Bozkurt, V.

Rappold, C.

Saito, T.

GSI Repository

New secondary vertex finding procedure for the HypHI projectC.Rappold∗1,2, T.R. Saito1,3,4, and V. Bozkurt11GSI, Darmstadt, Germany; 2Justus-Liebig-Universita¨t Giessen, Germany; 3Johannes Gutenberg-Universita¨t, Mainz,Germany; 4The Helmholtz Institute Mainz, Mainz, GermanyThe first experiment of the HypHI collaboration aimedto demonstrate the feasibility of the hypernuclear spec-troscopy by means of heavy ion beam induced reactions.Later, a second experiment of the HypHI collaboration wasperformed using a 20Ne beam at 2AGeV on a 12C tar-get. In the track reconstruction of the experimental dataof this second experiment, the track multiplicity is about5 to 10 times more that in the first experiment. New cri-teria for the track and event reconstruction had to be im-plemented in order to reduce the background contribution.In the event reconstruction, the vertex finding procedureis based on the geometrical closest distance approach be-tween the daughter track candidates. An additional algo-rithm has been implemented in order to have another cri-teria on the vertex quality. While, a vertex fitting proce-dure is the best step to judge the goodness of the vertexreconstruction, the speed of the vertex fitting procedure isrelatively slower than any vertex finding step. Thus a firststep is set to reduce the number of vertex candidates. Forthis purpose a more advance vertex finding step was im-plemented based on the algorithm described in publication[3]. The algorithm principle is the consider position co-variance matrix of the track, obtained after the track fitting,to create of tube representation in 3D around the track asa 1σ standard deviation in the 3D space. At a given z-position, an ellipse corresponding to the covariance is cal-culated. With this tube representation, a probability of be-ing close to the given track can be calculated, denoted asf(r) = exp(−0.5(r− rtrack)TC−1(r − rtrack)). Then avertex position vfrom where each considered track origi-nate will have to satisfy the criteria that it has to be close tothe track tubes. For this, a vertex function :V (v) =n∑i=0fi(v) −∑ni=0 f2i (v)∑ni=0 fi(v)(1)is used to defined a probability to be as close as possible tothe all n considered track tubes. By finding the maximumof the probability vertex distribution, the improved vertexposition can be determined. Additionally a covariance ma-trix of the maximization procedure can be inferred and as-sociated to the vertex position. During this maximization ofV (v) procedure, if the track set does not have a maximum,the vertex candidate is then rejected since it corresponds ofthe case where the track tubes are not close enough to de-fine an non-null value for V (v). Figure 1 shows the tracktubes for two different tracks and the obtained probabilitydistribution V (v). The calculation are performed in 3D,∗ c.rappold@gsi.dehowever for easier representation the projection to XY, XZand YZ global axis are shown in Figure 1.Figure 1: Profiles of the two 3D track tubes are shown inpanel (A), (B) and (C), and profile of vertex probability ob-tained by the algorithm of [3] are shown in panel (D), (E)and (F). In panel (A), (B) and (C), the solid black lines arethe tracks and the profiles corresponds to the tubes formedby the 1-σ standard deviation from the position covariancematrix of the associated track. In panels (D), (E) and (F),the dash lines represent the track, the cross marker the seedof the geometrical closest distance approach calculationand diamond marker the position of the highest probabil-ity obtained by the algorithm of [3]. In those panels, theprofile represents the probability distribution.References[1] C. Rappold et al., Nucl. Phys. A 913, 170 (2013).[2] C. Rappold et al., Phys. Rev. C 88, 041001 (2013).[3] D.Jackson et al. Nucl Instr. Meth. A 388, 247 (1997).GSI SCIENTIFIC REPORT 2014 MU-HADRONS-02DOI:10.15120/GR-2015-1-MU-HADRONS-02 109

New secondary vertex finding procedure for the HypHI project

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New secondary vertex finding procedure for the HypHI project

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