Observation of Z decays to four leptons with the CMS detector at the LHC

TL;DR

This work reports the first observation of Z boson decays to four leptons in proton–proton collisions at 7 TeV using the CMS detector, based on 5.02 fb⁻¹ of data. It defines a precise phase-space for Z→4ℓ, measures the production cross section times branching fraction and the branching fraction themselves, and finds results in agreement with SM predictions. The analysis also demonstrates that the Z→4ℓ peak can calibrate the four-lepton mass scale and resolution for Higgs searches in H→ZZ→4ℓ, underscoring its importance as a calibration channel in LHC analyses.

Abstract

The first observation of the Z boson decaying to four leptons in proton-proton collisions is presented. The analyzed data set corresponds to an integrated luminosity of 5.02 inverse femtobarns at sqrt(s) = 7 TeV collected by the CMS detector at the Large Hadron Collider. A pronounced resonance peak, with a statistical significance of 9.7 sigma, is observed in the distribution of the invariant mass of four leptons (electrons and/or muons) with mass and width consistent with expectations for Z boson decays. The branching fraction and cross section reported here are defined by phase space restrictions on the leptons, namely, 80 < m[4l] < 100 GeV, where m[4l] is the invariant mass of the four leptons, and m[ll] > 4 GeV for all pairs of leptons, where m[ll] is the two-lepton invariant mass. The measured branching fraction is B(Z to 4l) = (4.2 /+0.9/-0.8 (stat.) +/- 0.2 (syst.)) 10E-6 and agrees with the standard model prediction of 4.45 10E-6. The measured cross section times branching fraction is sigma(pp to Z) B(Z to 4 l) = 112 +23/-20 (stat.) +7/-5 (syst.) +3/-2 (lumi.) fb, also consistent with the standard model prediction of 120 fb. The four-lepton mass peak arising from Z to 4 l decays provides a calibration channel for the Higgs boson search in the H to ZZ to 4 l decay mode.

Figures (3)

• Figure 1: (Left) Diagram of the $\mathrm{Z}\rightarrow 4\ell$ process. (Right) Diagram of the $\mathrm{Z}\gamma^* \rightarrow 4\ell$ process for the irreducible background of $Z \to 2\ell$ production with the initial-state radiation undergoing an internal conversion $\gamma^* \to 2\ell$. Both Z and $\gamma^*$ are present in all propagators. The choice of propagators shown in the figures corresponds to the dominant contributions in the phase space $80 < m_{4\ell} < 100$$\,\text{Ge\spaceV}$.
• Figure 2: Four-lepton invariant mass distribution for events passing all selection requirements except that on $m_{4\ell}$. The data are shown by points. The filled histograms represent standard model expectations for $\mathrm{pp} \to \mathrm{Z}/\mathrm{Z}\gamma^{*} \to 4\ell$ and for reducible backgrounds. The three final states, $4\mathrm{e}$, $4\mu$, and $2\mathrm{e}2\mu$, are combined.
• Figure 3: (Left) Four-lepton mass distribution in simulation for $\mathrm{pp} \to 4\ell$, without the Higgs boson (light shaded histogram), $\mathrm{Z}+\mathrm{X}$ background (dark shaded histogram), and $\mathrm{pp} \to \mathrm{H} \to \mathrm{ZZ} \to 4\ell$ for a Higgs boson mass $m_{\mathrm{H}}=125$$\,\text{Ge\spaceV}$. The three contributions are stacked. The standard model cross section for the Higgs boson is scaled up by a factor of 5. (Right) Four-lepton mass distribution with data represented by the points with error bars. The three final states, $4\mathrm{e}$, $4\mu$, and $2\mathrm{e}2\mu$, are combined. The solid line represents a simultaneous fit to the background and Z boson peak.