Randall-Sundrum KK Gluon & Energetic Tops at the LHC

Randall-Sundrum KK Gluon & Energetic Tops at the LHC

Joseph Virzi, LBL

May 2007 Joseph Virzi UC Berkeley 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


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

(1)G tt


RS1 KK Gluon

• 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

Branching Ratio of KKG vs MKKG

May 2007 5

Signatures of RS KK Gluon

• 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

σ vs Invariant Mass

L/R Polarization vs Invariant Mass

SM Prediction


Introduction to PLR

• Look at the direction of the lepton in the top quark rest frame

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

θ


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

hadronic

leptonic

• Semileptonic (ttbar→bbjjℓν) channel most promising for this analysis.– BR(ttbar →{μ,e}) = 30%


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


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


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 eventsNumber of Jets

Nu

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of E

ven

ts


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


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


Invariant Mass Plots

• TopJet approach is vastly more statistically significant over the mass window

• The conventional method is more appropriate for lower energies

• Shape of the background

Where’s the peak?

TopJet Method

Conventional Method


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


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.

Taken from ATL-SOFT-2003-010

Jet Energy Scale for b & light jets

Lepton PT vs Invariant Mass


Efficiency of CutsOn Signal & Background

• RED survives all cuts

Signal (RS+SM)

W+JETSSINGLE TOP

May 2007 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


ConclusionsConclusions• 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 )


Backup Slides

May 2007 20

• Boost profile for com is central for large invariant mass

• Primary production is through qqbar

Motivates stiff PT cut

Kinematicstt

May 2007 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

Evolution of cuts for single top production

green is conventional mode

May 2007 22

PT of leptonic top after cuts


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

May 2007 24

Efficiencies of Cuts

• Conventional Reconstruction Method

• TopJet Reconstruction Method– Stiff PT cut provides the coup-de-grace

(discuss later)– Has high signal efficiency

RED are events passing all cutsBoth plots are drawn to same scale

GREEN is conventional reconstruction


Top Quark Pair Kinematics

Motivates stiff PT cut

The boost profile of the CMS is central for large invariant mass


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


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

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Randall-Sundrum KK Gluon & Energetic Tops at the LHC