CMS-PAS-EXO-12-033

CMS-PAS-EXO-12-033
Search for new physics using boosted Z bosons decaying to electrons in proton-proton collisions at $\sqrt{s} = $ 8 TeV
CMS Collaboration
February 2017

Abstract: A search for new heavy particles whose two-body decays involve boosted Z bosons is presented, based on the dielectron decay channel, using 19.7 fb$^{-1}$ of $\sqrt{s}= $ 8 TeV proton-proton collision data collected by the CMS detector in 2012. No evidence for non-standard-model physics is observed in the inclusive Z boson transverse momentum spectrum, and upper limits on the cross section for production of excited quarks decaying to qZ are obtained. The analysis excludes, at 95% confidence level, the production of excited quarks decaying to qZ for excited quark masses below 2.6 TeV (3.0 TeV) in gauge (contact) interaction production modes, under the standard assumption of $f=f'=f_s= $ 1.
Links: CDS record (PDF) ; inSPIRE record ; CADI line (restricted) ;

Figures	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Feynman diagrams for the boosted Z boson signatures of quark compositeness models at the LHC's proton-proton collisions; excited quark production via gauge interactions (left) and contact interactions (right).
png	Figure 1-a: Feynman diagram for the boosted Z boson signatures of quark compositeness models at the LHC's proton-proton collisions: excited quark production via gauge interactions.
png	Figure 1-b: Feynman diagram for the boosted Z boson signatures of quark compositeness models at the LHC's proton-proton collisions: contact interactions.
png pdf	Figure 2: The efficiency of the electron ID criteria [27] alone, combined with standard isolation and with modified isolation, for reconstructed Z bosons from simulated Drell-Yan events in which both electrons are within acceptance, shown as a function of Z boson transverse momentum.
png pdf	Figure 3: The predicted and observed transverse momentum spectra of reconstructed Z boson candidates passing selection criteria. In the upper plot, the points with errors bars represent data. The filled histograms show the contributions from standard model processes, as predicted by Monte Carlo simulation; the dark blue hatched band shows the systematic uncertainty on this prediction. The stacked open histograms show the additional contribution from the production of excited quarks (mass 2 TeV) through gauge interactions (GI) and contact interactions (CI). The lower plot shows the ratio of the observed event yield and that expected from standard model processes. All error bars represent the statistical uncertainty on the observed number of events.
png pdf	Figure 4: Upper limits on the product of the excited quark production cross section and branching fraction into dielectron final state, shown as a function of excited quark mass. The dotted-dashed black line shows the median expected limit based on the background-only hypothesis, and the shaded bands correspond to the central 68% and 95% quantiles. The solid black line shows the observed limit, calculated from the full 19.7 fb$^{-1}$ data set. The blue line shows the theoretical excited quark cross section and branching fraction for $f=f'=f_s= $ 1.
png pdf	Figure 4-a: Gauge interaction: Upper limits on the product of the excited quark production cross section and branching fraction into dielectron final state, shown as a function of excited quark mass. The dotted-dashed black line shows the median expected limit based on the background-only hypothesis, and the shaded bands correspond to the central 68% and 95% quantiles. The solid black line shows the observed limit, calculated from the full 19.7 fb$^{-1}$ data set. The blue line shows the theoretical excited quark cross section and branching fraction for $f=f'=f_s= $ 1.
png pdf	Figure 4-b: Contact interaction: Upper limits on the product of the excited quark production cross section and branching fraction into dielectron final state, shown as a function of excited quark mass. The dotted-dashed black line shows the median expected limit based on the background-only hypothesis, and the shaded bands correspond to the central 68% and 95% quantiles. The solid black line shows the observed limit, calculated from the full 19.7 fb$^{-1}$ data set. The blue line shows the theoretical excited quark cross section and branching fraction for $f=f'=f_s= $ 1.

Summary

A search for new heavy particles whose decays involve boosted Z bosons has been presented, based on the dielectron decay channel, using 19.7 fb$^{-1}$ of $\sqrt{s}= $ 8 TeV proton-proton collision data collected by the CMS detector. The observed Z boson $p_{\mathrm{T}}$ spectrum is consistent with expectations from the standard model, and upper limits have been set on the cross section times branching fraction for the production of excited quarks decaying to qZ. Excited quark production via gauge (contact) interaction mode is excluded at 95% confidence level for masses below 2.6 TeV (3.0 TeV), under the standard assumption of $f=f'=f_s=$ 1.

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