GArSoft Update: Hits and Tracks Tom Junk HPGTPC Meeting July 13, 2018
Two Zooms on a GENIE veCC Event in the Middle of the Detector Hit-finder is a simple threshold algorithm Hits are located at the pad (y,z) locations, and arrival time gives x. Grid pattern noticeable on tracks. Distribution in x is much narrower 2 July 13, 2018 Tom Junk | GArSoft Update
From June 22: Track Finder and Fitter • Start with Hits à Make Tracks • Pattern Recognition Step: Want homogeneity and isotropy - Sort hits in X (drift direction = E field direction – B field direction) - Loop over sorted hits. Add hits to tracks if hit is near the last one added to that tracks. Make a new track if hit is too far from any track in the list - Rather naive – follows kinks, may have delayed split, may get confused by delta rays • Extended Kalman filter track fitter - Initial version included in tracker1_module.cc - Five track parameters – as a function of x, have (y, z, curvature, phi, slope) 3 July 13, 2018 Tom Junk | GArSoft Update
Tracker To-Do from June 22 • Fit tracks in both directions. In a collider, we know which way the tracks go. Here, not so much. • Add parameters evaluated at both ends or along the way to the track data product. Add track-hit associations. • Look at displaying tracks in the event display • Cluster hits into "clusters" a la ALICE. Diffusion spreads out charge among many pads. Clusters are easier to track than pad hits. • Improve pattern recognition - Start new tracks if angle changes too much – especially important near primary vertex - Prune delta rays. - Deal with very low Pt curlers (are they really straight or corkscrews?) 4 July 13, 2018 Tom Junk | GArSoft Update
Initial Try at Pattern Recognition GArSoft's event display Curves are MC truth. Colored dots are patrec-clustered hits. draws tracks by drawing associated hits in different colors. No concept of helical track parameters in the event display (tracks in unmagnetized LAr are just collections of hits) Initial tracker does not add or subtract hits, just runs Kalman filter, and so evd just shows pattern-recognition step output. Not too bad, but the pair of close tracks gets split a lot 5 July 13, 2018 Tom Junk | GArSoft Update
Cluster Nearby Hits • ALICE clusters charge depositions on nearby channels before running tracking. Fewer objects to deal with, better resolution. • I added a step to the compressed hit finder (which just takes the zero-suppressed blocks and calls them "hits") by collecting together hits within 1 cm in x (along drift) and 5 cm in ( y,z ) (adjustable parameters), before creating hits for storage. • Charge centroids, charge sums, and RMS updated with component hits. • Hits are now no longer associated with a channel (channel = 0 now). • Could call the new objects "clusters" like ALICE, but there's already a cluster data product in GArSoft (we're not using it yet). • Hit clustering is switchable via fcl parameter 6 July 13, 2018 Tom Junk | GArSoft Update
Patrec on First Event with Clustered Hits • Much better! Curves are MC truth. Colored dots are patrec-clustered hits. Two views of the same event 7 July 13, 2018 Tom Junk | GArSoft Update
Still Not Perfect Near Vertex Zoomed further in. The green cluster overstays its welcome. It extends into the blue, purple, and yellow tracks, and continues along red and purple. Pattern recognition is a little too forgiving when tracks kink Also anisotropic – hits are sorted in x and stepped in just one direction 8 July 13, 2018 Tom Junk | GArSoft Update
Patrec Improvements • Address anisotropy and over-greediness of clusterer: - cluster in both directions in x - Look for disjoint subsets shared by both clusterings. - If a cluster is split in one direction and not the other, split it. - Now makes short clusters – to work on putting these back together! - Need to check the fitter first as extrapolating tracks to add hits requires precise track parameter measurements. - Track fitter has a minimum number of hits requirement • Should be a multi-stage approach: Find long, easy tracks first. Assign hits to them in crowded environments. Find vertex and use as a constraint if necessary. • Look for short tracks using remaining hits. 9 July 13, 2018 Tom Junk | GArSoft Update
Fitting Tracks • No event display of helical track parameters (yet). • 10 bars of gas à multiple scattering and energy loss are important. • Drawing a helix on top of hits will miss the points even when the parameters are correct at the end. • Which end? x , y , and z are in cm • Quote both! Kalman filter is the right tool as it gives track parameters at the end. Need to fit each track twice though. Maybe start with a single helix fit? • My own hacky track/hit display made from text dumps 10 July 13, 2018 Tom Junk | GArSoft Update
It didn't always work perfectly x , y , and z are in cm Curvature sign not correct on blue end. Red end – curvature probably too big. I had some bugs in my initial track parameter guesses to start off the Kalman fit. Debugged but still not quite perfect, especially when the track kinks at one end I checked ALICE's circle-from-three-points method – works! But the curvature sign was ALICE-specific Tunable parameters in the fit: uncertainties on the measured hit positions, and extrapolation uncertainties from one step to the next. 11 July 13, 2018 Tom Junk | GArSoft Update
Some Software Improvements • I added a new TrackPar class to have a place for tracking utilities - Method for finding the distance from a point to a helix • art data product design guide discourages adding lots of code to the class definitions of data to be stored on disk and tape. • I added a Vertex data product • I added 64-bit timestamps to RawDigit, Track, Vertex, and Cluster data products. Hit already has a time as a float (relative to some trigger). But tracks, clusters and vertices have their own times. • Near-term to do: write a vertex finder. Many examples from colliders assume some prior knowledge of where a vertex can be. 12 July 13, 2018 Tom Junk | GArSoft Update
Particle Charge • The charge of a particle depends on which way we propose it is going! • There's a two-fold ambiguity in how to specify track parameters in a detector where the tracks can start from any direction and go in any direction. • Need to assign a vertex first before one can assign a charge. • Event may have a primary vertex and a secondary interaction, making it at least a little ambiguous. • To think about – develop tools to provide interpretations of the event first. 13 July 13, 2018 Tom Junk | GArSoft Update
More Example Tracks x , y , and z are in cm 14 July 13, 2018 Tom Junk | GArSoft Update
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