Search for physics beyond the standard model in events with a Z boson, jets and missing transverse energy

Link to online version

The paper has been published in Physics Letters B, Volume 716, Issue 2, 19 September 2012, Pages 260-284. It is available on this sciencedirect page.

Abstract

A search is presented for physics beyond the standard model (BSM) in events with a Z boson, jets, and missing transverse energy (ETmiss). This signature is motivated by BSM physics scenarios, including supersymmetry. The study is performed using a sample of proton-proton collision data collected at √s = 7 TeV with the CMS experiment at the LHC, corresponding to an integrated luminosity of 4.98 fb−1. The contributions from the dominant standard model backgrounds are estimated from data using two complementary strategies, the jet-Z balance technique and a method based on modeling ETmiss with data control samples. In the absence of evidence for BSM physics, we set limits on the non-standard-model contributions to event yields in the signal regions and interpret the results in the context of simplified model spectra. Additional information is provided to facilitate tests of other BSM physics models.

Approved Plots ( click on plot to get .pdf )

Figure	Abbreviated Caption
	Limits on a neutralino LSP scenario with the JZB method: (left) signal efficiency, including acceptance, in the (mgluino,mneutralino) parameter space for the JZB>150 GeV region, normalized to the number of events with at least one leptonically decaying Z; (right) 95% CL upper limits on the cross section of the inclusive Z boson decay mode. The contribution from signal to the control regions is taken into account.
	Di-electron mass distribution for events passing the pre-selection (2 jets, 2 same-flavor opposite-sign leptons). Backgrounds from single top and W +jets are omitted since they are negligible. This plot compares distributions in data and Monte Carlo simulation; observed discrepancies arise due to incomplete modeling of resolutions, e.g., due to pile-up interactions. The backgrounds for the search are predicted from data as shown in other figures.
	Di-muon mass distribution for events passing the pre-selection (2 jets, 2 same-flavor opposite-sign leptons). Backgrounds from single top and W +jets are omitted since they are negligible. This plot compares distributions in data and Monte Carlo simulation; observed discrepancies arise due to incomplete modeling of resolutions, e.g., due to pile-up interactions. The backgrounds for the search are predicted from data as shown in other figures.
	Data and Monte Carlo yields for the di-lepton preselection with Njets ≥ 2.
	Data and Monte Carlo yields for the di-lepton preselection with Njets ≥ 3.
	Comparison between true and predicted JZB distributions in simulated samples for the background-only hypothesis. The lower plots show the ratio between true and predicted distributions.
	Comparison between true and predicted JZB distributions in simulated samples for the LM4-plus-background hypothesis. "MC B" and "MC S" denote the background and signal contributions to the true distribution, respectively. The lower plots show the ratio between true and predicted distributions.
	Comparison between true and predicted JZB event yields in SM MC simulation for the various search regions. Uncertainties on the true MC yields reflect the limited MC statistics. The first (second) uncertainty in the MC component.
	Comparison between the measured JZB distribution in the JZB > 0 region and that predicted from data control samples. The distribution from the LM4 MC is overlaid. The bottom plot shows the ratio between the observed and predicted distributions. The error bars indicate the statistical uncertainties in data only.
	Total number of events observed in the JZB search regions and corresponding background predictions. The first uncertainty is statistical and the second systematic. For the observed yield, the first (second) number in parentheses is the yield in the e+e- (μ+μ-) final state. The 95% CL upper limit (UL) on non-SM yield and the NLO yields for the LM4 and LM8 benchmark SUSY scenarios are also indicated. The contribution of LM4 and LM8 to the background control regions is taken into account in these yields.
	The observed missing ET distribution for events with Njets ≥ 2 for data (black points), predicted opposite-flavor background from MC normalized to the eμ yield in data (solid green histogram), WZ + ZZ background (solid purple histogram) and total background including the Z + jets predicted from γ + jets (red line) and QCD (blue line) missing ET templates. The ratio of observed yield to predicted background (data/pred) is indicated using the total background predicted from γ + jets Emiss templates, and the error bars indicate the statistical uncertainty in data only.
	Summary of results in the regions missing ET > 30, 60, 100, 200 and 300 GeV, for Njets ≥ 2. The total predicted background (total bkg) is the sum of the Z + jets background predicted from the γ + jets missing ET templates method (Z bkg), the flavor-symmetric background predicted from opposite-flavor events (OF bkg), and the WZ + ZZ background predicted from MC (VZ bkg). The uncertainties include both the statistical and systematic contributions. For the observed yield (data), the first (second) number in parentheses is the yield in the ee (μμ) final state. The 95% CL upper limit on the non-SM yield is indicated, using the average of the background predictions from the QCD and γ + jets templates methods and assessing half the difference between the 2 methods as an additional systematic uncertainty. The expected NLO yields for the LM4 and LM8 benchmark SUSY scenarios are indicated, including the uncertainties from lepton identification and isolation efficiency, trigger efficiency, hadronic energy scale, integrated luminosity, and MC statistics. These LM4 and LM8 yields have been corrected for the impact of signal contamination.
	The observed missing ET distribution for events with Njets ≥ 3 for data (black points), predicted opposite-flavor background from MC normalized to the eμ yield in data (solid green histogram), WZ + ZZ background (solid purple histogram) and total background including the Z + jets predicted from γ + jets (red line) and QCD (blue line) missing ET templates. The ratio of observed yield to predicted background (data/pred) is indicated using the average of the two total background predictions from γ + jets and QCD Emiss templates, and the error bars indicate the statistical uncertainty in data only.
	Summary of results in the regions missing ET > 30, 60, 100, 200 and 300 GeV, for Njets ≥ 3. The total predicted background (total bkg) is the sum of the Z + jets background predicted from the missing ET templates method (Z bkg), the flavor-symmetric background predicted from opposite-flavor events (OF bkg), and the WZ + ZZ background predicted from MC (VZ bkg). The Z + jets and total background predictions are quoted separately for the QCD and γ + jets missing ET templates. The uncertainties include both the statistical and systematic contributions. For the observed yield (data), the first (second) number in parentheses is the yield in the ee (μμ) final state. The 95% CL upper limit on the non-SM yield is indicated, using the average of the background predictions from the QCD and γ + jets templates methods. The expected NLO yields for the LM4 and LM8 benchmark SUSY scenarios are indicated, including the uncertain- ties from lepton identification and isolation efficiency, trigger efficiency, hadronic energy scale, integrated luminosity, and MC statistics.
	Summary of results in the exclusive missing ET bins 100-200, 200-300 GeV, and >300 GeV for Njets ≥ 2 used for the SMS exclusions. The total predicted background (total bkg) is the sum of the Z + jets background predicted from the γ + jets missing ET templates method (Z bkg), the flavor-symmetric background predicted from opposite-flavor events (OF bkg), and the WZ + ZZ background predicted from MC (VZ bkg). The uncertainties include both the statistical and systematic contributions. For the observed yield (data), the first (second) number in parentheses is the yield in the ee (μμ) final state.
	JZB distribution in Z+jets, top-antitop and signal MC simulation.
	Simplified model for the production of two gluinos decaying into a Z boson, neutralino and jets.
	Parameters of the JZB (top) and missing ET (bottom) response function. The parameter σ is the resolution, xthresh is the JZB or missing ET value at the centre of the efficiency curve and εplateau is the efficiency on the plateau.
	Limits on a neutralino LSP scenario with the MET method: (left) signal efficiency, including acceptance, in the (mgluino,mneutralino) parameter space for the MET>100 GeV region, normalized to the number of events with at least one leptonically decaying Z; (right) 95% CL upper limits on the cross section of the inclusive Z boson decay mode. The contribution from signal to the control regions is negligible.
	Reconstructed JZB selection efficiency as a function of the generator-level quantity, for the different search regions in LM4 simulation.
	Reconstructed MET selection efficiency as a function of the generator-level quantity, for the different search regions in LM4 simulation.
	Limits on a neutralino LSP scenario (x=0.75) with the JZB method: (left) signal efficiency, including acceptance, in the (mgluino,mneutralino) parameter space for the JZB>150 GeV region, normalized to the number of events with at least one leptonically decaying Z; (right) 95% CL upper limits on the cross section of the inclusive Z boson decay mode. The contribution from signal to the control regions is taken into account.
	Limits on a neutralino LSP scenario (x=0.25) with the MET method: (left) signal efficiency, including acceptance, in the (mgluino,mneutralino) parameter space for the MET>100 GeV region, normalized to the number of events with at least one leptonically decaying Z; (right) 95% CL upper limits on the cross section of the inclusive Z boson decay mode. The contribution from signal to the control regions is negligible.
	Limits on a neutralino LSP scenario (x=0.25) with the JZB method: (left) signal efficiency, including acceptance, in the (mgluino,mneutralino) parameter space for the JZB>150 GeV region, normalized to the number of events with at least one leptonically decaying Z; (right) 95% CL upper limits on the cross section of the inclusive Z boson decay mode. The contribution from signal to the control regions is taken into account.
	Limits on a neutralino LSP scenario (x=0.75) with the MET method: (left) signal efficiency, including acceptance, in the (mgluino,mneutralino) parameter space for the MET>100 GeV region, normalized to the number of events with at least one leptonically decaying Z; (right) 95% CL upper limits on the cross section of the inclusive Z boson decay mode. The contribution from signal to the control regions is negligible.
	Limits on a gravitino LSP scenario with the JZB method: (left) signal efficiency, including acceptance, in the (mgluino,mneutralino) parameter space for the JZB>150 GeV region, normalized to the number of events with at least one leptonically decaying Z; (right) 95% CL upper limits on the cross section of the inclusive Z boson decay mode. The contribution from signal to the control regions is taken into account.
	Limits on a gravitino LSP scenario with the MET method: (left) signal efficiency, including acceptance, in the (mgluino,mneutralino) parameter space for the MET>100 GeV region, normalized to the number of events with at least one leptonically decaying Z ; (right) 95% CL upper limits on the cross section of the inclusive Z boson decay mode. The contribution from signal to the control regions is negligible.
	Limits on a gravitino LSP scenario with the MET method. The upper limits on the cross section vs. gluino mass are plotted for three choices for the mass of the lightest neutralino, and compared to the predicted cross section computed at NLO. This plot contains the same information as the above MET templates gravitino LSP exclusion in the (mgluino,mneutralino) parameter space; here, the results are projected onto the mgluino axis

Links to documentation in CDS

• JZB analysis: CMS-PAS-SUS-10-010, CMS-PAS-SUS-11-019
• MET analysis: CMS-PAS-SUS-11-017

-- FredericRonga - 30-Jan-2012