semileptonic boosted tops

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Semileptonic Boosted Tops. Brock Tweedie Johns Hopkins University 10 July 09. K. Rehermann & B.T., To Appear. The Problem. b-jet. l. ~ m t / p t. n. Isolation probability ( D R bl > 0.4). Our Philosophy. Try to use these nonisolated leptons Avoid using MET for discrimination - PowerPoint PPT Presentation

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Semileptonic Boosted Tops

Brock Tweedie

Johns Hopkins University

10 July 09

K. Rehermann & B.T., To Appear

The Problem

~ mt / pt

b-jet

l

1 TeV 2 TeV

R 0.34 0.07

L 0.56 0.14

Isolation probability (Rbl > 0.4)

• Try to use these nonisolated leptons

• Avoid using MET for discrimination

• Do not b-tag

Our Philosophy

Leptons Inside of Jets

• Physics backgrounds– Heavy flavor (prompt and radiative)– + decays in flight

• Instrumental backgrounds

– 0 “e”

– + “”

Save for real experimentalists!

Event Generation

• PYTHIA and HERWIG ttbar continuum and generic dijet– Includes prompt heavy flavor, light meson

decays-in-flight in LHC-like detector volume

• Basic requirement: muon with Pt > 30 GeV– ~4% pass rate for dijet…per-jet probability

~2%

I will exclusively use PYTHIA plots / #s. HERWIG is practically identical.

+jets Event Reconstruction

• Set aside leading muon• Put remaining particles into perfect 0.1x0.1

calorimeter• Cluster with C/A

– Set R according to Ht in hemisphere opposite the muon

– Jet Pt > 50 GeV

• Leading jet == hadronic top• Jet closest to muon == b-jet from semilep

top

Semi-Leptonic Tops vs Light Jets

• + jet + MET + JUNK• Soft/collinear

singularities• Splittings more

common late in the shower (more gluons!)

b

bb

b

• + jet + MET

• hard and MET

• mT(+MET) ~ mW

• mass = mt

Mini-Isolation

W

B

t

R ~ mb / Ptb

R ~ mt / Ptt

B

B

R ~ mb / Ptb

R ~ ?

Mini-Isolation

• Many options for cone definition:– R ~ 1/Ptb ~ 1/Pt– R ~ 1/Ptt

– R = fixed #– …

• They all perform comparably

• R ~ 1/Pt convolves additional discriminating power from muon Pt distributions

Mini-IsolationIsolation cone R = (15 GeV) / Pt

Demand >90% isolated

top

light jet

W+jets

x

= 1 - mb2/mb

2

Demand x > 0.5

top

light jet

W+jets

(Thaler & Wang)

Mini-Isolation After xCut

top

light jet

W+jets

xAfter Mini-Isolation Cut

top

light jet

W+jets

Efficiencies of Leptonic Cuts (Pt ~ 1 TeV)

top light jet W+jets

Mini-iso 0.91 0.0038 0.95

x 0.89 0.0373 0.96

Combined 0.86 0.0014 0.93

Efficiencies of Leptonic Cuts (Pt ~ 2 TeV)

top light jet W+jets

Mini-iso 0.89 0.0026 0.95

x 0.84 0.0405 0.95

Combined 0.81 0.0012 0.92

Semi-Leptonic Tops vs W-strahlung

• + jet + MET

• hard and MET

• mT(+MET) ~ mW

• mass = mt

• + jet + MET

• hard and MET

• mT(+MET) ~ mW

• mW < mass < sqrt(s-hat)

b q’

W

q b

Ideal Top Mass Distributions

top

light jet

W+jets

Rb

• Wjj MadGraph 2 4

top

light jet

W+jets

Ideal-Mass vs Rb Discrimination

Efficiencies of Leptonic Cuts (Pt ~ 1 TeV)

top light jet W+jets

Rb 0.97 0.9970 0.45

Combined leptonic cuts

0.84 0.0013 0.39

Efficiencies of Leptonic Cuts (Pt ~ 2 TeV)

top light jet W+jets

Rb 0.95 0.9936 0.27

Combined leptonic cuts

0.76 0.0011 0.21

Backgrounds with Top-Mass Cut

Now use MET for global even reco. Define ==

Hadronic top-mass cut efficienciest ~ 85% / q/g ~ 25%

Backgrounds with Top-Tag

Resonance Efficiencies(incorporates +jets BR)

top-mass cut top-tag

Summary

• Assuming this all works in the detector, light QCD can be made negligible practically “for free”– In principle, rejection factors at the ~50,000

level– Allows for a comfortable margin of theory

error

• W-strahlung is still non-negligible– O(1) rejection “for free” by exploiting geometry

Summary

• Still various additional discriminators after incorporating MET– mT(top), m(top)

– Internal angular variables

• Possibilities for improvements in high-Pt t-tagging and b-tagging

Summary

• We will be seeing how these perform in full CMS simulation in the coming months

Extras

Resonances

Discovery Reach Estimates

= 0% = 15%

S/sqrt(B) > 5 & S > 15

Subjet Rates

* old PYTHIA results

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