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Randall-Sundrum KK Gluon & Energetic Tops at the LHC

Randall-Sundrum KK Gluon & Energetic Tops at the LHC. Joseph Virzi, LBL. Introduction. The formalism of the RS1 model leads to KK excitations We consider here the first excitation of the gluon, G (1) Experimental constraints favor masses of G (1) > 2TeV Case study: 3 TeV KK gluon

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Randall-Sundrum KK Gluon & Energetic Tops at the LHC

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  1. Randall-Sundrum KK Gluon & Energetic Tops at the LHC Joseph Virzi, LBL

  2. Introduction • The formalism of the RS1 model leads to KK excitations • We consider here the first excitation of the gluon, G(1) • Experimental constraints favor masses of G(1) > 2TeV • Case study: 3 TeV KK gluon • Will use 100 fb-1 of data (3 years at high luminosity @ LHC) • The work here reports on work performed with K. Agashe, A. Belyaev, T. Krupovnickas, G. Perez and JSV • hep-ph/612015 Joseph Virzi UC Berkeley

  3. Outline • Focus on detection of KKG using top quark pair production • Top reconstruction @ high PT • discuss associated challenges • propose approaches to address these challenges • Conclusions Joseph Virzi UC Berkeley

  4. RS1 KK Gluon Branching Ratio of KKG vs MKKG • Prefers decay into heavier quarks, especially to tops. • BR > 0.95 • Heavy quark couplings to G(1) are enhanced relative to the SM. • For tR ~5 • For tL & bL ~1. • Light quarks & bR couplings are suppressed by factor ~5. • SM gluon couplings vanish due to orthogonality conditions Joseph Virzi UC Berkeley

  5. Signatures of RS KK Gluon σvs Invariant Mass • The RS1 KK Gluon provides a resonance structure • Width ~0.2 MKKG ( 600 GeV ) • tR >> tL over the bump • Strategy • Correlate large L/R polarization asymmetry to the mass peak • G(1) contribution to PLR is large & opposite sign than SM L/R Polarization vs Invariant Mass SM Prediction

  6. Introduction to PLR • Look at the direction of the lepton in the top quark rest frame θ N+ & N- are the number of events where the lepton is forward (cos(θ) > 0.0) and where the lepton is backward, respectively in the top rest frame Joseph Virzi UC Berkeley

  7. Monte Carlo Simulation • To analyze the RS1 model with MKKG = 3 TeV, generated 100 fb-1 of signal ( which includes the SM ) • Invariant Mass > 1 TeV • σ(M>1TeV) = 30 pb • 20% b-tag efficiency • Atlas detector acceptance • Cone R=0.4 Jet Reconstruction • Used a customized version of the Sherpa MC • 100 fb-1 of each background sample (W+jets, single top) • Semileptonic (ttbar→bbjjℓν) channel most promising for this analysis. • BR(ttbar →{μ,e}) = 30% Joseph Virzi UC Berkeley

  8. Signal Reconstruction • Conventional Methods of Top Reconstruction at the LHC involve reconstruction of whole top decay chain • beats down background • Requires ≥4 jets, of which ≥2 are b-jets • The approach breaks down at energies ~ TeV • Jets collimate. Fewer events pass selection • We modified the methods to address deficiencies Joseph Virzi UC Berkeley

  9. Conventional Signal Reconstruction • Reconstruction of top pairs • Isolated lepton • Missing energy → neutrino • Top mass (174 GeV )is an input • b-jet + W reconstructs leptonic top • 2 light jets reconstruct hadronic side W • b-jet + W reconstructs hadronic top Joseph Virzi UC Berkeley

  10. Problem with Conventional Method • As the invariant mass of the ttbar event ↑ the jet multiplicity ↓ • Conventional approach works well here • Reconstruction efficiency is adversely affected @ high invariant mass • Very few 4 jet events Number of Events Number of Jets Joseph Virzi UC Berkeley

  11. TopJet Reconstruction • Leptonic top reconstruction • Isolation → MBL • Hadronic side reconstruction • Use the events where the decay products of the top are observed as a single jet • Impose a top-jet hypothesis on the hadronic side jet • remove b-tagging constraint on hadronic side • Stiff ( >600 GeV ) PT cut on the leptonic side top decimates background Joseph Virzi UC Berkeley

  12. Removing B Decay Leptons - MBL • MBL – the invariant mass between b-jet and lepton • B decay leptons have MBL ~ 5 GeV • Signal leptons have MBL ~ 50 GeV • 20% of b-jets contain leptons • descriminate against B decay leptons • Keep leptons from t → bW →bℓν Joseph Virzi UC Berkeley

  13. Invariant Mass Plots TopJet Method • TopJet approach is vastly more statistically significant over the mass window • The conventional method is more appropriate for lower energies • Shape of the background Conventional Method Where’s the peak? Joseph Virzi UC Berkeley

  14. Reconstruction Efficiency • Huge increase in efficiency • Efficiency turn on gradient due to stiff PT cut • Note: massive gluon produces central tops that survive PT cuts • peak not sculpted by ε curve Efficiency accounts for BR Joseph Virzi UC Berkeley

  15. More on Polarization Asymmetry The L/R polarization asymmetry will manifest itself in the lepton <PT> (A.T.Holloway) Additional Challenge • Jet Energy Corrections • Jet Energy ≠ Parton Energy • Vital to reconstructing quark cm frame for PLR • Adds uncertainty to reconstruction of cms kinematics. Jet Energy Scale for b & light jets Taken from ATL-SOFT-2003-010 Lepton PT vs Invariant Mass Joseph Virzi UC Berkeley

  16. Efficiency of CutsOn Signal & Background RED survives all cuts Signal (RS+SM) W+JETS SINGLE TOP Joseph Virzi UC Berkeley

  17. Results of Top Jet Approach • The peak becomes much more statistically significant • We correlate the mass peak to the PLR • Additionally, we can observe the <PT> of the lepton

  18. Conclusions • With new reconstruction technique, the signature(s) of the RS KK gluon becomes much more statistically significant • Combination of Topjet and Conventional techniques spans low to high MTT • The efficiency of reconstruction increases by O(5) • And turns out to stay relatively flat for increasing invariant mass ~4TeV • The W+jets and single top background is small • 100 fb-1 of data is a long time. • Depending on the mass of the KK gluon, efficiencies and fake rates, maybe we can get by with less data • Need to leave some wiggle room ( PDF & other uncertainties ) Joseph Virzi UC Berkeley

  19. Backup Slides Joseph Virzi UC Berkeley

  20. Boost profile for com is central for large invariant mass • Primary production is through qqbar Motivates stiff PT cut

  21. Single TopBackground • Sample used is single top production • Representing 100 fb-1 • MCMS > 1 TeV • PT > 50 GeV • 5 pb cross section • PT cut yields high background rejection • 97% light jet rejection • t-channel production is dominant green is conventional mode Evolution of cuts for single top production

  22. PT of leptonic top after cuts

  23. Reconstruction Efficiency • Huge increase in efficiency • Efficiency turn on gradient due to stiff PT cut • Note: massive gluon produces central tops that survive PT cuts • peak not sculpted by ε curve Efficiency accounts for BR Joseph Virzi UC Berkeley

  24. Efficiencies of Cuts Conventional Reconstruction Method • TopJet Reconstruction Method • Stiff PT cut provides the coup-de-grace (discuss later) • Has high signal efficiency GREEN is conventional reconstruction RED are events passing all cuts Both plots are drawn to same scale

  25. Top Quark Pair Kinematics The boost profile of the CMS is central for large invariant mass Motivates stiff PT cut Joseph Virzi UC Berkeley

  26. Spectrum of Hadronic SideReconstruction Modes • 2 light jet + 1 b jet events • b → semileptonic top • 2 light jets summed • 1 light jet + 2 b jet events • b → semileptonic top • hadronic top = b + j • 3 light jets + 1 b jet events • b → semileptonic top • hadronic top = j + j + j • 5+ jet events • In all cases, the jets on the hadronic side are summed to the top • Reconstruction modes are separated for different jet multiplicities • The final reconstruction depends weakly on jet reconstruction algorithm • Allows for weighing contribution from each mode Joseph Virzi UC Berkeley

  27. Feynman Diagrams • Relevant Tree Level Diagrams for our discussion • Primary production mechanism for top quark pairs • The gg→KKG vertex does not exist because of orthogonality arguments Joseph Virzi UC Berkeley

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