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Vertex reconstruction

Vertex reconstruction. Spiros Margetis (Kent State University) STAR Collaboration. Introduction. STAR needs several approaches Heavy Ion / High luminocity p-p Small pile-up vs Huge pile-up High / Low multiplicity Au+Au vs ‘no-touching’. Heavy Ion Program.

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Vertex reconstruction

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  1. Vertex reconstruction Spiros Margetis (Kent State University) STAR Collaboration

  2. Introduction • STAR needs several approaches • Heavy Ion / High luminocity p-p • Small pile-up vs Huge pile-up • High / Low multiplicity • Au+Au vs ‘no-touching’ STAR/ALICE Joint Software Workshop

  3. Heavy Ion Program • Vertex finder is based on a ‘seed’ finder plus an analytical LSM method combined with iterative outlier removal in each step (3 steps) • No simultaneous vertex/track fit • Quick/Simple but w/out sacrificing accuracy STAR/ALICE Joint Software Workshop

  4. Seed finder • Tracks are propagated to the beam-line • A density array is build in z steps (0.5cm) • The maximum is determined • The vertex ‘seed’ is put at ( 0, 0, z_max ) STAR/ALICE Joint Software Workshop

  5. LSM/outlier removal • Tracks are moved to dca (perigee or 3D) from seed or iteration vertex. This can be recalculated in every step • (Some) MCS is taken into account • Track model is linearized for short distances • A dca cut removes outliers from the fit • Weighted dca removal of single tracks in peripheral • A vertex and full error matrix is calculated based on a global minimization of track’s dca • Repeat three times STAR/ALICE Joint Software Workshop

  6. Primary tracks selection is based on a (relaxed) dca criterion (3cm) • FTPC and/or SVT can determine and store a vertex which is determined by them • Finder is ran on TPC only tracks to define a vertex for FTPC use • Track refit is performed in an independent fitting step STAR/ALICE Joint Software Workshop

  7. Performance • For central Au+Au and TPC only vertex resolutions in x,y,z are about 200 microns • For TPC+SVT drop to 50 microns • Relatively stable operation (lately!) STAR/ALICE Joint Software Workshop

  8. Plans • Merge low/high multiplicity finders • Upgrade outlier removal method • Take errors into account • Replace propagation routines with universal routines in a certain framework (Geane, Kalman …) • For fun only, we might use the ‘full’ fitting at some point STAR/ALICE Joint Software Workshop

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