetic field is applied, the bending of the track allows to constrain the particle’s momentum and electric charge. Calorimeters are optimised to measure the energy of the traversing particle. By stopping it in the calorimeter material and measuring the released energy very precisely the original energy of the particle can be reconstructed.
ATLAS [61] is a general-purpose detector, designed to study many different aspects of mod-ern particle physics. It is realised as a cylindrically symmetric magnetic spectrometer. The tracking detectors closest to the interaction point provide information on the particle trajec-tories and allow for an efficient vertex reconstruction, which is crucial for pile-up rejection and the identification of e.g. B-hadrons or tau leptons.
4.2.1 Coordinate System and Variable Definitions
The nominal interaction point where the proton beams are expected to collide defines the origin of the coordinate system used in ATLAS. The x-axis points towards the centre of the LHC, the z-axis is defined parallel to the beam circulating counter-clockwise and the y-axis to be orthogonal to both, such that a right-handed coordinate system is formed. The azimuthal angle φ in the x–y plane, defined relative to the x-axis, and the polar angle θ in the x–z plane, relative to the z-axis, are used to denote coordinates. Sinceθis not Lorentz-invariant, the pseudo-rapidity is often considered instead. It is defined as: η = −ln(tan(θ/2)). The rapidity y is subsequently given by:
y= 1
2lnE +pz
E−pz
, (4.2)
Chapter 4. The ATLAS Detector at the LHC 69 with E being the particle energy and pz being its longitudinal momentum. In the limit of massless particles, the rapidity equals the pseudo-rapidity. The distance∆R=p
∆η2+ ∆φ2 is often used to quantify how close two objects are to each other.
The transverse momentum pT is defined as pT = p
p2x+p2y, the transverse energy is given analogously. Since the incoming partons have no transverse momentum, the vectorial sum of transverse momenta of all produced objects in the collision has to be zero due to momentum conservation. This is not fulfilled experimentally if invisible or undetected particles are produced in the collision and the negative vectorial sum of the object pT’s is defined as missing transverse momentum ~pmissT . Its amplitude is given by ETmiss = q
p2x,miss+p2y,miss and is called missing transverse energy.
The so-called transverse mass mT targets leptonic W-boson decays. The expression mT = p2p`TETmiss(1−cos ∆φ(`, ETmiss)) aims at reconstructing the mass of a common parent par-ticle of neutrino and lepton. Since ETmiss can only be defined in the transverse plane, the obtained transverse mass is a lower bound of the true parent mass.
4.2.2 Detector Design
The ATLAS detector is characterised by a powerful muon system, motivated by the aim to discover and measure the Higgs Boson3 and calorimetry that covers almost the full solid angle of 4π, which allows to test multiple models of new physics, often characterised by large ETmiss. ATLAS is built in layers, as it is typical for general-purpose particle detectors.
The innermost layer is a tracking system surrounding the interaction point, immersed in a magnetic field. It is enclosed by electromagnetic and hadronic calorimeters as well as, in the outermost layer, a muon spectrometer. A schematic overview is given in Fig. 4.6. The detector can be sub-divided into thebarrel and theend-cap region. The barrel is cylindrically symmetric around the beam pipe and typically extends up to |η|<1.4. The endcaps “close”
the open sides of the barrel by cylindrical structures, extending the range to up to |η| <5.
The following description of the detector and its sub-systems is largely based on Ref. [61].
Inner Detector: The Inner Detector (ID) records information on the particle trajectories.
It is surrounded by a superconducting solenoid magnet that provides a 2 T field in which the particle tracks are bent. It consists of four sub-systems. Closest to the interaction point, the Insertable B-Layer (IBL) was installed during the shutdown between Run I and Run II. It is a very-high-resolution semiconductor pixel detector, extending up to |η| < 2.9. In order to improve vertex reconstruction and B-hadron identification as much as possible, it
3The Higgs decay channelH →4µis very important, since it is extremely clean to reconstruct while being presented with very low Standard Model background.
70 Chapter 4. The ATLAS Detector at the LHC
Figure 4.6: Schematic view of the ATLAS Detector and its sub-systems [62].
was installed as close as possible to the interaction point, around a new, thin beam pipe at a radial distance of only 3.3 cm from the beam axis. This requires the sensors to be very robust against ionising radiation. The IBL is surrounded by the Pixel Detector, consisting of three layers of semiconducting pixels in the barrel region and three discs in each end-cap.
It extends up to|η|<2.5between five and 12 cm radial distance from the interaction point.
Its high granularity requires 80 million read-out channels and leads to a spatial resolution of 10µm×115µm in R−φ×z. A silicon microstrip detector, the Semi-Conductor Tracker (SCT), is located at radii between 30–51 cm from the interaction point in the region of
|η|<2.5. Each of the four barrel layers and2×9end-cap disks contains two sub-layers with tilted strip orientations, providing a spatial resolution of 17µm×580µm inR−φ×z. The outermost part of the ID is the Transition Radiation Tracker (TRT), a system of gas-filled straws that are parallel to the beam pipe in the barrel and radially oriented in the end-caps.
It extends up to a radius of 108 cm and|η|<1.96. By exploiting the difference in emitted radiation between electrons and other particle species when they traverse the material, it allows for a very good separation of electrons and other particle types, pions in particular.
The lower spatial resolution (130µm) is compensated by the many hits provided per track (36) and the larger track length. An overview of the arrangement of the ID sub-systems is
Chapter 4. The ATLAS Detector at the LHC 71
Figure 4.7: Schematic view of the ATLAS Inner Detector [63]. The IBL (not shown here) is located between the beam pipe and the inner-most layer of the pixel detector.
given in Fig. 4.7. The ID ensures a precise tracking, enabling an efficient reconstruction of particle momenta and primary and secondary vertices. The latter is especially important to reject pile-up and in the identification of B-hadrons and tau leptons. A momentum resolution of σpT/pT = (0.05%·pT[ GeV] + 1%) [61] is targeted.
Calorimeter: An electromagnetic calorimeter, developed to contain and measure the showers of electrons and photons, is surrounded by a hadronic calorimeter. The electro-magnetic calorimeter is built as a sampling calorimeter, meaning that alternating layers of absorbing and active material are used. Liquid Argon is used as active material and is com-bined with lead absorbers. The absorbers, as well as the electrodes are accordion-shaped to prevent detection gaps in transverse direction. Its thickness ranges between 22 (central) and 24 (forward) radiation lengths to ensure that the full electromagnetic shower is con-tained. In the barrel region it consists of two half-barrels with 16 modules each and extends up to radii of 4 m and |η| < 1.475. An additional pre-sampler layer is added right after the Inner Detector to estimate the energy that has been lost before the particles enter the
72 Chapter 4. The ATLAS Detector at the LHC calorimeter. The end-cap regions are equipped with eight wedge-shaped modules each. It is designed to achieve an energy resolution ofσE/E = (10%/p
E[ GeV] + 0.7%) [61]. The high granularity in the first layers of this calorimeter and its longitudinal separation allows for a reconstruction of the photon direction and to disentangle close-by photons.
The outer hadronic calorimeter combines scintillating tiles with steel absorbers. It reaches a thickness of ten interaction lengths and is hence able to fully stop particles up to energies of several TeV.4 The tile calorimeter covers |η|<1.7and is supplemented at larger pseudo-rapidities of up to |η| < 3.2 by the hadronic end-cap calorimeter (HEC), a copper-liquid-Argon calorimeter, consisting of two discs per end-cap. An energy resolution of σE/E = (50%/p
E[ GeV] + 3%) [61] is aimed for.
The forward calorimeter covers3<|η|<4.9and is composed of copper-tungsten as absorber and liquid Argon as active material, combining electromagnetic and hadronic calorimetry.
The achieved energy resolution is expected to be: σ/E = (100%/p
E[ GeV] + 10%) [61].
The calorimeter system with its sub-detectors is sketched in Fig. 4.8.
Muon Spectrometer: The function of the ATLAS muon system is twofold. It provides precise measurements of trajectories of muons as well as trigger signals for events containing muon candidates. The muon momentum measurement is based on the bending of tracks in the field of superconducting toroid magnets. This is ensured in the range of|η|<1.4by the barrel toroid, providing a field of up to 0.5 T, between1.6<|η|<2.7, the smaller end-cap toroids provide a magnetic field of up to 1 T. Each toroid consists of eight coils, arranged with an eight-fold azimuthal symmetry around the calorimeters. While the solenoid around the ID causes a bending in the transverse plane orthogonal to the beam pipe, the toroids deflect the muon tracks in the longitudinal direction. The three cylindrical layers of precision tracking chambers consist of so-called Monitored Drift Tubes (MDTs), supplemented by Cathode Strip Chambers (CSCs) in the forward region beyond |η|>2.7. Due to support structures, there is an uninstrumented gap at η ∼ 0. The trigger chambers need to operate fast and rely on Resistive Plate Chambers (RPCs) and Thin Gap Chambers (TGCs). The muon system extends up to a radius of over 20 m from the interaction point and is the largest sub-detector by volume. An overview is given in Fig. 4.9. The design muon momentum resolution isσpT/pT = 10%at a muon pT of 1 TeV [61].
4For very energetic particles the energy might not be fully contained in the calorimeter in some cases and a signal in the muon system is observed, which is calledpunch-through.
Chapter 4. The ATLAS Detector at the LHC 73
Figure 4.8: Schematic view of the ATLAS calorimeters [64].