1 A Preliminary Model Independent Study of the Reaction pp  qqWW  qq ℓ qq at CMS  Gianluca CERMINARA (SUMMER STUDENT)  MUON group

25/09/2002Gianluca CERMINARA2 The Project A Study of the WW-Fusion Channel in a Model Independent way A Study of the WW-Fusion Channel in a Model Independent way  Purpose: The aim of my work is to verify if it’s possible to extract the signal from the background for the process pp  qqW L W L  qq ℓ qq  Tools: 1. PYTHIA for the event generation 2. CMSJET for the detector simulation 3. ROOT for the data analysis 4. … a complete simulation will follow…

25/09/2002Gianluca CERMINARA3 Why am I studying this channel? 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 M Higgs = M WW otherwise the cross section will deviate from the SM prediction. We want to know with which resolution we can study 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!!!

25/09/2002Gianluca CERMINARA4 What is the signal?  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:  Two jets in the central region with high Pt coming from the quarks of the W decay  One muon in the barrel with high p t 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!

25/09/2002Gianluca CERMINARA5 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. What are the main backgrounds?

25/09/2002Gianluca CERMINARA6 What are the main backgrounds? + W+jets background  Topology A quark and a gluon/anti-quark give a W and a quark/gluon.When the W decays leptonically, the final state is similar to the one of the signal.  Signature if during the hadronisation the gluon gives origin to more than one jet this background can be very “dangerous”.

25/09/2002Gianluca CERMINARA7 What are the main backgrounds? W T W T  ℓ qq background  Topology A quark and an anti-quark give two W bosons in the final state. One of them decays hadronically, the other decays in a muon and a neutrino.  Signature The final state is very similar to the one of the sinal: we have a muon two jets and missing energy from the neutrino. Only the two scattered quarks are missing.

25/09/2002Gianluca CERMINARA8 Event Generation Events generated with PYTHIA Monte Carlo package. Signal t-tbar W +jets WW  ℓ qq Cross section 18 pb 378 pb pb 11 pb Number of generated events PYTHIA Process code (“Isub”) 72, 77 81, 82 16, Only a little Monte Carlo statistics was available. PYTHIA 6.158

25/09/2002Gianluca CERMINARA9 Detector Simulation CMSJET Fast Simulation package.  The leptons can be identified in the range –2.4 <  < 2.4  Jet reconstruction: range –5 <  < 5. Cone algorithm with  R = 0.5 on the whole range of  of the detector. p t >10 GeV.  No event pile-up. CMSJET 4.801

25/09/2002Gianluca CERMINARA10 Signal Kinematics  The two scattered partons have pseudo-rapidity above 2.5 and they will give origin to forwad jets.  The quarks from the W decay are mainly produced in the central region of the detector. Analysis of the Parton Level kinematics.

25/09/2002Gianluca CERMINARA11 Event Reconstruction Muon The request of an isolated muon is unavoidable in order to identify a leptonic W decay.  Selection criteria: Max isolation. p t > 10 GeV  Efficiency: ~96% Leptons from W  Neutrino Reconstruction of neutrino four-momentum:  p t = system missing p t.  p z calculated imposing the constraint: m W = GeV  W = 2.14 GeV (p  + p ) 2 = m W 2  Smallest p z chosen.  Efficiency: ~82% ~82%

25/09/2002Gianluca CERMINARA12 Event Reconstruction  Selection criteria: |  | < 3 p t > 10 GeV Combination of 2 or 3 or 4 jets with invariant mass closest to m W = GeV  Efficiency: ~96% 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.

25/09/2002Gianluca CERMINARA13 Event Reconstruction  Selection criteria: |  | < 3 p t > 10 GeV Combination of 2 or 3 or 4 jets with invariant mass closest to m W = GeV  Efficiency: ~96% 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.

25/09/2002Gianluca CERMINARA14 Event Reconstruction  Selection criteria: p t > 10 GeV;  j1  x  j2  < 0; 1.5 < |  | < 5;  j1  –  j2  > 3.  Efficiency: ~50.8% Forward jet tagging. Tagging of two forward jets. (Parton Level) CUT

25/09/2002Gianluca CERMINARA15 Signal to Background ratio Efficiencies of applied cuts for the identification of the signal topology: Cuts Signal t-t bar W+ jets WW  ℓ q q Isolated muon p t > 10 GeV 96.0%91.5%91.6%93.6% Reconstruction neutrino p z 81.6%81.2%90.4%83.2% Reconstruction W from jets 96.3%99.9%35.2%83.6% Tagging jets 50.8%31.7%13.0%19.6% The forward jet tagging is a powerful criterion to improve the signal to background ratio.

25/09/2002Gianluca CERMINARA16 Signal to Background ratio  Pseudo-rapidity W which decays leptonically After the first set of cuts all the backgrounds are still important. For this reason others variables have been used for the discrimination: Applied cut: |  Wlept |  3 CUT

25/09/2002Gianluca CERMINARA17 Signal to Background ratio  Transverse momentum of the W boson which decays leptonically Applied cut: p t Wlept > 10 GeV  Transverse momentum of the W boson which decays hadronically Applied cut: p t Wqq > 10 GeV CUT

25/09/2002Gianluca CERMINARA18 Signal to Background ratio  Transverse momentum of the tagging jet with max P t Applied cut: p t jtag > 20 GeV  Invariant mass of the tagging jet system Applied cut: M (jt1+jt2) > 550 GeV/c 2 Cuts on the tagging jets. CUT

25/09/2002Gianluca CERMINARA19 Signal to Background ratio Efficiencies of applied cuts : Cuts Signal t-t bar W+ jets WW  ℓ q q |  Wlept |  %98.9%79.6%94.7% p t Wlept > 10 GeV 97.1%98.5%78.6%93.3% 95.8%98.3%86.8%92.6% 60 GeV < m Wqq < 100 GeV 85.9%95.9%74.2%90.9% p t jtag > 10 GeV 59.7%54.8%11.3%24.5% M (jt1+jt2) > 550 GeV/c %26.6%29.3%31.4% Total efficiency 15.6%3.4%0.3%1.1% Again the cuts on the forward tagging system are the most effective.

25/09/2002Gianluca CERMINARA20 Preliminary Results 1 year “Low Luminosity” = 10 fb -1 1 year “High Luminosity” = 100 fb -1 Even if the W+jets background is still important at high invariant masses the signal to background ratio is quite good. The high energy region is the most interesting for the new physics we are looking for. S/B = 2.9x10 3 /4.3x10 3 S/B = 2.1x10 4 /8.9x10 5

25/09/2002Gianluca CERMINARA21 Preliminary Results Resolution of the WW system invariant mass reconstruction. 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: ~10% M WW rec - M WW gen M WW ge n  r WW =

25/09/2002Gianluca CERMINARA22 Conclusions  Good signal to background ratio at high WW invariant masses S/B  1 S/B  1  Very good resolution on M WW  energy scale of the process ~10  15% ~10  15% At this stage the measurement appears possible:

25/09/2002Gianluca CERMINARA23 Future Projects A more complete study of this process using a “Full Simulation” of the CMS detector. …are coming soon!  20 k signal  30 k W+jet  20 k t-tbar

24 A Preliminary Model Independent Study of the Reaction pp  qqWW  qq ℓ qq at CMS  Gianluca CERMINARA (SUMMER STUDENT)  MUON group