Taller de Altas Energías Michael Holzbock September 7, LMU Munich

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1 Search for top squark pair production in final states with two τ leptons, jets, and missing transverse momentum in s = TeV pp-collisions with the ATLAS detector Taller de Altas Energías 6 Michael Holzbock September 7, 6 LMU Munich

2 (Short) Review of Supersymmetry Motivation shortcomings of SM no unification of coupl. const. no (cold) dark maer candidate loop corrections of Higgs mass can be solved by SUSY Implementation extend symmetries of Poincaré group operator Q relating fermions and bosons: Q Boson = Fermion and v.v. symmetry is broken by unknown mechanism

3 Signal Model Production & Decay direct production of scalar-top pairs -body decay of stop to b-quark, neutrino and stau stau decays into tau and gravitino (LSP) simplified model (BRs = ) lep-had final state Final State electron/muon hadronic tau p p t t τ τ b b ν τ τ ν ICHEP: ATLAS-CONF-6-8 b-jets large E miss T G G

4 Analysis Strategy CR, VR & SR define signal-enriched region check and normalize background predictions in CR extrapolate from CR to SR verify extrapolation in VR Implementation observable observable one SR, optimized for large stau masses background dominated by top anti-top events tau reconstruction challenging large fake (falsely reco. objects) rates two dedicated for t t CRs for true and fake τ s

5 Event Selection triggered by e/µ signatures one reconstructed isolated e/µ one reco. hadronic τ of OS at least two jets Discrimination against BG: m T p p p pt / p generalization of m T for FS with multiple invisible particles m T = min q +q =/p T [ max { m T (p, q ), m T (p, q ) }] m T (l, τ) falls off steeply for BG, broad distribution for signal m T = m p + (E p T/p T p T/p T )

6 SR Definition events / GeV 6.. ATLAS Internal s = TeV,. / fb preselection, normalized /6 (fake τ) t t (true τ) Z+jets W+jets +V +H ~ m( t, τ)=(7,9) GeV ~ m( t, τ)=(7,) GeV SR Definition N b-jet E miss T > 8 GeV p T (τ) > 7 GeV m T (l, τ) > GeV m T (l,τ) [GeV] verifying extrapolation of m T for true and fake τ s in two VRs correct normalization of t t with fake τ contribution is crucial crosschecks using adjusted selections normalization factor varies assign add. systematic uncertainty on this normalization 6

7 Systematic Uncertainties Detector related trigger and object identification/reconstruction efficiencies energy calibration and resolution luminosity Theory related factorization & renormalization scale, radiation and hard-scaering model cross sections and PDF sets Dominated by uncertainties on t t modeling and normalization 7

8 Results SR Observed events Total background.±.9 t t (fake τ).±.8 t t + V.6±.9 t t W + jets single-top.6±.6 t t + H.±..±. good agreement between observation and SM predictions update exclusion limits for this model 8

9 Exclusion Limits m(τ ) [GeV] 8 ~ ~ ~ t τ τ τg ~ t production, BR( t bν) =, BR( ) = SUSY Observed limit ± σtheory Expected limit (± σ exp ) - ATLAS 8 TeV,. fb LEP limit s = TeV,. / fb 6 m~ t < m b + m τ ~ m( t ) [GeV] 9

10 Summary & Outlook Summary search for direct stop-to-stau production with τ s in final state no excess above SM predictions observed increased exclusion limits for stop masses up to 87 GeV and for stau masses up to 7 GeV Outlook publication for Moriond 7 planned include had-had and lep-lep (reinterpretation) channels

11 BACKUP

12 CR and VR definitions Variable CR t t R CR t t F CR t t SS CR W N b-jet ET miss > 8 GeV > GeV > GeV > GeV p T (τ) > 7 GeV > 7 GeV > GeV < 9 GeV m T (l, τ) < 6 GeV GeV < m T (l, τ) < 6 GeV < 6 GeV < 6 GeV m T (l) > GeV < GeV < 8 GeV m(l, τ) > 8 GeV Variable VR t t R VR t t F VR W + jets p T (τ) VR W + jets m T (l, τ) N b-jet ET miss > 8 GeV > GeV > GeV > GeV p T (τ) > 7 GeV > 7 GeV > 9 GeV < 9 GeV m T (l, τ) 6 GeV < m T (l, τ) < GeV < GeV > 6 GeV m T (l) > GeV < GeV < 8 GeV < 8 GeV m(l, τ) > 8 GeV > 8 GeV

13 Model Independent Upper Limits Signal channel ɛσ 9 obs [fb] S9 obs S 9 exp CL B p(s = ) (Z) SR (.)

14 N- Plots of SR events / GeV s = TeV,. / fb t t (fake τ) + V SR W + jets H ~ m( t, τ ) = (7, ) GeV events / GeV s = TeV,. / fb SR t t (fake τ) + V W + jets H ~ m( t, τ ) = (7, ) GeV E miss T [GeV] m T(l,τ) [GeV] events s = TeV,./fb t t (fake τ) + V SR W + jets H ~ m( t, τ ) = (7, ) GeV events / GeV s = TeV,. / fb t t (fake τ) + V SR W + jets H ~ m( t, τ ) = (7, ) GeV N b-jets p T(τ) [GeV]

15 N- Plots of CR t t (fake τ) events / GeV s = TeV,. / fb CR Fake t t (fake τ) + V W + jets H events / GeV s = TeV,. / fb CR Fake t t (fake τ) + V W + jets H 6 8 E miss T [GeV] m T(l,τ) [GeV] events / GeV s = TeV,. / fb CR Fake t t (fake τ) + V W + jets H events / GeV s = TeV,. / fb CR Fake t t (fake τ) + V W + jets H m T [GeV] p T(τ) [GeV]

16 N- Plots of CR t t (true τ) events / GeV s = TeV,. / fb CR Real t t (fake τ) + V W + jets H events / GeV s = TeV,. / fb CR Real t t (fake τ) + V W + jets H 6 8 E miss T [GeV] m T(l,τ) [GeV] events / GeV s = TeV,. / fb CR Real t t (fake τ) + V W + jets H events / GeV s = TeV,. / fb CR Real t t (fake τ) + V W + jets H m T [GeV] p T(τ) [GeV] 6