Jet Prob optimization for the s-channel search

Hideki MaehashiUniversity of Tsukuba

7/28/2008

Contents1.Single Top Search2.Jet Probability3.Event Selection & Jet Probability threshold4.Results5.Summary

Single Top Search

• What we want to know Intensity of weak interaction (Wtb)• There are many events which include b-jets in

final state.(Background)• Single Top s-channel cross section is too small.

• To remove many background, we use some event selections and 2 b-jet tagging.

b-jet tagging (2 way)

• Secondary Vertex• Jet Probability

Because b-quark have a life time, b do not decay at a collision point of p and anti p

p

anti p

b

t

b-jet

primary vertex

secondary vertex

SV

JP

• Jet Probability indicate likelihood of light flavor and is depended on it’s threshold.

If it’s threshold is changed, sensitivity will change.

Jet Probability Optimization

In single top group we use 5% Jet Prob Threshold, but nobody estimated which threshold makes sensitivity the best, so this time I did.

Jet Probability Threshold

Number of Jet Probability Tag

5%

which is the best sensitivity?

Jet Probability Distribution

s-channel t-channel W+bb

tt W+jets W+cc

29 kinds of threshold = 0.1% 0.3% 0.5% 1% 2% 3% 4% 5% 8% 10% 12% 15% 20% 25% 30% 35% 40% 45% 50% 55% 60% 65% 70% 75% 80% 85% 90% 95% 100%

Ratio of JP, SV and at least 1SV tag

Event Selection• Exact one high PT Trigger lepton (PT > 20GeV)• Large missing Transverse Energy (ET > 25GeV)

• Two high PT jets with|η|<2.8

• Cosmic rejection, Dilepton rejection, Zboson rejection

• Choose 2 b-jet events• Likelihood Function

Discriminant (LFD >0.7)

threshold① 0.1% 0.3% 0.5% 1% 2% 3% 4% 5% 8% 10% 12%

15% 20% 25% 30% 35% 40% 45% 50% 55% 60% 65% 70% 75% 80% 85% 90% 95% 100%

② W + jets and nonW need Mistag Rate 0.1% 0.3% 0.5% 1% 2% 3% 4% 5% 10%

Signal and BackgroundSignal : s-channel

Background : t t W+bb W+charm W+jets WW WZ Zbb Zcc Z+jets ZZ nonW

t-channel

All backgrounds are predicted by standard model.

Signals, Backgrounds and Sensitivities

Sensitivities saturate around 10% Jet Probability threshold

LFD>0.7no mistag29 threshold

Sensitivities =Signal

Background

Signals, Backgrounds and Sensitivities

LFD>0.7mistag9 threshold

Sensitivities are maximized at 10% Jet Probability threshold

Including systematic uncertainty

Sensitivities =Signal

Bstat + (ΔBsys)2

When Jet Probability’s threshold is 5% sensitivity can be max

• s-channel : 18.1%• t-channel : 18.5%• tt : 21.7%• WW : 11.2%• WZ : 10.9%• ZZ : 11.6%

• Zjet : 40.6%• Zbb : 15.3%• Zcc : 15.5%• mistag : 71.2%• nonW : 80.0%

Including systematic uncertaintyno mistag29 kinds of threshold

mistag9 kinds of threshold

Conclusion

thre Sig BKG S/B S/sqrt(B) ΔB S/sqrt(B+ΔB ) 0.1 1.50 26.8 5.62×10 0.291 6.16 0.187 0.3 2.41 42.7 5.64×10 0.368 9.93 0.202 0.5 3.00 55.2 5.44×10 0.404 12.4 0.208 1 3.99 88.2 4.53×10 0.425 19.6 0.184 2 5.20 124 4.18 ×10 0.467 25.0 0.190 3 5.99 141 4.23×10 0.503 28.0 0.197 4 6.65 160 4.15×10 0.525 30.8 0.200 5 7.12 174 4.10×10 0.540 32.9 0.201 10 8.57 238 3.60×10 0.556 43.2 0.187

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• I did Jet Prob Optimization• I found which threshold makes sensitivity the best in

29 kinds (9 kinds : mistag) of threshold and estimated with systematic uncertainty.

• 5% threshold is the best threshold to make sensitivity max.

Jet Probability Distribution• I used Monte Carlo simulation of