The ATLAS Pixel detector and its use in a Search for Metastable Heavy Charged Particles

The discovery of the Higgs boson, the missing piece in the Standard Model puzzle, at the electroweak scale in 2012 by the ATLAS and CMS experiments, closed an important season of particle physics and a search lasted 50 years. Even though the discovery of the Higgs boson is a great achievement, the Standard Model is incomplete, since it does not include the gravitational field and can not explain some experimental measurements such as the dark matter observed in galaxy studies and the matter and anti-matter asymmetry observed in the universe. The experiments at LHC have the exciting goal to give answers to the SM open questions and make available the hint or the evidence that may allow to proceed beyond it. An introduction on the Standard Model and the LHC is provided in Chapter 1 where the ATLAS detector is also described. ATLAS is the largest of the detectors placed along the LHC ring and is able to detect products from pp and heavy ion collisions. The detector has a cylindrical geometry around the interaction point. The inner part of the ATLAS detector reconstructs trajectories of the charged particle that cross it, the position of the proton-proton collision and eventually the secondary vertices due to particle decays (e.g. $b-$hadrons). The silicon Pixel Detector is the innermost component of the tracker and is located few centimetres away from the interaction region. It provides at least three high accuracy space-point measurements per track and plays a crucial role for tracking, vertexing, and $\it{b}$-tagging capabilities of ATLAS. The reader can find some useful information on the interactions of the particles in matter and on semiconductor tracking detectors in Chapter 2. The LHC first beam circulated in 2008. To increase discovery potential, the luminosity\footnote{The luminosity is a measurement of the number of collisions that can be produced in a detector per unit of cross section and per second. LHC is designed to produce a luminosity $\mathscr{L}\sim10^{34}$ cm$^{-2}$s$^{-1}$.} of the machine has been boosted constantly and the detector has been improved to cope with the new luminosity conditions. During the first LHC long shutdown (LS1) in 2014, to maintain ATLAS performance, the Pixel Detector has been upgraded to a 4-layer detector with the addition of the Insertable B-Layer (IBL). IBL is a new detector installed between the existing pixel detector and a new beam-pipe of smaller radius in order to ensure and maintain excellent performance of tracking, vertexing and jet flavor tagging. My Master Thesis and part of my first Ph.D. year have been focused on the production, characterization and on the study of IBL modules. IBL uses different sensors and readout electronics than the old 3-layer detector. A description of IBL and some results from production are reported in the first part of Chapter 3. The different accuracy of IBL readout electronics and the different behaviour for large energy releases requires a non trivial track $dE/dx$ calculation algorithm. I have developed a new track $dE/dx$ computation algorithm that includes the IBL information. It has been validated and integrated in the ATLAS reconstruction framework (Athena), showing improvement in the $dE/dx$ resolution and reducing the residual amount of high-$dE/dx$ tails. A fit has been also performed with an empirical 5-parameters Bethe-Bloch formula based on Blum and Rolandi studies for the ALEPH TPC. It describes how the Most Probable Value of the specific energy loss (MPV$_{dE/dx}$) depends on the particle $\beta$$\gamma$. I worked also to increase the performance of the $dE/dx$ observable, developing a $dE/dx$ path correction thus correcting its dependence on the track pseudorapidity $\eta$. he Pixel track $dE/dx$ can be used to detect massive, highly ionizing particles such as charged $R$-hadrons (sparticles formed by squarks or gluinos and ordinary quarks) and charginos. These particles are predicted in many extensions of the Standard Model and they appear, for example, in both $R-$parity conserving and violating supersymmetry (SUSY) models and in universal extra dimensions theories. $R$-hadrons and charginos can be stable, or metastable\footnote{If a particle decays inside the detector volume is considered metastable}. If produced at the Large Hadron Collider, such heavy particles will be moving non-relativistically and therefore they will be identifiable through the measurement of an anomalously large specific energy loss or time of flight in various ATLAS subdetectors, including the Pixel Detector. In Run-1 a pixel-only analysis approach, whereof the analysis proposed in this thesis would be the continuation, has been also performed. The tracker-only approach has sensitivity to metastable particles, with a lifetime ranging from a fraction of a nanosecond to several tens of nanoseconds. After LS1 the LHC collision energy has been increased from 8 TeV to 13 TeV, thus enhancing the cross-section to generate heavy particles through the gluon-fusion mechanism. The larger the mass, the larger the cross section increase. As an example the pair production of 1.5 TeV gluinos is boosted by a factor close to 50, while the background increases much less (a factor between 2 and 3). Results on a search for massive charged long-lived particles performed with LHC $\sqrt{s}=$13 TeV data with an integrated luminosity of 3.2 fb$^{-1}$ are shown in Chapter 5. The Large Hadron Collider is working beyond expectations in terms of delivered instantaneous luminosity. The \emph{in time pile-up} produced by the multiple collisions in the same bunch crossing is problematic for the Pixel readout electronics, in particular in terms of readout bandwidth saturation. At the end of 2013, the number of Pixel Detector disabled modules were increased from an initial 2\% to 5$\%$. For these reasons during LHC LS1 the pixel data transmission system was completely renewed: new data cables, new electrical-to-optical converter boards, and new data fibres were installed. During the 2015-2016 LHC Winter Shutdown, the Pixel off-detector electronics (described in the second part of Chapter 3) has been also upgraded. In this contest, the last part of Chapter 3 is devoted to a description of the test developed to qualify the IBL Readout Driver (IBL-ROD) for the Pixel System, to which I have contributed. Finally Chapter 6 describes the last part of my work and shows some results and studies on the ATLAS Pixel Detector technologies for LHC upgrade in 2025, called High Luminosity LHC (HL-LHC). Here, the experiment will accumulate integrated luminosity up to 3000 fb$^{-1}$ and the inner (outer) pixel layers will be exposed to fluence close to 1.7$\times$10$^{16}$n$_{eq}$cm$^{-2}$ (1.4$\times$10$^{15}$ n$_{eq}$cm$^{-2}$). The HL-LHC luminosity conditions are too extreme for the current ATLAS tracking system. Therefore it will be completely replaced by a new system called Inner Tracker (ITk), an all-new all-silicon tracker to be built using only strip and pixel detectors. Three possible sensor types are considered for the pixel detector upgrade: Planars, 3Ds, CMOS. I have tested new 3D and CMOS sensors and I have studied new hybridization technologies, such as bump bonding at high density or capacitive coupling. For this purpose I have developed a laser setup, that has allowed the measurement of the charge collection inside the pixel cell with few $\mu$m resolution.