CERN's Discerning Detectors

In this thesis, my work in the Compact Muon Solenoid (CMS) experiment on the search for the neutral Minimal Supersymmetric Standard Model (MSSM) Higgs decaying into two muons is presented. The search is performed on the full data collected during the years 2011 and 2012 by CMS in proton-proton collisions at CERN Large Hadron Collider (LHC). The MSSM is explored within the most conservative benchmark scenario, m_h^{max}, and within its modified versions, m_h^{mod +} and m_h^{mod -}. The search is sensitive to MSSM Higgs boson production in association with a b\bar{b} quark pair and to the gluon-gluon fusion process. In the m_h^{max} scenario, the results exclude values of tanB larger than 15 in the m_A range 115-200 GeV, and values of tanB greater than 30 in the m_A range up to 300 GeV. There are no significant differences in the results obtained within the three different scenarios considered. Comparisons with other neutral MSSM Higgs searches are shown.

This thesis comprises a precision measurement of the inclusive \Zee production cross section in proton-proton collisions provided by the Large Hadron Collider (LHC) at a center-of-mass energy of $\sqrt{s}=7$~TeV and the absolute luminosity based on \Zee decays. The data was collected by the Compact Muon Solenoid (CMS) detector near Geneva, Switzerland during the year of 2010 and corresponds to an integrated luminosity of $\int\mathcal{L}dt = 35.9\pm 1.4$~pb$^{-1}$. Electronic decays of $Z$ bosons allow one of the first electroweak measurements at the LHC, making the cross section measurement a benchmark of physics performance after the first year of CMS detector and LHC machine operations. It is the first systematic uncertainty limited \Zee cross section measurement performed at $\sqrt{s}=7$~TeV. The measured cross section pertaining to the invariant mass window $M_{ee}\in (60,120)$~GeV is reported as: $\sigma(pp\to Z+X) \times \mathcal{B}( Z\to ee ) = 997 \pm 11 \mathrm{(sta t)} \pm 19 \mathrm{(syst)} \pm 40 \mathrm{(lumi)} \textrm{ pb}$, which agrees with the theoretical prediction calculated to NNLO in QCD. Leveraging \Zee decays as ``standard candles'' for measuring the absolute luminosity at the LHC is examined; they are produced copiously, are well understood, and have clean detector signatures. Thus the consistency of the inclusive \Zee production cross section measurement with the theoretical prediction motivates inverting the measurement to instead use the \Zee signal yield to measure the luminosity. The result, which agrees with the primary relative CMS luminosity measurement calibrated using Van der Meer separation scans, is not only the most precise absolute luminosity measurement performed to date at a hadron collider, but also the first one based on a physics signal at the LHC.

In light of the large quantity of data collected during the second operational run of the Large Hadron Collider (LHC) at CERN, which has made it possible to perform measurements at unprecedented energies with a high degree of statistical precision, the necessity of understanding and constraining the systematic effects on such measurements has become increasingly important. Precision measurements of jet observables in proton-induced collisions have proven instrumental in constraining the parton distribution functions (PDFs) which describe the internal structure of protons, and which remain one of the largest sources of uncertainty in many analyses performed at hadron colliders. This thesis presents the first triple-differential measurement of the dijet production cross section performed in proton-proton collisions at a center-of-mass energy of 13 TeV, based on a data sample of 35.9 fb$^{-1}$ recorded by the CMS experiment at the Large Hadron Collider at CERN. The cross section is measured using anti-$k_\text{T}$ jets with radius parameters of R = 0.4 and R = 0.8 as a function of the dijet rapidity separation $y^{*}$ , the total boost of the dijet system $y_\text{b}$ , and either the average transverse momentum $\langle p_\text{T}\rangle_{1,2}$ or the invariant mass $m_\text{jj}$ of the dijet system as the third variable. This choice of rapidity variables exploits the topology of the dijet system to achieve an increased sensitivity to the proton PDFs. After accounting for detector-induced systematic effects in a three-dimensional unfolding procedure, the measured spectra are compared to fixed-order theory predictions at next-to-next-to-leading order accuracy in perturbative quantum chromodynamics, obtained using several recent PDF sets. While the data are observed to be described by the theory within the experimental and theoretical uncertainties across a large portion of the phase space, potentially significant deviations are observed in areas where a heightened sensitivity to the PDFs is expected.

The Compact Muon Solenoid (CMS) is a general purpose detector at the Large Hadron Collider (LHC). The LHC luminosity is constantly increased through upgrades of the accel- erator and its injection chain. Two major upgrades will take place in the next years. The rst upgrade involves the LHC injector chain and allows the collider to achieve a luminosity of about 2 10 34 cm-2 s-1 A further upgrade of the LHC foreseen for 2025 will boost its luminosity to 5 10 34 cm-2 s1. As a consequence of the increased luminosity, the detectors need to be upgraded. In particular, the CMS pixel detector will undergo two upgrades in the next years. The rst upgrade (phase I) consists in the substitution of the current pixel detector in winter 2016/2017. The upgraded pixel detector will implement new readout elec- tronics that allow ecient data taking up to a luminosity of 2 10 34 cm-2s-1,twice as much as the LHC design luminosity. The modules that will constitute the upgraded detector are being produced at dierent institutes. Hamburg (University and DESY) is responsible for the production of 350 pixel modules. The second upgrade (phase II) of the pixel detector is foreseen for 2025. The innermost pixel layer of the upgraded detector will accumulate a radiation damage corresponding to an equivalent uence of eq = 2 10 16 cm-2 and a dose of 10 MGy after an integrated luminosity of 3000 fb-1. Several groups are investigating sensor designs and congurations able to withstand such high doses and uences. This work is divided into two parts related to important aspects of the upgrades of the CMS pixel detector. For the phase I upgrade, a setup has been developed to provide an absolute energy calibration of the pixel modules that will constitute the detector. The cali- bration is obtained using monochromatic X-rays. The same setup is used to test the buering capabilities of the modules' readout chip. The maximum rate experienced by the modules produced in Hamburg will be 120 MHz/cm 2 . For this rate the modules' eciency has been measured to be 99%. In view of the module production, the energy calibration procedure has been automated. The modules assigned to the Hamburg production center should be completed by the end of February 2016. For the phase II upgrade, thin silicon sensors with an active thickness of 100 m irradi- ated with protons up to eq = 1 : 3 10 16 cm-2 have been characterized. The charge collection eciency has been measured using pad diodes. Charge multiplication eects have been ob- served for both n- and p-bulk sensors. P-bulk strip sensors with an active thickness of 100 and 200 m have been characterized with a beam test. The signal of these sensors lies be- tween 4000 and 5000 e- after a uence of 1 : 3 10 16 cm-2. The 200 m thick sensors require a higher bias voltage than the 100 m thick sensors to reach this signal height. The threshold necessary to obtain 95% detection eciency is found to be around 2000 e- for the 100 m thick sensors

Radiation testing of electronics for the CMS endcap muon system

In LHC (Large Hadron Collider) Run 2, the center-of-mass energy for proton-proton collisions is 13 TeV and the instantaneous luminosity has doubled. This has made it more challenging to trigger on interesting events since the number of interactions per crossing (pileup) is significantly larger than in LHC Run 1. The Compact Muon Solenoid (CMS) experiment has installed, commissioned, and is operating the second stage of a two-stage upgrade to the Calorimeter Trigger. This upgrade has improved the trigger algorithms so that the trigger thresholds remain low and physics data is not compromised. The stage-1, which replaced the original CMS Global Calorimeter Trigger, operated successfully in 2015. The completely new stage-2 has replaced the entire calorimeter trigger in 2016 with AMC form-factor boards and optical links operating in a microTCA chassis. It required that updates to the calorimeter back-ends, the source of the trigger primitive data, were also fully installed and operational. The stage-2 system's boards use Xilinx Virtex-7 690T FPGAs and have hundreds of links operating at up to 10 Gbps to maximize data throughput. In addition, a new time-multiplexed architecture was implemented and extensive firmware and software development was necessary. The final commissioning, operation, and performance of the stage-2 calorimeter trigger in 2016 proton collisions are described.

The Compact Muon Solenoid (CMS) is one of the general purpose particle detectors at the Large Hadron Collider (LHC) at CERN. The challenging constraints on the design of one of its sub-detectors, the Electromagnetic Calorimeter (ECAL), required the development of a complex Detector Control System (DCS). In this paper the general features of the CMS ECAL DCS during the period of commissioning and cosmic running will be presented. The feedback from the people involved was used for several upgrades of the system in order to achieve a robust, flexible and stable control system. A description of the newly implemented features for the CMS ECAL DCS subsystems will be given as well.

Recent results on studies of hard scattering processes and jet quenching in nuclear collisions from the ATLAS and CMS experiments at the Large Hadron Collider (LHC) are reviewed. They are based on high statistics samples of PbPb collisions at √ sNN = 2.76 TeV collected at the LHC in 2011, and pp collisions at √ s = 2.76 TeV and pPb collisions at √ sNN = 5.02 TeV collected in 2013. New results for the jet nuclear modification factor, RAA, confirm a strong suppression of jet production. Modifications of jet fragmentation properties in PbPb collisions, are observed. In pPb collisions jets are not strongly modified, but some non-scaling behavior of jet rates in collisions with different centralities is observed by ATLAS. New measurements for the electroweak boson production in pPb collisions allow a more precise testing of parton distributions in lead nuclei with the conclusion that the EPS09 description of nuclear modifications of parton distribution functions is preferred.

This PhD thesis summarizes the work performed in the context of the ETH high-energy physics group in the CMS experiment at CERN’s Large Hadron Collider. The relevant theoretical framework is briefly described, starting with an introduction to the Standard Model and to the Monte Carlo techniques used to obtain numerical results. The main aspects of the Large Hadron Collider and of the CMS experiment are discussed, together with the reconstruction of physics objects, in order to characterize the final state of proton-proton collisions in terms of types of particles produced, their energy, momenta, charges and further properties. Then a description of the Z+jet angular correlation and event-shapes analysis at 7TeV is presented, starting from the selection up to the extraction of the results. With the higher luminosity available at 8TeV, the efforts then focused on the differential Z+jet over photon+jet cross-section ratio and on the differential cross-section for Higgs boson production, using the diphoton decay channel. These analyses take advantage of the knowledge learnt in the discovery of the Higgs boson, extending the extraction of the results to differential distributions, and of the expertise on Z boson and photon reconstruction developed in the 7TeV analysis within the ETH-IPP group. Finally, a brief presentation of object reconstruction studies and of a hardware research and development project are presented in the appendix sections. A discriminator capable of distinguishing between quarkand gluon-initiated jets, its performances in 8TeV data and a way to evaluate systematics uncertainties are presented and the development of a new data acquisition system, used in test beam in the CERN H4 area to study new prototypes for the future upgrade of the forward electromagnetic calorimeter of CMS is described.

The Large Hadron Collider (LHC) will undergo a major upgrade during its third Long Shutdown period, after which the High-Luminosity LHC era will begin. During the HL-LHC phase, proton-proton collisions are expected to be delivered at 5 to 7 times the nominal LHC instantaneous luminosity. Also, the number of collisions per proton bunch crossing will reach values up to 200. The CMS experiment will undergo a substantial upgrade as well. The harsher radiation environment and the much higher number of overlapping events will require CMS to upgrade both the detector technologies and electronics. The Drift Tubes (DT) chambers — the muon detectors instrumenting the barrel region — will remain the same throughout the HL-LHC era. However, legacy detector readout and trigger electronics are not capable of sustaining the trigger rate, which is expected to increase from present 100 kHz up to 750 kHz, and to tolerate the higher levels of radiation. The new On-detector Board for DT (OBDT) will primarily perform time-to-digital conversion forwarding up to 240 time digitization channels to the back-end system without data losses. A new back-end system will implement the DT trigger primitive algorithm capable of using the OBDT streamline data and performing the DT muon track reconstruction in the available latency and with a resolution similar to offline. A small fraction of the DT chambers have been already operated in parallel with both legacy and upgraded electronics during Run 3 data-taking. This early integration enabled CMS to validate the new electronics and evaluate its performances. This report will give an overview of the status of the DT electronics upgrade as well as a report on the upgraded electronics performance using latest Run 3 proton-proton collision data.

The Compact Muon Solenoid (CMS) experiment at CERN's Large Hadron Collider (LHC) is scheduled to come on-line in 2007. Fermilab will act as the CMS Tier-1 centre for the US and make experiment data available to more than 400 researchers in the US participating in the CMS experiment. The US CMS Users Facility group, based at Fermilab, has initiated a project to develop a model for optimizing movement of CMS experiment data between CERN and the various tiers of US CMS data centres and to design a WAN emulation facility which will enable controlled testing of unmodified or modified CMS applications and TCP implementations locally under conditions that emulate WAN connectivity. The WAN emulator facility is configurable for latency, jitter, and packet loss. The initial implementation is based on the NISTnet software product. In this paper we will describe the status of this project to date, the results of validation and comparison of performance measurements obtained in emulated and real environment for different applications including multistreams GridFTP. We also will introduce future short term and intermediate term plans, as well as outstanding problems and issues.

The Compact Muon Solenoid (CMS) experiment at the CERN Large Hadron Collider (LHC) uses Resistive Plate Chambers (RPCs) as a trigger-dedicated muon detector. The RPC subdetector presently consists of 912 chambers, providing a ≈3 ns time resolution for the pseudorapidity region |η| <; 1.6. Up to 18 on-detector Front-End Boards (FEBs) per RPC are responsible for signal processing. Each FEB is configured and monitored through its Inter-Integrated Circuit (I <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> C) interface to ensure correct and safe detector operation. The resulting ~1.2k I <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> C lines, serving 3 to 6 FEBs apiece, are accessible through Control Boards (CBs) in token rings. An online control and monitoring system has recently been developed to automatically configure and continuously monitor FEB parameters. A fine-grained software model of hardware and detector components, organized in a dual hierarchical tree structure, facilitates command and data flow. A novel web-based user-interface allows control of and navigation through the system for efficient problem tracing. This paper covers the front-end control and monitoring system for the RPC subdetector; the online software components that allow for flexible configuration and comprehensive monitoring are described.

The Compact Muon Solenoid (CMS) experiment is operating at the Large Hadron Collider (LHC) with proton–proton collisions at 13-TeV center-of-mass energy and at a bunch spacing of 25 ns. Challenging running conditions for CMS are expected after the High-Luminosity upgrade of the LHC (HL-LHC). We review the CMS ECAL crystal calorimeter upgrade and present results from the first test beam studies. Particular challenges at HL-LHC are the harsh radiation environment, the increasing data rates, and the extreme level of pile-up events, with up to 200 simultaneous proton–proton collisions. Precision timing can be exploited to reduce the effect of the pileup. We report on the timing resolution studies performed with test beams. We discuss the new readout and trigger electronics, which must be upgraded due to the increased trigger and latency requirements at the HL-LHC.

The synchronization of the trigger and data acquisition systems for the Cathode Strip Chambers (CSCs) in the Compact Muon Solenoid (CMS) detector at CERN is described. The CSC trigger system is designed to trigger CMS on muons with high efficiency (~99% per chamber) and is able to accurately identify its 25ns proton bunch crossing. To date, asynchronous cosmic ray data have been used to define the protocol and to refine timing algorithms, allowing synchronization to be realized within and between chambers to within ±10 ns. Final synchronization of the CSCs requires timing parameters to be accurate to 2 ns. This goal will be readily achieved from the cosmic ray baseline using data taken with the synchronous beam structure of the Large Hadron Collider.

The Data Quality Monitoring (DQM) of the Compact Muon Solenoid (CMS) silicon tracking detectors (Tracker) at the Large Hadron Collider (LHC) at CERN is a software based system designed to monitor the detector and reconstruction performance, to identify problems and to certify the collected data for physics analysis. It uses the framework provided by the central CMS DQM as well as tools developed especially for the CMS Tracker DQM. This paper describes aim, framework conditions, tools and work flows of the CMS Tracker DQM and shows examples of its successful use during the recent commissioning phase of the CMS experiment.