Cette thèse a étudié les performances de capteurs pixellisés et l’optimisation de la géométrie d’un nouveau trajectomètre interne multi-couche pour l’expérience BESIII. Les bons résultats initiaux obtenus avec le prototype FSBB, com-portant des pixels de grande taille, ont été confirmés par la caractérisation d’un circuit ultérieur, MIMOSA22THRb [7]. Il est ainsi validé, qu’un pixel de 39.0×50.8 µm2 permet d’atteindre une résolution spatiale intrinsèque de l’ordre de 10 µm avec une efficacité de détection proche de 100%. Cette con-firmation ouvre la voie pour le développement d’échelle de grande longueur, supérieure au mètre, dont BESSIII a besoin.
Un logiciel générique proposant une stratégie d’optimisation pour un trajec-tomètre pixellisé a été mis au point et appliqué au cas particulier du SPT de BESSIII. Il a permis de montrer qu’une configuration à trois couches simple-face optimise la résolution sur les trajectoire alors qu’une option incluant une couche double-face atteint une efficacité de reconstruction supérieure.
disposi-xxviii Résumé en Français
tifs de trajectométrie, notamment pour un futur collisionneur électron-positon en Chine (CEPC). Ils indiquent la possibilité de remplacer les dispositifs de détection gazeux usuellement employés à grand rayon mais limitant le taux d’interaction observables, par des dispositifs exploitant entièrement la tech-nologie des capteurs à pixels CMOS. L’outil logiciel donne également la pos-sibilité dans les études d’optimisation de considérer à la fois la résolution sur l’impulsion et celle sur l’extrapolation des trajectoires, qui sont deux quan-tités majeures pour l’identification robuste des états finals produits dans les collisions.
Introduction
In the field of τ -charm physics, the BEijing Spectrometer III (BESIII) experiment at Beijing Electron Positron Collider II (BEPCII) is remarkable for its fruitful results in-cluding a series of discoveries like four-quark bound states and precise measurements for R parameter, the τ mass and various decay modes of J/ψ, ψ′ and ψ(3770). One of the dom-inating sub-detector to bring the physics results is the multilayer drift chamber (MDC) which is the main tracker constructed with a spatial resolution better than 130µm on average and a high transfer momentum resolution around 0.5%@1GeV /c. Nevertheless, it suffers from ageing effects which lead to performance degrading in terms of detection efficiency, momentum resolution and spatial resolution. From the analysis in 2013, the efficiency of the MDC inner cell could be around 70% and its spatial resolution degraded from ∼ 120µm to ∼ 300µm in the worst case. This ageing effect would prevent BESIII to take as much data as its designed and would decrease the precision of measurements. The collaboration decided therefore to upgrade the original inner chamber with a newly designed one or with a tracker using more advanced technology. The project of a Silicon Pixel Tracker (SPT) was proposed in this perspective.
CMOS Pixel Sensor (CPS) also named as Monolithic Active Pixel Sensor (MAPS), first developed at IPHC, has offered an opportunity for nuclear and particle physics ex-periments to address the physics studies requiring accurate spatial resolution and low ma-terial budget. Therefore, it is possible now to open up a research frontier to replace a gas chamber with a CPS based silicon tracker. The CPS technology has been fully validated by the STAR experiment at the Relativistic Heavy Ion Collider (RHIC) in Brookhaven National Laboratory. New generations of CPS for particle physics applications are being researched and developed under the promotion of ALICE experiment at the large hadron collider (LHC) for the upgrade of its vertex detector. On the other hand, a more radia-tion tolerant technique of High voltage CMOS pixel sensor is favoured by detectors like
xxx Introduction
A Toroidal LHC ApparatuS (ATLAS) and the Compact Muon Solenoid (CMS), both of which were constructed for high luminosity p-p collisions. Benefiting from the rapid ex-pansion of CPS technology in high energy physics experiments, this thesis could address the development of BESIII inner tracker using CPS sensors to replace the original gas chamber.
The required spatial resolution for BESIII is less demanding than for vertex detectors like the Heavy Flavor Tracker (HFT) in STAR, because the generated D mesons are almost at rest and there is no production of a jet from hadronizations. Therefore, the spatial resolution of CPS sensors that are developed for vertex detectors exceeds the requirement of a tracker and leads to a waste of power consumption and data transmission bandwidth. The CPS with larger pixel size is then favoured by BESIII. As the main specification of a tracker, the momentum resolution of the inner tracker is chosen as the figure of merit to guide the geometry optimisation. In this thesis, several sensors with larger pixel pitches have been tested to validate the latest CPS technology and the general considerations of detector design problems in tracking theory have been discussed and summarized. With the answers of the mathematical optimisation problem for tracking, the potential of CPS applications in BESIII will be explored by evaluating and optimising several tracker designs.
The chapters composing the thesis are organised as below:
• In Chapter 1, Beijing spectrometer III (BESIII) experiment at Beijing Electron Positron Collider II (BEPCII) will be introduced. The physics goals and detector design of BESIII will be presented and the ageing effect of the multilayer drift chamber will be emphasised since it is the motivation for this thesis.
• In Chapter 2, CMOS pixel sensor technology and performance will be presented to demonstrate the possibility of CPS to meet the requirements of BESIII inner tracker and vertex detectors in future colliders. The mature rolling-shutter design architecture and the latest asynchronous readout strategy will be illustrated with the design of state-of-the-art MIMOSA-28 chip and the design of the frontier ALPIDE chip respectively. Sensor test facilities and their physics objectives will also be introduced.
• In Chapter 3, general considerations of sensor design optimisation for BESIII will be presented. To verify the latest CPS technology, the data taken from the beam test of single arm large area telescope (SALAT) and the test about full size building block
Introduction xxxi
(FSBB) sensors will be analysed. Considering the published results of MIMOSA-22THRb, the sensor performance with different pixel sizes and CMOS processes will be compared to show that the new TowerJezz 0.18 µm technology could make CPS work with larger pixel size, lower power consumption and higher readout speed than MIMOSA-28 in AMS 0.35 µm process, which offers more options for CPS designs. • In Chapter 4, the two widely used tracking algorithms, generalized least squares
(GLS) method and Kalman filtering/smoothing technique will be shown with deriva-tions and examples. The implicaderiva-tions of tracker parameters like the number of layers, the intrinsic resolution of each layer and the length of lever arm, will be illustrated using parabola model and a simple tracker geometry with uniform spac-ing. A Kalman filter implementation for beam trajectories will also be presented. Concerning track reconstruction efficiency, the method used in STAR experiment will be introduced for the related calculations.
• In Chapter 5, the general strategy to optimize barrel silicon pixelated trackers will be given using tracks in the bending plane, and the geometry optimization for vertexing will also be presented using telescope systems. In addition, the income of a double-sided ladder like the Pixel Ladders with Ultra-low Material Embedding (PLUME) on tracking performance, will also be discussed. The R&D is based on the algorithms described in Chapter 4.
• In Chapter 6, simulation tools and the relevant results will be presented. The standalone Geant4-based simulation software will be presented. It is developed for silicon tracker studies and the validation of the optimization strategy in Chapter 5. The full Monte-Carlo simulations processed in BESIII offline software system, will also be illustrated to show the overall performance of the silicon pixel tracker intensified with the outer gas chamber of BESIII.
• In the last chapter, the results of this thesis will be summarized with the main conclusions, and the perspectives of CPS based vertex detector at Circular Electron Positron Collider (CEPC) will be foreseen.