

· We report on a search for gaugino pairs using events with Z^{0} (to e^{+}e^{}), two or more jets and large missing E_{T }. This is a final state not previously studied. With 2.68/fb integrated luminosity, the number of Z^{0} (to e^{+}e^{})^{ }events is more than 100,000 and there are events with large missing E_{T}, which could indicate the presence of SUSY particles through a pair of gauginos, neutralino (χ^{0}_{2}) and chargino (χ^{±}_{1}), production ( > χ^{0}_{2} χ^{±}_{1} > Z^{0} χ^{0}_{1} W^{±}χ^{0}_{1}). This search requires the mass of χ^{0}_{2} (χ^{±}_{1}) to be larger than the lightest neutralino (χ^{0}_{1}) by Z^{0} (W^{±}) mass such that χ^{0}_{2 }(χ^{±}_{1}) can decay to real Z^{0} (W^{±}) and χ^{0}_{1}. The missing E_{T} is from χ^{0}_{1}. Since the mass of χ^{±}_{1 }is expected to be similar to χ^{0}_{2} in most SUSY models, χ^{0}_{2} >χ^{0}_{1}+ real Z^{0} automatically satisfies χ^{±}_{1} >χ^{0}_{1}+ real W^{±}. Using this channel, we set the limit on the pair gaugino production cross section at 95% confidence level as a function of χ^{0}_{2 }mass.
An event is considered if it has two electrons passing tight cuts with E_{T}> 20 GeV, at least two jets with E_{T}>20 GeV and ME_{T} > 40 GeV.
The event is processed if the invariant mass of two electrons (Mee) is within 85<Mee<97 GeV (called Z^{0} mass cut) and the invariant mass of any two jets (Mjj) is within 60<Mjj<95 GeV (called W mass cut). Three different ME_{T}^{ } cuts are tested, 40, 50 and 60 GeV for the 95% CL cross section calculation.
Background contributions from 8 standard model processes are taken into account for
the background estimation. Of these, 7 are evaluated by using
Left plot is a background stackup (color) and real data (+) of Mee distribution after W mass and ME_{T} >40 GeV cut. The green shaded areas indicate the background error bars, where the statistical and systematic errors are added quadratically.
Middle plot is a background stackup (color) and real data (+) of ME_{T} distribution after Z^{0} and W mass cut.
Right plot is background stackup (color) and real data (+) of Mjj distribution after Z^{0} mass and ME_{T} >40 GeV cut.

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Table: The comparison between data and background including all uncertainties (statistical and systematic errors) for three ME_{T} cuts: 40, 50 and 60 GeV.

The uncertainties included in the background calculation are:
·
Statistical uncertainty in the
· Jet Energy Scale uncertainty (JES)
· Cross section uncertainty
· Luminosity uncertainty
Signal acceptance is defined as signal detection efficiency times the integrated luminosity. This quantity is an input to the cross section limit calculations as described below. The uncertainties included are:
· Jet Energy Scale uncertainty (JES)
· Luminosity uncertainty
· Initial State Radiation uncertainty (ISR)
· Final State Radiation uncertainty (FSR)
· Parton Distribution Function uncertainty (PDF)
·
Statistical uncertainty in the
A Bayesian method is used to calculate 95% confidence level cross section limits. The calculation requires signal acceptance (signal detection efficiency times integrated luminosity), estimated background, and observed data as input. To calculate signal detection efficiency, signal MC Pythia events are generated as a function of M_{1/2} for μ<0 and μ>0 separately while the other three SUGRA parameters are fixed at default values. For the range of M_{1/2} values considered here, the mass of χ^{0}_{2} and χ^{±}_{1 }is directly proportional to M_{1/2} and the mass difference between χ^{0}_{2} and χ^{±}_{1 } is less than 0.5 GeV for a given M_{1/2 }and μ. Also, the χ^{0}_{2 } mass for μ<0 is about 6 GeV less than in the μ>0 case for a fixed M_{1/2}. The cross section is from Prospino (NLO) and the branching ratio is from Pythia generator.
The limit is calculated with three ME_{T}^{ } cuts, 40, 50 and 60 GeV for a given χ^{0}_{2 }mass. From the three limits,
the one giving the best expected limit is chosen and plotted as a function of χ^{0}_{2
}mass.
Left: Cross section x branching ratio limits at 95% CL for the gaugino pair production. Red line is expected limit. Yellow band is ±1 sigma of expected limit. Green band is ±2 sigma of expected limit. Black line with dots is data limit. Blue line is theoretical cross section from NLO Prospino. The branching ratio is from Pythia. The dip near 230 GeV is due to the appearance of χ^{0}_{2} >h^{0} χ^{0}_{1 }decay mode.
Right: Cross section x branching ratio limits at 95% CL for the gaugino pair production. Red line is expected limit. Yellow band is ±1 sigma of expected limit. Green band is ±2 sigma of expected limit. Black line with dots is data limit. Blue line is theoretical cross section from NLO Prospino. The branching ratio is from Pythia.


Using the events with Z^{0} (>e^{+}e^{}), two or more jets and large ME_{T}, we searched for a signature of gaugino pair production ( > χ^{0}_{2} χ^{±}_{1} > Z^{0} χ^{0}_{1} W^{±}χ^{0}_{1}) with 2.68/fb of data. The number of events passing our default cuts is 7 while the background prediction is 6.41 ± 0.69 (stat. error) ± 0.64 (sys. error). With the 50 GeV ME_{T} cut, the data is 2 and the background is 3.76 ± 0.48 ± 0.33. With the 60 GeV cut, the data is 1 and the background is 2.02 ± 0.30 ± 0.13. Without any evidence of excess, the 95% CL limits on the pair gaugino production cross section have been calculated as a function of χ^{0}_{2}^{ }mass.