Janice Drohan, Simon Dean, Nikos Konstantinidis

UCL Christmas Meeting

12/12/2005

Search for Low Mass SM Higgs in the Channel ttH(Hbb)

Simon Dean, UCL Low Mass Higgs Search: ttH(Hbb) Christmas Meeting 12/12/2005

Topics

• Overview• Evolution of ttH(Hbb)

– TDR analysis– Recently redone– Changes at UCL

• Hadronic W rescaling

Simon Dean, UCL Low Mass Higgs Search: ttH(Hbb) Christmas Meeting 12/12/2005

Overview of Channel• One of three channels

important for Higgs search in low mass region (110-150 GeV)– Also Hγγ and qqHqqττ

• LEP search limit– 114 GeV < mH (SM) < 237 GeV

(95% C.L.)

• Hbb is the dominant decay channel for mH<135 GeV

• Direct production difficult to extract from the huge QCD 2-jet background

• Associated production.

Simon Dean, UCL Low Mass Higgs Search: ttH(Hbb) Christmas Meeting 12/12/2005

Event Topology

4 b-jets

2 light jets

1 isolated lepton

Missing momentum

Simon Dean, UCL Low Mass Higgs Search: ttH(Hbb) Christmas Meeting 12/12/2005

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

Simon Dean, UCL Low Mass Higgs Search: ttH(Hbb) Christmas Meeting 12/12/2005

TDR Analysis

Detector and Physics Performance Technical Design Report (1999)

• Preselection– 1 isolated lepton, pT

e>20GeV, pTμ>6GeV, |η|<2.5

– At least 6 jets, pT>15GeV, |η|<5 – Exactly 4 tagged as b-jets

• W->lν– constrain invariant mass of lv to W mass, solve for pz

v

• W->jj – Pairs of non b-jets with mjj=mW±25GeV

• Reconstruction of 2 top quarks– Minimise– Reconstructed top masses must be within ±20GeV of mt

222tjjbtlvb mmmm

Simon Dean, UCL Low Mass Higgs Search: ttH(Hbb) Christmas Meeting 12/12/2005

TDR Analysis

• Analysis done with Pythia, CTEQ2L• Results for 30fb-1:

Higgs Mass (GeV) 80 100 120Signal S 81 61 40ttZ 7 8 2Wjjjjjj 17 12 5ttjj 121 130 120Total Background B 145 150 127S/√B 6.7 5.0 3.6

Simon Dean, UCL Low Mass Higgs Search: ttH(Hbb) Christmas Meeting 12/12/2005

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:

MH (GeV) S/√B (30fb-1)

TDR Updated

100 5.0 3.4

120 3.6 1.9

140 - 0.7

Simon Dean, UCL Low Mass Higgs Search: ttH(Hbb) Christmas Meeting 12/12/2005

Studies Made At UCL• Hadronic W decaying to three

jets

• Ejet/Eb-quark distribution is much worse when the b-quark decays semi-leptonically (Br(blvc) ~20%)– Small correlation observed

between jet and neutrino energies

– pT dependant correction to jet energy and equivalent correction to missing ET

2

2

2

2

2

22 )()()(

jj

Wjj

lvb

tlvb

jjb

tjjb mmmmmm

• 29% of events with correct jets assigned - low 2 replaced with 2

• Likelihood technique

Simon Dean, UCL Low Mass Higgs Search: ttH(Hbb) Christmas Meeting 12/12/2005

Hadronic W Rescaling

• Simple rescaling sets mjj = mW(PDG) by scaling jj vector

• Gaussian + polynomial fit to EW

reco/EWMC

improves but RMS wider– Partially effective

Simon Dean, UCL Low Mass Higgs Search: ttH(Hbb) Christmas Meeting 12/12/2005

Hadronic W Rescaling

• Allow each light jet to scale independently 2 minimisation to improve hadronic W reconstruction

• Uses Minuit

Simon Dean, UCL Low Mass Higgs Search: ttH(Hbb) Christmas Meeting 12/12/2005

Results• Improved and RMS• Improved mass distribution

Simon Dean, UCL Low Mass Higgs Search: ttH(Hbb) Christmas Meeting 12/12/2005

Rescaling Parameters

• Both always raised or lowered

• Follows from form of 2 and mjj = 2E1E2(1-cos)

Simon Dean, UCL Low Mass Higgs Search: ttH(Hbb) Christmas Meeting 12/12/2005

That's It

Merry Christmas UCL HEP group!