Inner Detector

tan θ

2

. (3.1)

A cross-section of the ATLAS Inner detector in units ofη is shown in Figure 3.3. Pseudo-rapidity is used as a variable in relativistic hadron colliders due to the fact that a shift in pseudorapidity is Lorentz invariant. This means the expected QCD multijet particle rate per shift in pseudorapidity (∆η) is constant. The second variable used at ATLAS is the azimuthal angle, denoted by φ.

The final variables used at ATLAS are the transverse components of momentum (p_T) and energy (E_T). Since the collider uses protons, which contain multiple quarks and gluons traveling at various fractions of the proton’s momentum, the initial longitudinal energies and momenta are unknown for the colliding particles. As a result, the trans-verse components are used since the total initial transtrans-verse energy and momenta are zero.

The resulting imbalance in transverse momenta allows for undetectable particles, such as neutrinos, to be quantified by the missing transverse energy (6ET).

3.2.2. Inner Detector

The first section of the ATLAS detector, and closest to the interaction point, is the Inner detector [94]. The inner detector is designed to perform precision tracking and vertex measurements for charged particles, up to a precision of 5 cm on the vertex position. The section itself is subdivided into several components: the pixel detector, the semiconductor tracker (SCT), and the transition radiation tracker (TRT). The three components are

3. Experimental Setup

Figure 3.3.: The ATLAS inner detector denoted by sections of pseudorapidity (η). The variable η begins at 0 for an upwards trajectory and continues to ∞ when pointing horizontally. The useful region for object reconstruction within the detector extends to an|η| ≈2.5. Figure taken from [93].

shown in Figure 3.4. The three components of the inner detector each contain a barrel and end-cap region. A computer generated image shows the relative distances of each subsection of the inner detector to the beam of protons in Figure 3.5.

Figure 3.4.: The ATLAS inner detector and its main components. The innermost ponent is the pixel detector made of silicon pixel sensors. The second com-ponent, moving outwards from the interaction point, is the silicon strip detector. Finally, located at the outermost section of the inner detector are the transition radiation tracker tubes. Figure taken from [93].

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3.2. The ATLAS Detector

Figure 3.5.: A computer generated image of the inner detector and the three subsections’

distances from the beam. The Inner detector has three components which lie at certain distances away from the beam in order to get a precise position measurement of the particle interaction vertex and the direction and charge of the outgoing charged particles. Figure taken from [93].

The pixel detector is composed of tiny pixel sensors (50×400µm²). The sensors are made of silicon, which is a semiconductor detector, where timing and position is read-out.

The pixel detector has three cylindrical layers which lie between 50 mm and 123 mm from the centre of the beam axis. The detector extends to anη range of 2.6. In total, there are over 80 million read-out channels. The resolution of the pixel detector is up to 12 µm.

The second component is the silicon strip detector. The SCT is similarly made of silicon semiconductors, however instead of pixel sensors, long strips are built to cover a larger region. The strips themselves are aligned with a 40 mrad angle to one another in order to reduce ghost hits when multiple charged particles hit the same strips. A single strip covers an area of 80 µm by 12.6 cm. The SCT is setup in four double-layer sections, totaling over 6.2 million read-out channels. The resolution of the SCT detector is up to 17 µm.

The final component of the inner detector is the transition radiation tracker. The TRT is comprised of about 400 000 Xenon gas filled straw tubes. Each straw tube has a radius of 2 mm and a length of 7 m. The gas is comprised of 70 % Xe, 27 % CO₂ and 3 % O₂. At the centre of each tube is a tungsten anode wire. The outside of the tube acts as a cathode. The straw tubes are placed from about 55 cm to 110 cm away from the beam.

The TRT is used to differentiate between charged particles, especially electrons, which can be distinguished from heavier charged particles due to the different amounts of radiation they produce inside the tube. Different materials with differing dielectric constants are placed around the straws. As the charged particles traverse the material, they produce

3. Experimental Setup

transition radiation. The resolution of the TRT is up to 130µm.

All charged particles will leave tracks within the inner detector. It is important that the tracks can be properly reconstructed and that they can be traced back to the vertex.

The reconstructed track efficiency and impact parameter (d₀) are shown in Figure 3.6 for minimum bias √

s = 7 TeV collision events. The reconstructed track efficiency is calculated by the number of matched tracks divided by the number of generated tracks in a certain p_T and η bin. It can be seen that the track efficiency is dependent on p_T, and that the efficiency reaches a plateau of about 80% at track pT of about 1 GeV. Therefore, tracks can be reconstructed in the inner detector beginning at about a pT of 1 GeV. The same plateau is seen as a function of η. The best reconstruction efficiency, about 80%, can be seen in the barrel region of the inner detector. The transverse impact parameter, denoted by d0, is very important for selectingb-jets decaying from t¯tevents. This impact parameter is calculated by the transverse displacement of a reconstructed track compared to the jet axis, to which the track belongs. The d0 is signed positive if the track crosses the jet axis in front of the primary vertex and negative if it crosses behind the primary vertex. This will be explained in further detain in Section 4.1.3, where it is described in the context ofb-jet identification. The performance of the inner detector in data is shown to be as expected by MC simulations. The performance of the inner detector also depends on the magnet system at ATLAS, which helps identify charged particles. The magnet system is described in Section 3.2.5.

Figure 3.6.: (Top): Impact parameter (d0), (left): track efficiency in bins of pT and, (right): track efficiency in bins of η of reconstructed tracks from minimum bias events from√

s= 7 TeV collision events. The plots show very good MC to data agreement for reconstructed track efficiency and d₀ displacement.

Figures taken from [95].

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3.2. The ATLAS Detector