2.5 Beyond the Standard Model
2.5.2 Extra Dimensions
Another set of theories propose the existence of extra dimensions beyond the 3+1 space- time dimensions we directly observe in order to incorporate gravity into the SM [49]. The fields of the SM only couple weakly to the extra dimensions while gravity (via the graviton) couples strongly, reducing the strength of the gravitational force relative to the electroweak and strong forces and solving the hierarchy problem. In order for the extra dimensions to be hidden, they are proposed to be small and compact. The phenomenology of the various models with extra dimensions depends on the number and geometry of the additional dimensions but most models result in so called Kaluza-Klein particles. These additional resonances could potentially provide a dark matter candidate [58] or modify Higgs production at the LHC [59].
Chapter
3
The ATLAS Detector
The ATLAS experiment [60] is one of four large experiments located at the LHC. The ATLAS detector is a multi-purpose particle detector with roughly 4π coverage and consists of four main detector components: the Inner Detector (ID), the calorimeter system consisting of liquid Argon (LAr) and Tile calorimeters, the Muon Spectrometer (MS), and the magnet system consisting of both solenoidal and toroidal magnets. Furthermore, a triggering system, which is essential due to the high luminosity of the LHC, is integrated into the calorimeter and MS systems. A cut-away view of the ATLAS detector is shown in Figure 3.1. The full detector is roughly 44 m in length and 25 m in diameter, weighing nearly 7000 metric tons [60].
The ATLAS coordinate system is defined as follows: the nominal interaction point is defined as the origin of the coordinate system, while the anti-clockwise beam direction defines thez-axis and thex-yplane is transverse to the beam direction. The positivex-axis is defined as pointing from the interaction point to the center of the LHC ring and the positive y-axis is defined as pointing upwards. The azimuthal angle φ is measured around the beam axis and the polar angle θ is the angle from the beam axis. The pseudorapidity is defined as η = −ln(tan(θ/2)). The positive z side of the detector is designated the A-side, while the negative z side of the detector is designated the C-side. The transverse momentum pT and
Figure 3.1: Cut-away view of the ATLAS detector [60].
transverse momentumET are the components of the momentum or energy in thex-y plane.
The separation of two objects in angular space is defined as ∆R=p∆η2+ ∆φ2.
The ID and its subdetectors are described in detail in Section 3.1 as the geometry is relevant in Chapter4. A brief description of the calorimeters is given in Section3.2and the Muon Spectrometer is described in Section3.3. The triggering scheme is described in Section
3.4.
3.1
Inner Detector
The ATLAS Inner detector (ID) is designed to measure the trajectories of charged particles withpT>500 MeV within|η|<2.5. The ID consists of three separate sub-detectors with an
outer radius of 1.15 m, all contained within a 2 Tesla solenoid magnet. Each sub-detector is divided into barrel and end-cap elements and a cut-away view is shown in Figure 3.2. The
3. The ATLAS Detector
layout of the sensitive detector elements in both the barrel and end-caps of the ID with a pT= 10 GeV track passing through them can be seen in Figure3.3.
At the innermost radius is the Pixel detector, a silicon pixel tracker which provides high resolution position and vertexing measurements very close to the interaction point. The Semi- Conductor Tracker (SCT) is a silicon strip tracker located outside the Pixel detector which provides more precision position measurements of the track trajectory. Generally speaking, silicon sensors are composed of thin, high-purity doped silicon wafers. As a charged particle traverses the wafer, energy is deposited into the silicon and electron-hole pairs are created. A bias voltage is applied to the silicon and these electron-hole pairs are collected as currents read out by the front-end electronics on the surface of the silicon. Due to detector material and budgetary concerns, silicon trackers generally rely on a small number of very high precision measurements (usually at the micron level) to measure track trajectories.
At the outermost radius of the ID is the Transition Radiation Tracker (TRT), a straw-tube tracker which provides additional position measurements and particle identification capabili- ties. Each straw in the TRT is strung with a wire which is held at a positive electric potential with respect to the straw wall (see the diagram in Figure4.1). As a charged particle traverses the straw, gas molecules within the straw are ionized. The ionization electrons accelerate towards the wire due to the electric field in the straw. As they gain energy, the electrons ionize additional gas molecules and an avalanche of electrons is created, which is read out as a current on the wire in the front-end electronics. Gas-based detectors generally provide a much less precise hit position measurement as compared to silicon trackers, but in the TRT this is compensated by the large number of recorded hits.
Figure 3.2: Cut-away view of the Inner Detector [60].
Figure 3.3: Schematic view of the Inner Detector active elements in the barrel on the left and the end-cap on the right [60].
3. The ATLAS Detector