Fig. 1. Central part of the PHOBOS detector - no data possible - drawing only Fig. 2 Determination of the vertex position using all combinations ... - no data possible - drawing only Fig. 3 The correlation of the difference between the position of the hit and the position of the primary vertex - 3D plot - practically no data possible Fig. 4 Distributions of the distance between the primary vertex and the hits - example histograms for illustration only Fig. 5 The vertex reconstruction efficiency and fraction of wrongly reconstructed events a) for specrometer 2D+1D Zv efficiency wrong events fraction -52.5 0.0242826 0.0463576 -47.5 0.353535 0.19697 -42.5 0.382932 0.131291 -37.5 0.447983 0.07431 -32.5 0.454545 0.0885167 -27.5 0.516667 0.0619048 -22.5 0.578125 0.0379464 -17.5 0.590129 0.0128755 -12.5 0.594937 0.035865 -7.5 0.624242 0.020202 -2.5 0.682875 0.00845666 2.5 0.620455 0.00454545 7.5 0.621749 0.00472813 12.5 0.640082 0.0 17.5 0.478673 0.0 22.5 0.0904977 0.0 b) for vertex detector Zv efficiency wrong events fraction -22.5 0.377232 0.0334821 -17.5 0.56867 0.0300429 -12.5 0.723629 0.0443038 -7.5 0.751515 0.131313 -2.5 0.788584 0.175476 2.5 0.770455 0.159091 7.5 0.761229 0.0992908 12.5 0.736196 0.0449898 17.5 0.620853 0.0308057 22.5 0.391403 0.0294118 c) for octagon Zv efficiency wrong events fraction -57.5 0.839002 0.00453515 -52.5 0.935982 0.0110375 -47.5 0.934343 0.020202 -42.5 0.958425 0.0131291 -37.5 0.96603 0.00636943 -32.5 0.954545 0.00956938 -27.5 0.952381 0.0119048 -22.5 0.928571 0.015625 -17.5 0.935622 0.0128755 -12.5 0.953587 0.0105485 -7.5 0.955556 0.0141414 -2.5 0.930233 0.00634249 2.5 0.936364 0.0227273 7.5 0.955083 0.0070922 12.5 0.9591 0.00817996 17.5 0.936019 0.0260663 22.5 0.934389 0.0135747 27.5 0.949153 0.0105932 32.5 0.944568 0.00886918 37.5 0.970852 0.00672646 42.5 0.930693 0.0247525 47.5 0.937811 0.0174129 52.5 0.929782 0.0145278 57.5 0.836559 0.00430108 Fig. 6 The vertex reconstruction efficiency as a function of the number of primary charged particles present in the subdetector used ... spectrometer (solid line) primaries efficiency 1 0.00941176 2 0.0956522 3 0.641667 4 0.814433 5 0.907692 6 0.911765 7 0.980392 8 0.953488 9 0.947368 10 0.977273 11 1.0 12 1.0 13 1.0 14 1.0 15 1.0 16 1.0 17 1.0 18 1.0 19 1.0 20 1.0 21 1.0 22 1.0 23 1.0 24 1.0 25 1.0 26 1.0 27 1.0 28 1.0 29 1.0 30 1.0 31 1.0 32 1.0 33 1.0 34 1.0 35 1.0 36 1.0 37 1.0 38 1.0 39 1.0 40 1.0 vertex detector (dashed line) primaries efficiency 1 0.0 2 0.0 3 0.0581395 4 0.465753 5 0.737705 6 0.87037 7 0.875 8 0.828571 9 0.709677 10 0.806452 11 0.8 12 0.857143 13 0.866667 14 0.941176 15 1.0 16 1.0 17 1.0 18 1.0 19 1.0 20 1.0 21 1.0 22 1.0 23 1.0 24 1.0 25 1.0 26 1.0 27 1.0 28 1.0 29 1.0 30 1.0 31 1.0 32 1.0 33 1.0 34 1.0 35 1.0 36 1.0 37 1.0 38 1.0 39 1.0 40 1.0 octagon (points) primaries efficiency 4.5 0.26087 6.5 0.413793 8.5 0.807692 10.5 0.733333 12.5 0.8 14.5 0.863636 16.5 0.956522 18.5 0.923077 20.5 0.862069 22.5 0.857143 24.5 1.0 26.5 0.9 28.5 0.941176 30.5 1.0 32.5 0.923077 34.5 0.928571 36.5 1.0 38.5 1.0 Table 1 Vertex position accuracy and efficiency for Au+Au collisions at sqrt(sNN)=200 GeV in the vertx range limited to |Zv|<10 cm. 15% most central events: Method err(Xv) err(Yv) err(Zv) efficiency (%) Spectrometer 3D 0.015 0.022 0.020 100 Spectrometer 2D+1D 0.025 0.022 0.030 100 Vertex detetector - 0.015 100 Octagon - 0.500 100 30% most peripheral events: Method err(Xv) err(Yv) err(Zv) efficiency (%) Spectrometer 3D 0.350 0.100 0.300 4 Spectrometer 2D+1D 0.150 0.150 0.250 7 Vertex detetector - 0.030 0.023 28 Octagon - - 1.100 85 Table 2 The efficiency of the vertex reconstruction algorithms for different collision typews Efficiency of the method (%): System energy [GeV] Nch spec. 2D+1D vertex octagon Au+Au 200 947 64.2 76.8 94.4 Au+Au 19 322 57.5 72.7 89.3 Cu+Cu 200 296 49.0 64.9 88.9 d+Au 200 66 12.4 39.9 86.4 p+p 200 16 1.4 6.4 53.4