1. tt+jets simulation comparisons
Kenneth Wraight
tt+jets simulation comparisons
generator level
Fast&Full simulation

2. semileptonic ttbar decays
Looking for 4 jets of interest: 2 b quarks and 2 hadronic W daughters
Trigger on lepton: e (>25GeV), μ (>20GeV).
(↑Tevatron’s favourite (~85%))
LHC: ~95% gluon-gluon production.

3. Study top quark properties: charge, spin, etc.
Motivations
Study top quark properties: charge, spin, etc.
Background to associated Higgs production and multi-jet SUSY decays (~1 ttbar event per second for σ=833pb at L=1033cm-2s-1): must be controlled.
Ultimate test of generation and simulation software: multiscale QCD calculations

4. exact predictions for fixed orders
First Comparison: & Alpgen Generator level
exact predictions for fixed orders
higher order MEs with NLO accuracy to describe inclusive rates and LO accuracy for +1 jet
Alpgen:
approximate predictions for all orders (LL accuracy for FS + N jets)
Consistent merging of LO MEs with shower MCs for multiparton FSs
~25k 5200 sample (red):
Fully hadronic removed (es&μs)
Default cuts
Alpgen ~50k homemade (blue):
Semileptonic (es only)
ptjet ≥ 20GeV
ηjet ≤ 6.0, ηbjet ≤ 5.0, ηcjet ≤ 5.0
ΔRjet ≥ 0.7
plots area normalised...

5. Pt & η of generator level tops & Ws

6. Gen level bs & W daughters kinematics

7. Comparing Full Simulation and Fast Simulation
Second Comparison: Fast & Full Simulation
Comparing Full Simulation and Fast Simulation
Atlfast ~4-5 orders of magnitude faster than Full Simulation
Replaces detector simulation and jet reconstruction part of simulation chain with smears on MC truth info
Matching GL to Simulation:
Begin by finding partons of interest in GL using mother & daughter information
Match to shower jets using ΔR≤0.4 (take lowest value)
veto any double matches
repeat procedure for shower to simulation jets.
~25k 5200 sample
Fullsim Pjets (red)
Fastsim AFjets (blue) …no longer area normalised.

8. Relative finishing positions of matched b and W daughter jets
Pt-Ordered position
within the event of the
highest Pt b-quark
Pt-Ordered position
within the event of the
highest Pt W daughter

9. Pt of matched b and W daughter jets
Pt spectrum of
b-quark with
highest Pt
Pt spectrum of
W daughter with
highest Pt

10. Other jet relative finishing positions and pt
91 corresponds to cluster fragments

11. Reconstruction recipe
poi
1st
2nd
3rd
4th
5th
6th
other W1 W2 b1 b2
¬poi
For triggered events, use jets within η≤3.2, ptjet>20GeV
Cut on 5 hardest jets to reduce combinatoric background
Reconstruct W using di-jet combinations
Uses generator level info for b-vetos
Reconstruct top using di-jets in W mass peak to make tri-jet combinations by adding btagged jets
Simple reconstruction i.e. no calibration.

12. Fitted top & W reconstructed masses (with ideal btagging)
Fullsim: W: mass , signal , background
top: mass , signal , background
FastSim: W: mass , signal , background
top: mass , signal , background
Fitted top & W reconstructed masses (with ideal btagging)

13. Fitted P top & W masses and pt spectra with realistic btagging
Using fullsim b-weights cut=4.0
Fitted P top & W masses and pt spectra with realistic btagging
W: mass , signal , background
top: mass , signal , background
with additional pt<150GeV cut on top candidates:
top: mass , signal , background

14. Extra jet
Extra jet pt:
Jets from
“well-reconstructed
top” events:
b-vetoed and
not part of top
tri-jet

15. Alpgen partons are slightly harder at generator level
Summary
Presented:
Alpgen partons are slightly harder at generator level
Fullsim & Fastsim comparable for
Fullsim provides higher mass W&top on reconstruction
So Far:
Compared Alpgen & semileptonic tt+jets samples
All justify cut on 5 hardest jets top reduce combinatorics
similar study possible for semileptonic associated Higgs production
To do:
Crosscheck and compare with Sherpa
Look into calibration and reconstruction efficiencies.

16. ALPGEN: Finishing positions
Back up Alpgen slides...
FastSim (blue), FastSim Alpgen (pink)
poi
1st
2nd
3rd
4th
5th
other W1 W2 b1 b2
¬poi
Cut on 5 hardest jets is justified!

17. ALPGEN: Gen Level W daughters matched to AF jets
Pt-ordered position
within the event of the
highest Pt W daughter
Pt spectrum of
W daughter with
highest Pt
Pt spectrum of
Other W daughter

18. ALPGEN: Gen Level b partons matched to AF jets
Pt-ordered position
within the event of the
highest Pt b-quark
Pt spectrum of
b-quark with
highest Pt

19. ALPGEN: Gen Level partons matched to AF jets
Other jets’ finishing positions:
First-finishing other jets seem to originate form quarks(1-5), gluons(21), photons(22) and cluster fragmentation(91) 

20. Extra jet
Extra jet pt:
Jets from
“well-reconstructed
top” events:
b-vetoed and
not part of top
tri-jet
Of the original 53,286 event inclusive sample (comparable to ~43,000 ttbar event in first 2 months):
18603 matched from Gen.Level to AtlFast (~35%)
13867 were reconstructed (~26%)