Galatyuk, T.

Krebs, E.

Stroth, J.

GSI Repository

Background rejection in the dilepton analysis with the CBM-Micro VertexDetector∗E. Krebs1, T. Galatyuk2,3, and J. Stroth1,3 for the CBM collaboration1Goethe-Universita¨t, Frankfurt, Germany; 2Technische Universita¨t Darmstadt, Germany; 3GSI Darmstadt, GermanyThe light vector mesons ρ, ω and φ are excellent probesof the strongly interacting matter under extreme conditions.Their leptonic decay channels of interest as leptons leavethe hot and dense fireball without strong interaction andmay reveal information on the characteristics of the mattercreated in the collisions. Single electron or positron tracksfrom incompletely detected γ-conversions and Dalitz de-cays of pi0-mesons are the most abundant source contribut-ing to the significant combinatorial background. This studyaims at exploring the use of MVD hits topology to reducethis background, despite of the fact that additional back-ground is produced due to the material budget of the MVD.To do so, electron pairs from meson decays have beensimulated from a thermal source for central Au+Au re-actions at SIS-100 energies such that the meson spectraare consistent with pT and rapidity distributions measuredby NA49 [1]. The decays of various sources simulatedwith the Pluto[2] event generator are embedded into thehadronic environment calculated with UrQMD transportmodel calculations. The magnetic field was set to 100%strength and δ-electrons with energies above 1 MeV havebeen added equivalent to a 10 kHz interaction rate.The strategy of background rejection comprises severalsteps. In order to identify leptons from photon conversionsthat were produced outside of the target region, each recon-structed track is extrapolated to the primary decay vertexand removed from the analysis depending on its deviationto the vertex. One characteristic for conversion pairs is theirsmall opening angle. A wedge cut is applied taking into ac-count the opening angle of an identified electron to its clos-est neighbor with particle identification and product of themomenta of the two tracks. As lepton tracks from back-ground sources can predominantly be found at low trans-verse momenta such tracks are rejected as well [3].The MVD of the CBM experiment can further contributeto reduce this background by including points from theMVD into the track reconstruction. An improved rejec-tion of pairs originating from the target region could be ob-served. Previous studies have shown that the MVD stationsare also a source for γ conversions which can not be effec-tively rejected by the vertex extrapolation cut, especiallyin the first two stations closest to the target. Extrapolatingtracks to the first MVD station and requiring that they arein its acceptance helped to better suppress off-vertex tracksfrom γ-conversions and resulted in an improved signal-to-background ratio for the low mass vector mesons ρ, ω and∗This work has been supported by BMBF (05P12RFFC7), HIC forFAIR, HGS-HIRe, H-QM, and GSI.Mass Range[GeVc2]MVDStationsS/B S/√S + B0− 0.15 0 6.56 31.34 8.27 29.40.15− 0.6 0 0.10 3.74 0.14 3.70.6− 1.2 0 0.15 4.94 0.21 5.4ω → e+e− 0 0.67 7.44 0.96 8.0φ→ e+e− 0 0.13 0.744 0.19 0.86Table 1: Signal-to-background ratios and significance fordilepton decays in different mass regions.φ as depicted in Tab. 1. The invariant mass spectrum of thefull cocktail after all analysis steps is presented in Fig. 1.]2cGeV [eeM0 0.2 0.4 0.6 0.8 1 1.2 1.4-1 ]2 [GeV/ceedN/dM-610-510-410-310-210Signal with MVDSignal without MVDBackground with MVDBackground without MVDωρφη-Dalitzω0piFigure 1: Invariant mass spectrum after all cuts are appliedfor central AuAu collisions at 8AGeV.There have been major updates to the CBM softwarewith more realistic digitization and geometries for theCBM detectors, including the MVD [4]. The effects onthe dielectron reconstruction are under investigation.References[1] C. Alt et al. (NA49 Collaboration), Phys. Rev. C 78, 044907(2008)[2] I. Froehlich et al., arXiv:0708.2382[3] T. Galatyuk, PhD Thesis, Goethe-Universita¨t, Frankfurt amMain, (2009)[4] S. Amar-Youcef et al., Status of the CBM MVD simulationmodel, this reportGSI SCIENTIFIC REPORT 2014 MU-NQM-CBM-23DOI:10.15120/GR-2015-1-MU-NQM-CBM-23 59

Background rejection in the dilepton analysis with the CBM-Micro Vertex Detector

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Background rejection in the dilepton analysis with the CBM-Micro Vertex Detector

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