Sellheim, P.

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Backtracking algorithm for lepton reconstruction with HADES ∗P. Sellheim1 for the HADES Collaboration1Goethe-Universita¨ Frankfurt, Frankfurt am Main, GermanyEvents recorded in Au+Au collisions at a beam en-ergy of 1.23 GeV/u have the highest multiplicities mea-sured with HADES [1]. The track reconstruction andparticle identification in the high track density envi-ronment are challenging. In case of dileptons: (i) ARing Imaging Cherenkov detector (RICH) [2] is essen-tial since it is the most important detector componentfor single lepton identification; (ii) one has to cope withlarge combinatorial background stemming from γ con-version and pi0 Dalitz decays.In order to further improve the purity and efficiency of anelectron sample, a new backtracking algorithm using infor-mation provided by tracking and Time-of-Flight detectorsis applied. Furthermore in the special case of close tracks,with opening angles smaller than 4◦, rings are overlappingand do not have a perfect ring shape. Thanks to its ex-cellent double hit resolution, by using the expectation ofnearby tracks backtracking will be able to resolve such pat-terns and identify both leptons.Figure 1: Schematic side view of the RICH detector. Apositron track shows usage of angular information to deter-mine a ring center.In detail, only tracks preselected by velocity informationprovided by Time-of-Flight detectors and energy loss mea-sured in Multi-wire drift chambers are used as possible lep-ton candidates. Those candidates are used to predict a pos-sible ring on the RICH pad-plane based on angular trackinformation ( see Fig.1). Out of it the centroid position onthe RICH photon detection plane is fixed. Using a simula-tion based ring parametrization, potential areas with a highphoton hit probability are predicted.An example of a ring prediction is shown in Figure 2. Twopredicted rings and photon hits measured by the photon de-tection plane are plotted. The most basic observables arethe sum of charge, pads and clusters. Pads where horizon-tal and vertical neighbors have a lower charge than the pad∗This work has been supported by BMBF (05P12RFGHJ), GSI,Helmholtz Alliance EMMI, HIC for FAIR, HGS-HIRe and H-QM.itself are called maxima and are summed up as well. Ad-ditionally a ring quality based on the difference betweenthe actual photon hit and the most probable hit position iscalculated. For special cases of overlapping rings, informa-tion of shared maxima is also considered. An application ofFigure 2: Overlapping rings predicted by ring parametriza-tion. Fired RICH Pads are indicated by dark red boxes andthe maximum position of clusters is marked with crosses.the backtracking algorithm to simulated and real data hasshown that the number of pads per ring and maxima perring are the most powerful to distinguish between leptonsand hadrons. Especially maxima are a good measure sincethey correspond to the number of photons per ring.Finally, backtracking was also tested for close pair rejec-tion in simulated data. Above 2◦ ring predictions are notoverlapping anymore. Therefore backtracking informationis useful to identify the close partner of a lepton in orderto reject close pairs. Figure 3 demonstrates a further re-jection of combinatorial background due to a reduction ofclose pairs. In contrast the signal of ω pairs is not reducedsignificantly.Figure 3: Cut efficiency of close pair cuts. Additionallyto a close neighbor cut demonstrated in the second columnbacktracking information is used in columns 3 and 4 forfurther close pair rejection.References[1] G. Agakishiev and the HADES collaboration , Eur.Phys.J.A41 (2009) 243-277[2] K. Zeitelhack and the HADES collaboration, Instr. Meth. A433 (1999) 201-206MU-NQM-HADES-07 GSI SCIENTIFIC REPORT 20148 DOI:10.15120/GR-2015-1-MU-NQM-HADES-07

Backtracking algorithm for lepton reconstruction with HADES

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Backtracking algorithm for lepton reconstruction with HADES

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