Mykola Savitskyi

A bstract A search for new top quark interactions is performed within the framework of an effective field theory using the associated production of either one or two top quarks with a Z boson in multilepton final states. The data sample corresponds to an integrated luminosity of 138 fb − 1 of proton-proton collisions at $$ \sqrt{s} $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msqrt> <mml:mi>s</mml:mi> </mml:msqrt> </mml:math> = 13 TeV collected by the CMS experiment at the LHC. Five dimension-six operators modifying the electroweak interactions of the top quark are considered. Novel machine-learning techniques are used to enhance the sensitivity to effects arising from these operators. Distributions used for the signal extraction are parameterized in terms of Wilson coefficients describing the interaction strengths of the operators. All five Wilson coefficients are simultaneously fit to data and 95% confidence level intervals are computed. All results are consistent with the SM expectations.

A new pixel detector for the CMS experiment was built in order to cope with the instantaneous luminosities anticipated for the Phase~I Upgrade of the LHC. The new CMS pixel detector provides four-hit tracking with a reduced material budget as well as new cooling and powering schemes. A new front-end readout chip mitigates buffering and bandwidth limitations, and allows operation at low comparator thresholds. In this paper, comprehensive test beam studies are presented, which have been conducted to verify the design and to quantify the performance of the new detector assemblies in terms of tracking efficiency and spatial resolution. Under optimal conditions, the tracking efficiency is $99.95\pm0.05\,\%$, while the intrinsic spatial resolutions are $4.80\pm0.25\,\mu \mathrm{m}$ and $7.99\pm0.21\,\mu \mathrm{m}$ along the $100\,\mu \mathrm{m}$ and $150\,\mu \mathrm{m}$ pixel pitch, respectively. The findings are compared to a detailed Monte Carlo simulation of the pixel detector and good agreement is found.

Measurements of normalized differential top-quark-pair ($t\bar{t}$) production cross sections are performed using final states with two leptons ($e^+e^-$, $μ^+μ^-$, and $e^\pmμ^\mp$) in proton-proton ($pp$) collisions at $\sqrt{s}$ = 13 TeV at the CERN LHC. The data were recorded in 2015 with the CMS detector and correspond to an integrated luminosity of 2.2 fb$^{-1}$. The $t\bar{t}$ production cross section is measured as a function of kinematic properties of the top quarks and the $t\bar{t}$ system in the full phase space, as well as of the jet multiplicity in the event in the fiducial phase space. Several perturbative quantum chromodynamics (QCD) calculations are confronted with the data and are found to be broadly in agreement with the measured results.

Measurements of normalized differential top-quark-pair ($t\bar{t}$) production cross sections are performed using final states with two leptons ($e^+e^-$, $\mu^+\mu^-$, and $e^\pm\mu^\mp$) in proton-proton ($pp$) collisions at $\sqrt{s}$ = 13 TeV at the CERN LHC. The data were recorded in 2015 with the CMS detector and correspond to an integrated luminosity of 2.2 fb$^{-1}$. The $t\bar{t}$ production cross section is measured as a function of kinematic properties of the top quarks and the $t\bar{t}$ system in the full phase space, as well as of the jet multiplicity in the event in the fiducial phase space. Several perturbative quantum chromodynamics (QCD) calculations are confronted with the data and are found to be broadly in agreement with the measured results.

Several measurements of top quark properties are presented using data collected by the CMS experiment at the centre-of-mass-energy of 8 TeV.The properties are mostly probed in production of the top quarks.The charge asymmetry is measured inclusively and differentially probing anomalous couplings to the gluons at production mode.These measurements are extended searching for asymmetries in CP-odd operators in production and decay.The measurement of the spin density matrix in different final states (lepton+jets and dileptons) is discussed as well as the measurement of the top quark polarization in tt events.The results are compared to the state-of-the-art predictions, when available, and re-interpreted as searches for new physics inducing deviations from the standard model predictions.