Minuit vertex finder recent development

1. Matthew Cervantes Texas A&M University Minuit vertex finder recent development STAR Analysis Meeting: S&C

2. I. Importance of primary vertex reconstruction - Physical quantities used in analyses anchored to vertex location - Associating the event vertex to the event that fired a “trigger” II. Electronic readout limits of STAR detector - TPC (80 micro-seconds) and EMC (10 nano-seconds) readouts - High luminosity and collision rates lead to “vertex pile-up” III. STAR uses two vertex finding algorithms - Minuit Vertex Finding (MinuitVF)‏ - Pile-Up Proof Vertexer (PPV)‏ Introduction STAR Analysis Meeting: S&C

3. I. High-quality tracks for vertex reconstruction - Vertex candidates found via z-distribution of DCA to beam-line. - Tracks require radial DCA to the beam-line less than 2 cm. II. Crude peak-search performed w/ these tracks - Candidate vertices have at least 5 tracks that extrapolate back to within 6 cm of z. III. Minuit minimization routine and vertex rank - Minimize DCA and robust potential used in 2 to reduce outlier track contributions. - After stable point reached (or max iterations) rank is assigned. MinuitVF Method STAR Analysis Meeting: S&C

4. IV. <dip angle>track - The angle between a track and z-axis. - Vertices reconstructed near the edge of TPC have fewer hits than those from the center: skewed <dip angle>track V. The ranking of a vertex candidate - <dip angle>track vs. z-position of vertex (see upper right) - # of tracks matched to energy deposits in the BEMC - # of tracks which cross the central membrane of the TPC MinuitVF Method (continue)‏ STAR Analysis Meeting: S&C

5. I. Detector readout and bunch crossings - Peak collision rates for dAu 2008 roughly 300 Hz. - TPC: up to 20 separate minbias collisions per event recorded. - A TPC readout less than the collision rate will lead to “pile- up”. - Pile-up vertices are proportional to instantaneous luminosity. - Increased luminosity will only exacerbate the pile-up issue. II. There are three types of pile-up that occur - Pre-crossing pile-up, post-crossing pile-up and “bucket” pile-up. - Bucket pile-up vertices are due to collisions within the same bunch crossing. Can not be distinguished/removed from data. Detector limits, collisions and Pile-Up STAR Analysis Meeting: S&C

6. The three types of pile-up vertices cartoon: - fig. c: Bucket pile-up is not separable - fig. b: Pre-Crossing occurs before triggered vertex. - fig. a: Post-Crossing (red pile-up vertex occurs after the blue triggered vertex). Pre and Post-Crossing Tracks STAR Analysis Meeting: S&C

7. I. PCT rejection currently implemented into the PPV package. II. PCT rejection needs to be implemented into the MinuitVF package and then tested. III. Inspection of PPV package and “Vertex threads” have yielded insight into how to go about doing this. Towards MinuitVF PCT rejection STAR Analysis Meeting: S&C

8. I. We have put a piece of code similar to the PPV function “isPostCrossingTrack()” into the Minuit VF, then ran the bfc.C() over a 2008 dAu 200 GeV daq file. bfc.C'(1,100,"DbV20080712 B2008a ITTF IAna hitfilt ppOpt VFMinuit l3onl emcDY2 fpd ftpc trgd ZDCvtx NosvtIT NossdIT Corr4 analysis OSpaceZ2 OGridLeak3D beamLine debug logger”, "/star/data05/scratch/mcc/VertexFinder/rosi_dAu2008_daqfile/st_physics_9026071_raw_1010017.daq")' II. MuDst Tree from the daq output then used to create an ntuple of interesting quantities (see Rosi's previous “vertex blog”) for future study. III. Need to study rejected PCT vertex distributions vs. un-rejected PCT vertex distributions. MinuitVF PCT rejection STAR Analysis Meeting: S&C

9. I. Comparison of 100 dAu events appears to shift the “# of vertices” distribution in the expected direction (decrease). II. An in-depth study of the effects of PCT vertex rejection will follow as I learn the details of how MinuitVF works. MinuitVF PCT rejected vertex STAR Analysis Meeting: S&C