CMS-EXO-14-006

CMS-EXO-14-006 ; CERN-EP-2016-295
Search for leptophobic Z' bosons decaying into four-lepton final states in proton-proton collisions at $\sqrt{s} = $ 8 TeV
CMS Collaboration
5 January 2017
Phys. Lett. B 773 (2017) 563
Abstract: A search for heavy narrow resonances decaying into four-lepton final states has been performed using proton-proton collision data at $\sqrt{s} = $ 8 TeV collected by the CMS experiment, corresponding to an integrated luminosity of 19.7 fb$^{-1}$. No excess of events over the standard model background expectation is observed. Upper limits for a benchmark model on the product of cross section and branching fraction for the production of these heavy narrow resonances are presented. The limit excludes leptophobic Z' bosons with masses below 2.5 TeV within the benchmark model. This is the first result to constrain a leptophobic Z' resonance in the four-lepton channel.
Links: e-print arXiv:1701.01345 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Leading order Feynman diagram for the production and cascade decay of a Z' resonance to a four-lepton final state.
png pdf	Figure 2: The $ {m_{4\ell}} $ spectrum for the combination of the five studied channels. The points with vertical bars represent the data and the associated statistical uncertainties; the histograms represent the expectations from SM processes; "Top quark'' denotes the sum of the events for $ {\mathrm{t} {}\mathrm{\bar{t}}}$, $\mathrm{t} \mathrm{W} $, ${\mathrm{t} {}\mathrm{\bar{t}}} \mathrm{Z} $ processes; "EW'' denotes the sum of the events from $\mathrm{ W } \mathrm{ W } $, $\mathrm{ W } \mathrm{ Z } $, $\mathrm{ W } \mathrm{ W } \gamma $, $\mathrm{ W } \mathrm{ W } \mathrm{ Z } $, $\mathrm{ W } \mathrm{ Z } \mathrm{ Z } $, and $\mathrm{ Z } \mathrm{ Z } \mathrm{ Z } $ processes. The inset shows the expectation from the benchmark model for a signal at $ {m_{\mathrm{Z'}}} = $ 2.5 TeV with $ {m_{\varphi}} = $ 50 GeV.
png pdf	Figure 3: The 95% CL upper limit on the cross section times branching fraction as a function of $ {m_{4\ell}} $ for the combination of the five channels: expanded view of the low mass region. The shaded green (yellow) band indicates the one (two) sigma uncertainty in the expected limits.
png pdf	Figure 3-a: The 95% CL upper limit on the cross section times branching fraction as a function of $ {m_{4\ell}} $ for the combination of the five channels: full mass range. The shaded green (yellow) band indicates the one (two) sigma uncertainty in the expected limits. The blue dashed line represents the theoretical predictions for the benchmark model [29] for $ {m_{\varphi}} = $ 50 GeV.
png pdf	Figure 3-b: The 95% CL upper limit on the cross section times branching fraction as a function of $ {m_{4\ell}} $ for the combination of the five channels: expanded view of the low mass region. The shaded green (yellow) band indicates the one (two) sigma uncertainty in the expected limits.

Tables
png pdf	Table 1: Summary of the observed yield and expected backgrounds for all channels, where $N_\text {obs}$ is the number of observed events in data. The total background ($N_\text {tot}$) is the sum of three different backgrounds that are estimated using MC simulations; $N_{\mathrm{Z} \mathrm{Z}}$ refers to the background from $\mathrm{Z}\mathrm{Z} $ events; $N_{\mathrm{t}}$ is the background from ${\mathrm{t} {}\mathrm{\bar{t}}}$, single top quark, and ${\mathrm{t} {}\mathrm{\bar{t}}} \mathrm{Z} $ production; $N_\mathrm {EW}$ is the background from $\mathrm{W} \mathrm{W} $ and $\mathrm{W} \mathrm{Z} $, and triple gauge boson production. The quoted uncertainties are statistical only.
png pdf	Table 2: The 95% CL lower limits (in TeV) on $ {m_{\mathrm{Z'}}}$ for the five separate channels and for their combination. Results are presented for the benchmark assumption $ {m_{\varphi}} = $ 50 GeV, and for the five different values of the ratio $ {m_{\varphi}} / {m_{\mathrm{Z'}}} $.

Summary

Results have been presented from a search for heavy narrow resonances decaying into four-lepton final states via intermediate scalar particles $\varphi$, where the branching fraction of $\varphi \to \ell\ell$ ($\ell = $ e or $\mu$) is set to 1. These results are based on a sample of proton-proton collision data at $\sqrt{s} = $ 8 TeV, corresponding to an integrated luminosity of 19.7 fb$^{-1}$. The four-lepton invariant mass spectra are consistent with the standard model predictions. Masses of Z' bosons have been excluded at 95% confidence level for a specific benchmark model with ${m_{\varphi}} = $ 50 GeV, and for five different assumptions for the ratio ${m_{\varphi}} /{m_{\mathrm{Z'}}} ({m_{\varphi}} /{m_{\mathrm{Z'}}} = $ 5, 10, 20, 30 and 40%). Five decay channels (${\mu\mu\mu\mu} $, ${\mu\mu\mu \mathrm{ e }} $, ${\mu\mu \mathrm{ e }\mathrm{ e }} $, ${\mu\mathrm{ e }\mathrm{ e }\mathrm{ e }} $, ${\mathrm{ e }\mathrm{ e }\mathrm{ e }\mathrm{ e }} $) are considered in this analysis. Combining all channels, a lower limit on the Z' mass of 2.5 TeV is obtained for the benchmark model, and 2.6 TeV for each of the models assuming a fixed ratio between ${m_{\varphi}} $ and ${m_{\mathrm{Z'}}} $. This is the first result to constrain a leptophobic Z' resonance in the four-lepton channel.

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