Differential Top-Quark-Pair Cross Sections in pp Collisions at $\sqrt{s} = 7$ TeV with CMS and Charge Multiplication in Highly-Irradiated Silicon Sensors

Author: 
Jörn Lange
Date: 
May 2013

Thesis Type:

Modern particle-physics experiments like the ones at the Large Hadron Collider (LHC) are global and interdisciplinary endeavours comprising a variety of different fields. In this work, two different aspects are dealt with: on the one hand a top-quark
physics analysis and on the other hand research and development towards radiationhard silicon tracking detectors.
The high centre-of-mass energy and luminosity at the LHC allow for a detailed investigation of top-quark-pair (tt) production properties. Normalised differential $t\bar{t}$ cross sections $\frac{1}{\sigma} \frac{d \sigma_{t\bar{t}}}{dX}$ are measured as a function of nine different kinematic variables X of the $t\bar{t}$ system, the top quarks and their decay products (b jets and leptons). The analysis is performed using data of proton-proton collisions at $\sqrt{s} = 7$ TeV recorded by the CMS experiment in 2011, corresponding to an integrated luminosity of 5 fb$^{-1}$. A high-purity sample of tt events is selected according to the topology of the lepton+ jets decay channel. Lepton-selection and trigger eciencies are determined with data-driven methods. The top-quark four-vectors are reconstructed using a constrained kinematic fit. The reconstructed distributions are corrected for background and detector effects using a regularised unfolding technique. By normalising the differential cross sections with the in-situ measured total cross section, correlated systematic uncertainties are reduced, achieving a precision of typically 4-11%. The results are compared to standard-model predictions from Monte-Carlo event generators and approximate nextto-
next-to-leading-order (NNLO) perturbative QCD calculations. A good agreement is observed.
A high-luminosity upgrade of the LHC (HL-LHC) is envisaged for 2022, which implies increased radiation levels for the silicon tracking detectors. The innermost pixel layer is expected to be exposed to a 1-MeV-neutron-equivalent uence in the order of 10$^{16}$ cm$^{2}$. The novel effect of radiation-induced charge multiplication (CM) is studied as an option to overcome the expected signal-to-noise degradation due to radiation damage (mainly due to charge-carrier trapping). Epitaxial silicon pad diodes
of 75-150 $\mu$m thickness and of standard and oxygen-enriched materials are investigated after irradiation with 24 GeV protons up to equivalent uences of 1016 cm$^{2}$. Charge collection in response to different radiation (670, 830, 1060nm laser light, $\alpha$ and $\beta$ particles) is studied with the transient-current technique and a $^{90}$Sr $\beta$ setup. The
different penetration properties of the radiation types are used to localise the CM region. The dependence of CM on voltage, fluence, thickness, material, temperature and annealing time is studied, as well as its proportionality, spatial uniformity and longterm stability. The absolute amount of charge in response to particles is measured, and the impact of CM on noise, signal to-noise and the charge-spectrum width is investigated. Implications for realistic segmented devices at the HL-LHC are discussed.

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