A study of the decay
class="mathmlsrc">class="formulatext stixSupport mathImg" data-mathURL="/science?_ob=MathURL&_method=retrieve&_eid=1-s2.0-S0370269316301022&_mathId=si1.gif&_user=111111111&_pii=S0370269316301022&_rdoc=1&_issn=03702693&md5=a27d90cd9ed1c8a9305ee68cb10567d6" title="Click to view the MathML source">D0→K−π+μ+μ−class="mathContainer hidden">class="mathCode"> is performed using data collected by the LHCb detector in proton–proton collisions at a centre-of-mass energy of 8 TeV,
corresponding to an integrated luminosity of
class="mathmlsrc">class="formulatext stixSupport mathImg" data-mathURL="/science?_ob=MathURL&_method=retrieve&_eid=1-s2.0-S0370269316301022&_mathId=si2.gif&_user=111111111&_pii=S0370269316301022&_rdoc=1&_issn=03702693&md5=aa553350039047030cfe0c9c588b103c" title="Click to view the MathML source">2.0 fb−1class="mathContainer hidden">class="mathCode">. Decay candidates with muon pairs that have an invariant mass in the range
class="mathmlsrc">class="formulatext stixSupport mathImg" data-mathURL="/science?_ob=MathURL&_method=retrieve&_eid=1-s2.0-S0370269316301022&_mathId=si102.gif&_user=111111111&_pii=S0370269316301022&_rdoc=1&_issn=03702693&md5=9ca79b3c364a17f07386b953b9201081" title="Click to view the MathML source">675–875 MeV/c2class="mathContainer hidden">class="mathCode"> are considered. This region is dominated by the
class="mathmlsrc">class="formulatext stixSupport mathImg" data-mathURL="/science?_ob=MathURL&_method=retrieve&_eid=1-s2.0-S0370269316301022&_mathId=si11.gif&_user=111111111&_pii=S0370269316301022&_rdoc=1&_issn=03702693&md5=ed6c8ea4c95c2a6afc807fff99e1cc17" title="Click to view the MathML source">ρ0class="mathContainer hidden">class="mathCode"> and
ω resonances. The branching fraction in this range is measured to be
class="formula" id="fm0010">
This is the first observation of the decay
class="mathmlsrc">class="formulatext stixSupport mathImg" data-mathURL="/science?_ob=MathURL&_method=retrieve&_eid=1-s2.0-S0370269316301022&_mathId=si1.gif&_user=111111111&_pii=S0370269316301022&_rdoc=1&_issn=03702693&md5=a27d90cd9ed1c8a9305ee68cb10567d6" title="Click to view the MathML source">D0→K−π+μ+μ−class="mathContainer hidden">class="mathCode">. Its branching fraction is consistent with the value expected in the Standard Model.