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Quark Compositeness With Di-Photon Final State at LHC:Update Sushil S. Chauhan Prof. Debajyoti Choudhury, Dr. Satyaki Bhattacharya & Prof. Brajesh C. Choudhary Department of Physics & Astrophysics University of Delhi, India India-CMS meeting 21st -22nd January 007
Outline • Brief Introduction • Last Presentation • Discriminating variables • Kinematical & Isolation cuts • Confidence Limit (CL) Calculation • Systematic Error • Future Plans
Motivation • At what scale quark substructure is possible? • Compositeness scale Λ provides estimation of quark substructure scale (e.g., ΛQCD gives the distance scale for quarks inside the proton) • Two photons final state gives a clean signal compared to other channels • No excited quark study exist with two photon final state
Feynman Diagrams: Signal • For signal one need to add SM q-qbar qγγcontribution coherently to the q* signal • Compositeness scale Λ and mass of q*, M* are free parameters of the theory
Feynman Diagrams :Background All these backgrounds have the same final state and very large cross section compared to q* signal
Event Generation with PYTHIA • For generation of events the matrix element has been included in PYTHIA (CMKIN) with showering and hadronization effects • Cross-Section for q* Signal with PT (hat) >190 GeV (TeV)M* (TeV) ª( fb ) • 0.7 0.5 106.90 • 1.0 0.5 95.41 • 2.0 0.5 81.94 • 3.0 0.5 78.69 • 5.0 0.5 77.11 • 100.0 0.5 76.04 • SM ------------------------------------- > 76.04 • For fixed values of M* & sqrt (s) the x-section decreases with increasing Λ, hence it becomes difficult to extract the signal ªFor standard parametrization
Photon Finding Algorithm • Using 10x10 clustering algotithm to “reconstruct” the photon at the generator level • Select a seed with P,e±T> 5GeV and • Look around the seed in 10x10 crystal size in φ and η directions • Where Δφ=0.09 and Δη=0.09 • Add the 4-momentum vectorialy • Only e+/e- and are selected as seed and inside 10x10 crystal size around the seed • Vector additions provides EGamma Super-Cluster or Photon Candidate • Compare this algorithm with actual detector simulation for fake and direct photons - found to be in good agreement
Generator Level Resonstruction Vs Detector level Simulation For leading Photon Candidates ( +Jet sample) Δη & Δφ Distributions Δη For Next-To-Leading Photon Candidates of (+Jet sample) Δφ ( radians) η φ( radians)
Discriminating Variables • Variables considered: • ET sum in a cone around photon • # of stable charge tracks around photon in a cone (from +/- , p+/- , K+/-, e+/-) • PT of the highest track in a cone around photon • PT sum of tracks in a cone around photon • Vector PT sum of tracks in a cone around photon • PT of first few nearest tracks in a cone around photon • Have studied these variables for a number of cone sizes
Selection of Cuts: Track min. PT Highest PT Track Signal Efficiency increases with increase in min. Pt of track by ~ 50 % for Ntrk =0 (Histograms are normalized to unity)
Final cuts • Kept gamma+Jet nearly ~1 % & large signal efficiency • Analysis points for signal are chosen which have similar x-section as SM process With these cuts JJ background estimated to be ~ 3.5 events* at Lum. of 1 fb-1 (* if we assume same efficiency as gamma +Jet background)
Confidence Limit Calculations • Due to statistical fluctuation we can not say whether the data is from signal or from background. We interpret results in terms of “ Confidence Limits (CL)” and test whether data is consistent with signal or background from theory • Using Frequentist approach • Hypothesis: (S+B) -Type OR B-Type only. The observed data can be of S+B type or B type. • Generating “Gedenken” experiment to put 95 % CL and “5σ Discovery Limit”. • Estimator is Log Likelihood Ratio (LLR): LLR= - 2 * ln X
Signal vs Background Distributions Kinematical variables can be used to estimate the CL
Exclusion: Λ- Mq* Parameter Space Cos ө* used as test variable Work in Progress: Generating points for 300 fb-1 of luminosity
Systematic • Scale variation: We varied the scale by a factor of 0.5 and 2.0 from the central scale. Also estimated x-section with other scales like t-hat, PT etc. The maximum variation found to be 1.6 % in the cross-section • PDF uncertainty: We have used CTEQ5L. Taking CTEQ6M as reference we compared CTEQ5M1, MRST2001 & CTEQ5L. The maximum uncertainty of ~7 % found with CTEQ5L. MRST2001 and CTEQ5M1 shows 2.3 % and 3.5 % of uncertainty • Luminosity error: Expected to be 3 % above 30 fb -1 • Effect of systematic on C.L : Still to be done
Summary & Plans • Combining two discriminating variable (PT and Cos ө* ) will give better limits (3-6 % CL ). Effect of systematic need to be evaluate • Preliminary results show that we can probe up to a distance scale of ~ 10-20 m at LHC with this channel ( ~10-19 m excluded by Tevatron: ATLAS-TDR ) • Propose this channel in BSM group, some results were presented at the BSM meetings at CERN in Nov. 06 • Publication: To be subimmited very soon
Compositeness scale Compositeness scale: • Λ >> sqrt (s-hat) : Contact interaction • Λ << sqrt (s-hat) : Excited state • Λ ~ sqrt (s-hat) : Model Dependent
Could be Useful!!! • MC@NLO interface with CMKIN_6_1_0. Available at, http://schauhan.web.cern.ch/schauhan/MCNLO_Interface/mcatnlocmk.tar.gz
Generator Level Reconstruction Vs FAMOScont.. For Next-To-Leading Photon Candidate
Generator Level Reconstruction Vs FAMOScont.. • Those events where EGamma Super Clusters < Generated EGamma Super Clusters
Matrix Element for q-qbarq*γγ SM Piece For Standard Parametrization f1=1, n1=1. Is the compositeness scale and m is the mass of q*
Available Literature For Quark & Lepton compositeness: • Dijet channel (Phys.Rev. D-03110, Robert Harris hep-ph/9609319) • Drell -Yan (S. Jain et. al.hep-ex/0005025 ) • Gamma+Jet final State: ATLAS collaboration (ATL –PHYS-99-002). (No such study exists for CMS) • Two photon final state: Some phenomenological studies have been done without complete SM background e.g.,Thomas G. Rizzo PRD v51,Num-3 (No such study exists for CMS) • Existing Limit at the LHC’s center of mass energy, with two photon final state is: ~Λ >2.8 TeV for contact interaction (depends on kinematical cuts and luminosity)
Present Limit on M* Limits from Tevatron: • CDF: M* > 80 GeV (q*q ) • CDF: M* > 150 GeV (q* q W ) • CDF (All channels): M* >200 GeV • D0: M*> 200 GeV • Simulation study: Mass reach up to 0.94 TeV at Tevatron ( 2 TeV, 2 fb-1, q*q-qbar) • ATLAS Study: upto 6.5 TeV at LHC ( f=fs=1, q*q )
Motivation • Are quarks fundamental particles? OR Do they have sub-structure? • Replication of three generation of quarks and leptons suggests the possibility that may have composite structures made up of more fundamental constituents • Large Hadron Collider (LHC) will explore physics “Beyond the Standard Model” @ the TeV scale • Excited quark state represents signal for substructure of quarks and physics beyond the SM
Effects of Different Cuts So far best variables to discriminate the signal from background are, Cut A: Riso< 0.35, ETsum< 5.0GeV Cut B: Riso< 0.35, Highest Tracks PT < 4.0 GeV Cut C: Riso< 0.10, # of Tracks < 2 For L= 1 fb-1
Effects of Different Cuts ….. Cut A: Riso< 0.35, ETsum< 5.0GeV Cut B: Riso< 0.35, Highest Tracks PT < 2.0GeV Cut C: Riso< 0.10, # of Tracks < 2 For L= 1 fb-1
Why Two photon final state? • Two photon final state provides a cleaner signal compared to other channels • CMS ECAL energy resolution is very good • Not much studies have been done with this channel without detector effects • Disadvantages with other channels, like energy correction scale with jets • Large background with lepton final state e.g., Drell -Yan etc.