Measurement of the ZZ production cross section and Z to l+l-l'+l'- branching fraction in pp collisions at sqrt(s) = 13 TeV

TL;DR

This study measures ZZ production and the Z→4ℓ branching fraction in proton-proton collisions at $\sqrt{s}=13$ TeV using CMS data corresponding to $2.62\pm0.07\ \mathrm{fb}^{-1}$. It employs stringent four-lepton event selections, data-driven background estimates, and data-driven efficiency calibrations to extract fiducial and total cross sections, along with the Z→4ℓ branching fraction, all compared to SM predictions. The results, including $\sigma(pp\to ZZ) = 14.6^{+1.9}_{-1.8}\ \mathrm{pb}$ and $\mathcal{B}(Z\to \ell^+\ell^-\ell'^+\ell'^-) = (4.9^{+0.8}_{-0.7})\times 10^{-6}$, are consistent with SM expectations and provide a stringent test of higher-order QCD calculations. The analysis demonstrates the precision of the four-lepton channel for probing diboson production and calibrating mass measurements in related Higgs analyses.

Abstract

Four-lepton production in proton-proton collisions, pp to (Z/gamma*)(Z/gamma*) to l+l-l'+l'-, where l, l' = e or mu, is studied at a center-of-mass energy of 13 TeV with the CMS detector at the LHC. The data sample corresponds to an integrated luminosity of 2.6 inverse femtobarns. The ZZ production cross section, sigma(pp to ZZ) = 14.6 +1.9/-1.8 (stat) +0.5/-0.3 (syst) +/- 0.2 (theo) +/- 0.4 (lumi) pb, is measured for events with two opposite-sign, same-flavor lepton pairs produced in the mass region 60 < m[l+l-], m[l'+l'-] < 120 GeV. The Z boson branching fraction to four leptons is measured to be B(Z to l+l-l'+l'-) = 4.9 +0.8/-0.7 (stat) +0.3/-0.2 (syst) +0.2/-0.1 (theo) +/- 0.1 (lumi) x E-6 for the four-lepton invariant mass in the range 80 < m[l+l-l'+l'-] < 100 GeV and dilepton mass m[l+l-] > 4 GeV for all opposite-sign, same-flavor lepton pairs. The results are in agreement with standard model predictions.

Figures (5)

• Figure 1: Distributions of (top) the four-lepton invariant mass $m_{\ell^+\ell^-\ell^{\prime+}\ell^{\prime-}\xspace}$ and (bottom) the invariant mass of the dilepton candidates in all selected four-lepton events, including both ${Z}_1$ and ${Z}_2$ in each event. Points represent the data, while shaded histograms represent the SM prediction and background estimate. Hatched regions around the predicted yield represent combined statistical, systematic, theoretical, and integrated luminosity uncertainties.
• Figure 2: (top) The distribution of the reconstructed mass of the ${Z}_1$ candidate. Points represent the data, while shaded histograms represent the SM prediction and background estimate. Hatched regions around the predicted yield represent combined statistical, systematic, theoretical, and integrated luminosity uncertainties. (bottom) The reconstructed $m_{{Z}_2}$ plotted against the reconstructed $m_{{Z}_1}$ in data events, with distinctive markers for each final state.
• Figure 3: (top) The distribution of the reconstructed four-lepton mass $m_{\ell^+\ell^-\ell^{\prime+}\ell^{\prime-}\xspace}$ for events selected with $m_{\ell^+\ell^-\ell^{\prime+}\ell^{\prime-}\xspace} < 110\,\text{Ge\spaceV}\xspace$. Points represent the data, while shaded histograms represent the SM prediction and background estimate. Hatched regions around the predicted yield represent combined statistical, systematic, theoretical, and integrated luminosity uncertainties. (bottom) The reconstructed $m_{{Z}_2}$ plotted against the reconstructed $m_{{Z}_1}$ in data events selected with $m_{\ell^+\ell^-\ell^{\prime+}\ell^{\prime-}\xspace}$ between 80 and 100$\,\text{Ge\spaceV}$, with distinctive markers for each final state.
• Figure 4: Distributions of (top) the four-lepton invariant mass $m_{\ell^+\ell^-\ell^{\prime+}\ell^{\prime-}\xspace}$ and (bottom) dilepton candidate mass for four-lepton events selected with both ${Z}$ bosons on-shell. Points represent the data, while shaded histograms represent the SM prediction and background estimate. Hatched regions around the predicted yield represent combined statistical, systematic, theoretical, and integrated luminosity uncertainties.
• Figure 5: The total ${Z}{Z}\xspace$ cross section as a function of the proton-proton center-of-mass energy. Results from the CMS and ATLAS experiments are compared to predictions from matrix and mcfm with NNPDF3.0 PDF sets and fixed scales $\mu_F = \mu_R = m_{Z}$. Details of the calculations and uncertainties are given in the text. Measurements at the same center-of-mass energy are shifted slightly along the x-axis for clarity.