Search for heavy right-handed Majorana neutrinos in t̄t decays in proton-proton collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

In the Standard Model of particle physics, neutrinos are considered massless.
However, neutrino oscillation experiments have demonstrated that neutrinos have non-zero mass. This discovery challenges the fundamental assumptions of the
Standard Model. To explain the smallness of neutrino masses, physicists have proposed theories beyond the Standard Model, such as the seesaw mechanism.
This mechanism introduces the concept of heavy right-handed neutrinos to account for the low mass of neutrinos in the Standard Model. The neutrinoless
double beta decay process serves as a crucial research avenue for investigating heavy neutrinos.
This thesis uses the full Run-2 data collected by the ATLAS detector at the Large Hadron Collider to search for right-handed Majorana neutrinos through top quark pair decay processes. The full Run-2 data refers to proton-proton collision data at a center-of-mass energy of  $\sqrt{s}$ = 13 TeV, with an integrated luminosity of 140.1 $fb^{-1}$. The physical target is to investigate the neutrinoless double beta decay of a W boson in the final state of top quark pair events, where the W boson decays to produce either an electron or muon and a heavy right-handed Majorana neutrino. Subsequently, the heavy neutrino further decays into another charged lepton of the same flavor and charge, along with an off-shell W boson, resulting in a final state with same-sign di-electrons or di-muons.
This thesis employs a multivariate analysis approach based on boosted decision trees to distinguish signal from background. The analysis achieves significant
enhancements in sensitivity and stability through comprehensive optimization of input variables and hyperparameters. Additionally, by comparing truth-level information, leptons from different sources are classified. This information is used to define control regions, and template fit methods are applied to estimate the
contributions of dominant backgrounds. Subsequently, all systematic uncertainties in the analysis are estimated and incorporated into the fitting process to enhance the reliability of the results. Finally, the thesis uses a maximum likelihood estimation method to fit the signal and control regions to estimate the
signal strength of the heavy neutrinos at different mass points.
No significant excess over the expected background is observed in the data. Therefore, upper limits on the coupling parameters to electrons and muons are set for Majorana neutrinos in the mass range of 15–75 GeV. For a heavy neutrinowith the mass of 40 GeV, the final results are $\left |V_{e,N} \right |^{2}< 2.0\times 10^{-4}$ and $|\left |V_{\mu ,N} \right |^{2}< 6.8\times 10^{-5}$. This thesis extends the ATLAS search for heavy neutrinos to a mass range of up to 75 GeV and is the first search for heavy right-handed neutrinos through top quark pair decay events. These results provide reliable evidence for future searches of heavy neutrinos and the exploration of new physics.