Measurement of WW/WZ cross section <br> in lνjj using 4.3 fb^-1

Measurement of WW/WZ cross section <br> in lνjj using 4.3 fb^-1 Measurement of WW/WZ cross section
in lνjj decay using 4.3 fb^-1

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We present a search for the standard model WW/WZ → lνjj processes. The data used in this analysis were taken with the upgraded CDF (CDF II) detector at the Fermilab Tevatron collider, and correspond to an integrated luminosity of 4.3 fb^-1. We consider a data sample of high-p_T electrons and muons to reconstruct the leptonically decaying W boson. We look for another boson candidate in the event by selecting two additional jets. In order to extract our diboson signal, a fit to the invariant mass distribution M_jj of the two jets is performed. We found 1582 ± 275 (stat.) ± 107 (syst.) WW/WZ → lνjj events , corresponding to a statistical significance of 5.24 σ (expected 5.1 σ) . We measure the production cross-section to be σ(WW/WZ) = 18.1 ± 3.3(stat.) ± 2.5(syst.) .

The results have been approved as of November the 25^th, 2009.

CDF - WW+WZ Cross Section using 4.3 fb^-1

At CDF using 4.3 fb-1 of data we measure the WW+WZ production cross section to be:

Expected signal and background event yield

Events are selected which contain one high-p_T lepton (electron or muon) and significant missing transverse energy. This signature is characteristic of a W bosons decaying leptonically. We further select events containing at least 2 jets. We use the two jets with the highest transverse energy to reconstruct the second boson (W or Z) candidate.
We consider two classes of processes as our background:

W + jets process.
Generic QCD jet production that mimics WW/WZ → lνjj topology.
Z + jets process.
top production (ttbar and single top).

The expected number of signal and background events is shown in the table below along with the number of events observed in data. This table includes statistics uncertainties only.

MC estimate of the expected number of events for signal and each background component for m_jj [28,200] GeV/c².
The Z+jets and ttbar background yields are estimated starting from the measured cross-section. The W+jets and QCD background yields are obtained from data. We estimate them with a fit to the missing transverse energy (MET) in the event. The MET separates multi-jet events (QCD) with small MET from electroweak events with large MET associated with the leptonic decay of the W boson. The QCD and W+jets estimate here are used for a preliminary data/MC comparison. Both normalization will be re-determined in the final fit (see below).

Signal extraction

The diboson signal is extracted from the background using a Χ² fit of the m_jj distribution separately for the electron and muon samples. This simple method allows to search for a signal peak over a smooth background. The fit is performed in the m_jj region [28,200] GeV/c² and estimates the fractions of signal, QCD, W+jets, Z+jets and top backgrounds using m_jj templates obtained from the CDF full simulation (signal and EWK) and data (QCD). The total W+jets contribution is a free parameter of the fit while Z+jets and top contribution are constrained to the measured cross section. The QCD contribution is gaussian constrained to the value previously obtained from the MET fit. Finally, the total number of events in data is a free parameter to be determined by the fit.

We estimate:

m_jj projection of the fit result for the electron and muon samples separately.

Systematics

We consider several sources of systematic uncertainty, taking into account their effect on both the signal acceptance and on template shapes for the signal extraction fit. The uncertainty on the normalization of the backgrounds is taken as part of the statistical uncertainty. The largest sources of systematics are due to the shape uncertainty of the EWK and QCD templates. Other sources of systematics considered are the jet energy scale (JES) and resolution (JER), initial and final state radiation (ISR and FSR), PDFs, jet energy resolution and trigger and lepton identification efficiency. Finally, another important contribution to the cross section measurement systematics is due to luminosity measurement uncertainty.

We found:

Final Results

We estimate the number of signal events in the muon and electron samples, including systematics on the signal extraction, to be:

After correcting for efficiencies, acceptance and including the additional systematic uncertainties, we measure the cross section in the electron and muon samples to be, respectively:

Combining the electron and muon results, we estimate 1582 ± 275 (stat.) ± 107 (syst.) WW/WZ → lνjj events , corresponding to a statistical significance of 5.24 σ (expected 5.1 σ) and measure the WW/WZ cross section to be:

Electrons and muon combined result

Result of the fit for the combined electron+muon data sample.

Below are the eps and png versions of all figures meant for download.

Figure : di-jet invariant mass fit (electron + muon data) (.eps) (.png)
Figure : di-jet invariant mass fit (muon data) (.eps) (.png)
Figure : di-jet invariant mass fit (electron data) (.eps) (.png)
Figure : background subtracted signal (muon data) (.eps) (.png)
Figure : background subtracted signal (electron data) (.eps) (.png)
Figure : expected background composition (electron data) (.eps) (.png)
Figure : expected background composition (muon data) (.eps) (.png)
Figure : MET fit for electron sample. (.eps) (.png)
Figure : MET fit for CMUP sample. (.eps) (.png)
Figure : MET fit for CMX sample. (.eps) (.png)