CMS-BPH-16-001

CMS-BPH-16-001 ; CERN-EP-2018-131
Observation of the $\mathrm{Z} \to \psi \ell^{+} \ell^{-}$ decay in pp collisions at $\sqrt{s} = $ 13 TeV
CMS Collaboration
11 June 2018
Phys. Rev. Lett. 121 (2018) 141801
Abstract: This Letter presents the observation of the rare Z boson decay $\mathrm{Z} \to \psi \ell^{+} \ell^{-}$. Here, $\psi$ represents contributions from direct $ \mathrm{J}/\psi$ and $\psi\mathrm{(2S)}\to\mathrm{J}/\psi\,X$, ${\ell^+\ell^-} $ is a pair of electrons or muons, and the $ \mathrm{J}/\psi$ meson is detected via its decay to ${\mu^{+}\mu^{-}} $. The sample of proton-proton collision data, collected by the CMS experiment at the LHC at a center-of-mass energy of 13 TeV, corresponds to an integrated luminosity of 35.9 fb$^{-1}$. The signal is observed with a significance in excess of 5 standard deviations. After subtraction of the $\psi\mathrm{(2S)}\to\mathrm{J}/\psi\,X$ contribution, the ratio of the branching fraction of the exclusive decay $\mathrm{Z} \to \mathrm{J}/\psi \ell^{+} \ell^{-}$ to the decay $\mathrm{Z} \to \mu^{+}\mu^{-}\mu^{+}\mu^{-}$ within a fiducial phase space is measured to be $\mathcal{B}(\mathrm{Z} \to \mathrm{J}/\psi \ell^{+} \ell^{-}) / \mathcal{B}(\mathrm{Z} \to \mu^{+}\mu^{-}\mu^{+}\mu^{-}) = $ 0.67 $\pm$ 0.18 (stat) $\pm$ 0.05 (syst).
Links: e-print arXiv:1806.04213 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Feynman diagram for the dominant production process and leading-order decay of the $ {{\mathrm {Z}}\to \mathrm{J}/\psi\ell ^+\ell ^-} $.
png pdf	Figure 2: Distribution of invariant masses $m_{{{{\mu ^+}} {{\mu ^-}}}}$ vs. $m_{{{{\mu ^+}} {{\mu ^-}}} {\ell ^+\ell ^-}}$ for the selected candidates. The values in the legend give the numbers of candidates per bin.
png pdf	Figure 3: Invariant mass distributions for the $\psi $ muon pairs (left) and for ${\psi \,\ell ^+\ell ^-}$ (right), for ${{\mathrm {Z}}\to \psi \, {{\mu ^+}} {{\mu ^-}}}$ (upper) and ${{\mathrm {Z}}\to \psi \, {\mathrm {e}^+} {\mathrm {e}^-}} $ (lower) candidates. In each histogram the data are represented by the points, with the vertical bars showing the statistical uncertainties, and the solid curve is the overall fit to the data. The shaded region corresponds to the signal yield, while the dashed lines are the $\psi $ meson signal from the Z boson background (left) and the Z boson signal from the $\psi $ meson background (right). The dotted line represents the combinatorial background.
png pdf	Figure 3-a: Invariant mass distribution for the $\psi $ muon pairs, for ${{\mathrm {Z}}\to \psi \, {{\mu ^+}} {{\mu ^-}}}$ candidates. The data are represented by the points, with the vertical bars showing the statistical uncertainties, and the solid curve is the overall fit to the data. The shaded region corresponds to the signal yield, while the dashed lines are the $\psi $ meson signal from the Z boson background. The dotted line represents the combinatorial background.
png pdf	Figure 3-b: Invariant mass distribution for ${\psi \,\ell ^+\ell ^-}$, for ${{\mathrm {Z}}\to \psi \, {{\mu ^+}} {{\mu ^-}}}$ candidates. The data are represented by the points, with the vertical bars showing the statistical uncertainties, and the solid curve is the overall fit to the data. The shaded region corresponds to the signal yield, while the dashed lines are the Z boson signal from the $\psi $ meson background. The dotted line represents the combinatorial background.
png pdf	Figure 3-c: Invariant mass distribution for the $\psi $ muon pairs, for ${{\mathrm {Z}}\to \psi \, {\mathrm {e}^+} {\mathrm {e}^-}} $ candidates. The data are represented by the points, with the vertical bars showing the statistical uncertainties, and the solid curve is the overall fit to the data. The shaded region corresponds to the signal yield, while the dashed lines are the $\psi $ meson signal from the Z boson background. The dotted line represents the combinatorial background.
png pdf	Figure 3-d: Invariant mass distribution for ${\psi \,\ell ^+\ell ^-}$, for ${{\mathrm {Z}}\to \psi \, {\mathrm {e}^+} {\mathrm {e}^-}} $ candidates. The data are represented by the points, with the vertical bars showing the statistical uncertainties, and the solid curve is the overall fit to the data. The shaded region corresponds to the signal yield, while the dashed lines are the Z boson signal from the $\psi $ meson background. The dotted line represents the combinatorial background.

Tables
png pdf	Table 1: Definition of the fiducial phase space for the measurement of the ratio of branching fractions. Here, $\ell $ refers to a prompt muon or electron from the signal decay, or to either of the two muons from the higher invariant-mass pair in the reference-channel decay, and ${{\mu}}$ refers to a $ \mathrm{J}/\psi$ daughter or a member of the lower invariant-mass pair in the reference-channel decay. The symbol $\ell _1$ ($\ell _2$) refers to the prompt lepton having the higher (lower) value of ${p_{\mathrm {T}}}$. The $ {p_{\mathrm {T}}} ^{\mathrm{J}/\psi}$ threshold is applied to the signal and the $m({{\mu} ^\pm {{\mu}}^\mp})$ requirement to the reference channel.
png pdf	Table 2: The contributions to the systematic uncertainty in the ratio of branching fractions for the prompt muon, prompt electron, and combined samples, in percent. The last row gives the sum in quadrature of all components.

Summary

In summary, a new decay mode of the Z boson into a $\psi$ meson, where $\psi$ represents the contributions from direct $ \mathrm{J}/\psi$ and $\psi\to\mathrm{J}/\psi\,X$, and an additional pair of leptons (muons or electrons), is observed with a statistical significance greater than 5 standard deviations. Using data from proton-proton collisions collected with the CMS detector at $\sqrt{s}= $ 13 TeV, corresponding to an integrated luminosity of 35.9 fb$^{-1}$, 13.0 $\pm$ 3.9 events of the $\mathrm{Z} \to \mu^{+}\mu^{-}$ and 11.2 $\pm$ 3.4 events of the $\mathrm{Z} \to \mathrm{e^{+}e^{-}}$ decay are obtained. This is the first observed Z boson decay to a vector meson and two oppositely charged same-flavor leptons. The ratio of the branching fraction for this decay to the one for the reference channel $\mathrm{Z} \to \mu^{+}\mu^{-}\mu^{+}\mu^{-}$ in the fiducial phase space of the measurement, as defined in Table 1, after subtracting the $\psi$ feed-down, is $\mathcal{R}_{\mathrm{J}/\psi\,\ell^{+} \ell^{-}} = $ 0.67 $\pm$ 0.18 (stat) $\pm$ 0.05 (syst). Using the known branching fraction for $\mathrm{Z} \to \mu^{+}\mu^{-}\mu^{+}\mu^{-}$ results in a branching fraction for $\mathrm{Z} \to \mathrm{J}/\psi \ell^{+} \ell^{-}$ consistent with standard model predictions.

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