Gaugino Search using Z^0+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 E_T . 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 E_T, 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 (χ^±₁) to be larger than the lightest neutralino (χ⁰₁) by Z⁰ (W^±) mass such that χ⁰₂ (χ^±₁) can decay to real Z⁰ (W^±) and χ⁰₁. The missing E_T 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.

• Authors: Seog H. Oh, Chiho Wang, Jared Yamaoka,
• Data : RunII, 2.68/fb
• Internal Documentation: CDF note 9727 PDF.
• Public Note: CDF note 9791 PDF.

Plots and Tables:

Comparison with SM background:

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 Z⁰ 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 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 E_T. 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 ME_T), the SM background estimation is either straightforward or small. The QCD contribution is small because of two fake electrons and ME_T requirements. The contribution from di-boson events (WZ and ZZ) is also small because both bosons decay without neutrinos (low ME_T). 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 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 stack-up (color) and real data (+) of ME_T distribution after Z⁰ and W mass cut.

Right plot is background stack-up (color) and real data (+) of Mjj distribution after Z⁰ mass and ME_T >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, M_1/2=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 ME_T 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 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 χ⁰₂ and χ^±₁ is directly proportional to M_1/2 and the mass difference between χ⁰₂ and χ^±₁ is less than 0.5 GeV for a given M_1/2 and μ. Also, the χ⁰₂ 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.

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

The limit is calculated with three ME_T 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 ME_T, 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 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 χ⁰₂ mass.