Galatyuk, T.

Krebs, E.

Markert, J.

Stroth, J.

GSI Repository

Employing the CBM Micro Vertex Detector for Background Rejection inDilepton Analyses ∗E. Krebs1, T. Galatyuk2, J. Markert1, and J. Stroth1,3 for the CBM collaboration1Goethe-Universita¨t, Frankfurt, Germany; 2Technische Universita¨t Darmstadt, Germany; 3GSI Darmstadt, GermanyLow mass electron (muon) pairs are considered to beexcellent probes of the processes taking place in the in-terior of extreme states of matter formed in the collisionzone of heavy-ions. However, the reconstructed distribu-tion of of electron pairs contains in addition contributionsfrom mesons decaying after freeze-out and from combina-torial pairs. Single electron or positron tracks from incom-pletely detected γ-conversions and Dalitz decays of π 0-mesons are the most abundant source contributing to thesignificant combinatorial background. The excellent po-sition resolution of the Micro-Vertex Detector (MVD) ofthe CBM experiment and its proximity to the target offersa chance to reject efficiently the close pairs. This holdsin particular for the abundant case, in which the magneticfield of CBM bends the low momentum partner out of theCBM acceptance while the high momentum partner con-tributes to the combinatorial background of the invariantmass spectrum. We tried to improve the reconstruction thelow momentum partner by including points from the MVDinto the track reconstruction and to reject the pair based onthe reconstructed opening angle.Not Accepted Track Fragment Track Segment Global TrackTracks [%]01020304050607080 No Mvd2 Mvd 10/203 Mvd 5/10/204 Mvd 5/10/15/205 Mvd 8.5-24.5Figure 1: Track topology of dielectrons from π 0-Dalitz de-cays assuming different configurations of the MVD, i.e.no MVD or a MVD featuring different amount of sensorplanes, see text.Emission from a thermal source was assumed to simulateelectron pairs from meson decays for Au+Au reactions atSIS-100 and SIS-300 energies. The simulation parameterswere chosen such that the meson spectra are consistent withpT and rapidity distributions measured by NA49 [1]. Thepairs simulated with the Pluto[4] event generator are em-bedded into hadronic final states calculated with UrQMD.Two versions of the hadronic cocktail were used: one cock-tail simulates vector meson decay in vacuum, the secondone assumes an in-medium modification of the ρ0.∗This work is supported by BMBF (05P12RFFC7), HIC for FAIR, H-QM and GSI.]cGeV [reco pep0 0.5 1 1.5 2 2.5 3]° [Θ00.511.522.53Yield00.20.40.60.811.21.41.6-10×(a) ρ0]cGeV [reco pep0 0.5 1 1.5 2 2.5 3]° [Θ00.511.522.53Yield012345-10×(b) π0-DalitzFigure 2: Opening angle vs. momentum of an identifiedelectron and its nearest reconstructed track for pairs from(a) ρ0 decays and (b) π0-Dalitz decays.Figure 1 shows the track type of the low momentum part-ner of an e+/e−-pair from a π0-Dalitz decay, where thehigh momentum partner was reconstructed and identifiedin the RICH. We distinguish track fragments (only indi-vidual hits are seen), track segments (also charge and mo-mentum were reconstructed) and global tracks (also parti-cle ID was reconstructed). The number of reconstructedtrack segments was found to increases substantially withthe amount of MVD stations. This comes with draw backsin terms of increasing computing time. In Figure 2, theopening angle between an identified electron and its nearesttrack segment is correlated to the product of the momentaof these two tracks. A wedge cut can be applied to reducethe background [2] [3]. The additional number of recon-structed low-momentum electrons increases the number ofbackground pairs, which is rejected by this cut. This sug-gests that the MVD might help to reduce the related back-ground and that this capability is getting more pronouncedwith an increasing number of MVD-stations.Further studies are needed to conclude on the optimalnumber and positions of MVD stations and the optimalmagnetic field configuration. The impact of the numerousδ-electrons emitted from the target remains to be taken intoaccount. Finally, the rejection strategy for γ-conversionsoccurring in the additional MVD stations has to be opti-mized.References[1] C. Alt et al. (NA49 Collaboration), Phys. Rev. C 78, 044907(2008)[2] T. Galatyuk, PhD Thesis, Goethe-Universita¨t, Frankfurt amMain, (2009)[3] E. Lebedeva, GSI-SR2012-PHN-NQM-EXP-54, (2012)[4] I. Froehlich et al., arXiv:0708.2382GSI SCIENTIFIC REPORT 2013 NQM-CBM-39doi:10.15120/GR-2014-1-NQM-CBM-39 55

Employing the CBM Micro Vertex Detector for Background Rejection in Dilepton Analyses

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Employing the CBM Micro Vertex Detector for Background Rejection in Dilepton Analyses

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