CMS-SMP-18-014

CMS-SMP-18-014 ; CERN-EP-2022-088
Precision measurement of the Z boson invisible width in pp collisions at $\sqrt{s} = $ 13 TeV
CMS Collaboration
14 June 2022
Phys. Lett. B 842 (2023) 137563
Abstract: A precise measurement of the invisible width of the Z boson produced in proton-proton collisions at a center-of-mass energy of 13 TeV is presented using data recorded by the CMS experiment at the LHC, corresponding to an integrated luminosity of 36.3 fb$^{-1}$. The result is obtained from a simultaneous fit to kinematic distributions for two data samples of Z boson plus jets: one dominated by Z boson decays to invisible particles and the other by Z boson decays to muon and electron pairs. The invisible width is measured to be 523 $\pm$ 3 (stat) $\pm$ 16 (syst) MeV. This result is the first precise measurement of the invisible width of the Z boson at a hadron collider, and is the single most precise direct measurement to date, competitive with the combined result of the direct measurements from the LEP experiments.
Links: e-print arXiv:2206.07110 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; Physics Briefing ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Distributions of ${\mathcal {U}}$ in the regions of $\mu$+jets (left) and e+jets (right). Ratios are taken of the data with respect to both the SM post-fit and SM pre-fit results from a simultaneous likelihood fit is performed to assess the consistency between the $\mu$+jets and e+jets channels. Pull is the difference between the data and the SM post-fit results, normalized by their uncertainties summed in quadrature. The 'Minor' background includes the sum of all the contributions from processes such as diboson events, top pair production, and single top production.
png pdf	Figure 2: Distributions of ${\mathcal {U}}$ in the regions of $\mu$+jets (left), e+jets (center), and $\tau_{\text{h}}$+jets (right). Ratios are taken of the data with respect to both the SM post-fit and SM pre-fit results from a likelihood fit performed to verify the consistency between the combined $\mu$+jets and e+jets region with the $\tau_{\text{h}}$+jets region. The pulls are the difference between the data and the SM post-fit results, normalized by their uncertainties summed in quadrature. The 'Minor' background includes the sum of all the contributions from processes such as diboson events, top pair production, and single top production.
png pdf	Figure 3: Distribution of ${\mathcal {U}}$ for the $p_{\text {T}}^{\text {miss}}$+jets (left), ${{\mathrm {Z}} /\gamma^{} \to \mu \mu}$ (center), and ${{\mathrm {Z}} /\gamma^{} \to \mathrm{ee}}$ (right) regions. Ratios are taken of the data with respect to both the SM post-fit and SM pre-fit results. The pulls are the difference between the data and the SM post-fit results, normalized by their uncertainties summed in quadrature. The 'Minor' background includes the sum of all the contributions from processes such as diboson events, top pair production, single top production that are determined from simulation.
png pdf	Figure 4: Direct measurements of the Z invisible width by the LEP experiments and the result from the CMS experiment presented here. Also shown is the prediction from the SM. The inner (outer) error bars show the statistical (total) uncertainty.

Tables
png pdf	Table 1: Relative uncertainties (in %) on the final measurement from different sources.
png pdf	Table 2: SM post-fit predictions for the $p_{\text {T}}^{\text {miss}}$+jets, ${{\mathrm {Z}} /\gamma^{} \to \mu \mu}$, and ${{\mathrm {Z}} /\gamma^{} \to \mathrm{ee}}$ regions and the total number of events measured in data.

Summary

In summary, this Letter presents the first precise measurement of the invisible width (${\Gamma_{\text{inv}}} $) of the Z boson at a hadron collider. The measurement is performed using proton-proton collisions at $\sqrt{s} = $ 13 TeV and data corresponding to an integrated luminosity of 36.3 fb$^{-1}$ recorded with the CMS detector, yielding a result of ${\Gamma_{\text{inv}}} =$ 523 $\pm$ 3 (stat) $\pm$ 16 (syst) MeV. It represents the single most precise direct measurement of ${\Gamma_{\text{inv}}}$ to date and is competitive with the combined direct measurement from LEP and compatible with the value expected in the standard model.

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