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Top-partner mass reconstruction by using jets

Top-partner mass reconstruction by using jets. Michihisa Takeuchi （ KEK,YITP ）. In collaboration with Mihoko M. Nojiri (KEK), Naotoshi Okamura (KEK). Introduction. There are several models: MSSM,LHT,UED, … Common signal at the LHC: Many high jets, leptons, Large

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Top-partner mass reconstruction by using jets

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  1. Top-partner mass reconstruction by using jets Michihisa Takeuchi （KEK,YITP） In collaboration with Mihoko M. Nojiri (KEK), Naotoshi Okamura (KEK)

  2. Introduction There are several models: MSSM,LHT,UED, … Common signal at the LHC: Many high jets, leptons, Large Lepton 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

  3. What is the jet ? Theoretically : Hadrons which are showered from a parton Parton shower results in a set of collinear hadrons Experimentally : Hadrons which have collinear momenta We should define jets and give the procedure to obtain a set of jets from a set of hadrons’ momenta. Cone algorithms Most activities are within a cone which has some radius. ⊿Ｒ＝0.4 is standard The simplest cone algorithm implemented in AcerDET • Find the particle with the maximum pt. • Draw a cone which center is the selected direction. • Sum all momenta within the cone, and back to step 2 and repeat until the cone converge. SISCone More sophisticated cone algorithm 2

  4. Clustering algorithms The activity decreases, when it goes away from the center of activity. • Calculate “distances” between all the pairs of particles. • Pick up the pair with minimum distance, merge two as one and back to step1 until all distances are larger than some threshold. kt Cambridge kt SISCone Cambridge AcerDET Jet definition is not unique. It is important to understand what is stable and what is sensitive to algorithms. We have implemented 4 algorithms into our code by using FastJet and can compare them. 3

  5. Plan of talk • Introduction (Jets are important) • Top-partner mass reconstruction by using jets in the LHT (Cambridge) 3． Comparison among jet reconstruction algorithms 4. Summary 4

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

  7. 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 Missing momentum BG Point: tops from the top-partner decays are highly boosted. 5

  8. hemisphere 1 hemisphere 2 Hemisphere analysis The method objects into 2 groups called hemispheres. t t Collinear objects are grouped in the same hemisphere Decay products from a boosted top has collinear momenta Tops from ttbar are not boosted, this grouping dose not work well. 6

  9. Event selection Mass spectra Generated by HERWIG Summary of Cuts To drop ttbar contributions 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. 7

  10. Top reconstruction To produce large , at least one top decays leptonically. 150<m<190GeV Difficult to reconstruct in We regard hemispheres momenta as top momenta 8

  11. Mass measurement of Top partner Now we have 2 top momenta from production events and . The correct momenta is substituted Defined only by transverse variables and masses We can reconstruct top-partner mass using jets The correct splitting is included 10

  12. 3． Comparison among jet reconstruction algorithms

  13. KT KT ACER ACER SIS SIS CB CB 3． Comparison among jet reconstruction algorithms Invariant mass distribution in distribution 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. 11

  14. Effect of underlying events Generated by JIMMY of HERWIG With underlying events With no underlying event CB CB KT KT 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. 12

  15. 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. 13

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