CMS-PAS-BPH-16-001

CMS-PAS-BPH-16-001
Observation of the decay $\mathrm{Z}\to\psi\,\ell^+\ell^-$ in pp collisions at $\sqrt{s}= $ 13 TeV
CMS Collaboration
March 2018

Abstract: The observation is presented of the Z boson rare decay to a $\psi$ meson and two oppositely charged same-flavor leptons, $\ell^+ \ell^-$, where $\psi$ represents the sum of $\mathrm{J}/\psi$ and $\psi(\mathrm{2S})\to\mathrm{J}/\psi\,X$, and $\ell=\mu$, e. The data sample of proton-proton collisions at a center-of-mass energy of $ \sqrt{s}= $ 13 TeV corresponds to an integrated luminosity of 35.9 fb$^{-1}$ accumulated by the CMS experiment at the CERN LHC. The signal is observed with a significance in excess of 5 standard deviations. Removing contributions from $\psi(\mathrm{2S})$ decays to $\mathrm{J}/\psi$, the signal is interpreted as being entirely from $\mathrm{Z}\to\mathrm{J}/\psi\,\ell^+\ell^-$, with its fiducial branching fraction relative to that of the decay $\mathrm{Z}\to\mu^+\mu^-\mu^+\mu^-$ measured to be $ {\mathcal{B}(\mathrm{Z}\to\mathrm{J}/\psi\,\ell^+\ell^-)}/{\mathcal{B}(\mathrm{Z}\to\mu^+\mu^-\mu^+\mu^-)}= $ 0.70 $\pm$ 0.18 (stat) $\pm$ 0.05 (syst). This result is obtained with the assumption of no $ \mathrm{J}/\psi$ polarization. Extreme polarization scenarios give a variation of the fiducial branching fraction measurement of ($ - $ 22 to $+$ 24)%.
Links: CDS record (PDF) ; inSPIRE record ; CADI line (restricted) ; These preliminary results are superseded in this paper, PRL 121 (2018) 141801. The superseded preliminary plots can be found here.

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Leading-order diagram for the $ {{\mathrm {Z}} \to {{\mathrm {J} / \psi}} + \ell ^+\ell ^-} $ process.
png pdf	Figure 2: Distribution of invariant masses $m_{{\mu ^+} {{\mu ^-}}}$ vs. $m_{{\psi + \ell ^+\ell ^-}}$ for selected candidates.
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 with filled circles, and the solid line curve is the overall fit to the data. The shaded region corresponds to the signal yield, while the dashed lines are the $\psi $ signal from the Z background (left) and the Z signal extracted from the $\psi $ background (right).
png	Figure 3-a: Invariant mass distribution for the $\psi $ muon pairs, for $ {{\mathrm {Z}} \to \psi + {{\mu ^+}} {{\mu ^-}}}$ candidates. The data are represented with filled circles, and the solid line curve is the overall fit to the data. The shaded region corresponds to the signal yield, while the dashed lines are the $\psi $ signal from the Z background.
png	Figure 3-b: Invariant mass distribution for for ${\psi + \mu ^+\mu ^-}$, for $ {{\mathrm {Z}} \to \psi + {{\mu ^+}} {{\mu ^-}}}$ candidates. The data are represented with filled circles, and the solid line curve is the overall fit to the data. The shaded region corresponds to the signal yield, while the dashed lines are the $\psi $ signal from the Z signal extracted from the $\psi $ background.
png	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 with filled circles, and the solid line curve is the overall fit to the data. The shaded region corresponds to the signal yield, while the dashed lines are the $\psi $ signal from the Z background.
png	Figure 3-d: Invariant mass distribution for ${\psi + \mathrm{e} ^+ \mathrm{e} ^-}$, for ${{\mathrm {Z}} \to \psi + {\mathrm {e}^+} {\mathrm {e}^-}}$ candidates. The data are represented with filled circles, and the solid line curve is the overall fit to the data. The shaded region corresponds to the signal yield, while the dashed lines are the $\psi $ signal from the Z signal extracted from the $\psi $ background.

Tables
png pdf	Table 1: Selection definitions for the branching fraction ratio. Here, $\ell $ refers to a prompt muon or electron from the signal decay, or to either of the two leading muons from the reference decay. Subscripts give the lepton ${p_{\mathrm {T}}}$ rank in decreasing order.
png pdf	Table 2: The contributions to the systematic uncertainty in the ratio of the branching fractions for the prompt muon and prompt electron channels, and their sum, in percent.
png pdf	Table 3: Variations in percent of the ratio of branching fractions for different ${{\mathrm {J} / \psi}}$ polarizations with respect to the value found assuming an unpolarized scenario.

Summary

In summary, a new decay mode of the Z boson into a $\psi$ meson, where $\psi$ is the sum of $\mathrm{J}/\psi$ and $ \psi(2\text{S})\to\mathrm{J}/\psi\,X$, and an additional pair of leptons (muons or electrons) is observed with a statistical significance greater than 5$\sigma$. Using 35.9 fb$^{-1}$ of pp collisions collected with the CMS detector at $\sqrt{s} = $ 13 TeV, 13.0 $\pm$ 3.9 events of the decay $\mathrm{Z}\to\psi\,\mu^{+}\mu^{-}$ and 11.2 $\pm$ 3.4 events of the decay $\mathrm{Z}\to \psi\,\mathrm{e^{+}}\mathrm{e^{-}}$ are observed. This is the first observation of the 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 branching fraction for the reference channel $\mathrm{Z} \to 4 \mu $ in the fiducial phase-space of the experiment is $\mathcal{R}_{\mathrm{J}/\psi\,\ell^+\ell^-} = $ 0.70 $\pm$ 0.18 (stat) $\pm$ 0.05 (syst). Extreme $ \mathrm{J}/\psi $ polarization scenarios imply $\approx$20% variations of this result. These variations can be reduced in the future with larger data samples and improved calculations of the kinematics of the signal process.

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