ATLAS Top Mass reconstruction in ttbar events leptonjets

ATLAS Top Mass reconstruction in ttbar events (lepton+jets channel) Regina Moles-Valls Maria Moreno Llácer Carlos Escobar Ibañez Vicente Lacuesta Miquel Andreas Wildauer IFIC – Universitat de València

Top Mass Physics Motivations o Top quark mass is a fundamental parameter of the SM particle physics o Top quark was discovered by CDF & D 0 in 1995. Top Physics field was opened !! o Top properties: o Charge= (2/3)e , Spin = 1/2 o Large mass (mtop > 35 mbottom) o Present world average value measured by CDF & D 0 with Lint=5. 8 fb-1 , is: mtop = (173. 2 ± 0. 56 stat ± 0. 76 syst) Ge. V (ar. Xiv: 1107. 5255 v 1 [hep-ex] 26 Jul 2011 TOP QUARK MASS INTERESTING FIELDS: • Top quark mass is an important parameter for the EW precision measurement due to: • Large contributions in radiative corrections: • Constraint the Higgs Boson Mass • Increase the sensitivity to physics beyond SM 18/05/2021 --- XXXIII Bienal de Física (Santander) --- 2

Top Quark at LHC Top quark Production o. Top quark is produced predominantly in pairs: o LHC at √s = 7 Te. V σtop= 165+11 -16 pb (theoretical NLO+NNLO) o Precision measurements and new searches Quark-Anti. Quark scattering Gluon-gluon fusion 15% at LHC 85% at LHC Top quark Decay o Top quark decays take place almost exclusively t W+b W decay? b-jet always hadronize 18/05/2021 TRIG BKG Dileptonic (11%) • 2 leptons • 2 neutrinos • 2 bbar lep+Etmiss Semileptonic (44%) • 1 lepton • 1 neutrino • 2 light jets • 2 bbar Full. Hadronic (44%) • 4 light jets • 2 bbar lep+Etmiss multi-jets --- XXXIII Bienal de Física (Santander) --- 3

Top Mass Analysis in ATLAS (lepton+jets) The most precise top mass measurements can be achieved using semi-leptonic channel: Electron: Energy deposits in EM calorimeter with associated well reconstructed track in the ID Muon: Track segments in different layers of muon chambers are combined and matched with ID track Jets: Anti-kt 4 algorithm is used starting from energy cluster of adjacent calorimeters jets. Calibration is done using EM+HAD calorimeter information. b-jet tagger used ID in formation too B-jet Trigger: Trigger is based on signatures of electron muons and jets, as well as missing transverse energy. Full Detector Working !!! 18/05/2021 --- XXXIII Bienal de Física (Santander) --- ETmiss Muon 4

Top Mass Analysis in ATLAS (lepton+jets) The main methodology used to determine mtop at hadron collider consists in measuring the invariant mass of the decay products of the top-quark candidate (mtopreco) and reconstructing top mass using different methods. EVENT SELECTION: o Single electron or muon trigger fired o Only one reconstructed lepton: • electron: p. T>25 Ge. V • muon: p. T>30 Ge. V o Transverse energy: • electron: Etmiss>35 Ge. V & m. T(W)>25 Ge. V • muon: ETmiss>20 Ge. V & ETmiss+m. T(W)>60 Ge. V o At least 4 jets with p. T>25 Ge. V |η|<2. 5 o One of these jets tagged as b-jet (SV 0>5. 85) Background: o The background is under control 18/05/2021 --- XXXIII Bienal de Física (Santander) --- 5

Top Mass Analysis in ATLAS (lepton+jets) Hadronic top mass reconstruction: o How the jet association is done? • Jet triplet with high p. T and at least 1 b-jet • ΔR constraint between two light jets • W mass window cut to improve the purity of the jet triplet Muon channel MW cut = ± 30 o Agreement Data/MC is important to understand the combinatorial background Muon channel o Precise knowledge of the Jet Energy Scale is needed to forbid possible shifts in the mtopreco: • jet re-calibration o Optimization of the estimator to limit systematic errors 18/05/2021 --- XXXIII Bienal de Física (Santander) --- 6

Top Mass Analysis in ATLAS (lepton+jets) Muon channel o How the b-jet association is done? • Jet triplet with high p. T (usually p. Tmax method used) • ΔR distance between b-jet and W bosson (closer b-jet) Leptonic Top mass reconstruction: o The most difficult challenge comes from the kinematic of the neutrino o E miss comes from undetected neutrino: E miss = p ν T T T ν o pz chosen using W boson mass o b-jet assignment 18/05/2021 --- XXXIII Bienal de Física (Santander) --- 7

Top Mass Analysis in ATLAS (lepton+jets) o Last ATLAS top mass results: o 2 -dimensional template analysis o. Lint= 0. 7 fb-1 o Systematic errors Electron Channel Muon Channel (electron) mtop = (173. 9 ± 1. 2 stat ± 3. 1 syst) Ge. V (muon) mtop = (175. 9 ± 1. 0 stat ± 2. 7 syst) Ge. V 18/05/2021 --- XXXIII Bienal de Física (Santander) --- 8

Summary of the Top Mass Analysis in ATLAS o ATLAS has rediscovered and re-measured the top quark mass • 2010 with a Lint=35. 3 pb-1 : (electron) mtop = (173. 8 ± 6. 7 stat ± 4. 8 syst) Ge. V (muon) mtop = (166. 7 ± 5. 0 stat ± 5. 0 syst) Ge. V • 2011 with a Lint=0. 7 fb-1 : (electron) mtop = (173. 9 ± 1. 2 stat ± 3. 1 syst) Ge. V (muon) mtop = (175. 9 ± 1. 0 stat ± 2. 7 syst) Ge. V mtop= (175 ± 0. 9 stat ± 2. 7 syst) Ge. V Statistic is increasing The uncertainty of the results is dominated by the systematic uncertainties: • JES Uncertainty for b-jets and light-jets • Initial and final state radiation • Some aspects of the MC modelling 18/05/2021 --- XXXIII Bienal de Física (Santander) --- 9

Backup 18/05/2021 --- XXXIII Bienal de Física (Santander) --- 10

2 -D Template Method o This method determines simultaneously mtop and Jet Energy Scale Factors o JSF based on observed differences between mwreco mwpredicted (η, p. T) o χ2 fit used to obtain parton scale factors (α) o Signal template for mtop, mw and background are built for different mass points. o. Template hostograms fitted by (sum of) Gaussian/Landau functions o Parameters of the Gaussian/Landau functions fitted as a function of (mtop, JES) o For any set of (m , m ) a Likelihood can be maximize with respect to m , JSF top W top and fraction background 18/05/2021 --- XXXIII Bienal de Física (Santander) --- 11