Backtracking algorithm for lepton reconstruction with HADES

Events recorded in Au+Au collisions at a beam energy of 1.23 GeV/u have the highest multiplicities measured with HADES [1]. The track reconstruction and particle identification in the high track density environment are challenging. In case of dileptons: (i) A Ring Imaging Cherenkov detector (RICH) [2] is essential since it is the most important detector component for single lepton identification; (ii) one has to cope with large combinatorial background stemming from γ conversion and π Dalitz decays. In order to further improve the purity and efficiency of an electron sample, a new backtracking algorithm using information provided by tracking and Time-of-Flight detectors is applied. Furthermore in the special case of close tracks, with opening angles smaller than 4◦, rings are overlapping and do not have a perfect ring shape. Thanks to its excellent double hit resolution, by using the expectation of nearby tracks backtracking will be able to resolve such patterns and identify both leptons.