1. Semileptonic ttH(Hbb) studies at UCL
Lily Asquith, Catrin Bernius, Sebastian Boeser, Simon Dean, Nikos Konstantinidis
Atlas UK Meeting, Higgs Session
11/01/2008

2. Atlas UK Meeting, Higgs Session
Cut-Based Analysis
Preselection
1 isolated lepton, pTe>25GeV, pTμ>20GeV, |η|<2.5
At least 6 jets, pT>15GeV, |η|<5
At least 4 tagged as b-jets
Wl
constrain invariant mass of l to W mass, solve for pzv
Wjj
Pairs of non b-jets with mjj=mW±25GeV
Reconstruction of 2 top quarks
Minimise 2 with top masses
Reconstructed top masses must be within ±20GeV of mt
Reconstruction of Higgs
Use the two highest-pT remaining b-jets
Simon J.H. Dean
Atlas UK Meeting, Higgs Session

3. Atlas UK Meeting, Higgs Session
Studies Shown Here
Jets in the crack region
Background invariant mass shape
Fast simulation jets
Generator samples
Simon J.H. Dean
Atlas UK Meeting, Higgs Session

4. Jets in the Crack Region (Lily Asquith)
Uninstrumented angular region = "crack"
Considered 1.4 < || < 1.6
Hadronic tile calorimeters
EM accordion calorimeters
Hadronic LAr endcap calorimeters
Forward LAr calorimeters
Simon J.H. Dean
Atlas UK Meeting, Higgs Session

5. Jets in the Crack Region
Abundance of jets in ttH final state
Poor jet reconstruction may seriously affect overall event reconstruction
Study uses cone 0.4 jets in release ttH and ttbb samples (5870, 5871 & 5855)
Matched to truth-jets using dR < 0.2
In ||-bins of 0.2, Gaussian fits made to ET(reco) / ET (true)
Linearity = mean
Resolution = width / mean
Simon J.H. Dean
Atlas UK Meeting, Higgs Session

6. Atlas UK Meeting, Higgs Session
Light Jet Performance
Low and high energy jet samples
~4% linearity dip in crack region
Resolution worse at lower pT
Deviations at high eta due to low fit stats
Simon J.H. Dean
Atlas UK Meeting, Higgs Session

7. Atlas UK Meeting, Higgs Session
B-Jet Performance
No b-tagging outside Inner Detector (|| < 2.5)
~4% dip again observed in linearity
Resolution also worse at lower pT
More work to show impact on hadronic W, tops and Higgs
Simon J.H. Dean
Atlas UK Meeting, Higgs Session

8. Background Shape (Catrin Bernius)
Sinead M. Farrington, "B Physics Lessons from the Tevatron", YETI'08
signal
J. Drohan
Broad bb mass peak
Kinematic selection produces similar signal and background shape
Would like better discrimination
Started by considering ttH vs. ttbb (QCD)
Simon J.H. Dean
Atlas UK Meeting, Higgs Session

9. Background Shape Formulated "tight selection" on Higgs b-jets
at low pt more jets in ttbb than ttH
Tight selection: pT(b1Higgs)> 40 GeV, pT(b2Higgs)> 30 GeV
Simon J.H. Dean
Atlas UK Meeting, Higgs Session

10. Tight Selection
ET(b2higgs) vs ET(b1higgs) in mass window 90GeV<mbb<150GeV to check cut correlation
Cut out
Tight selection: pT(b1Higgs)> 40 GeV, pT(b2Higgs)> 30 GeV
Simon J.H. Dean
Atlas UK Meeting, Higgs Session

11. Atlas UK Meeting, Higgs Session
Shape Comparison
Normalised to 30 fb-1 ("combined sample")
Simon J.H. Dean
Atlas UK Meeting, Higgs Session

12. Correct/Incorrect Combinations in ttH Signal
Correct combinations: b-jets (from Higgs) correctly matched
Fit Gaussian to correct combinations, Landau to incorrect combinations
Simon J.H. Dean
Atlas UK Meeting, Higgs Session

13. Fitting Background & Signal shapes
Fit gaussian + landau to ttH signal sample
Landau also describes ttbb background very well
Simon J.H. Dean
Atlas UK Meeting, Higgs Session

14. Atlas UK Meeting, Higgs Session
Combined Fit
Fit background + signal shape to the combined sample, keeping parameters fixed but with scaling factors applied
Standard Selection
Tight Selection
Mass window: GeV:
signal (gauss+landau) = 437.9
ttbb(QCD) (landau) =
signal/(signal+ttbb) = 0.12
signal(gauss)/(all signal) = 0.28
Mass window: GeV:
signal (gauss+landau) = 377.9
ttbb(QCD) (landau) =
signal/(signal+ttbb) = 0.15
signal(gauss)/(all signal) = 0.31
Simon J.H. Dean
Atlas UK Meeting, Higgs Session

15. Next Steps For Shape Study
Include ttbb(EW) and ttjj backgrounds
Explore behaviour of combined fit when varying analysis cuts
eg... b-weight cut at different values
Simon J.H. Dean
Atlas UK Meeting, Higgs Session

16. Atlas UK Meeting, Higgs Session
Fast Simulation Jets
Fast simulation uses different cone jet finder than accessed by JetRec
Atlfast currently only supplies the results of cone 0.4 jet finder in AOD output
Can also run using kT algorithm
Done via KtEvent interface, same as JetRec
In principle, re-running Atlfast using kT and comparing with full-chain kT jets should give closer agreement
Simon J.H. Dean
Atlas UK Meeting, Higgs Session

17. Atlas UK Meeting, Higgs Session
cone jets
Full Fast
7000 signal events considered
Cone 0.4
Included for both full and fast in AOD files
Noticeably higher number of jets from Atlfast
Overestimation of jet efficiency at low pT, as seen in other analyses
Consistent with Bill Murray (RAL)
Simon J.H. Dean
Atlas UK Meeting, Higgs Session

18. Atlas UK Meeting, Higgs Session
kT jets
Full Fast
Full chain kT jets included in AODs, fast not
Atlfast run directly from evgen files used in standard production
R-parameter = 0.4
Shift to higher pT in fast
Cone jets should be more consistent from r14 as FastKtJet interface will be adopted by Atlfast (deliverable)
Simon J.H. Dean
Atlas UK Meeting, Higgs Session

19. Generator Studies (Sebastian Boeser)
ttHTruthCollector tool used to inspect signal and background truth samples
identify the hard scatter vertex, i.e. the one with either an outgoing higgs or two outgoing tops
identify any additional b-partons at the hard scatter vertex
follow the top-branches to identify the b-partons and W's
follow the W-branches to identify the jets or lepton-neutrino pairs
Simon J.H. Dean
Atlas UK Meeting, Higgs Session

20. Atlas UK Meeting, Higgs Session
tWs decays
38950 ttH events
Missing b-quarks from top decays compensated by finding s-quarks
The relative number of tWs/tWb is in good agreement with |Vts|2 as given by the PDG
tWs decays supported in PYTHIA sample, neglected in other generators
Simon J.H. Dean
Atlas UK Meeting, Higgs Session

21. Atlas UK Meeting, Higgs Session
W decays
5000 ttjj events
7.4% missing qq pairs from W decays
Equally high number of -pairs is found in their place.
W being accepted as "hadronic" decay in the semi-leptonic sample.
(In some cases there are extra b-quarks at the hard-scatter vertex. Happens if the hard scatter is connected to the underlying event with a b-quark)
Simon J.H. Dean
Atlas UK Meeting, Higgs Session

22. Next Steps in Generators Study
Understand effect on analysis of additional W decays
Understand correct way of dealing with ttbb component in ttjj sample, to avoid double-counting events (not covered here)
Simon J.H. Dean
Atlas UK Meeting, Higgs Session