Eric COGNERAS LPC Clermont-Ferrand Prospects for Top pair resonance searches in ATLAS Workshop on Top Physics october 2007, Grenoble

2 Theoritical motivations  Top quark is a very special quark : mass of the order of EW scale WHY ?  No ttbar resonance in SM  Many BSM models predict ttbar resonances o Top Color (C. Hill, S. Park, PRD49, 4454, 1994 ) o Extradimension ( J. Rosner, CERN-TH/96-169, 1996, B. Lillie, hep-ph/ ) o Extended gauge theories ( A. Leike, Phys. Rep. 317, 143, hep- ph/ ) 20/10/2007 Workshop on Top Physics, Grenoble Is the 3rd generation special ? Is it a sign of New Physics ?

3 Analysis scheme 20/10/2007 Workshop on Top Physics, Grenoble  Search for resonance from : o ttbar mass spectrum  look for bump in spectrum o Modification of the cross section  comparison with SM predictions  Search for narrow resonance o Model independant o Width dominated by detector effects  ttbar Channels o Lepton+jets o Dilepton  Two strategies : o Exclude resonance mass for a given theory (when data) o Discovery potential (when no data)

4 CDF/DØ : Event Selection  Analysis performed in the lepton+jets channel  Event selection :  1 high P T lepton  High missing transverse energy  At least 4 high E T jets  Signal and background : o SM ttbar, W+jets ~90 % or more o QCD, Diboson, Single Top Minor background 20/10/2007 Workshop on Top Physics, Grenoble

5 CDF/DØ : ttbar invariant mass reconstruction (RunII)  Unknow variables : o 4 quark momenta  how to assign particles? o neutrino 3-momentum  DØ solution : Use a kinematical fit to find the most likely combination using kinematical constraints (W & Top mass).  CDF solution : Use the Matrix Element Method (see yesterday talks) to define the best combination. 20/10/2007 Workshop on Top Physics, Grenoble Invariant W mass constraint Invariant W mass constraint (2 solutions) m top =175 GeV Mass constraint 1 b-tag jet 12 jet-parton assignment + 2 P z : Choose lowest  ² among 24 combinations

6 Results from CDF/DØ (RunII)  Z’ →ttbar such as  Z’ =0.012 M Z’  CDF o 327 events 20/10/2007 Workshop on Top Physics, Grenoble M Z’ < 720 GeV 95 % C.L. CDF Note 8675 L =955 pb -1

7 Results from CDF/DØ (RunII)  Z’ →ttbar such as  Z’ =0.012 M Z’  DØ o 197 events 20/10/2007 Workshop on Top Physics, Grenoble M Z’ < 720 GeV 95 % C.L. M Z’ < 680 GeV 95 % C.L. DØ Note 5443_CONF L =0.9 fb -1

8 Search of Top pair LHC  LHC advantage wrt Tevatron : o Luminosity X 10 o Ttbar x-section X 100  Analysis performed in the lepton+jets channel  Event selection : similar to Tevatron  1 electron (muon) with P T >20 (25) GeV [trigger]  E T miss > 20 GeV  At least 4 jets with P T > 40 GeV (2 b-tagged) 20/10/2007 Workshop on Top Physics, Grenoble 1 LHC  3 Tevatron  lower statistical error

9 ATLAS : ttbar invariant mass reconstruction 20/10/2007 Workshop on Top Physics, Grenoble Had. W Reconstructed from 2 nearest jets Had. Top Reconstructed from W and nearest b-jet Lep Top Choose P z leading to the nearest lep. M top from hadronic side mean value

10 ATLAS : ttbar invariant mass reconstruction 20/10/2007 Workshop on Top Physics, Grenoble Apply additional cuts on W and Top mass spectra   2  on M W   3  on M Top Had. W Had. Top Lep Top

11 What we could see  For a 700 GeV resonance o SM ttbar is background o W+jet background negligible (< 1% due to 2 b-tagged jet requirement) 06/07/2016 T7 note meeting 700 GeV Z’ resonance on SM ttbar mass spectrum 700 GeV Z’ resonance SM ttbar background What is the minimal production rate of a resonance to be seen ? In this plot,  Z’ →ttbar = 140 pb L =1 fb -1

12 Discovery potential calculation  Discovery potential of a narrow resonance decaying into Top pair  Sliding window over the invariant mass spectrum  Compute the number of resonance events required for discovery (n X = 5  deviation on n SM )  Method requirements : o Reconstruction efficiency o Purity of final sample o Resolution on M tt 20/10/2007 Workshop on Top Physics, Grenoble Standard Model n SM nXnX Use resonance samples from 700 GeV to 3 TeV

13 Discovery potential calculation  Discovery potential of a narrow resonance decaying into Top pair  Sliding window over the invariant mass spectrum  Compute the number of resonance events required for discovery (n X = 5  deviation on n SM )  Method requirements : o Reconstruction efficiency o Purity of final sample o Resolution on M tt :  M tt )=40 GeV for M tt = 700 GeV  M tt )=60 GeV for M tt = 2 TeV 20/10/2007 Workshop on Top Physics, Grenoble Resolution on resonance mass Fit function

14 Results expected by ATLAS  Discovery potential of a narrow resonance decaying into Top pair  Fast simulation 20/10/2007 Workshop on Top Physics, Grenoble ATL-PHYS-PUB TeV resonance Discovery if : (  ×Br) ≥ 1 pb with 30 fb -1 (3 year low lumi.) Combinatorial background L =4.6 fb -1

15 Results expected by ATLAS  Discovery potential of a narrow resonance decaying into Top pair  Full simulation 20/10/2007 Workshop on Top Physics, Grenoble Note in progress 1 TeV resonance Discovery if : (  ×Br) ≥ 1 pb with 10 fb -1 (1 year low lumi.) (  ×Br) ≥ 0.6 pb with 30 fb -1 (3 year low lumi.) At high Mass, Jet overlap (leak of recons. evts)  other strategy to be performed Better than previous analysis (improvement of particle preselection, reconstruction method) L =1 fb -1

16 Model dependant point of view  If exist, TopColor leptophobic Z’ will be LHC [hep-ph/ v1]  KK gluon (extradimension model) [hep-ph/ v1] : ATLAS could exclude M KK < ~2 TeV with 1 fb -1 20/10/2007 Workshop on Top Physics, Grenoble

17 Other research axis 20/10/2007 Workshop on Top Physics, Grenoble  Top P T spectrum o Many exotic models predict sizeable enhancements in the tt x-section at high P T (P T >200 GeV/c) [T.G. Rizzo, hep-ph/ ; K. Lane, Phys. Rev. D52, 1546 (1995)]  Measure in p T bins  Results (RunI) : [Phys.Rev.Lett. 87, (2001)] (SM expects 0.025) o Nothing yet from RunII  Using Di-lepton channel o Prospected at Tevatron (RunI) o Could be interesting to investigate at LHC

18 Conclusion  Classical ttbar reconstruction methods used to search for resonances  CDF/DØ o No evidence for a new resonance o Limit for Z’ narrow resonance (leptophobic topcolor assisted technicolor): ~ % C.L. o Now, including more data (currently ~1 fb -1 )  LHC o High x-section and luminosity  higher statistics : Better sensitivity to new physics o High energy : direct observation of resonance ? 20/10/2007 Workshop on Top Physics, Grenoble

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