Janice Drohan, Simon Dean, Nikos Konstantinidis UCL Christmas Meeting 12/12/2005 Search for Low Mass SM Higgs in the Channel ttH(H  bb)

Christmas Meeting 12/12/2005 Simon Dean, UCL Low Mass Higgs Search: ttH(H  bb) Topics Overview Evolution of ttH(H  bb) –TDR analysis –Recently redone –Changes at UCL Hadronic W rescaling

Christmas Meeting 12/12/2005 Simon Dean, UCL Low Mass Higgs Search: ttH(H  bb) Overview of Channel One of three channels important for Higgs search in low mass region ( GeV) –Also H  γγ and qqH  qq ττ LEP search limit –114 GeV < m H (SM) < 237 GeV (95% C.L.) H  bb is the dominant decay channel for m H <135 GeV Direct production difficult to extract from the huge QCD 2-jet background Associated production.

Christmas Meeting 12/12/2005 Simon Dean, UCL Low Mass Higgs Search: ttH(H  bb) Event Topology 4 b-jets 2 light jets 1 isolated lepton Missing momentum

Christmas Meeting 12/12/2005 Simon Dean, UCL Low Mass Higgs Search: ttH(H  bb) Backgrounds Irreducible Backgrounds –ttZ – much lower cross-section than signal –ttbb Reducible Backgrounds –Misidentified b-jets –ttjj, Wjjjjjj, Wbbjj etc –Latter two suppressed by reconstruction of top decays ttjj is the main background

Christmas Meeting 12/12/2005 Simon Dean, UCL Low Mass Higgs Search: ttH(H  bb) TDR Analysis Detector and Physics Performance Technical Design Report (1999) Preselection –1 isolated lepton, p T e >20GeV, p T μ >6GeV, |η|<2.5 –At least 6 jets, p T >15GeV, |η|<5 –Exactly 4 tagged as b-jets W->lν –constrain invariant mass of lv to W mass, solve for p z v W->jj –Pairs of non b-jets with m jj =m W ±25GeV Reconstruction of 2 top quarks –Minimise –Reconstructed top masses must be within ±20GeV of m t

Christmas Meeting 12/12/2005 Simon Dean, UCL Low Mass Higgs Search: ttH(H  bb) TDR Analysis Analysis done with Pythia, CTEQ2L Results for 30fb -1 : Higgs Mass (GeV) Signal S ttZ782 Wjjjjjj17125 ttjj Total Background B S/√B

Christmas Meeting 12/12/2005 Simon Dean, UCL Low Mass Higgs Search: ttH(H  bb) Updated Analysis Brought up-to-date in 2003/4 Updates included –CTEQ5L –AcerMC to model ttbb and ttZ backgrounds (matrix elements) –Lepton pre-selection updated for new trigger menus 25GeV electron, 20GeV muon, 20GeV jets Results: M H (GeV)S/√B (30fb -1 ) TDRUpdated

Christmas Meeting 12/12/2005 Simon Dean, UCL Low Mass Higgs Search: ttH(H  bb) Studies Made At UCL Hadronic W decaying to three jets E jet /E b-quark distribution is much worse when the b-quark decays semi-leptonically (Br(b  lvc) ~20%) –Small correlation observed between jet and neutrino energies –p T dependant correction to jet energy and equivalent correction to missing E T 29% of events with correct jets assigned - low  2 replaced with  2 Likelihood technique

Christmas Meeting 12/12/2005 Simon Dean, UCL Low Mass Higgs Search: ttH(H  bb) Hadronic W Rescaling Simple rescaling sets m jj = m W (PDG) by scaling jj vector Gaussian + polynomial fit to E W reco /E W MC  improves but RMS wider –Partially effective

Christmas Meeting 12/12/2005 Simon Dean, UCL Low Mass Higgs Search: ttH(H  bb) Hadronic W Rescaling Allow each light jet to scale independently  2 minimisation to improve hadronic W reconstruction Uses Minuit

Christmas Meeting 12/12/2005 Simon Dean, UCL Low Mass Higgs Search: ttH(H  bb) Results Improved  and RMS Improved mass distribution

Christmas Meeting 12/12/2005 Simon Dean, UCL Low Mass Higgs Search: ttH(H  bb) Rescaling Parameters Both always raised or lowered Follows from form of  2 and m jj = 2E 1 E 2 (1-cos  )

Christmas Meeting 12/12/2005 Simon Dean, UCL Low Mass Higgs Search: ttH(H  bb) That's It Merry Christmas UCL HEP group!