1. Experimental and theoretical Group Torino + Moscow
A Preliminary Model Independent Study of the Reaction ppqqWWqqℓnqq at CMS
Experimental and theoretical Group
Torino + Moscow
Gianluca CERMINARA

2. We are looking for everything that may happen in this channel!!!
ppqqWLWLqqℓnqq
A Model Independent Study of the WW Fusion in order to clarify the Symmetry Breaking Mechanism.
The Standard Model predicts that without a Higgs the scattering amplitude of the WW fusion process violates unitarity at about 1.5 TeV…
…for this reason something must happen:
In the Higgs case we will observe a resonance at MHiggs = MWW
otherwise
the cross section will deviate from the SM prediction.
We want to know with which resolution we can measure the cross section as a function of the invariant mass of the two scattered W bosons!
We are looking for everything that may happen in this channel!!!
14/02/2003
Gianluca CERMINARA

3. Model Independent
Standard Model Cross Section (PYTHIA) to generate the signal sample for the study.
NO non-standard Symmetry Breaking Mechanism was assumed!!!
Measurement of the s(WW) vs M(WW) up to ~2 TeV
14/02/2003
Gianluca CERMINARA

4. What is the signal? fwd q m p W n q sVV fwd q Topology
Two quarks of the proton emit a W boson, the two bosons interact giving in the final state two other W bosons: one of them decays leptonically and the other decays into a pair of quarks.
Signature:
One muon in the barrel with high pt and missing energy of the neutrino (from the W decay)
two jets in the forward region of the detector (from the parton hadronisation) .
6 fermion final state! W p
fwd q q m n
sVV
Two jets in the central region with high pt from the W decay
fwd q
14/02/2003
Gianluca CERMINARA

5. What are the main backgrounds?
pp tt WWbb
t-tbar background
Topology
From the interaction of a quark and an anti-quark or of two gluons of the proton we have a t and anti-t quark pair.They immediately decay into a b and a W.
Signature
Since the W decays both leptonically and hadronically we have the same final state as the signal: 4 jets (2 from the b and 2 from the W) and a muon.
ℓnℓ qq
14/02/2003
Gianluca CERMINARA

6. What are the main backgrounds?
W+2jets background
Topology
Two quarks/gluons from the protons give a final state with a W and two quarks/gluons.When the W decays leptonically, the final state is similar to the one of the signal.
pp W+q/g+q/g mn
Signature
if during the hadronisation quarks or gluons give origin to more than one jet this background can be very “dangerous”. m n W p
q/g
14/02/2003
Gianluca CERMINARA

7. What are the main backgrounds?
pp W
ℓnℓ
qq-bar
Single W background
Topology
All processes with a single W boson in the final state can be considered backgrounds for our study.
Signature
From the W decay we can have two jets in the central region of the detector or a lepton with missing Et from the neutrino. p W
14/02/2003
Gianluca CERMINARA

8. Event Generation
Events generated with PYTHIA and CompHEP Monte Carlo packages.
Signal
t-tbar
W+2jets
Single W
Cross section
18 pb
624 pb
770 pb pb
Number of generated events
24153
46229
300000
596618
Monte Carlo
PYTHIA
CompHEP
PYTHIA 6.158
CompHEP 41.10
14/02/2003
Gianluca CERMINARA

9. PYTHIA vs CompHEP
Comparison the two generators for W+jets backgrounds.
PYTHIA: it calculates diagrams for 2  2 processes (e.g. qq  gW and qq  qW ). Other diagams like W+many partons are taken into account with showering procedure;
CompHEP is able to calculate a complete set of diagrams W+1,2,3 partons (W+4 with showering procedure)
pt distribution W
Eta distribution W
Harder pt(W)
More central W
A. Krokhotine + CompHEP group
14/02/2003
Gianluca CERMINARA

10. Detector Simulation CMSJET CMS detector Fast Simulation package.
The leptons can be identified in the range
–2.4 < h < 2.4
Jet reconstruction:
range –5 < h < 5.
Cone algorithm with DR = 0.5 on the whole range of h of the detector.
pt >20 GeV.
No event pile-up.
CMSJET 4.801
14/02/2003
Gianluca CERMINARA

11. Event Reconstruction Leptons from Wmn Muon
The request of an isolated muon is unavoidable in order to identify a leptonic W decay.
Selection criteria:
Max isolation.
DR > 0.1
pt > 20 GeV
Efficiency:
~77%
Neutrino
Reconstruction of neutrino four-momentum:
ptn = system missing pt.
pz calculated imposing the constraint:
mW = GeV
GW = 2.14 GeV
(pm + pn )2 = mW2
Smallest pz chosen.
Efficiency:
~61%
14/02/2003
Gianluca CERMINARA

12. Event Reconstruction Jets from the W decay : Wqq
The possibility of a good reconstruction of the W boson is basically linked to the number of detected jets.
Selection criteria:
|h| < 3
pt > 20 GeV
Combination of 2 or 3 or 4 jets with invariant mass closest to mW = GeV
Efficiency:
~90%
14/02/2003
Gianluca CERMINARA

13. Event Reconstruction Jets from the W decay : Wqq
The possibility of a good reconstruction of the W boson is basically linked to the number of detected jets.
Selection criteria:
|h| < 3
pt > 20 GeV
Combination of 2 or 3 or 4 jets with invariant mass closest to mW = GeV
Efficiency:
~90%
14/02/2003
Gianluca CERMINARA

14. Event Reconstruction CUT Forward jet tagging.
Tagging of two forward jets.
Selection criteria:
pt > 20 GeV;
hj1 x hj2 < 0;
1.5 < | h1 | < 5 or 1.5 < | h2 | < 5
| hj1 – hj2| > 3.
Efficiency:
~41%
(Parton Level)
14/02/2003
Gianluca CERMINARA

15. Efficiencies
Efficiencies of applied cuts for the identification of the signal topology:
Cuts
Signal
t-tbar
W+2jets
Single W
Isolated muon pt > 20 GeV
76.7%
34.3%
84.5%
52.0%
Reconstruction neutrino pz
60.9%
24.7%
70.5%
45.8%
Reconstruction W from jets
90.1%
98.2%
86.4%
21.0%
Tagging jets
40.6%
24.2%
11.0%
2.7%
The forward jet tagging is a powerful criterion to improve the signal to background ratio.
14/02/2003
Gianluca CERMINARA

16. Signal to Background ratio
CUT
Transverse momentum of the W boson
Applied cut:
ptWlept > 10 GeV
ptWqq > 15 GeV
Pseudorapidity of the W boson which decays hadronically:
|hWlept| < 3
Very powerful cut on higher Pt
ptWlept < 130 GeV
Not to apply in high mass resonance case!!!
14/02/2003
Gianluca CERMINARA

17. Signal to Background ratio
CUT
Cuts on the tagging jets.
Difference of pseudo-rapidity between muon from W and tagging jets (rapidity gap cut)
Applied cut:
Dhm-jt > 1
CUT
Invariant mass of the tagging jet system
Applied cut:
M(jt1+jt2) > 550 GeV/c2
14/02/2003
Gianluca CERMINARA

18. Efficiencies |hWlept| < 3 Efficiencies of applied cuts : Cuts
Signal
t-tbar
W+2 jets
Single W
|hWlept| < 3
97.7%
99.4%
100%
89.3%
ptWlept > 10 GeV
97.8%
98.6%
90.8%
ptWadron. > 15 GeV
94.1%
96.2%
96.9%
74.8%
60 GeV < mWqq < 100 GeV
82.1%
86.4%
70.2%
71.0%
M(jt1+jt2) > 550 GeV/c2
66.5%
30.0%
46.8%
11.5%
Dh(m-jt) > 1
79.5%
49.0%
46.3%
57.1%
ptWlept < 130 GeV
87.6%
75.8%
38.4%
75.0%
Total efficiency
6.7%
0.4%
0.01%
Again the cuts on the forward tagging system are the most effective.
14/02/2003
Gianluca CERMINARA

19. With higher pt cut (ptWlept < 130 GeV)
Preliminary Results
With higher pt cut (ptWlept < 130 GeV)
Even if the W+2jets background is still important at high invariant masses, the signal to background ratio is quite good.
1 month “low” luminosity
Without higher pt cut (ptWlept < 130 GeV)
1 month “low” luminosity
S/B  0.41
S/B  12/~0
The high energy region is the most interesting for the new physics we are looking for.
S/B  0.32
S/B  0.23
14/02/2003
Gianluca CERMINARA

20. Preliminary Results
With the 20k WLWL events generated with PYTHIA the maximum M(WW) achieved is 2 TeV
Assuming a flat efficiency vs M(WW) we can estimate to have:
10 events with 1.5 TeV < M(WW) < 1.6 TeV in 1.5 years at “low” luminosity.
10 events with 1.8 TeV < M(WW) < 2.0 TeV in 3 years at “low” luminosity.
But New Physics will manifest itself through increasing s(WW) for the signal at the energy scale (M(WW)) where the Symmetry is broken.
1 year “Low Luminosity” = 20 fb-1
1 year “High Luminosity” = 100 fb-1
14/02/2003
Gianluca CERMINARA

21. Preliminary Results
Resolution of the WW system invariant mass reconstruction.
MWWrec - MWWgen
MWWgen
drWW =
The resolution in the reconstruction of the WW invariant mass at this stage is :
~15%
Fitting the histogram with the sum of two gaussian curves the resolution in the central peak is:
~8%
14/02/2003
Gianluca CERMINARA

22. Conclusions At this stage the measurement appears possible:
Good signal to background ratio at high WW invariant masses
S/B  good
Very good resolution on MWW  energy scale of the process
~10 15%
2 TeV region reached in 3 years at “low” luminosity
14/02/2003
Gianluca CERMINARA

23. Future Projects
A more complete study of this process using a “Full Simulation” of the CMS detector.
A new Monte Carlo for our signal (by A. Ballestrero, E. Accomando, E. Maina –Torino group-)
New backgrounds: W+1 jet, W+3jets, W+4jets (Moscow + CompHEP Group)
…are coming !
14/02/2003
Gianluca CERMINARA

24. Experimental and theoretical Group Torino + Moscow
A Preliminary Model Independent Study of the Reaction ppqqWWqqℓnqq at CMS
Experimental and theoretical Group
Torino + Moscow
Gianluca CERMINARA