Agenda : Electron-Id meeting -------------------------------- Wednesday, July 26th 1995 10:30 AM Farside 1) CC/EC cog biases E. Flattum 2) EM vertexing P. Tamburello 3) EM_FIX package status M. Narain ============================================================================= 1) CC/EC cog biases : < text not yet available> if you need a copy of the transparencies please send mail to fnald0::meena ============================================================================= 2) EM vertexing in D0FIX and vertex reconstruction efficiency (P. Tamburello) It has been shown that D0RECO incorrectly selects the primary vertex in more than 20% of Z->ee events with two or more reconstructed vertices. Consequently, some electrons are not reconstucted properly because the road around the EM calorimeter cluster and false vertex does not include the electron's track. Often the correct track exists and can be identified outside the road. Furthermore, such tracks provide important information about the location of the true primary vertex [1]. This note describes results obtained using a routine written for the D0FIX package to select the reconstructed vertex preferred by electron candidates. The procedure is as follows. For a given CC (EC) EM cluster all available CDC (FDC) tracks are examined and the one which best matches the calorimeter cluster position is chosen. The z-intercept of the electron's trajectory is determined using the center of mass position of the track and the calorimeter cluster position. The resolution of the z-intercept computed in this way is about 2 cm compared to 3 cm using the track theta. If the nearest reconstructed vertex is within 10 cm of the z-intercept that vertex is flagged. The efficiency of this procedure was studied in three data sets. The first consists of 490 events of ISAJET/FULL_GEANT/RECO_12.15 Monte Carlo. The second is the same as the first set with one additional interaction per event added from an ISAJET MIN_BIAS/TWO_JET mixture. The third set is approximately 65 pb-1 of run 1b data from the top group di-electron micro- dst sample. For this analysis good EM clusters are PELC or PPHO objects with Pt > 20 GeV (as computed by D0RECO and CORRECTEM), chi2 < 100 and isolation fraction < 0.15. Good tracks have a track match significance < 10. Events with two good CC EM clusters and at least two reconstructed vertices are selected and classified according to whether one, both or neither of the clusters have good tracks. The number of Z events with mass between 80 and 100 GeV is estimated with a sideband subtraction. Before the fix algorithm is applied only tracks in the original road are considered and four momenta are calculated based on the 1st vertex. Table 1a shows the numbers of Zs found in this way. Also shown is the efficiency to find a good track based on the ratio of 2, 1 and 0 track events to the total. After including all tracks the mass is calculated using the vertex chosen by the first cluster, if any, otherwise the vertex chosen by the second cluster and, failing that, the first vertex. These results are given in Table 1b. Comparison shows that the total number of events identified as Zs increases from 984 to 1135 after the correction despite there being the same total events in each case. This can be attributed to an improvement in the resolution obtained by choosing the correct vertex more often. The fraction of clusters with good tracks increases from 68% to 75%. After the fix, for each event with two good tracks the average z-intercept is calculated for events in which the two intercepts are within 10 cm of each other. If this average z is within 10 cm of the 1st reconstructed vertex then that vertex is assumed to be correct. Table 1c shows how many Z events have the correct vertex found depending on whether the clusters were originally designated PELC or PPHO. The vertex selection efficiency is about 73%. This is comparable to the value of 78% found in [1] with a similar study. The analogous results for the Monte Carlo with a second interaction are given in Table 2. The vertex selection efficiency from Table 2c is (77 +/- 8)%. The first reconstructed vertex is within 10 cm of the isajet vertex n (81 +/- 4)% of the events with two good clusters. The vertex flagged by good EM clusters is the closest one to the Monte Carlo vertex more than 99% of the time. References 1. S. Glenn, Electron ID Meeting, 7/12/95 Table 1a. Conventional CC/CC, No. Vertices > 1 NO. OF ZS FOUND TRACKING EFFICIENCY 2 Tracks 494.0000 +- 5.477226 0.7085425 +- 0.1239131E-01 1 Track 353.0000 +- 7.280110 0.7657632 +- 0.3131397E-01 0 Tracks 137.0000 +- 11.22497 0.6268678 +- 0.1986971E-01 Total 984.0000 +- 14.45683 No. of 2 Track Events with ZNEAR_ID=1 465.0000 +- 5.291502 94.12955 % Table 1b. Corrected CC/CC, No. Vertices > 1 NO. OF ZS FOUND TRACKING EFFICIENCY 2 Tracks 638.0000 +- 6.082763 0.7497430 +- 0.1067492E-01 1 Track 425.0000 +- 7.071068 0.7505501 +- 0.3060314E-01 0 Tracks 72.00000 +- 9.380832 0.7481347 +- 0.2105950E-01 Total 1135.000 +- 13.22876 Table 1c. Events with a Z selected vertex 1st Vertex Selected Total PELC/PELC 424.0000 +- 5.196152 468.0000 +- 5.291502 PELC/PPHO 10.00000 +- 1.000000 46.00000 +- 1.000000 PPHO/PPHO 0.1000000E-02+- -1.000000 82.00000 +- 1.414214 Totals 434.0010 +- 5.385165 596.0000 +- 5.567764 Table 2a. Conventional CC/CC, No. Vertices > 1, Monte Carlo NO. OF ZS FOUND TRACKING EFFICIENCY 2 Tracks 24.00000 +- 1.000000 0.8944123 +- 0.5069048E-01 1 Track 6.000000 +- -1.000000 0.8873026 +- 0.1018309 0 Tracks 0.1000000E-02+- 1.732051 0.9942266 +- 5.000583 Total 30.00100 +- 2.236068 No. of PELC/PELC with ZNEAR_ID=1 21.00000 +- 1.000000 87.50000 % Table 2b. Corrected CC/CC, No. Vertices > 1, Monte Carlo NO. OF ZS FOUND TRACKING EFFICIENCY 2 Tracks 28.00000 +- 1.000000 0.8819049 +- 0.4307679E-01 1 Track 8.000000 +- -1.000000 0.8726822 +- 0.9807481E-01 0 Tracks 0.1000000E-02+- -1.000000 0.9947296 +- 2.636439 Total 36.00100 +- 1.732051 Table 2c. Events with a Z selected vertex 1st Vertex Selected Total PELC/PELC 19.00000 +- 1.000000 20.00000 +- 1.000000 PELC/PPHO 1.000000 +- -1.000000 3.000000 +- -1.000000 PPHO/PPHO 0.1000000E-02+- -1.000000 3.000000 +- -1.000000 Totals 20.00100 +- 1.732051 26.00000 +- 1.732051 ============================================================================= 3) EM_FIX package status (M. Narain) < text not yet available> if you need a copy of the transparencies please send mail to fnald0::meena