Next-to-next-to-leading order event generation for Z -boson production in association with a bottom-quark pair

We consider the production of a <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mi>Z</a:mi></a:math> boson decaying to leptons in association with a bottom-quark pair in hadronic collisions. For the first time, we compute predictions at next-to-next-to-leading order (NNLO) in QCD, and we combine them with the all-order radiative corrections from a parton-shower simulation (<c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:mrow><c:mi>NNLO</c:mi><c:mo>+</c:mo><c:mi>PS</c:mi></c:mrow></c:math>). Our method represents the first approach to <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:mrow><e:mi>NNLO</e:mi><e:mo>+</e:mo><e:mi>PS</e:mi></e:mrow></e:math> event generation applicable to processes featuring a color singlet and a heavy-quark pair in the final state. The novel two-loop corrections are computed for massless bottom quarks, and the leading mass corrections are restored through a small-mass expansion. The calculation is carried out in the four-flavor scheme, and we find that the sizable NNLO QCD corrections lift the long-standing tension between lower-order predictions in four- and five-flavor schemes. Our predictions are compared to a CMS measurement for <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"><g:mi>Z</g:mi></g:math> boson plus <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"><i:mi>b</i:mi></i:math>-jet production, achieving an excellent description of the data.