ATLAS Overview

ATLAS is designed for the general purpose analysis of proton–proton collisions. Therefore, it was designed to provide nearly 4π angular coverage around the collision point with systems to determine the kinematics of the collision products. The detector has been designed in such a way as to be able to observe, as much as possible, the potential new phenomena that may appear at the TeV scale. These considerations are outlined below.

The physics case for the general-purpose LHC experiments, ATLAS and CMS, was primarily to find and study the SM Higgs boson, if it existed. Confirming that a new resonance is the Higgs, and not some other particle, requires the discovery and precise measurement of several of the Higgs’ production and decay mechanisms. The study of these mechanisms provided benchmarks for the performance of all of the detector sub-systems. For example, the decay mode H→γγ, while expected to be small for a

1The VELO is retracted out when the beam is initially filled and needs to be focused, and in once beams have been declared stable.

SM Higgs, is one of the golden modes for Higgs discovery because of the extreme rarity of photons in proton–proton collisions. To identify and measure the photons, however, requires excellent particle tracking and highly performant calorimetry. The discovery of this channel is also helped by the longitudinal segmentation of the EM calorimeter, which allows the direction of the photon to be (roughly) measured and matched to the hard collision process. Similarly, the decay channel H→ZZ^∗→ℓ⁻ℓ⁺ℓ⁻ℓ⁺ requires excellent muon and electron identification and charge determination over a large pT range and H→bb requires excellent jet and secondary vertex reconstruction to identify b decays from the predominantly light quark backgrounds. The H→W W channel has large missing transverse momentum from the neutrinos from the W decay. Measurements in this channel therefore require good transverse momentum reconstruction, which implies hermetic electromagnetic and hadronic calorimetry with high resolution. The large luminosity requires that the detector components have fast, radiation-hard electronics and sensors with high granularity to handle the particle flux and reduce the influence of overlapping events. All of these requirements come into play in top pair reconstruction, which requires lepton identification, jet and missing transverse momentum reconstruction, and good tracking for the identification of b-jets from their secondary vertices.

The ATLAS detector is designed as a series of coaxial cylindrical subdetectors enclosing the proton–proton interaction point (the barrel), with a forward-backward symmetric series of disc-shaped subdetectors to complete the 4π coverage (the end-caps). From inside out, the detector contains: a tracking system designed to detect and track charged, high energy particles coming from the central collision point, a calorimeter designed to measure the direction and energy of the outgoing particles, and a muon spectrometer designed to explicitly detect and identify muons from the collision event. The detector also has a three level, configurable triggering system, which works to identify and then store to disk the events of physics interest out of the massive number of collisions.

Figure 3.3 shows a computer-generated image of the ATLAS detector in cut-away view, displaying all the detector sub-systems. Figure 3.4 shows an example of a dileptonic top pair event, illustrating the response of the ATLAS detector to these events. In particular, it illustrates the way the different sub-systems respond differently to the various particles produced in these events. The general performance goals of the detector are listed in table 3.2.

ATLAS uses a standard coordinate system and notation to describe the particles emerging from the interaction. The nominal interaction point, which is the centre of

Table 3.2: General performance goals of the ATLAS detector. Note that, for high-pT muons, the muon-spectrometer performance is independent of the inner-detector system.

The units for E and p_T are in GeV.

Detector component Required resolution η coverage Measurement Trigger

Tracking σ_pT/p_T = 0.05%p_T ⊕1% ±2.5

EM calorimetry σ_E/E = 10%/√

E⊕0.7% ±3.2 ±2.5

Hadronic calorimetry

barrel and end-cap σE/E = 50%/√

E⊕3% ±3.2 ±3.2

forward σE/E= 100%/√

E⊕10% 3.1<|η|<4.9 3.1<|η|<4.9 Muon Spectrometer σp_T/pT = 10% at pT = 1 TeV ±2.7 ±2.4

Figure 3.3: Overview of the ATLAS detector, including most of its subsystems shown in a cut-away view. From [92]

Figure 3.4: Event display of a top paire-µ dilepton candidate with twob-tagged jets. The electron is shown by the green track and calorimeter cluster in the 3D view, and the muon by the long red track intersecting the muon chambers. The twob-tagged jets are shown by the purple cones, whose sizes are proportional to the jet energies.

The inset shows the XY view of the vertex region, with the secondary vertices of the twob-tagged jets indicated by the orange ellipses. Taken from [99]

the detector, defines the origin of the coordinate system. Thez-axis is defined to be in the direction of the beam at the origin, with the positive side in the counter-clockwise direction of the LHC ring. The x-y plane is defined so that the coordinate system is right-handed, with they-axis pointing upward and thex-axis pointing to the centre of the LHC ring. Thex−y plane is commonly referred to as the transverse plane, as it is transverse with respect to the beamline. Typically, rather than using the polar angle from the z-axis θ, the pseudo-rapidity η=−ln tan^θ₂ is used. This is because in the limit of a massless particle, the pseudo-rapidity is equivalent to the rapidity y = ¹₂ln^E+p_E−p^z

z, which is a quantity invariant with respect to Lorentz boosts along the z-axis. Solid angle distances ∆R are then measured using the difference in pseudorapidity and the azimuthal angle φ giving ∆R=p

∆φ²+ ∆η².

Another important concept is the “fiducial volume” of the detector. This is defined as the kinematic acceptance of the detector for the various objects that are reconstructed.

Since ATLAS is (approximately) cylindrically symmetric (so has complete azimuthal acceptance) and final state objects are approximately massless compared to their momenta, this usually refers to theη andpT acceptances. This is analysis dependent and the ranges of the objects considered in the analysis of this thesis are presented in section 4.1.

Envelopes

1771.4 2115.2 2505 2720.2 0

400.5 495 580 650 0

Figure 3.5: Schematic view of the ATLAS Inner Detector (ID). The diagram shows ar−z slice of the cylindrical barrel, disc endcaps along with the support tubes and solenoid providing charged particle bending inside the ID. Lines of constantη are indicated. Taken from [92].