Search for type-III seesaw heavy leptons with the ATLAS detector at the LHC

Novak, Tadej (author), Kerševan, Borut Paul (mentor)

The thesis presents a search for the pair production of heavy leptons as predicted by the type-III seesaw mechanism. The search uses proton-proton collision data at a centre-of-mass energy of 13 TeV, corresponding to 139 fb$^{-1}$ of integrated luminosity recorded by the ATLAS detector during Run 2 of the Large Hadron Collider. The analysis focuses on the final state with two light leptons (electrons or muons) of different flavour and charge combinations, with at least two jets and large missing transverse momentum. A cut-based approach is extended using a multivariate analysis. No significant excess over the Standard Model expectation is observed. The results are translated into exclusion limits on heavy lepton masses, and the observed lower limit on the mass of the type-III seesaw heavy leptons is 790 GeV using cut-based and 950 GeV using multivariate approach at 95% confidence level.

• Language: English
• Keywords: seesaw mechanism, type-III seesaw, fermion triplet, machine learning, boosted decision trees, neural networks, ATLAS, CERN, LHC Run 2, dilepton, two jets, missing transverse energy
• Work type: Doctoral dissertation
• Typology: 2.08 - Doctoral Dissertation
• Organization: FMF - Faculty of Mathematics and Physics
• Year: 2020
• PID: 20.500.12556/RUL-120457
• COBISS.SI-ID: 32414467
• Publication date in RUL: 20.09.2020

Secondary language

• Language: Slovenian
• Title: Iskanje težkih leptonov v gugalničnem mehanizmu tipa III z detektorjem ATLAS na trkalniku LHC

Abstract

V doktorskem delu bom predstavil iskanje težkih leptonov v gugalničnem mehanizmu tipa III z detektorjem ATLAS na Velikem hadronskem trkalniku. Za analizo so bili uporabljeni podatki, zajeti pri trkih protonov s težiščno energijo 13 TeV med drugim obdobjem trkov in meritev (Run 2). Skupna integrirana luminoznost znaša 139 fb$^{-1}$. Pri analizi sem se osredotočil na končna stanja z dvema lahkima leptonoma (elektroni ali mioni) v različnih možnih kombinacijah okusa in naboja, ob tem pa sta v dogodku še vsaj dva pljuska hadronov in visoka manjkajoča transverzalna gibalna količina. V sklopu dela sem razširil pristop pravokotnih rezov s strojnim učenjem. Statistična obdelava podatkov ne nakazuje nobene sledi novih delcev. Iz rezultatov sem izpeljal spodnje meje za mase težkih leptonov po gugalničnem mehanizmu tipa III, ki znašajo 790 GeV pri uporabi pravokotnih rezov in 950 GeV z uporabo strojnega učenja.

• Keywords: gugalnični mehanizem, gugalnični mehanizem tipa III, triplet fermionov, strojno učenje, odločitvena drevesa, nevronske mreže, ATLAS, CERN, LHC Run 2, dva leptona, dva pljuska hadronov, manjkajoča transverzalna količina