Myriam Schönenberger

The observation of Higgs boson production in association with a top quark-antiquark pair is reported, based on a combined analysis of proton-proton collision data at center-of-mass energies of √s = 7, 8, and 13 TeV, corresponding to integrated luminosities of up to 5.1, 19.7, and 35.9  fb^(-1), respectively. The data were collected with the CMS detector at the CERN LHC. The results of statistically independent searches for Higgs bosons produced in conjunction with a top quark-antiquark pair and decaying to pairs of W bosons, Z bosons, photons, τ leptons, or bottom quark jets are combined to maximize sensitivity. An excess of events is observed, with a significance of 5.2 standard deviations, over the expectation from the background-only hypothesis. The corresponding expected significance from the standard model for a Higgs boson mass of 125.09 GeV is 4.2 standard deviations. The combined best fit signal strength normalized to the standard model prediction is 1.26^(+0.31)_(−0.26).

This paper presents the measurement of the bidirectional reflectance distribution function of tungsten (W) samples and the resulting reflection models in the nuclear fusion device WEST (tokamak). For this, an experimental gonio-spectrophotometer was developed to fully characterize the material’s optical and thermal-radiative properties of metallic samples with different roughnesses. Ray-tracing photonic simulation was then carried out to predict the photon behavior in a fully metallic environment as a function of reflectance measurement. Low emissivity (0.1 at 4 μm) and highly specular reflectance (fitting with a Gaussian distribution around the specular direction with a small width lower than 10°) are found for W samples. These measurements have been used as input for the photonic simulation, and the resulting synthetic image reproduced the reflection features well on the upper divertor, detected in WEST infrared experimental images.

A sampling calorimeter using cerium fluoride scintillating crystals as active material, interleaved with heavy absorber plates, and read out by wavelength-shifting (WLS) fibers is being studied as a calorimeter option for detectors at the upgraded High-Luminosity LHC (HL-LHC) collider at CERN. A prototype has been exposed to electron beams of different energies at the INFN Frascati (Italy) Beam Test Facility. This paper presents results from the studies performed on the prototype, such as signal amplitudes, light yield and energy resolution.

A prototype for a sampling calorimeter made out of cerium fluoride crystals interleaved with tungsten plates, and read out by wavelength-shifting fibres, has been exposed to beams of electrons with energies between 20 and 150 GeV, produced by the CERN Super Proton Synchrotron accelerator complex. The performance of the prototype is presented and compared to that of a Geant4 simulation of the apparatus. Particular emphasis is given to the response uniformity across the channel front face, and to the prototype׳s energy resolution.

A sampling calorimeter using cerium fluoride scintillating crystals as active material, interleaved with absorber plates made of tungsten, and read out by wavelength-shifting fibres has been tested with high-energy electron beams at the CERN SPS H4 beam line, as well as with lower-energy beams at the INFN Frascati Beam Test Facility in Italy. Energy resolution studies revealed a low stochastic term (<10%/E). This result, combined with high radiation hardness of the material used, marks this sampling calorimeter as a good candidate for the detectors׳ forward regions during the high luminosity phase of LHC.

The High-Luminosity phase of the Large Hadron Collider at CERN (HL-LHC) poses stringent requirements on calorimeter performance in terms of resolution, pileup resilience and radiation hardness. A tungsten-CeF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> sampling calorimeter is a possible option for the upgrade of current detectors. A prototype, read out with different types of wavelength-shifting fibers, has been built and exposed to high energy electrons, representative for the particle energy spectrum at HL-LHC, at the CERN SPS H4 beam line. This paper shows the performance of the prototype, mainly focussing on energy resolution and uniformity. A detailed simulation has been also developed in order to compare with data and to extrapolate to different configurations to be tested in future beam tests. Additional studies on the calorimeter and the R&D projects ongoing on the various components of the experimental setup will be also discussed.

Searches for physics beyond the Standard Model (SM) are a main focus of the physics program at the Large Hadron Collider at CERN. I present in this thesis two searches for supersymmetry (SUSY), using data collected with the Compact Muon Solenoid detector. The first search is a broad range search for SUSY in the tails of the stransverse mass distribution MT2. Data collected in the year 2016 corresponding to an integrated luminosity of 35.9/fb are used to obtain data driven estimates of the \zinv, lost leptons and QCD multijet backgrounds. No sign of SUSY has been found. Upper limits on the production cross section of simplified models of SUSY are set. Gluino masses up to 2 TeV are excluded for a massless lightest supersymmetric particle (LSP). Squark masses up to 1 TeV(1.6 TeV) for one (four) light squark type(s) for a massless LSP are excluded. The second search explores strong and electroweak SUSY production through the final state of a Higgs boson decaying to a photon pair analyzing 77.5/fb of integrated luminosity collected in 2016 and 2017. The nonresonant diphoton background is estimated from fits to the data, while the resonant background from the SM Higgs boson production is estimated from simulation. To increase sensitivity with respect to the backgrounds, categories with leptons, jets, b-tagged jets, MT2 and the transverse momentum of the Higgs boson are employed. No significant excess over the SM prediction is found. Upper limits on the production cross section of simplified models of SUSY are set. Bottom squark masses up to 530 GeV are excluded for a massless LSP. For wino-like chargino-neutralino production masses below 220 GeV for a gravitino LSP mass of 1 GeV are excluded. For the higgsino-like chargino-neutralino production with 100% branching fraction LSP→Hgravitino masses up to 275 GeV and with 50% branching fraction to LSP→Hgravitino and LSP→Zgravitino masses up to 190 GeV are excluded for a gravitino mass of 1 GeV.