First look of ME studyon ttH(bb) channel

Univ. of Tokyo

Y.Kataoka

Feb.21.2003 @CERN

Previous Analysis (1) We introduced modified method

to solve the Combinatorial BG. In this method, we used values based on mass information,

likelihood output of angle information.

Reconstructed Higgs mass plot (MH=120GeV)

ttbb BG is dominant.

Mbb

30fb-1

• Details are in the previous talk (Dec.12,2002).

Previous Analysis (2)

(true comb.)

Number of events (30fb-1)

Combinatorial BGIs still large

Matrix Element Study

Modified 2 method is not effective

to solve Combinatorial BG.

We start a new study based on

“Matrix Element”.

This ultimate method may solve the problems.

• Is it possible to improve the current result ?

Event Selection

Number of isolated lepton = 1for electron, pT>25GeV, ||<2.5

for muon, pT>20GeV, ||<2.5

Number of light flavor jet >= 2pT>20GeV, ||<2.5

pT>40GeV for at least one jet

Number of b-jet = 4pT >30GeV

( The same as the previous study )

Decay mode H b b t l b t j j b

Matrix Element After the event selection, we calculate an event weight: “-log(ME)”, and use this weight to solve the Combinatorial BG.

We can choose the most probable combination.

Matrix element is calculated by Grace (2.2.0).

Input: 4-vector of 8 final fermions(Hbb, tj j b, tl b )

80 diagrams and their interference

Detector Resolutions• In order to take detector resolutions into accounts,

we set Higgs, top, W decay widths to 20GeV (temporary).• In the parton level, we check the outputs of grace.

• Blue line is distribution with natural decay width.• Red line is distribution with smearing.

If we have good energy resolution,we have very good performance.

-log(ME) Small |ME|2Large |ME|2

Wrong comb.Correct comb.

Parton Level StudyFirst, we use the parton level 4-momenta

of the ttH signal sample (MH=120GeV). 1. all partons are correctly assigned (red line) 2. b and b-bar exchanged (blue line)

ME method has the power of discrimination between correct and wrong assignments.

-log(ME)

• Wide widths are included for W, t , H

Detector Level StudyNext, we use the observed 4-vectors (Atlfast output).

1. jets and leptons are correctly assigned (red line)

2. b and b-bar exchanged (blue line)

-log(ME)

The discrimination power is decreased.

Parton level study shows:ME method is powerful

to discriminate Combinatorial BG.

Detector level study shows:

smearing in the detector level

decreases the discrimination power.

More studies are necessary.

Conclusions We apply the ME method

to solve the Combinatorial BG. We find the ME method

has discriminating power. More studies on detector smearing

are necessary to apply this method. We also hope to apply this method to

BG rejection (ttbb,tt).

Future Plans• We also start to study the ME method based on 7-body final particles.

8-body H b b t l b t j j b

7-body H

t l b t j j b

• We expect this 7-body ME has more power on Combinatorial BG than the current 8-body ME.

We will report new results in the very near future.