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Search for diboson (W+W- or W±Z°) resonances in electron + missing Et + 2 jets final state

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We report on a search for resonances decaying into a pair of gauge bosons, W+W- or W±Z°. In this search mode, one W decays through a leptonic (electron) mode and the other boson decays into two jets. Based on ~3/fb data, three resonance hypotheses, G* (R/S graviton), Z’ and W’, are tested and their cross section limits at 95% confidence level are calculated.

Plots and Tables:

WW Invariant mass distribution:

Before an event is considered for the WW reconstruction process, it is required to have only one electron in the central region with Et > 30 GeV, a missing Et > 30 GeV, 2 or 3 jets with |eta| < 2.5 and jet Et > 30 GeV, and an overall Ht > 150 GeV. Ht is defined as the sum of the electron Et, the missing Et and the jet Et of all jets with raw Et > 8 GeV and |eta| < 2.0. Further optimization using Et cuts is applied in cross section limit calculations (see  below). The plots and tables shown below are before the optimization.

 

The WW invariant mass is constructed by first combining the electron with the missing Et assuming that they are from a W. Due to unknown missing Ez, if there are 2 real solutions, both solutions are kept.  Next, two jets are combined to form the other W candidate with the requirement that 65 GeV < M(jj) < 95 GeV. When there are more than 2 jets, the pair that gives a M(jj) closest to the W mass is selected. Finally, the WW candidates are combined for WW invariant mass.

 

Left plot is a stack-up of the reconstructed WW invariant mass distributions from various SM backgrounds overlayed with the observed data (+ symbols).

Right plot is a comparison of data and background integrated over ±1.5 sigma (horizontal error bar) of reconstructed G* signal. The sigma is determined by fitting the reconstructed mass shape with a Gaussian function. The vertical error bars for the data and the signal are staggered for clarity. The background error bars include statistical and systematic errors. 

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WZ Invariant mass distribution:

WZ invariant mass reconstruction is similar to WW’s except that the two-jet invariant mass is required to be within 75 GeV < M(jj) < 105 GeV.  When there are more than 2 jets, the pair that gives a M(jj) closest to the Z mass is selected.  

 

Left plot is a stack-up of reconstructed WZ invariant mass distributions from various SM backgrounds overlayed with observed data (+ symbols).

Right plot is a comparison of data and background integrated over ±1.5 sigma (horizontal error bar) of reconstructed W’ signal. The sigma is determined by fitting the mass shape with a Gaussian function. The vertical error bars for the data and the signal are staggered for clarity. The background error bars include statistical and systematic errors. 

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Background compositions:

Contributions from 12 standard model processes are taken into account for the background estimation. Of these, 11 are evaluated by Monte Carlo simulation. The background from QCD jet events is estimated using jet triggered data and the fake rate (a jet faking an electron) as a function of Et.     

 

Left table:  Data and breakdown of background processes for WW invariant mass.

Right table: Data and breakdown of background processes for WZ invariant mass.

The systematic error includes 6% luminosity uncertainty. 

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Background uncertainties:

The uncertainties included in background calculation in decreasing importance are:

·         Jet Energy Scale uncertainty (JES)

·         Cross section uncertainty

·         Luminosity uncertainty

·         Statistical uncertainty in the Monte Carlo

The dominating uncertainty is JES at ~ 13%. The cross section and luminosity uncertainties are ~6% each.

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 in decreasing importance 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

The dominating uncertainties are JES and luminosity (6%). For JES, it varies from 12% (170 GeV mass) to 6% (700 GeV mass) for G*, 13% (170 GeV) to 5% (700 GeV) for Z’, and 9% (190 GeV) to 6% (700 GeV) for W’. 

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, MC events for G*, Z’ and W’ are generated for a set of selected mass points and a mass window of ±1.5 sigma of the reconstructed signal is used. The data and background invariant mass plots shown above (on the right side of WW and WZ mass plots) reflect this set of mass points with ±1.5 sigma mass windows.

 

Further optimization is carried out to improve sensitivities at higher mass regions. Additional higher Et cuts for constituent particles (electron, missing Et, jets) are applied in two different ways: One is to require a higher Et cut on all 4 participant particles ranging from 40 GeV to 80 GeV in 10 GeV steps. The other is to require a higher Et cut on one of each boson’s decay daughters, i.e. either the electron or the missing Et (W -> e nu) and one of the jets (W/Z -> jj). The increased Et runs from 40 to 120 GeV in 10 GeV steps. These 2 sets of cuts make up a total of 15 variations of Et cuts including the original Et > 30 GeV cut. At each mass point, the expected cross section limits are calculated for 15 cuts. The cut that gives the best expected limit is chosen to give the final result. The following plots are results from this optimization. 

R/S graviton, G*, (k/mp=0.1) 95% confidence level cross section limit:

Left: Cross section x branching ratio limits at 95% CL for G*. 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 of G* from  Pythia 6.216 and using a constant k factor 1.3 (Phys. Rev. Lett.  99, 171801 (2007)).

Right: Cross section limits divided by theoretical cross section. Mass regions with ratio below 1 are excluded. Expected cross section limits exclude G* mass region < 632 GeV. Data exclude mass region < 607 GeV. 

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Table: Optimal cuts applied to G*, where “2, 40” denotes the “2 particles, Et > 40 GeV” cut as described above, “4, 50” denotes the “all 4 particles, Et > 50 GeV” cut.

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Typical G* Signal/Background/Data at a Selected Cut: WW invariant mass plot with optimal cut (2,120) for 600 GeV G* (see table above). Expected G* signals at 600 GeV is shown in the blue hatched histogram.

Left Plot: In linear scale.

Middle Plot: In log scale

Right Table: Data and background compositions of WW for this cut (2,120): { (Et(ele) || missing Et) && (one of the jets) } > 120GeV.

 

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Sequential Z’ 95% confidence level cross section limit

Left: Cross section x branching ratio limits at 95% CL for Z’. 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 of Z’ from Pythia 6.216 and using a constant k factor 1.3 (Phys. Rev. Lett. 102. 031801 (2009)).

Right: Cross section limits divided by theoretical cross section. Mass regions with ratio below 1 are excluded.  Expected cross section limits exclude 257 GeV < MZ’ < 630 GeV. Data exclude mass region 247 GeV < MZ’ < 545 GeV. 

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Table: Optimization cuts applied to Z’, where “2, 40” denotes the “2 particles, Et > 40 GeV” cut as described above, “4, 50” denotes the “all 4 particles, Et > 50 GeV” cut.

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Typical Z’ Signal/Background/Data at a Selected Cut: WW invariant mass plot with optimal cut (4,60) for 600 GeV Z’ (see table above). Expected Z’ signals at 600 GeV is shown in the blue hatched histogram. Note that this is a different cut from G* at the same mass energy.

Left Plot: In linear scale.

Middle Plot: In log scale

Right Table: Data and background compositions of WW for this cut (4,60): { (Et(ele) && missing Et && (both jets) } > 60 GeV.

 

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Sequential W’ 95% confidence level cross section limit

Left: Cross section x branching ratio limits at 95% CL for W’. 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 of W’ from a NLO calculation (Z. Sullivan, Phys. Rev. D 66 075011, (2002)).

Right: Cross section limits divided by theoretical cross section. Mass regions with ratio below 1 are excluded.  Expected cross section limits exclude 381 GeV < MW’ < 420 GeV. Data exclude mass region 284 GeV < MW’ < 515 GeV. 

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Table: Optimal cuts applied to W’, where “2, 40” denotes the “2 particles, Et > 40 GeV” cut as described above, “4, 50” denotes the “all 4 particles, Et > 50 GeV” cut.

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Typical W’ Signal/Background/Data at a Selected Cut: WZ invariant mass plot with optimal cut (4,60) for 600 GeV W’ (see table above). Expected W’ signals at 600 GeV is shown in the blue hatched histogram. Note that although this is the same cut as Z’ at 600 GeV this is WZ invariant mass instead of WW.

Left Plot: In linear scale.

Middle Plot: In log scale

Right Table: Data and background compositions of WZ for this cut (4,60): { (Et(ele) && missing Et && (both jets) } > 60 GeV.

 

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Result summary:

The following table summarizes mass exclusion regions derived from cross section limit plots above.

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