CDF Logo Search for New Physics in Like-Sign Dilepton Using the Inclusive High-Momentum Lepton Sample Exotics Logo

We describe a search for New Physics in 1/fb of Run II data, using a sample of two identified leptons of the same charge (``like-sign dileptons.'') This search is sensitive to New Physics with three or more leptons, such as SUSY trilepton signatures, or signals with Majorana particles, such as gluino pair production signatures with decays into leptons.

Please see the note above for details about the analysis.

Plots and Tables:

Crosschecks

Control regions

To test our ability to estimate Standard Model backgrounds, we define the following control regions:

Control regions summary

The plots below show the summary of the numerical agreement in the control regions, for various lepton categories. In short, we see good agreement in all cases.

EPS or GIF Data vs MonteCarlo agreement for the e-e control regions, for electrons with Si hits
EPS or GIF Data vs Monte Carlo agreement for the e-e control regions, where one electron is has hits in the Si detector and the other does not.
EPS or GIF Data vs Monte Carlo agreement for the mu-mu control regions

Cross Sections

We also measure the cross-sections of the following important background processes:

The following kinematical distributions show that we have our modeling of shapes under control.

Kinematics of the control region and the modeling of different contributions
Missing transverse energy in the EWK control region. GIF, EPS.
Leading lepton transverse momentum in the Z boson control region. GIF, EPS.
Momentum of the lepton in the W+γ selection GIF, EPS.
Invariant mass of the two leptons in the Z+γ selection. GIF, EPS.

Results in the signal region

Having checked our control regions, we now look in the region of two same-sign leptons to see if we can find an excess. We consider two selections: inclusive, where no additional requirements are made beyond two high-momentum same-sign leptons, and tighter, where we additionally apply a Z veto (no two same-flavor, opposite leptons in the event whose invariant mass is 66 <m_ll<116) and MET>15.

The tables below shows our results.

Inclusive: Results for two leptons of the same sign, without a MET or Z veto requirement. Errors are statistical and systematic, combined.
Category Observed Predicted ±σ Drell Yan Diboson ttbar Fakes
e_si e_si 11 6.5 0.8 3.2 1.4 0.4 0.0 1.4
ee 3 1.3 0.3 0.9 0.1 0.0 0.0 0.2
e_si e 9 9.1 1.2 6.4 1.6 0.1 0.0 1.1
e_si μ 11 7.0 0.8 0.8 2.8 1.1 0.0 2.2
e μ 5 6.5 0.9 3.4 1.9 0.2 0.0 1.0
μ μ 5 3.3 0.3 0.1 1.4 0.8 0.0 0.9
sum 44 33.7 3.5 14.9 9.3 2.5 0.1 6.9
Tighter: Results for two leptons of the same sign, with a MET>15 GeV and Z veto requirement. Errors are statistical and systematic, combined.
Category Observed Predicted ±σ Drell Yan Diboson ttbar Fakes
e_si e_si 1 1.3 0.2 0.4 0.6 0.0 0.0 0.4
ee 1 0.1 0.0 0.0 0.1 0.0 0.0 0.0
e_si e 2 1.5 0.2 0.1 1.2 0.0 0.0 0.2
e_si μ 4 1.7 0.2 0.0 1.0 0.1 0.0 0.7
e μ 4 2.3 0.4 0.6 1.4 0.0 0.0 0.2
μ μ 1 0.9 0.1 0.0 0.5 0.1 0.0 0.4
sum 13 7.9 1.0 1.1 4.7 0.2 0.0 1.9

Kinematic distribution of the events found in the signal box

Inclusive selection (no Z veto or MET requirement)

GIF or EPS Transverse momentum of the leading lepton. Note the excess at high transverse momentum. GIF or EPS Transverse momentum of the sub-leading lepton.
GIF or EPS Missing transverse momentum. GIF or EPS Invariant mass of the lepton pair, without MET or Z veto requirement.

Tighter selection (Z veto and MET>15 GeV requirement)

GIF or EPS Transverse momentum of the leading lepton. Note the excess at high transverse momentum. GIF or EPS Transverse momentum of the sub-leading lepton.
GIF or EPS Missing transverse momentum. GIF or EPS Invariant mass of the lepton pair, without MET or Z veto requirement.

Event Displays

We show here the event displays of three events: the two highest-E_T events, which are electron-electron, and of an e-μ event.

GIF Two electrons above 100 GeV each. In the same event we have a photon of 15GeV, Met of 25GeV and a third electron of 5GeV that does not pass the calorimeter isolation GIF same event, lego view
GIF same event, rz view
GIF This event has more than 100GeV Met. There are lots of piled-up interactions. As one can see from the next figure, the third electron does not come from the same interaction vertex. GIF same event in rz view
GIF One of the e-mu events GIF same event in lego view

Conclusion

We have performed a search for new physics in 1/fb of data from CDF's Run II. In our tighter selection, We expect 7.9±1.0 events from Standard Model background sources and observe thirteen. In our inclusive selection, we expect 33.7±3.5 events and we observe 44. We observe an excess of events in the high momentum leading lepton distribution.