semileptonic boosted tops
DESCRIPTION
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 PresentationTRANSCRIPT
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