Searches for new Physics in the Top quark decay 정 연 세 University of Rochester

Outlook 2  Introduction to charged Higgs search  Tevatron & CDF detector  Event Selection & Reconstruction  Dijet mass improvement study  H + search in CDF II data  Placing upper limits on B(t  H + b)  Extending limits to generic charged boson  Summary & Prospect; FCNC in Top decay

What is Charged Higgs boson?  In standard model, a scalar complex doublet field is responsible for the electro- weak symmetry breaking (EWSB)  Results in massive weak bosons and predicts a neutral scalar Higgs boson  No observation of a SM Higgs boson to date  Extend Higgs sector beyond the SM  The simplest extension of the Higgs sector: two-Higgs-doublet model (2HDM)  Predicts two charged (H ± ) and three neutral (h 0, H 0, A 0 ) Higgs bosons  Minimal Supersymmetric Standard Model (MSSM):  Employs type-II 2HDM One complex doublet couples to up-type fermions and the other to down-type fermions  Higgs bosons expressed by two key parameters: tan β – ratio of vacuum expectation values of two doublets m(H + ) – mass of the charged Higgs  Discovery of the charged Higgs is evidence for new physics beyond SM  No analogous particle in the SM 3

Physics at Tevatron  Measured cross sections agree with SM theory prediction  Direct charged Higgs production cross section is much smaller than other SM processes  Impossible to separate such small signal in an enormous number of SM events

Charged Higgs Tevatron Direct production cross-section ~ fb level. ~1/1000 of top pair production Associated production with top quark Light Higgs Light Higgs: M(H + )<M(t)-M(b) Heavy Higgs Heavy Higgs: M(H + )>M(t)+M(b) Prog.Part.Nucl.Phys.50:63-152,2003 5

Charged Higgs in Top Quark Decays  Charged Higgs search is doable in high and low tan β assumption in MSSM  For tan β > 7,  H  dominant  challenging to reconstruct τ  For low tan β < 1,  dominant for H + mass ≤130 GeV  full reconstruction is possible 6 Phys. Rev. Lett. 96, (2006)

Charged Higgs in Top Quark Decays tt protonanti-proton W-W-W-W- W-W-W-W- b q(c) e-,μ-e-,μ-e-,μ-e-,μ- ν W + (H + )  Top quarks are mainly produced in pairs in proton- antiproton collisions.  Use lepton+jets channel  Decays to lepton, neutrino, 4 quarks  Best signal to background ratio  Expect a charged Higgs boson in place of the hadronic W in the final state  W and H + are separated by the invariant mass of the two jets (dijet mass) 7

Tevatron at Fermilab  The most powerful Collider to data  ppbar collision at center of mass energy of 1.96 TeV  Two Collision points (D0 and CDF)  Deliver 60 pb -1 /week  Delivered 6.8 fb -1  Acquired 5.6 fb -1  This analysis uses 2.2 fb -1 8 CDF

Tevatron Performance Used 2.2 fb -1 data taken by 08/2007 Cross section σ: 60 mb at 1.96 TeV (1b = cm 2 )

Muon Chambers Electromagnetic Calorimeter Hadron Calorimeter 1.4 T Solenoid Silicon Vertex Detectors Tracking Chamber Beamline 39ft. 48ft. Weight 5000t Interaction points CDF II detector

Event Selection the most energetic four jets ( leading jets) Use the most energetic four jets ( leading jets) Veto events including  Z boson (  l + l - )/two leptons/Cosmic muon/Conversion( γ  e + e - ) Final State ObjectRequirement A lepton (e/ μ )p T ≥20 GeV/c, | η |<1 Missing transverse energy≥20 GeV Number of Jets≥4≥4 Jet E T ≥20 GeV, | η |<2 Number of b-jets≥2≥2 11

Event Reconstruction M t =175GeV M W =80GeV Γ : natural width of W and top quark M t =175GeV M W =80GeV 12  Assign Selected objects to ttbar decay particles  Kinematic fitter minimizes the  2 by  Constraining top quark and leptonic W masses  Varying jet energies in the fitter  Unclustered energies to have corrected missing E T

Event Reconstruction M t =175GeV M W =80GeV Γ : natural width of W and top quark M t =175GeV M W =80GeV 13  Assign Selected objects to ttbar decay particles  Kinematic fitter minimizes the  2 by  Constraining top quark and leptonic W masses  Varying jet energies in the fitter  Unclustered energies to have corrected missing E T Look at dijet invariant mass of the final state hadronic boson Search for a second peak in the dijet mass spectrum

Energy Loss from Radiation  Low mass tails  From final state gluon radiation  An extra jet in addition to the four leading jets  radiation jet from Higgs boson Final State Radiation (FSR) Initial State Radiation (ISR) Pythia MC 14

Angular distribution of the 5 th jet  Use the 5 th energetic jet with E T >12 GeV and | η |<2.4  Calculate angular distance between the 5 th jet and closest leading jet  The radiation jet (5 th jet) is close to its mother BLEP Initial quarkH+H+ BHAD Close to b-jet Close to Higgs jet 15

Merging the 5 th jet  Merge the 5 th jet to the closest leading jet if Δ R < 1.0  The leading jet recovers its energy by merging with the hard radiation jet  Merging is done before ttbar reconstruction  Better energy fit in the ttbar final state  Higgs boson mass resolution is improved in events with more than four jets About ¾ among 5 th jets added to the Higgs turn out to be a real FSR from Higgs 103.3±21.8 GeV  105.7±20.8 GeV in events with more than four jets true mass = 120 GeV in MC ΔRΔR Reconstruct di-jet mass after merging nearby 5 th jet 16

Dijet mass spectrum (2.2 fb -1 ) Dijet compositionFraction (%) (assuming =6.7 pb)91.6 Non- Events8.4 W + heavy flavor jets4.5 W + light flavor jets1.1 Single Top1.0 Non-W (QCD)0.9 Diboson (WW/ZZ/WZ)0.4 Z + light flavor jets0.3 Estimated using Pythia Monte Carlo simulation samples and data Observed 200 candidates 17

Extracting B(t  H + b)  Use maximum binned likelihood method  N bkb (Non-ttbar) is constrained using Gaussian statistics  Event composition in dijet mass is estimated using templates :  W, H +, non-ttbar distribution  Maximum LH returns  N(W), N(H + ), N(non- )  Calculate B(t  H + b) using N(W) and N(H + )  B(t  H + b)+B(t  W + b)=1.0  B(H +  csbar)=1.0 N(W)=N(H+)=N(non-ttbar) 18

Systematic Uncertainty  Dominant systematic errors come from dijet mass shape perturbation due to:  Jet Energy Scale Correction  Monte Carlo Program (Pythia vs. Herwig)  More/Less Initial/Final state radiation  W+jet production scale (Q 2 )  Negligible uncertainties from top mass constraints, b-jet tagging efficiency  Individual systematic uncertainties are combined in quadrature 19

Likelihood Smearing  Include the systematic uncertainty on B(t  H + b)  The likelihood is smeared out according to Gaussian statistics  The total systematic uncertainty is used for the Gaussian error. B(t  H + b) Maximum Likelihood  Observed B(t  H + b) before/after LH smearing 20

Upper Limits on B(t  H + b)  The upper limit is placed where the integration reaches 95% of positive branching ratio area  A negative branching ratio (B(t  H + b)<0) is unphysical  use positive branching ratio region  With 95% confidence level (C.L.) the observed B(t  H + b) is less than the upper limit 95% 5% 21

Upper Limits on B[t  H(  csbar)b]  Observed limits from 2.2 fb -1 data agree with the SM expectation  SM expected upper limits are obtained from 1000 SM pseudoexperiments M(H + )>80 GeV from LEP data 22

Expanding searches  The charged Higgs search is performed independent of any model specific parameters  Assumptions for charged Higgs (H + ):  Scalar charged boson  Narrow width Which is predicted for the light H + in MSSM  H +  csbar 100%  The search is extended to  Mass below W boson (down to 60 GeV)  Same analysis is performed for another two jets decay mode (udbar) 23

Analysis with udbar Decays  Dijet mass of udbar decays is narrower than that of csbar decays  Better mass separation between W and charged boson (  udbar) results in lower upper limits by ~10% 24 The upper limits for decays can be used conservative limits for any generic non-SM scalar charged boson in top quark decays

Model Independent Upper Limits Applicable to generic non-SM scalar charged boson in top quark decays Assumptions: Spin-0 particle Hadronic decays Narrow width 25

Summary  The MSSM charged Higgs has been searched in lepton+jets top quark decays  This is the first attempt of H  csbar search using fully reconstructed mass  No evidence of the charged Higgs in 2.2 fb -1 CDF II data  Based on simulation, upper limits on B(t  H + b) can be used as model independent limits for generic scalar charged boson production in top quark decays  This analysis is about to be submitted to PRL soon  The binned likelihood method using kinematic distribution  for branching ratio measurement (or ratio of decay branching ratio) of new particles  especially in early LHC data analysis 26

Prospect for future H + analysis  With doubled CDF II data the limits would be improved by 25%.  LHC is a top factory (Expect X100 more top pair production)  The precision study of top quark decay branching ratio available  In the first 200 pb -1 data at 10 TeV energy beam, the limits will be around 0.04~0.14 assuming similar systematic uncertainty with CDF  Sizable cross section for the direct charged Higgs production associated with top quark at LHC  Dominant decay is predicted H +  tb for heavier H + than top quark  Top quark is still the key to the charged Higgs search at LHC 27

Flavor Changing Neutral Current  Search in the Z+4jet channels B(t  Zq) < 95% C.L. World’s best limit. Improved previous limit L3) by a factor of 3.5