A study of the decay thmlsrc">text 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−thContainer hidden">thCode">th altimg="si1.gif" overflow="scroll">D0tretchy="false">→K−π+μ+μ−th> 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 thmlsrc">text 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−1thContainer hidden">thCode">th altimg="si2.gif" overflow="scroll">2.0text> fbtext>−1th>. Decay candidates with muon pairs that have an invariant mass in the range thmlsrc">text 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/c2thContainer hidden">thCode">th altimg="si102.gif" overflow="scroll">675text>–text>875text> MeVtext>tretchy="false">/c2th> are considered. This region is dominated by the thmlsrc">text 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">ρ0thContainer hidden">thCode">th altimg="si11.gif" overflow="scroll">ρ0th> and ω resonances. The branching fraction in this range is measured to be
This is the first observation of the decay thmlsrc">text 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−thContainer hidden">thCode">th altimg="si1.gif" overflow="scroll">D0tretchy="false">→K−π+μ+μ−th>. Its branching fraction is consistent with the value expected in the Standard Model.