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Trilepton search for Chargino-Neutralino Production at CDF
Inclusive low-pT dimuon+lepton analysis with 1 fb-1
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M. Gold, V. Rekovic, J. Strologas

University of New Mexico


Abstract:  We present a search for chargino-neutralino production at 976 pb-1, using low-pT dimuon triggers. We investigate the trilepton signature mu+mu+l, where l can be an electron or a muon. In this analysis we extend the search to transverse momenta as low as 5 GeV, for all leptons. This inclusive low-pT channel increases our reach and presents new challenges, due to the presence of heavy-flavor background at low transverse momenta, a background that is estimated using data. We investigate 19 dilepton and trilepton control regions, and we note good agreement between observation and Standard Model (SM) predictions. At the same time, we remained statistically unbiased at the signal region, by not looking at the data there, until the very end of the analysis. Upon examining the data yield in the signal region, we observed one very interesting trimuon event, while we expected 0.4 +/- 0.1 SM events. An upper limit on the production of chargino-neutralino will be set.

Analysis strategy:  Study the SM backgrounds and CDF data in well defined control regions.  Remain statistically unbiased at the signal region.

SUSY signals:  Two points are investigated: 

Our benchmark Point (SIG1)
m0=100, m1/2=180, A0=0, tanβ=5, μ > 0
σ*BR = 0.642 * 0.22 pb (Prospino)

The D0 point (SIG2)
m0=  74, m1/2=168, A0=0, tanβ=3, μ > 0
σ*BR = 1.023 * 0.5 pb (Prospino)

Luminosity: 976 pb-1.

Triggers: Low pT dimuon triggers (two muons with pT>4 GeV).

Dimuon selection:  Require central (|eta|<1) muons passing the standard muon ID cuts and having transverse energy of at least 5 GeV.  In addition, we require minimum dimuon mass of 10.5 GeV (exclusion of Y), 3-d deltaPhi less that 178,
and DeltaR between the muons more than 0.4.

Trilepton selection: Require a third central (|eta|<1) muon passing the standard muon ID cuts , or a central electron passing the  standard electron ID cuts.  A minimum pT of 5 GeV for the third lepton and additionally ET>5 GeV, if it is an electron are required.  The third lepton is also DeltaR>0.4 away from the two leading muons.

Jet counting: Cone size of 0.4, minimum level-5-corrected energy of 20 GeV, emf<0.9

Trilepton Signal Region: Require three leptons, MET>15 GeV, dimuon mass greater than 15 GeV, with the exclusion of the Z mass region (76 to 106 GeV), and no more than one jet.

Monte-Carlo-estimated backgrounds: Drell-Yan (DY), diboson (WW,WZ,ZZ), boson+parton (W+q, W+c, W+b) and top-antitop backgrounds are estimated with Monte Carlo.

Fakes estimation: Fakes (jets faking electrons and tracks faking muons) are estimated with the application of a fake rate on jets and tracks in real data.

Heavy Flavor Background estimation:  A HF-rich data sample was created by reversing the impact parameter cut for at least one of the two leading muons.  We fit the DY+HF dimuon mass to the data (DY is absolutely normalized using the luminosity) for opposite-sign and same-sign muons separately.  The results of the fits are used to weight the HF-rich data sample  and treat it as HF, with a dimuon upper mass cut at 35 GeV. The method gives good agreement in yields and shape of the HF in our control regions, as can be seen here.

Control Regions investigated: They are parts of the mass vs. met phase space, for low and high jet multiplicities, defined here.

Control Regions yields:
yields for all control and signal regions for dimuons and trileptons. The overview of the control and signal regions can be found here.

Systematics study:
systematics in the signal region. Tables for all control regions: systematics and fractional systematics (breakdown of systematics for background/signal dileptons/trileptons and all control regions)

Control Regions Plots:

CON_Z
CON_UNM
CON_E
CON_F
CON_G
CON_H
CON_I
CON_J
SIG_A
SIG_A2

Observed trimuon event: We observed one very interesting trimuon event. Kinematic distributions for the trilepton signal region can be found here.