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Gaugino Search using Z0+W±+missing ET Channel

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·         We report on a search for gaugino pairs using events with Z0 (to e+e-), two or more jets and large missing ET . This is a final state not previously studied. With 2.68/fb integrated luminosity, the number of Z0 (to e+e-) events is more than 100,000 and there are events with large missing ET, which could indicate the presence of SUSY particles through a pair of gauginos, neutralino (χ02) and chargino (χ±1), production (  -> χ02 χ±1 -> Z0 χ01 W±χ01). This search requires the mass of χ02±1) to be larger than the lightest neutralino (χ01) by Z0 (W±) mass such that χ02 ±1) can decay to real Z0 (W±) and χ01. The missing ET is from χ01. Since the mass of χ±1 is expected to be similar to χ02 in most SUSY models, χ02 ->χ01+ real Z0 automatically satisfies χ±1 ->χ01+ real W±. Using this channel, we set the limit on the pair gaugino production cross section at 95% confidence level as a function of χ02 mass.

 

Plots and Tables:

Comparison with SM background:

An event is considered if it has two electrons passing tight cuts with ET> 20 GeV, at least two jets with ET>20 GeV and MET > 40 GeV.  

 

The event is processed if the invariant mass of two electrons (Mee) is within 85<Mee<97 GeV (called Z0 mass cut) and the invariant mass of any two jets (Mjj) is within 60<Mjj<95 GeV (called W mass cut). Three different MET  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 Monte Carlo data. The background from QCD jet events is estimated using 20 GeV jet triggered data and the fake rate (a jet faking an electron) as a function of ET. The fake rate is also used to estimate the contribution from W+jet events. Because of unique requirements (Z and W mass cuts and large MET), the SM background estimation is either straightforward or small. The QCD contribution is small because of two fake electrons and MET requirements. The contribution from di-boson events (WZ and ZZ) is also small because both bosons decay without neutrinos (low MET). This is a relatively simple and clean channel for a discovery.

 

Left plot is a background stack-up (color) and real data (+) of Mee distribution after W mass and MET >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 stack-up (color) and real data (+) of MET distribution after Z0 and W mass cut.

Right plot is background stack-up (color) and real data (+) of Mjj distribution after Z0 mass and MET >40 GeV cut.

 

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In each plot above, the expected signal distribution (multiplied by ten) from a pair of gaugino production is also displayed with an o symbol. The Pythia MC event generator with five SUGRA parameters M0=1000 GeV, M1/2=275 GeV, tanβ=10, A=0 and μ<0 is used for the signal generation. This set is referred to as the default set.

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

 

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Background uncertainties:

The uncertainties included in the background calculation are:

·         Statistical uncertainty in the Monte Carlo

·         Jet Energy Scale uncertainty (JES)

·         Cross section uncertainty

·         Luminosity uncertainty

Signal acceptance uncertainties:

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 Monte Carlo

Cross section limits calculation:

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 M1/2 for μ<0 and μ>0 separately while the other three SUGRA parameters are fixed at default values. For the range of M1/2 values considered here, the mass of χ02 and χ±1 is directly proportional to M1/2 and the mass difference between χ02 and χ±1  is less than 0.5 GeV for a given M1/2 and μ.  Also, the χ02  mass for μ<0 is about 6 GeV less than in the μ>0 case for a fixed M1/2. The cross section is from Prospino (NLO) and the branching ratio is from Pythia generator.

95% confidence level cross section limit for  -> χ02 χ±1 -> Z0 χ01 W±χ01.

The limit is calculated with three MET  cuts, 40, 50 and 60 GeV for a given χ02 mass. From the three limits, the one giving the best expected limit is chosen and plotted as a function of χ02 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 χ02 ->h0 χ01 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.

 

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Summary:

Using the events with Z0 (->e+e-), two or more jets and large MET, we searched for a signature of gaugino pair production ( ->  χ02 χ±1 -> Z0 χ01 W±χ01) 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 MET 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  χ02 mass.