A new fast track-fit algorithm based on broken lines - Desy

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1. Track measurement in particle physics experiments e Run 270530 Event 157876 25/04/2000 e’ p 2 m K + π − π − 5 cm 5 mm D − 50 cm e’ Charm event in the H1 detector at HERA with drift chamber and vertex detector. LHC (2007 –): 1000 <strong>track</strong>s per event, several events per bunch crossing (bunch spacing 25 ns) Fast <strong>track</strong> reconstruction is essential. V. Blobel – University of Hamburg A <strong>new</strong> <strong>fast</strong> <strong>track</strong>-<strong>fit</strong> <strong>algorithm</strong> <strong>based</strong> on broken lines page 2
Track parametrization Ideal parametrization: a helix (κ, dca, φ, z0, tan λ) in a homogeneous magnetic field Bz with xy- or rφ-plane: circle, residual εi of a measured point (xi, yi) from a particle trajectory εi = 1 2 κ � x 2 i + y 2 i + d 2 � ca − (1 + κdca) (xi sin φ − yi cos φ) + dca . sz-plane: straight line, residual εi of a measured point (si, zi) εi = z0 + (tan λ) · si − zi with si = <strong>track</strong> length in xy . . . but there are random and non-random perturbations: Multiple scattering (msc): Elastic scattering of charged particles in the Coulomb field of the nuclei in the detector material. The mean of the projected deflection angle θ after traversing a material layer is zero and the distribution has a variance of Multiple scattering deflections will influence all downstream measurement in a correlated way, and delimits the momentum measurement at low mo- variance V [θ] ∝ t β 2 p 2 where t = ∆s/X0 menta. si−1 si si+1 si+2 × yi−1 ± σi−1 Energy loss, Radiation of electrons, Field inhomogeneity, . . . × yi ± σi × yi+1 ± σi+1 × yi+2 ± σi+2 V. Blobel – University of Hamburg A <strong>new</strong> <strong>fast</strong> <strong>track</strong>-<strong>fit</strong> <strong>algorithm</strong> <strong>based</strong> on broken lines page 3
Page 1: Zürich 3rd - 7th October 2005 A <s
Page 5 and 6: 2. Track fitting m
Page 7 and 8: 3. Track-fit <stro
Page 9 and 10: Two phases in the track</st
Page 11 and 12: First phase: “Straight line” tr
Page 13 and 14: (b) Fit of circle residuals versus
Page 15 and 16: Second phase: Track parameters The
Page 17 and 18: Accuracy vs momentum Lutz detector
Page 19 and 20: Backup slides • 200 MeV/c <strong
Page 21 and 22: . . . with broken-line fit<
Page 23 and 24: Pulls of position measurement The p
Page 25 and 26: Outlier example 1 200 MeV/c, 6σ ou
Page 27 and 28: Outlier example 3 200 MeV/c, 6σ ou
Page 29 and 30: A large-angle scatter A 1 GeV/c <st

A new fast track-fit algorithm based on broken lines - Desy

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