1. Higgs → t+t- in Vector Boson Fusion
S. Burdin, M. Campanelli, S. Farrington,
C. Hays, A. Larner, K. Leney,
J. Monk, C. Taylor
11th January 2008

2. Higgs Physics Where to look?
Indirect evidence points to low mass (<160GeV) Higgs boson
Which production mechanism?
Which decay?
Decay rate
Vector Boson
Fusion
Production rate

3. Signal Significance at 30fb-1
In low mass region, Vector Boson Fusion H  tt is one of the most promising search modes
Signal
Significance

4. Event Topology Distinctive experimental signature t  t 
Necessitates understanding of:
Electron/muon ID
Tau ID and tau-tau mass resolution
Trigger efficiencies
Missing ET
Jets (including jet energy scale, jet algorithms, forward calorimeter performance)
Backgrounds
Two tau leptons
Forward jets
Missing Et

5. Event Selection
In UK we focus on lepton-hadron and di-lepton modes at the moment.
Trigger: 20GeV Muon / 25 GeV Electron
Cuts: jets not back-to-back
missing Et >40GeV
Dh between tag jets >4.4
Tag jets invariant mass >700GeV
Central jet veto: no third jet in |h| <3.2 with pt >20GeV
tau candidates should be between tag jets
Cut flow:

6. UK Efforts tt background studies: Liverpool/Oxford Jet studies: UCL
VBF Production of Z bosons: Oxford

7. tt Background Studies (Liverpool/Oxford)
W decays mimic t decays
W leptonic decay to e and m resemble leptonic t decay
W hadronic decay to jets resemble hadronic t decay
b jets resemble VBF tagging jets

8. Cross Sections: Signal/tt Background
Apply baseline H→ tt cuts to signal and tt background samples
Signal: Herwig VBF120tautaulh 5334, VBF120tautaull 5333
tt: ttbar Liverpool samples. No all hadronic decays.
Not enough events to calculate total efficiency for ttbar
separate the cuts into three independent sets
tau correlated, jet correlated, and a third set correlated to both tau and jet properties
Multiply the three sets of efficiencies together

9. tt Background Studies Efficiencies Lepton-hadron Lepton-Lepton
Tau cuts
1.7x10-4
6.4x10-3
Jet cuts
1.4x10-2
Angular cut -
0.46
Centrality
0.81
0.79
Central jet veto
0.14
0.21
Total Efficiency
3x10-7
7x10-6
Effective stt
0.11fb
3.2fb
Effective sH→tt
0.35fb
0.43fb

10. tt Background Studies
Using truth information we can see which W decay modes contribute the most background after cuts
Lepton-hadron mode:
After requiring two leptons in the event still have significant contribution from lepton + jets W decays
Suggests lepton from b decay may be reconstructed as leptonic t
Using truth information we see that in cases where a jet “fakes” a muon, ~70% of the time it is matched to a b jet and is a real muon
problem is ameliorated by isolation cut on electron
looking into applying muon isolation cut

11. Reduction of tt background
Charge Correlation helps to reject background
Lepton hadron mode: a jet faking a tau may
come from either b
Lepton lepton mode: either b hadron can
decay to a lepton
The proportion of wrong sign events is known (BRs + b mixing) so number of wrong sign events in the data is a way to calibrate the size of this background
Veto on wrong sign events rejects 20% of ttbar background in lepton
hadron mode, 25% in dilepton mode

12. Reduction of tt background
b jets in ttbar decays can mimic the forward jets in the VBF Higgs signal
b jet veto helps in rejecting background
Apply veto to any jet in the event, not just those tagged as VBF
anti b-tag veto has been tuned: require weight <1 (default b-tag selection cut is at 6.75)
S/√(S+B)
b tag weight
b tag weight
Veto on b jets rejects a further 60% of ttbar background in lepton
hadron and dilepton modes
After charge correlation and b jet veto the signal loss is 4% in lepton hadron mode,
10% in lepton lepton mode

13. Reduction of tt and Z+jets background
Studies of Log Likelihood variable combining jet cuts indicate great improvements in background rejection (note: ttbar and Z+≥ 2 jets background)
Preliminary studies indicate powerful background suppression (factor 10) but this is performed with low statistics samples (few events are left after the other cuts have been applied)
Includes: Dh between jets,
mass(jets),
central jet veto h requirement

14. Jet Studies (UCL)
Identifying VBF signature requires understanding forward jets
Which algorithms work well in forward region?
Do jets behave as we expect?
Several jet algorithms studied
(using Spartyjet to compare them)

15. Truth/Reconstructed Jet Matching
Compare W+jets MC with VBF Higgs MC:
VBF jets are in the more forward regions of the detector
Calorimeter geometry is not projective
QCD jets truth/reco match slightly better than VBF jets
KT 04 and SIS Cone 04 algorithms show best matching h h

16. Truth/Reconstructed Jet Matching
Matching efficiency as function of h
t jets in central
region are included
in this plot
W+jets h h
VBF
Liquid Argon calorimeter endcap region

17. Truth/Reconstructed Jet Matching
Jet resolution as function of pt and h
5% pt correction required
only in endcap pt h

18. Jet Studies Work in progress studies of central jet veto
Pythia vs Herwig comparisons
inclusion of pile-up and multiple interactions

19. VBF Production of Z Bosons (Oxford)
Seek Vector Boson Fusion signature for Z ee,mm,(tt)
Never observed
Expect ~4x the Higgs rate
Will look almost exactly like the Higgs signature
Useful way to study backgrounds, as well as being an interesting first observation in itself /Z t

20. Available MC’s VBF Z production not implemented in Herwig/Pythia
MC’s on the market which we can use: Madgraph, VBFNLO, SHERPA
Comparison of MC’s taking place
see large differences in predicted cross sections
working with MC authors to understand these
Moving towards generating full simulation samples

21. Outlook All of this work is included in the CSC note
Now looking beyond CSC note
Optimisation of cuts under realistic conditions, including all background contributions
Jet triggers are under study
There is now significant UK contribution to a fascinating Higgs mode

22. tt Background Studies
Di-lepton mode: