bTagging Issues in t t H(H-> b b)

bTagging Issues in ttH(H->bb)

Lily Asquith

23 Nov 07

ATLAS UCL

The Importance of bTagging

Pre-selection: ttH ttjj

1 hard lepton +At least 6 jets ~1200 ~850000

At least 4 bTagged jets ~100 ~800

~10% ~0.1%

Before pre-selection: signal :background ~ 1:1000

B quarks’ large mass and low CKM matrix elements make it’s lifetime long, giving us the opportunity to identify it using tracking in the Inner Detector ( |eta|<2.5) and ‘bTag’ it.

ttH(H->bb): final state has 4b jets and two light jets

Jet-heavy backgrounds swamp the signal, but 4 bjet final states not common

Intro: Impact Parameter

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.





Impact parameter a0 (z0) a0 / sigma(a0) Weight of track =Pb/Pu

bTagging used: ‘IP3D + SV1’

IP3D uses transverse IP a0 and longitudinal IP z0

3d Impact parameter gives probability of being a b versus probability of being a u



Weight(jet) = sum of weights of tracks. No tracks-> weight = - 20.72

Excellent description: http://cdsweb.cern.ch/record/686346

Intro: Secondary Vertex

Energy in vertex/ Jet Energy Mass of pcles in SV Number of 2-track vertices

bTagging used: ‘IP3D + SV1’

SV1 uses secondary vertex information

Uses distance from beam pipe to cut out SVs due to material interaction and K0 and Lambda decays



No SV-> weight = - 1.2

Excellent description: http://cdsweb.cern.ch/record/686346


are needed to see this picture.QuickTime™ and a

TIFF (Uncompressed) decompressorare needed to see this picture.


are needed to see this picture.+ +

bTagging Efficiency

(Numbers for efficiency and rejection are dependant on the choice of bweight cut.

These numbers are for a bweight cut of 4)

E( b )~ 60% If it’s a b, we will call it a b ~60% of the time

R( u,d,s ) ~ 100 If it’s a u,d,s we’ll call it a b ~1% of the time

R( c ) ~ 8 If it’s a c, we’ll call it a b ~12% of the time

Naively expect both light jets from W to be tagged as bjets roughly 8 % of time, and all 4 bjets to be tagged as a bs (0.6)4: 13% of the time

These numbers do not take into account the bias introduced by our pre-selection cuts.

Our bTagging efficiency

(These numbers are for events where I have been able to match all 6

quarks (of status 3) to jets within dR <0.2.)

28% of events have at least one jet from the W tagged as a bjet

34% of events have either a light jet or a bjet mistagged

Current analysis separates jets into light jets / bjets based on tagging.

Light jets only are used for W reconstruction: we scupper our chances of correctly reconstructing the higgs 28% of the time at this point.

Want to keep more jets for making W.

Would like a more efficient way of deciding which jets should be used to reconstruct W,tops, higgs…

Segregation: to what extent…

None: use all jets to try and make a decent hadronic W mass. Use leftovers for tops (lose none)

Sort Jets in terms of bweight, exclude highest weighted jets from W reconstruction

• exclude top four weighted jets (lose ~27%)

• exclude top three weighted jets (lose ~10%)

• exclude top two weighted jets (lose ~4%)

Aims:

To improve significance of channel (more signal events contributing to higgs reconstruction wrt background)

To improve higgs mass peak (want to make the Higgs out of the right 2 jets)

To look at:

Questions for me:

? Effect of larger collection for W on top reconstruction (play with W mass window)

- Marseille showed bjets successful in making a W in mass window 10% of time

? Efficiencies for W, tops, higgs

- standard cut-based analysis:(of events passing pre-selection)

- 67% hadronic W in mass window

- 44% both tops reconstructed

- 17% higgs in mass window

? Effect on significance - higgs in mass window correct, incorrect, background

-e.g.: currently 17% of pre-selected events have higgs in mass window, but less than half of these use the right pair of bjets

bTagging Issues in t t H(H-> b b)

Documents

Transcript of bTagging Issues in t t H(H-> b b)