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Randall-Sundrum KK Gluon & Energetic Tops at the LHC. K. Agashe, A. Belyaev, T. Krupovnickas, G. Perez and JV / hep-ph/612015 Work in Progress with K.Agashe, T.Han, G.Perez. Joseph Virzi, LBL. Outline. Brief Introduction to Randall-Sundrum (RS) Model

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randall sundrum kk gluon energetic tops at the lhc

Randall-Sundrum KK Gluon & Energetic Tops at the LHC

K. Agashe, A. Belyaev, T. Krupovnickas, G. Perez and JV / hep-ph/612015

Work in Progress with K.Agashe, T.Han, G.Perez

Joseph Virzi, LBL

outline
Outline
  • Brief Introduction to Randall-Sundrum (RS) Model
  • Focus on detection of KKG using top quark pair production
  • Top reconstruction @ high PT
    • discuss associated challenges
    • propose approaches to address these challenges
  • Polarization asymmetry measurement
  • Background Analysis & Discussion
  • Conclusions

Joseph Virzi UC Berkeley

randall sundrum model with sm fields propagating in the bulk
Randall-Sundrum Modelwith SM fields propagating in the bulk

solves the hierarchy problem for

  • motivation for model is hierarchy problem – vast difference between the weak and Planck scales

4D metric

Joseph Virzi UC Berkeley

slide4

Warp Factor

  • SM phenomenology constrains profiles in 5th dimension
  • Yukawa couplings are given by overlap with Higgs on TeV brane

Joseph Virzi UC Berkeley

particle profiles
Particle Profiles

LIGHT

HEAVY

  • Couplings to tops are enhanced and parity violating
  • Dominant coupling to tR because of pheno’ constraints

Joseph Virzi UC Berkeley

analysis
Analysis
  • 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)

Joseph Virzi UC Berkeley

rs1 kk gluon
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

feynman diagrams
Feynman Diagrams
  • Relevant Tree Level Diagrams for our discussion
  • The gg→KKG vertex does not exist because of orthogonality arguments
  • Primary production mechanism for top quark pairs

+

Joseph Virzi UC Berkeley

signatures of kk gluon
Signatures of KK Gluon
  • The RS1 KK Gluon provides a resonance structure
    • Width ~0.2 MKKG ( 600 GeV )
  • total cross section 850 pb
  • ΔσRS = O(100 fb)

σvs Invariant Mass

signatures of kk gluon cont d
Signatures of KK Gluon (cont’d)
  • The excess production will have more tR than tL
  • Strategy
    • G(1) contribution to PLR is large & opposite sign than SM
    • Correlate large L/R polarization asymmetry to the mass peak

L/R Polarization vs Invariant Mass

RS prediction

SM prediction

l r polarization asymmetry introduction to p lr
L/R Polarization Asymmetry 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

slide12

Dileptonic channel → 2 neutrinos

    • Difficulty resolving neutrino
    • 10% BR
  • Fully hadronic decay
    • Background more difficult
    • 60% BR
  • Semileptonic (ttbar→bbjjℓν) channel most promising for this analysis.
    • BR(ttbar →{μ,e}) = 30%

Joseph Virzi UC Berkeley

monte carlo simulation strategy
Monte Carlo Simulation Strategy
  • Used a customized version of the Sherpa MC
    • Full spin correlations in top decays
  • 100 fb-1 of signal ( SM/RS ) with MKKG = 3 TeV
    • Invariant Mass > 1 TeV
    • σ(M>1TeV) x 0.3 semileptonic BR = 8.8 pb
  • 100 fb-1 of W+jets sample
    • Invariant Mass > 1 TeV & PT > 300 GeV
    • σ (M>1TeV) = 6.5 pb
  • 100 fb-1 of single top production sample
    • Invariant Mass > 1 TeV & PT > 50 GeV
    • σ (M>1TeV)= 5 pb

Joseph Virzi UC Berkeley

signal reconstruction overview
Signal Reconstruction Overview
  • 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. We will discuss later
  • We overhauled the methods to address deficiencies

Joseph Virzi UC Berkeley

conventional signal reconstruction
Conventional Signal Reconstruction
  • Reconstruction of top pairs
    • ≥4 jets, 2 are b-tagged
    • Isolated lepton - ΔR
    • Missing energy → neutrino
    • Top mass (174 GeV ) is an input
    • 1 b-jet + W reconstructs leptonic top
    • 2 light jets reconstruct hadronic side W
    • Other b-jet + W reconstructs hadronic top

Joseph Virzi UC Berkeley

problem with conventional method
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

topjet reconstruction
TopJet Reconstruction
  • Hadronic side – giving up
    • 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
  • Modify leptonic top reconstruction
    • Lepton isolation difficult (next)

Joseph Virzi UC Berkeley

removing b decay leptons m bl
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

invariant mass plots
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

efficiency plot
Efficiency Plot

Huge increase in reconstruction efficiency

The efficiency & mass curves are shaped by the physics

The mass curve is not shaped by the efficiency curve

Reconstruction Efficiency vs Invariant Mass

Joseph Virzi UC Berkeley

slide21
Boost profile for com is central for large invariant mass
  • Primary production is through qqbar

Motivates stiff PT cut

slide22
JETS

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jet p t over mass peak
Jet PT over mass peak
  • Distributions are normalized to unit area
  • <PT> of b-jets = 555 GeV
  • 50% of b-jets have PT > 300 GeV

Joseph Virzi UC Berkeley

b tagging @ high p t
B-tagging @ high PT
  • Important Issue but still relatively uncertain
    • Best estimates at low energies place ε = b-tag efficiency = 60%
  • Best estimates are approximately 20% at upper end of the PT spectrum
    • March, Ros, Salvachua ATL-PHYS-PUB-2006-002
  • Remain conservative & use 20% throughout
  • Conventional reconstruction methods depend on 2 b-tags. Quadratic dependence on ε
  • New approach described here only requires 1 b-tag. Linear dependence on ε

Joseph Virzi UC Berkeley

light jet rejection
Light Jet Rejection
  • Ensuring that we do not label jets from lighter partons as b-jets
    • especially important for W+jets background
  • Current estimates
    • March, Ros & Salvachua ATL-PHYS-PUB-2006-002
    • Rc = 30. Ru = 130
  • This analysis is performed with a uniform rejection ratio Rq=30

Joseph Virzi UC Berkeley

l r polarization asymmetry
L/R Polarization Asymmetry

Challenges

  • 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

Joseph Virzi UC Berkeley

l r polarization asymmetry cont d lepton p t distribution
L/R Polarization Asymmetry Cont’dLepton PT Distribution

The L/R polarization asymmetry will manifest itself in the lepton <PT> (A.T.Holloway)

Lepton PT vs Invariant Mass

Joseph Virzi UC Berkeley

background analysis
Background Analysis

Joseph Virzi UC Berkeley

efficiency of cuts on signal background
Efficiency of CutsOn Signal & Background

RED survives all cuts

Signal (RS+SM)

W+JETS

SINGLE TOP

Joseph Virzi UC Berkeley

results of top jet approach
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
lhc reach
LHC Reach
  • Our reconstruction efficiency remains relatively flat to 4 TeV
  • Current estimates place the reach of the LHC for our signal to 4 TeV

Joseph Virzi UC Berkeley

conclusions
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 )
  • Preliminary analysis using more realistic reconstruction techniques shows consistency with the results herein

Joseph Virzi UC Berkeley

backup slides
Backup Slides

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summary of cuts
Summary of Cuts

Joseph Virzi UC Berkeley

jet p t distributions from signal sample
Jet PT Distributionsfrom signal sample
  • B-jet spectrum is harder than for light jets

Joseph Virzi UC Berkeley

single top background
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

w jets background
W+JETS background
  • Sample is W+jets
    • representing 100 fb-1
    • MCMS > 1.5 TeV
    • PT > 300 GeV
    • cross section 6.5 pb
    • Light jet rejection → 97%

Evolution of cuts for W+jets background

efficiencies of cuts
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

conventional reconstruction
Conventional Reconstruction

Joseph Virzi UC Berkeley

topjet cut statistics
TopJet Cut Statistics

Joseph Virzi UC Berkeley

efficiencies of reconstruction using different modes
Efficiencies of Reconstructionusing Different Modes

Leptonic top

PT>600 GeV

Joseph Virzi UC Berkeley

efficiencies of reconstruction using different modes1
Efficiencies of Reconstructionusing Different Modes

Leptonic top

PT>400 GeV

Joseph Virzi UC Berkeley

w jets background1
W+JETS background
  • I focus here on background most likely to do damage
    • Invariant mass > 1.5 TeV
    • PT > 300 GeV
    • Cross section 6.5 pb
  • The background plot looks at all combinations of 2, 3 and 4 jets which pass the indicated cuts on the leptonic side.
    • Superset of actual background
    • No b-tagging / light jet rejection assumptions

Evolution of cuts for W+jets background

Joseph Virzi UC Berkeley

single top background1
Single TopBackground
  • Sample used is single top production
    • MCMS > 1.5 TeV.
    • PT > 100 GeV
    • 5 pb cross section
  • The background plot looks at all combinations of 2, 3 and 4 jets which pass the indicated cuts on the leptonic side.
    • Superset of actual background
    • No b-tagging / light jet rejection assumptions

Evolution of cuts for single top production

Joseph Virzi UC Berkeley

spectrum of hadronic side reconstruction modes
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

single top s channel
single tops-channel

PT(leptonic top)

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single top t channel
Single top t-channel

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slide56

jlkajsdkθ

θ

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w jets jet pt distribution
W+Jets Jet PT Distribution

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