Top-partner mass reconstruction by using jets

Michihisa Takeuchi （ KEK,YITP）

In collaboration with Mihoko M. Nojiri (KEK), Naotoshi Okamura (KEK)

IntroductionThere are several models: MSSM,LHT,UED, …Common signal at the LHC: Many high jets, leptons, LargeLepton channels are established.

However, the analyses in lepton channels depend on the lepton BR.

If the lepton BR is small, few events can be observed at the LHC.

Jet BR is large, much events with only jets will be observed.

To Understand Jets are important toward the LHC.1

What is the jet ?

Experimentally : Hadrons which have collinear momenta

Theoretically : Hadrons which are showered from a parton

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to obtain a set of jets from a set of hadrons’ momenta.

Most activities are within a cone which has some radius.Cone algorithms

⊿ Ｒ＝ 0.4 is standard

Parton shower results in a set of collinear hadrons

We should define jets and give the procedure

1. Find the particle with the maximum pt.

2. Draw a cone which center is the selected direction.

3. Sum all momenta within the cone, and back to step 2 and repeat until the cone converge.

The simplest cone algorithm implemented in AcerDET

SISCone More sophisticated cone algorithm

Clustering algorithms

We have implemented 4 algorithms into our code by using FastJet and can compare them.

It is important to understand what is stable and what is sensitive to algorithms.

kt

Cambridge

1. Calculate “distances” between all the pairs of particles.

2. Pick up the pair with minimum distance, merge two as one and back to step1 until all distances are larger than some threshold.

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Jet definition is not unique.

The activity decreases,

when it goes away from the center of activity.

kt CambridgeAcerDET SISCone

Plan of talk

1. Introduction (Jets are important)

2. Top-partner mass reconstruction by using jets

in the LHT (Cambridge)

3 ． Comparison among jet reconstruction algorithms

4. Summary

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2. Top-partner mass reconstruction by using jets in the LHT

Top partner in Littlest Higgs model with T-paity• Studied in S.Matsumoto, M.M. Nojiri, D.Nomura PRD75,(2007) The simplest cone algorithm (AcerDET ） is used for reconstructing jets.

• AcerDET treats jet masses as 0. Then invariant masses of jet systems are underestimated.

• We have reanalysed the same process by using kt, Cambridge, SISCone.

Signal

Point: tops from the top-partner decays are highly boosted.

Missing momentum

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BG

Hemisphere analysis

hemisphere 2

hemisphere 1

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Decay products from a boosted top has collinear momenta

The method objects into 2 groups called hemispheres.

Collinear objects are grouped in the same hemisphere

t

t

Tops from ttbar are not boosted, this grouping dose not work well.

Event selection

Cut the SM events

Forbid semi-leptonic decay of tops

Require boosted tops

Drop the events with other QCD jets or 2 tops in 1 hemisphere.

Mass spectraGenerated by HERWIG

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Summary of CutsTo drop ttbar contributions

Top reconstructionTo produce large ,

at least one top decays leptonically.

Difficult to reconstruct in

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We regard hemispheres momenta as top momenta

150<m<190GeV

Mass measurement of Top partner

The correct splitting is includedWe can reconstruct top-partner mass using jets

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Defined only by transverse variables and masses

The correct momenta is substituted

Now we have 2 top momenta from production events and .

3 ． Comparison among jet reconstruction algorithms

3 ． Comparison among jet reconstruction algorithms

Acerdet has a dull peak below and a tail below the peak.

Behaviors of KT and CB are similar.

Mass measurement with works well for all algorithms.

distributionInvariant mass distribution in

ACER KT

SISCB

ACER KT

SISCB

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Effect of underlying events

Conesize R=0.4. CB: black line KT: red line

KT & CB behaviors are similar with no underlying event.

With underlying events, the peak position of KT move above 175GeV.

With no underlying event 　 With underlying events

Generated by JIMMY of HERWIG

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CBKT

CBKT

Summary• To understand Jets is important at the LHC.

• We have reconstructed top momenta by using jets

and measure the mass of top-partner using .

• We have studied the dependence of jet algorithms. * mass measurement with is not so sensitive to algorithms.

* Cambridge algorithm is stable to underlying events,

kt algorithm is sensitive to them.

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