Measurement of the associated production of a Z boson with charm or bottom quark jets in proton-proton collisions at $\sqrt{s} =$ 13 TeV
Ratios of cross sections, $\sigma$(Z+c jets) / $\sigma$(Z+jets), $\sigma$(Z+b jets) / $\sigma$(Z+jets), and $\sigma$(Z+c jets) / $\sigma$(Z+b jets) in the associated production of a Z boson with at least one charm or bottom quark jet are measured in proton-proton collisions at $\sqrt{s} =$ 13 TeV. The data sample, collected by the CMS experiment at the CERN LHC, corresponds to an integrated luminosity of 35.9 fb$^{-1}$, with a fiducial volume of $p_\mathrm{T}$ $>$ 30 GeV and $|\eta|$ $<$ 2.4 for the jets, where $p_\mathrm{T}$ and $\eta$ represent transverse momentum and pseudorapidity, respectively. The Z boson candidates come from leptonic decays into electrons or muons with $p_\mathrm{T}$ $>$ 25 GeV and $|\eta|$ $<$ 2.4, and the dilepton mass satisfies 71 $\lt$ $m_\mathrm{Z}$ $\lt$ 111 GeV. The measured values are $\sigma$(Z+c jets) / $\sigma$(Z+jets) = 0.102 $\pm$ 0.002 $\pm$ 0.009, $\sigma$(Z+b jets) / $\sigma$(Z+jets) = 0.0633 $\pm$ 0.0004 $\pm$ 0.0015, and $\sigma$(Z+c jets) / $\sigma$(Z+b jets) = 1.62 $\pm$ 0.03 $\pm$ 0.15. Results on the inclusive and differential cross section ratios as functions of jet and Z boson transverse momentum are compared with predictions from leading and next-to-leading order perturbative quantum chromodynamics calculations. These are the first measurements of the cross section ratios at 13 TeV.