1. Higgs detection in vector bosons fusion process
VV → WW → qq eνe
Gianluca Petrillo I.N.F.N. Turin

2. Software Two different Monte Carlo simulation programs have been used:
PHANTOM (was PHASE; Accomando, Ballestrero, Maina) generates events with 6 fermions in final state including processes with order α6EW and α4EWα2S and using exact matrix element calculation; official production data has been used to generate purely α6EW qq → qqqqeνe events
AlpGen (Mangano, Moretti, Piccinini, Pittau, Polosa) is a collection of generators dedicated to different classes of physical processes; it implements a jet matching procedure which, working with fragmentation and hadronization programs like PYTHIA and Herwig, avoids double-counting events in different jet multiplicity samples from the same process class; it was employed in generation of W + jets and tt + jets samples
The CMS fast detector simunation and reconstruction program, FAMOS, has been used to reconstruct the high level objects used in the analysis (jets, calorimeter clusters etc.)
11/19/05

3. PHANTOM: signal q't,1 ℓ q1 ℓ' V qV q2 q'V q't,2
The goal of this work is the detection of Higgs boson as a resonance in vector boson fusion process.
The required decay chain for Higgs boson is through two W bosons into a quark pair and an electron/neutrino pair, which is commonly called semi-leptonic channel. q1 q2
q't,1
q't,2 qV ℓ ℓ'
q'V ? V V H0 W- W+ V
γ,V,H0 W W- V W+
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4. PHANTOM: irreducible backgroundq1
q'1 V W
γ,W±,Z0
q'2 q2
Together with the signal events, PHANTOM also generates events whose dominant component is not coming from V-V fusion process diagrams.
Eventa are classified, according to their kynematics, as belonging to signal or irreducible background sample: q1 q2 W V q1
q'1 V W
q'2 q2
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5. AlpGen: backgrounds j b p W qV t p j W
AlpGen Monte Carlo generator has been used to generate “QCD” background with contestual emission of a W boson decaying into e/νe pair. The two most probable processes have been considered: p W j p b W qV t j
Note: cross section reported here is before double-counting suppression.
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6. Samples summary
The third column reports the cross section ratio of procesess respect to the signal process with Higgs boson mass of 500 GeV/c2; the last column reports the factor for scaling the number of events to the target integrated luminosity of 60 fb-1, which corresponds to 3 years of data with LHC at low luminosity (2∙1033 cm-2s-1).
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7. Electron Samples are normalized to 1.
Irreducible backgrounds in all Higgs scenarios have been omitted; they are very similar to tt + 1 jet sample though.
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8. Calorimeter superclusters
The candidate electron is chosen among the electromagnetic calorimeter (ECAL) superclusters.
A ECAL cluster is a group of calorimeter crystals around a “seed” crystal which has gathered energy.
A super-cluster is a set of neighbouring clusters along φ direction, whilc allows to collect an electron cluster, bent along φ by the magnetic field, and emitted photon which are less bent the sooner they have been emitted.
Superclusters have been required to be isolated both from other superclusters (ΔR > 0.2) and from jets (ΔR > 0.5), except the closest if it has Ej/ESClus < 2 (in which case it's considered the HCAL tail of an electron).
Efficiency in considering only isolated clusters: 94.4% (mH = 500 GeV/c2, P.U.), 91.7% (irr. bkgr., P.U.), 89.1% (tt)
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9. Electron: reconstruction resolution
The distance in ΔR between the direction of the generated electron and the closest supercluster is representative of spatial resolution of electron reconstruction.
Energy resolution is computed as the relative error of energy of reconstructed supercluster respect to the one of the generated electron, when these are less than ΔR = 0.5 far from each other.
Both the resolutions are not affected by selection cuts.
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10. Neutrino Samples are normalized to 1.
Irreducible backgrounds in all Higgs scenarios have been omitted; they are very similar to tt + 1 jet sample though.
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11. Missing E⊥: reconstruction resolution
(left) relation between generated neutrino and reconstructed missing transverse energy; for ΔR, the recognition of the electron is required
(below) resolution on transverse missing energy as measurement of νe transverse energy
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12. Lepton-decaying W boson
Matching criteria:
electron: E┴ > 15 GeV; isolated (ΔRee > 0.2 , ΔRej > 0.5); H/E < 2;
neutrino: MET > 30 GeV, MET/SET > 7%
pz,ν set by m(eν) = mW constraint
“best m┴” for choosing the electron
m┴ ∈ [ 40 ; 105 ] GeV/, m ∈ [ 60; 110 ] GeV
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13. Quarks: distributions (I)
Samples are normalized to 1;
all quarks are mixed together.
All irreducible backgrounds in all Higgs scenarios are very similar: only the one for no-Higgs scenario is plotted.
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14. Quarks: distributions (II)
Samples are normalized to 1.
The peaks on the left are Δη between central jets, while the short, bottomed plots are Δη among tag jets.
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15. Jets: reconstruction resolution
An iterative cone algorithm with radius ΔR = 0.5 is used to compose jets; energy is calibrated by a γ-jet calibration simulation.
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16. Jet selection: cuts Criteria for selecting central and tag jets are:
only 5 jets with the highest p┴ are considered;
tag jets: p┴ > 30 GeV, mj > 10 GeV, ηFW > -1, ηBW < +1
most central jet: p┴ > 25 GeV, mj > 5 GeV, |η| < 2
other central jet: p┴ > 20 GeV mj > 5 GeV, |η| < 2.5
mtags > 200 GeV, mcentral ∈ [ 60 ; 110 ] GeV
Central jets are chosen first, as the pair whose mass is closest to W one; then tag jets are chosen as the ones with highest invariant mass.
Event is then required to have |Δηtags| > 3, |Δηc| < 1 and central jets between tag ones in η, or it's discarded.
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17. Hadronic-decaying W boson
matching cuts
Matching cuts are expected to improve resolution on mass of decaying hadronically W boson since mass itself undergoes a cut.
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18. Cuts summary
After merging leptons and jets selection, I eventually have a complete selection of events with well defined roles.
The table shows that the rejection power of this selection is quite big; efficiency for signal is around 8-9% in the different Higgs mass scenarios, while irreducible background has ε ~ 3%; tt + 1 jet is already the thoughest background to be rejected, but it's W + jets the most annoying one since it has huge cross section.
In signal, purity is shown to be π ~ 50% at most; this means that half those events could contribute to the noise nevertheless.
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19. mWW resolution
Bosons invariant mass resolution is around 17%, but a tail is evident on the left of the peak.
The value of mass itself is shifted toward lower values of 5%.
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20. mWW after matching Plot for Ldt = 60 fb-1
At this point, a W+W object can be reconstructed; there is no hint of any resonance, which is sunk by background.
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21. min{mWj} cut Samples are normalized to 1.
This plot shows the minimum mass among the combinations of the two reconstructed vector bosons with the two tag jets.
Signal has this quantity neatly bigger than the other samples.
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22. mWW after min{mWj} cut Cut applied: min{mWj} > 200 GeV
Plot for Ldt = 60 fb-1
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23. mWWjj cut Samples are normalized to 1.
This plot shows the total energy of hard process as reconstructed from its products.
Signal has this quantity neatly bigger than the other samples.
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24. mWW after mWWjj cut Cut applied: mWWjj > 1500 GeV
Plot for Ldt = 60 fb-1
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25. Top cut
Top could be cut away checking the invariant mass of hadronic W with one of the tag jets (second column) or even with any of the jets that were candidate to be tags but were worse than the one which was finally chosen. Such a cut improves top rejection but heavily affects signal efficiency.
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26. Final mWW Plot for Ldt = 60 fb-1
Events with hadronic W plus any were-candidate tag jet which nake an invariant mass in [125;225] GeV/c2 range are discarded.
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27. Conclusions
the most dangerous backgrounds are W + 4 jets and tt + 1 jet, which have a big cross section and one jet more than the other considered backgrounds; this additional jet increases the chance of passing kinematical cuts by chance;
b-tagging, which is not stictly bound to event kinematics, could be a good way to reduce tt + 1 jets background;
more aggressive cut thresholds could be employed, at the cost of lower signal efficiency and increased need for luminosity, events, run time;
even without a formal statistical analysis, it's positive that background fluctuations can cover the signal, expecially using huge weights for background samples;
the statistical analysis will come very soon nevertheless;
I need sleep, badly.
11/19/05

28. (do you really want this?)
Backup
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