Search for top squarks with the ATLAS detector Nicolas Khler Max - PowerPoint PPT Presentation

Search for top squarks with the ATLAS detector Nicolas Köhler Max Planck Institute for Physics (Werner-Heisenberg-Institut) Thursday 20 th July, 2017

Why searching for Supersymmetry? 07/20/2017 Nicolas Köhler - Search for top squarks with ATLAS 2/13

Introduction ˜ t → 0 ℓ analysis Top reconstruction Results Summary Shortcomings of the Standard Model Unification of gauge couplings Dark Matter candidate Hierarchy problem Neutrino masses Quantum Gravity (10d string theory) Gauge Higgs Gauginos Squarks Quarks bosons bosons χ 0 χ 0 u 1 , 2 ˜ c 1 , 2 ˜ ˜ t 1 , 2 ˜ ˜ 1 2 u c t γ h , H 0 χ 0 χ 0 ˜ d 1 , 2 ˜ s 1 , 2 ˜ b 1 , 2 ˜ ˜ 3 4 s d b Z A χ ± χ ± e 1 , 2 ˜ ˜ µ 1 , 2 ˜ τ 1 , 2 ˜ ˜ 1 2 e µ τ W ± H ± g ν µ ˜ ν e ˜ ˜ ν τ ˜ ν e ν µ ν τ g Sleptons Leptons → Supersymmetry can explain all those questions 07/20/2017 Nicolas Köhler - Search for top squarks with ATLAS 2/13

Introduction ˜ t → 0 ℓ analysis Top reconstruction Results Summary Shortcomings of the Standard Model Unification of gauge couplings Dark Matter candidate Hierarchy problem Neutrino masses Quantum Gravity (10d string theory) Gauge Higgs Gauginos Squarks Quarks bosons bosons χ 0 χ 0 u 1 , 2 ˜ c 1 , 2 ˜ ˜ t 1 , 2 ˜ ˜ 1 2 u c t γ h , H 0 χ 0 χ 0 ˜ d 1 , 2 ˜ s 1 , 2 ˜ b 1 , 2 ˜ ˜ 3 4 s d b Z A χ ± χ ± e 1 , 2 ˜ ˜ µ 1 , 2 ˜ τ 1 , 2 ˜ ˜ 1 2 e µ τ W ± H ± g ν µ ˜ ν e ˜ ˜ ν τ ˜ ν e ν µ ν τ g Sleptons Leptons → Supersymmetry can explain all those questions 07/20/2017 Nicolas Köhler - Search for top squarks with ATLAS 2/13

Why searching for top squarks at the LHC? 07/20/2017 Nicolas Köhler - Search for top squarks with ATLAS 3/13

Introduction ˜ t → 0 ℓ analysis Top reconstruction Results Summary Why searching for top squarks at the LHC? The Higgs is unnaturally light! m H = 125 GeV with m 2 H = ( m 2 H ) bare + δ m 2 Remaining contribution after SUSY H breaking: f ∼ − λ 2 ( ) H H δ m 2 H ∼ − y 2 t m 2 Λ stop ln f m stop ¯ (one-loop-level) f ˜ f where Λ is the energy scale up to which the theory is renormalizable ∼ λ s → m stop should be in the TeV range H H → Fermionic and scalar contributions cancel exactly if SUSY is unbroken 07/20/2017 Nicolas Köhler - Search for top squarks with ATLAS 3/13

JHEP 1209 (2012) 035 Introduction ˜ t → 0 ℓ analysis Top reconstruction Results Summary Simulated SUSY processes The MSSM has 105 free parameters → Use simplified models where m ˜ t , m ˜ 1 and m ˜ 1 are in the TeV χ ± χ 0 range, but the other sparticles are decoupled with higher masses Stops described by these models are accessible to searches at the LHC Assume R -parity conservation Assume m ˜ t 2 ≫ m ˜ t 1 07/20/2017 Nicolas Köhler - Search for top squarks with ATLAS 4/13

Introduction ˜ t → 0 ℓ analysis Top reconstruction Results Summary Top squark ( ˜ t 1 ) decays Assume only 2 decays are possible: b t W + p p ˜ ˜ χ 0 t χ 0 t ˜ ˜ 1 χ + 1 ˜ 1 χ − ˜ 1 χ 0 χ 0 ˜ ˜ ¯ ¯ ˜ ˜ 1 1 t t p p W − ¯ ¯ t b χ ± χ 0 t → t + ˜ t → b + ˜ BR (˜ 1 ) = 1 − BR (˜ 1 ) → Search for signatures with either 0, 1 or 2 Leptons 07/20/2017 Nicolas Köhler - Search for top squarks with ATLAS 5/13

CombinedSummaryPlots/SUSY/index.html Introduction ˜ t → 0 ℓ analysis Top reconstruction Results Summary Top squark searches in ATLAS during LHC Run 1 t p ∼ ∼ ∼ ∼ ~ ~ ~ ~ ~ ~ → χ 0 → χ 0 → χ 0 → χ 0 ˜ t t production, t b f f’ / t c / t W b / t t χ 0 t ˜ 1 1 1 1 1 1 1 1 1 1 1 [GeV] 450 ATLAS -1 [GeV] s =8 TeV, 20 fb 40 ∼ χ 0 ~ → χ 0 ¯ ˜ t t t0L/t1L combined ˜ 30 1 1 t 400 ~ ∼ 1 0 0 1 → χ t t t2L, SC 0 1 p ∼ χ 1 ∼ χ m 1 ∼ ∼ m ~ 20 → χ 0 χ 0 t W b /b f f’ WW 1 ~ ∼ 1 1 0 → χ ¯ t W b t1L, t2L 10 t 350 1 ∼ 1 ~ → χ 0 t c tc 1 0 ~ 1 ∼ 0 → χ tc, t1L 170 180 190 200 210 t b f f’ 1 b 1 m ~ [GeV] t 300 1 W + Observed limits Expected limits All limits at 95% CL p W ∼ 0 + m ,t) < m χ 1 ˜ ) < 0 t χ 0 ˜ 250 ) < m χ + 1 ∼ 0 b ~ χ ˜ 1 t 1 , m( ~ 1 m( t 1 ∼ 0 χ ∆ 1 ~ , ∆ t χ − ˜ m( 1 1 χ 0 200 ∆ ˜ ∼ ¯ χ 0 ˜ 1 c t 1 p W − 150 ¯ b ∼ 0 χ b f f’ 1 100 c b p W 50 ∼ ˜ 0 t χ W b 1 χ 0 χ ± ˜ ˜ 1 1 χ ∓ 0 ˜ χ 0 ˜ 200 300 400 500 600 700 800 1 1 ˜ t m [GeV] p W ~ from https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/ t b 1 ; c 07/20/2017 Nicolas Köhler - Search for top squarks with ATLAS 6/13

Introduction ˜ t → 0 ℓ analysis Top reconstruction Results Summary χ 0 t → bqq ˜ The full-hadronic decay channel ( ˜ 1 ) q b q p t W ˜ t Lepton veto leads to good background χ 0 ˜ 1 suppression χ 0 ˜ 1 ˜ t W Remaining backgrounds: t p q -- t ¯ t + Z ( Z → νν ) q b -- Z +jets ( Z → νν ) b -- t ¯ t → bbqq ℓν (missing the ℓ ) q p Full reconstruction of top quark due to q ˜ full-hadronic W -decay t χ 0 ˜ W 1 χ ± ˜ 1 χ ∓ ˜ χ 0 χ ± → Both ˜ t → t + ˜ 1 and ˜ t → b + ˜ 1 are 1 χ 0 ˜ ˜ t 1 accessible q p W q b 07/20/2017 Nicolas Köhler - Search for top squarks with ATLAS 7/13

Introduction ˜ t → 0 ℓ analysis Top reconstruction Results Summary Select events of interest q b q p t ˜ W t χ 0 ˜ 1 Veto against leptons χ 0 ˜ Large missing transverse energy 1 ˜ t W t ( E miss p > 250 GeV) q T q At least 4 jets ( R = 0 . 4 ) with b p T > (80 , 80 , 40 , 40) GeV At least 2 b -jets → Reconstruct top quark + More discriminating variables 07/20/2017 Nicolas Köhler - Search for top squarks with ATLAS 8/13

Introduction ˜ t → 0 ℓ analysis Top reconstruction Results Summary Select events of interest q b q p t ˜ W t χ 0 ˜ 1 Veto against leptons χ 0 ˜ Large missing transverse energy 1 ˜ t W t ( E miss p > 250 GeV) q T q At least 4 jets ( R = 0 . 4 ) with b p T > (80 , 80 , 40 , 40) GeV At least 2 b -jets → Reconstruct top quark + More discriminating variables 07/20/2017 Nicolas Köhler - Search for top squarks with ATLAS 8/13

Introduction ˜ t → 0 ℓ analysis Top reconstruction Results Summary Select events of interest q b q p t ˜ W t χ 0 ˜ 1 Veto against leptons χ 0 ˜ Large missing transverse energy 1 ˜ t W t ( E miss p > 250 GeV) q T q At least 4 jets ( R = 0 . 4 ) with b p T > (80 , 80 , 40 , 40) GeV At least 2 b -jets → Reconstruct top quark + More discriminating variables 07/20/2017 Nicolas Köhler - Search for top squarks with ATLAS 8/13

Introduction ˜ t → 0 ℓ analysis Top reconstruction Results Summary Select events of interest q b q p t ˜ W t χ 0 ˜ 1 Veto against leptons χ 0 ˜ Large missing transverse energy 1 ˜ t W t ( E miss p > 250 GeV) q T q At least 4 jets ( R = 0 . 4 ) with b p T > (80 , 80 , 40 , 40) GeV At least 2 b -jets → Reconstruct top quark + More discriminating variables 07/20/2017 Nicolas Köhler - Search for top squarks with ATLAS 8/13

Introduction ˜ t → 0 ℓ analysis Top reconstruction Results Summary Select events of interest q b q p t ˜ W t χ 0 ˜ 1 Veto against leptons χ 0 ˜ Large missing transverse energy 1 ˜ t W t ( E miss p > 250 GeV) q T q At least 4 jets ( R = 0 . 4 ) with b p T > (80 , 80 , 40 , 40) GeV At least 2 b -jets → Reconstruct top quark + More discriminating variables 07/20/2017 Nicolas Köhler - Search for top squarks with ATLAS 8/13

Introduction ˜ t → 0 ℓ analysis Top reconstruction Results Summary Select events of interest q b q p t ˜ W t χ 0 ˜ 1 Veto against leptons χ 0 ˜ Large missing transverse energy 1 ˜ t W t ( E miss p > 250 GeV) q T q At least 4 jets ( R = 0 . 4 ) with b p T > (80 , 80 , 40 , 40) GeV At least 2 b -jets → Reconstruct top quark + More discriminating variables 07/20/2017 Nicolas Köhler - Search for top squarks with ATLAS 8/13

ATLAS-CONF-2017-020 Introduction ˜ t → 0 ℓ analysis Top reconstruction Results Summary Select events of interest q b q p t ˜ W t χ 0 ˜ 1 Veto against leptons χ 0 ˜ Large missing transverse energy 1 ˜ t W t ( E miss p > 250 GeV) q T q At least 4 jets ( R = 0 . 4 ) with b Events / 50 GeV ATLAS Preliminary p T > (80 , 80 , 40 , 40) GeV 6000 Data -1 s =13 TeV, 36.1 fb SM Total m b ,min preselection + >50 GeV t t T Single Top At least 2 b -jets t t +V 4000 W Z Diboson ~ ∼ χ 0 20 x ( t , )=(600,300) GeV 1 1 ~ ∼ χ 0 100 x ( t , )=(1000,1) GeV 2000 1 1 → Reconstruct top quark + More discriminating variables 0 2.0 Data / SM 1.5 1.0 0.5 0.0 0 200 400 600 b m ,min [GeV] T 07/20/2017 Nicolas Köhler - Search for top squarks with ATLAS 8/13