3.2 The ATLAS detector
The ATLAS detector is a cylindrical-shaped multi-purpose detector, with length of 44 m, diameter 25 m and weight of approximately 7000 tons. A longitudinal view of the detector is shown in Fig.3.4.
The purpose of ATLAS detector is to measure properties of particles. Due to the variability of particles produced in ppcollisions and the manner of their interactions, ATLAS is composed of a system of different sub-detectors, a magnet system and a trigger system. There are three main sub-detectors: the inner detector, the calorimeter and the muon spectrometer. The ATLAS coordinate system and a brief description of the individual detectors is provided in the following sections based on Ref. [90].
Fig. 3.4: Longitudinal view of the ATLAS detector [90].
3.2.1 ATLAS coordinate system
We first introduce the coordinate system and some of the quantities typically used in high-energy collider physics. ATLAS uses a right-handed coordinate system, where thez-axis points in the direction of the beam pipe. The x− y plane is perpendicular to the beam pipe, with x coordinate pointing towards the centre of the LHC and ypoints upwards.
A spherical coordinate system is introduced, whereφis the azimuthal angle in the x−yplane with respect to the x-axis direction. The direction with respect to the z-axis is denoted by angle θ. It is customary to use pseudorapidityη instead of θ for relativistic particles, due to ∆η being Lorentz-invariant under boosts along thez-axis. Pseudorapidity is defined as follows:
η =−log tan θ 2
! !
. (3.2)
Distances between particles or other reconstructed objects are characterised via distance metric
3. The LHC and the ATLAS experiment
∆R=q
(∆η)2+(∆φ)2. This quantity is also Lorentz-invariant underz-axis boosts.
Due to the random nature of the total longitudinal momentum in theppcollision, it is typical to refer totransversequantities, such as thetransverse momentumpT, defined as:
pT=q
p2x+py2= |~p|sinθ (3.3)
The sum of transverse momenta of all particles produced in appcollision is expected to be zero due to momentum conservation and the interacting partonspT ≈0.
We will now briefly describe the individual detector subsystems of the ATLAS and its magnet and trigger systems.
3.2.2 The inner detector
The inner detector (ID) [91–93] is the closest detector to the interaction point, consisting of several types of detectors: semiconductor pixel detector, micro-strip semiconductor tracker (SCT) and a straw tube detector, theTransition Radiation Tracker(TRT). The purpose of the inner detector is the reconstruction of charged particle tracks and vertices, measurement of particle momentum and electron identification. The layout of the inner detector is shown in Fig.3.5. The ID is placed within a 2 T magnetic field to allow measurement of electric charge and momentum of particles. The pseudorapidity coverage of the ID is|η| <2.5. The momentum resolution is
σ
pT =0.05%×pT⊕1%(1), (3.4)
wherepTis in GeV.
The pixel and SCT detectors are silicon-based semiconductor detectors, which are designed for reconstruction of particle tracks and vertices. The tracks are reconstructed fromhits, electron-hole pairs produced by the passage of the charged particles in the detector’s sensitive volume, and the electric signal is processed by the read-out electronics. To reduce the amount of noise from thermal electron-hole pairs, and to remove the heat produced by additional leakage currents from radiation damage, the semiconductor detectors are cooled down by an evaporative system.
The pixel detector is the closest detector to the interaction point. It was composed of three layers of silicon pixel detectors and for Run-II an additional layer called IBL was added for the purpose of improving identification of secondary vertices fromB-hadrons. The nominal pixel size is 50×400 µm2, each having a separate readout. The pixel detector thus contains more than 80 million readout channels.
90 % of the channels have spatial resolution 50 µm in thex−yplane and 400 µm inz-axis direction.
Surrounding the pixel detector is the SCT, a micro-strip detector with similar purpose and technology as the pixel detector, however it is composed of four layers of longer rectangular strips rather than pixels. Each strip measures 80 µm×6−12cm. To resolve ambiguities, hits are reconstructed from energy deposits in two layers. The SCT contains over 6 million SCT readout channels. The spatial resolution of SCT is 16 µm inx−yplane and 580 µm inz.
Surrounding the silicon tracking detectors is the TRT detector composed of straw-tube detectors.
Charged particles passing through the TRT pass through environments with different refraction indices,
(1)The⊕operator designates sum of operands in quadrature.
3.2. The ATLAS detector
Fig. 3.5: The layout of the ATLAS inner detector. [94]
emitting transition-radiation photons. These photons are absorbed in a mixture of gasses inside the straw tubes. The amplitude of the signal depends on the Lorenz factorγ. This factor is typically very large for lightest particles like electrons and positrons, thus the signal from TRT is not only used for enhancing momentum resolution but also for electron/pion discrimination and fast triggering. In contrast to other components of the ID, the TRT covers region of only up to |η| < 2.0. The TRT consists of almost 30 000 straws, where each straw is 144 cm long (37 cm for the end caps), with a diameter of 4 mm. Each straw contains a 30 µm diameter gold-plated Wolfram-Rhenium wire that collects ionised electrons from the absorbed transition radiation and ionization produced by charged particles passing through the straw. The straws are filled with a gas mixture of 70% Xe, 27% CO2and 3% O2. The electron collection time is 48 ns with a drift-time accuracy of 130 µm.
3.2.3 Calorimeters
In addition to momentum measurement for charged particles, the measurement of energy of both charged and neutral particles is necessary. The calorimeters in ATLAS are designed for this purpose. The calorimeter system covers large pseudorapidity range of|η| < 4.9 and is divided into electromagnetic (EM) calorimeter and a hadronic calorimeter that surrounds the EM calorimeter. The EM calorimeter has finer granularity than the hadronic calorimeter, allowing for high precision measurements of electron and photon energy. Both calorimeters are sampling-type, which means they contain layers of active medium sensitive to the energy of the particle, as well as passive absorbers. This design is a compromise between the size and cost of the calorimeters and the requirement to absorb the full energy of showers induced by the particles produced inppcollisions.
Two types of calorimeters are employed within ATLAS in terms of detection material, the liquid
3. The LHC and the ATLAS experiment
Argon (LAr) calorimeter [95], which uses LAr as the active medium and lead absorber plates of an accordion shape. The particle-induced showers produced in the absorber ionize liquid argon atoms and the collected electrons produce a signal read out by electronics. The second type of calorimeter used in ATLAS is the Tile calorimeter [96], which uses alternating passive layers of iron and tiles of plastic scintillator as the active medium. The shower particles interacting with scintillator produce visible light which is converted in photomultiplier tubes into electric signal.
The EM calorimeter consists of the barrel calorimeter, which is a LAr calorimeter covering
|η| < 1.375, and the end-cap LAr calorimeters covering 1.475 < |η| < 3.2. The size of the EM calorimeter is such that it achieves absorbing capacity of approximately 22 radiation lengths(2). The barrel EM calorimeter consists of three layers, where the first layer has increased granularity for discriminating isolated photons from non-prompt photons from hadron decays. Its granularity is
∆η×∆Φ= 0.0258 ×0.1 for|η| <1.4. The barrel and end-cap EM calorimeters have 101760 and 62208 readout channels, respectively.
The hadronic calorimeter consists of several parts surrounding the EM calorimeter, and is designed to measure the energy of jets produced by strongly-interacting particles. To absorb the showers, it provides the thickness of approximately 11 radiation lengths. The barrel and the extended barrel Tile calorimeters cover regions of|η| < 1 and 1 < |η| < 1.7, respectively. The end-cap hadronic calorimeters use LAr technology with copper plate absorbers, and cover regions of 1.5 < |η| <3.2.
The Tile calorimeter has 5769 readout channels in the long barrel and 4092 channels in the extended barrel and the granularity of 0.1×0.1.
In addition, the forward calorimeter covering 3.1< |η| < 4.9 is a LAr calorimeter made of three layers where the first layer is used as an EM calorimeter and the rest of the layers as hadronic calorimeter.
The absorber layers are made of a single layer with copper and the rest of the layers made of tungsten.
Both the forward and the end-cap calorimeters employ LAr as the active medium due to its larger radiation resistance, which is necessary due to the higher activity in forward regions of the detector.
Finally, the most important characteristic of a calorimeter is its energy resolution. For sampling calorimeters, it is typically parametrised as follows:
σE E = S
√E ⊕ N
E ⊕C, (3.5)
whereS, N andC are the stochastic, noise and constant terms, respectively, andE is in GeV. The stochastic term describes the fluctuations in the particle multiplicities of the shower in the active medium. The noise term describes the contributions of noise from the readout electronics, and is not energy-dependent. The constant term cumulates all of the systematic effects, which can range from imperfections in the detector structure or readout system, to radiation damage. The constant term is the dominant contribution to the energy resolution at high energies.
The EM calorimeter was designed to achieve S = 10 %, N = 170 MeV,C = 0.7 %. The Tile calorimeter was designed to achieveS=50 % andC =3 %, with a negligible noise term thanks to the precise photomultiplier readout. The end-cap and forward hadronic calorimeter achieveS=100 % andC=10 %.
(2)The radiation length is the thickness of calorimeter that decreases the energy of the particle inducing the shower by a factor ofe−1.
3.2. The ATLAS detector
3.2.4 Muon spectrometer
Due to their large mass compared to electrons, muons traverse calorimeters with minimum energy loss. A standalone system dedicated to their detection and measuring their momentum is installed, surrounding the calorimeters. The muon spectrometers are placed in a magnetic field generated by toroids in order to measure momentum based on the muon track curvature. They cover the region of|η| <2.7. The momentum measurement is performed by combining information from the muon detectors and the ID, achieving a combined momentum resolution of 2-3 %, except for very high momenta (> 1 TeV) where the resolution is 10 %. The muon spectrometer consists of several parts: the cathode strip chambers(CSC), themonitored drift tube(MDT)chambers, theresistive plate chambers (RPC) and thethin gap chambers(TGC) [97].
MDTs together with CSCs are used for precise measurement of particles momenta. MDT chambers cover range|η| <2, and are composed of layers of drift tubes with a diameter of 3 cm. The drift tubes are proportional chambers filled with a mixture of Ar and CO2gasses and contain a central wire anode.
The position of a crossing muon with respect to the anode is determined from the ionised electron cloud drift time. The average position resolution per tube is 80 µm and the maximum drift time is 500 ns.
CSCs cover the end-cap region of 2.0< |η|< 2.7 and are radially oriented in planes perpendicular to the beam pipe. CSCs are multi-wire proportional chambers with fast readout, with cathode planes segmented into strips in orthogonal directions. This enables CSC to measure both coordinates of charged particles and to cope with the larger particle multiplicities in the forward region. The resolution of chambers is 40 µm in the bending plane and 5 mm in the transverse plane.
RPCs and TGCs are used for fast trigger decisions due to their extremely fast operation. RPCs are very thin gas chambers with parallel Bakelite resistive plates, covering the barrel region of|η| <1.05).
Their fast gathering of ionised electron achieve readout time of 1.5 ns. The TGCs are multi-wire proportional chambers that cover the end-cap regions (1.05 < |η| < 2.4). Their fast operation is achieved by using a gas mixture with high quenching abilities, reducing the amount of ionised electrons that are collected by the anode, and thus achieving readout time< 25 ns. The fast readout of both RPCs and TGCs is also used to detect beam crossing.
3.2.5 Magnet systems
As we have mentioned, the ATLAS magnet system [98] forms a crucial part of the detector systems, for particle momenta and charge measurement. It consists of three parts: inner solenoid [99], barrel toroid [100], and end-cap toroids [101]. All of the magnet systems are based on super-conducting niobium-titanium cooled down by liquid Helium to a temperature of 4 K.
Theinner solenoidsurrounds the inner detector, creating mostly homogeneous 2 T magnetic field.
The inner solenoid is very thin to absorb as little particle energy as possible, for that reason its width comprises only a 0.8-times radiation length. This is achieved partially by sharing the cryogenic system of the solenoid with the EM calorimeter.
Thebarreland theend-capair-filled toroids provide magnetic field for the muon spectrometers.
Each of the toroids is built from 8 radially-arranged rectangular coils. The magnetic field varies in strength up to 2 T in the barrel region, and up to 4 T in the end-cap regions.
3. The LHC and the ATLAS experiment
3.2.6 Triggers
The trigger system plays an important role in the data acquisition process, because the event rate at ATLAS is far beyond any technology available to process. The trigger system reduces the rate by picking events with interesting signatures. Compared to Run-I, the trigger system in Run-II underwent a significant upgrade to accommodate rates up to 5×higher due to the increased LHC luminosity [102–104]. The trigger systems consists of hardware-level L1 trigger [105] and software high-level trigger (HLT) [106].
L1 trigger is a hardware trigger that searches for high pT muons, electrons, photons and jets.
It combines the information from the fast muon spectrometer subsystems and reduced-granularity information from the calorimeter, while maintaining a latency of 2.5 µs. It reduces the data rate from approximately 40 MHz to around 100 kHz. In each event L1 also identifies one or more Regions-of-Interest (ROIs), which are coordinates inη andΦwith a potentially interesting activity. The ROI information is passed to the HLT. The L1 trigger does not attempt to use information from track reconstruction.
HLT is a software trigger using a large computer farm, which uses information from L1 trigger. In contrast to L1 trigger, the HLT also includes tracking information and all other detector information with full granularity within the ROIs, applying algorithms similar to the offline reconstruction of events.
The HLT reduces the trigger rate to approximately 1 kHz with a latency of approximately 200 ms.