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Nectarios Ch. Benekos CERN/ATLAS

Performance of the ATLAS ID Reconstruction. Nectarios Ch. Benekos CERN/ATLAS. EESFYE - HEP 2003 Workshop, NTUA, April 17-20, 2003. OUTLINE ATLAS Inner Detector Pattern Recognition Programs xKalman iPatRec Fitting Method in iPatRec Material Tuning Performance studies Conclusions.

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Nectarios Ch. Benekos CERN/ATLAS

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  1. Performance of the ATLAS ID Reconstruction Nectarios Ch. Benekos CERN/ATLAS EESFYE - HEP 2003 Workshop, NTUA, April 17-20, 2003

  2. OUTLINE • ATLAS Inner Detector • Pattern Recognition Programs • xKalman • iPatRec • Fitting Method in iPatRec • Material Tuning • Performance studies • Conclusions Nectarios Ch. Benekos EESFYE – HEP 2003 Workshop, NTUA, April 17-20, 2003 1

  3. Diameter 25 m Barrel toroid length 26 m Endcap end-wall chamber span 46 m Overall weight 7000 Tons ATLAS Coordinates XYZ right handed coordinate system with Z in beam direction Nectarios Ch. Benekos EESFYE – HEP 2003 Workshop, NTUA, April 17-20, 2003 2

  4. The ATLAS ID ATLAS Tracker A side view ID layout • Requirements of the ID Reconstruction: • to reconstruct efficiently the tracks and vertices in an event • to perform, together with the calorimeter and muon systems, electron,pion and muon identification • to find short lived particle decay vertices. Nectarios Ch. Benekos EESFYE – HEP 2003 Workshop, NTUA, April 17-20, 2003 3

  5. The updated ATLAS ID layout • Updated ID Layout: • main change is insertable pixel layout: • to accommodate construction delayed1 year later installation • consequences: • increased structural material (> 6m long cylinders) • >double material at low radius (insertable + realism) • b-layer: same modules as outer layers • pixel size increased from 50x300 mm2(TDR)50x400 mm2 • change of the b-layer radial position 4350.5 mm (due to the change in outer diameter beam pipe 5069.2 mm) • SCT small changes to forward layout • to increase inner radius in order to allow insertable pixels • TRT reduced straw length(occupancy) in endcaps • the continuous tracking of the TRT is approximated using 4 discrete layers The updated initial layout (low lumi) has: • only 2 pixel layers + • 2(+/-) pixel wheels instead of • 3 pixel layers + 3(+/-) pixel wheels Nectarios Ch. Benekos EESFYE – HEP 2003 Workshop, NTUA, April 17-20, 2003 4

  6. Requirements of any track reconstruction algorithm • Find the tracks of particles in the detector • Introducing the minimum number of fake tracks • Give best estimation of the tracks’ • actual momenta • direction, slope (cot (q)) of the track • Vertex finding • impact parameter estimation pattern recognition track fitting • Track fitting to minimize c2 • measures how close the measured parameters are to what they are assumed to be from a particular fit hypothesis (e.g., helical trajectory) • Track fitting would be trivial if it was not for complications arising because: • of multiple scattering • energy loss • non-uniform magnetic filed, ….and of course • IF we understood our detectors PERFECTLY. Nectarios Ch. Benekos EESFYE – HEP 2003 Workshop, NTUA, April 17-20, 2003 5

  7. ATLAS ID Pattern recognition algorithms • Two inner detector pattern recognition and track reconstruction packages based on two different techniques are existing in ATLAS: • xKalman is a pattern recognition package based upon a Kalman –filter smoother formalism for finding and fitting tracks in the inner detector. • iPatRec uses a helix fitting method. • Its basic strategy is to initiate track finding from space-points and fit these tracks using an iterative method based on Newton-Raphson technique Nectarios Ch. Benekos EESFYE – HEP 2003 Workshop, NTUA, April 17-20, 2003 6

  8. xKalman Nectarios Ch. Benekos EESFYE – HEP 2003 Workshop, NTUA, April 17-20, 2003 7

  9. iPatRec Nectarios Ch. Benekos EESFYE – HEP 2003 Workshop, NTUA, April 17-20, 2003 8

  10. iPatRec Searches for tracks using SP formed in Pixel and SCT • Reconstruction is performed within a “narrow canonical raod” joins Vxregion to a Sdregion on the outer surface of ID • Seeds can be: • e/g candidates from EM calo, • jets from HAD and, • muon tracks found in the external muon detectors. Tracks extension into TRT detector after passing quality cuts Track fitting using c2 minimization fit also TRT hits are included by a histogramming method in a narrow road around the reconstructed helix of the track Nectarios Ch. Benekos EESFYE – HEP 2003 Workshop, NTUA, April 17-20, 2003 1 Nectarios Ch. Benekos EESFYE – HEP 2003 Workshop, NTUA, April 17-20, 2003 8

  11. iPatRec: stand alone pattern recognition (cont.) • form space points from matching f and z hits : • find up to 7 space-points on different layers that might form a track • The points are required: • to be close enough azimuthally • to lie in a “conical narrow road” defined as a+b/pT • (multiple scattering term) • tracks extension into TRT detector after passing • quality cuts Nectarios Ch. Benekos EESFYE – HEP 2003 Workshop, NTUA, April 17-20, 2003 9

  12. Introduction to Track Fitting • The trajectory of a particle moving in a uniform magnetic field • with no multiple scattering and negligible bremsstrahlung radiation • is described by ahelix. • Basically a helix can be decoupled into: • moving along a circle in the xy-plane • (3 points needed to define it) and • in the rz plane by a straight line: • (2 points needed to define it) Nectarios Ch. Benekos EESFYE – HEP 2003 Workshop, NTUA, April 17-20, 2003 10

  13. Fitting a model to data using c2 minimization • In order to start fitting a track, one needs two things: • a model which approximates the trajectory of the tracks • an understanding of the detector accuracy(resolution) • Track fitting :is a procedure to determine the helix parameters by fitting a set of • coordinates(measurements) measured in a tracking detector to a helix. • We want to fit a model : • with M parameters aj • to a set of N uncorrelated measurements yi with error si. • fi(a) is the expected i-th coordinate when the helix parameter vector is • a[q/pT,tanq…] for yi Minimizing the c2 to determine the values of aj Nectarios Ch. Benekos EESFYE – HEP 2003 Workshop, NTUA, April 17-20, 2003 11

  14. Fitting a model to data using c2 minimization (cont.) • for a linear model : • the solution is independent of the starting estimator and • NO iteration is needed • for a non-linear model (helix) one needs to iterate. • it gives the correct answer • i.e. converges to the global minimum, if is • sufficiently close to • so called Newton-Raphson method Nectarios Ch. Benekos EESFYE – HEP 2003 Workshop, NTUA, April 17-20, 2003 12

  15. Generalization • This method is global in the sense that it fits all the measurements at the same time • IF all measurements are independent of each other, the execution • time is ~ number of measurements (n) • BUT • IF we have correlations between measurements the covariance matrix will contain non-diagonal terms • and inverting it becomes VERY time consuming for large n Nectarios Ch. Benekos EESFYE – HEP 2003 Workshop, NTUA, April 17-20, 2003 13

  16. Particle Interactions with matter - Energy Loss • The trajectory of a charged particle is affected by any material • several types of secondary interactions between particles and • material may occur. • Therefore energy loss and multiple scattering have to be applied to the track fitting. • at low energiesionization (described by Bethe-Bloch formula) dominates: • at high energies, bremsstrahlung dominates • Radiation length: • Mean distance over which • a high energy e- loses all but • 1/e of its energy • by bremsstrahlung. Nectarios Ch. Benekos EESFYE – HEP 2003 Workshop, NTUA, April 17-20, 2003 14

  17. Multiple Scattering in iPatRec c2-fit Mostly due to Coulomb scattering from nuclei Thickness of the scattering material in radiation lengths For small angles roughly Gaussian distribution • Multiple Scattering(MS) in Track Fitting • MS at the detector planes introduces additional parameters pMS, • i.e. the two (fitted) deflections (Df,Dcotq) at each detection plane: • pMS=(Df1,Dcotq1, Df2,Dcotq2,…,Dfn,Dcotqn) • Scattering centres are expensive typically # parameters = 2N+5 (5 track params + 2 x N scat. angles/scattering centre) • (instead of 5 params ,ignoring material effect) • The scattering processes in the different planes(centres) are independent • from each other The multiple scattering angles pMS + Helix pareameters p Full description of the path of a particle through the detector Nectarios Ch. Benekos EESFYE – HEP 2003 Workshop, NTUA, April 17-20, 2003 15

  18. Tuning Multiple Scattering in iPatRec • Method : • pulls on 5 perigee parameters • residual for a track parameter a: where atrack is the result of the fit • pull for a track parameter a is defined as: • tune material to give : • mean=0 (dE/dx) • sigma=1 (X0) IF the fit is reasonable and errors are correctly described Nectarios Ch. Benekos EESFYE – HEP 2003 Workshop, NTUA, April 17-20, 2003 16

  19. Tuning Multiple Scattering in iPatRec • Procedure : • need lowest Etrack material effects dominate • high statistics (to cut on limited region with uniform material) • start with tuning inner layers then work outwards • reduce # of layers  lower PT for material to dominate • start with barrel as already ~ 1/3 of phase-space (uniform material) • (|h|<0.8 , total acceptancy to 2.5) • Plots in the following using first 7 layers (Pixels + SCT) only • 1/PT • 1/PT pull • a0 (impact parameter d0) • a0 pull • Increase material - tuned to give all 5 parameters fitting correctly in barrel so plotting pulls can see IF errors are correct or over/under estimated ! Nectarios Ch. Benekos EESFYE – HEP 2003 Workshop, NTUA, April 17-20, 2003 16

  20. D(1/pT)/(1/pT) |h|<0.8 1.6<|h|<2.5 Well centered • single muons tracks pT =200 GeV/c • Pixel + SCT using iPatRec D(1/pT)/(1/pT) ~ 9% (~7% in TDR) in barrel ~ 20% (~15% in TDR) in endcap Nectarios Ch. Benekos EESFYE – HEP 2003 Workshop, NTUA, April 17-20, 2003 17

  21. D(1/pT)/(1/pT) |h|<0.8 1.6<|h|<2.5 D(1/pT)/(1/pT) ~ 1.8% in barrel ~ 2.7% (~3% in TDR) in endcap • single muons tracks pT =1 GeV/c • Pixel + SCT using iPatRec Increased material thickness ! Systematic shifts on mean dE/dX underestimated Nectarios Ch. Benekos EESFYE – HEP 2003 Workshop, NTUA, April 17-20, 2003 18

  22. Impact parameter resolution N N |h|<0.8 1.6<|h|<2.5 DRf DRf • single muons tracks pT =200 GeV/c • Pixel + SCT using iPatRec Impact parameter ~ 13-15 mm (TDR 11 mm) Nectarios Ch. Benekos EESFYE – HEP 2003 Workshop, NTUA, April 17-20, 2003 19

  23. Impact parameter resolution N N |h|<0.8 1.6<|h|<2.5 DRf DRf • single muons tracks pT =1 GeV/c • Pixel + SCT using iPatRec Impact parameter ~ 100 mm / √(sinθ) (TDR 73 mm / √(sinθ) Nectarios Ch. Benekos EESFYE – HEP 2003 Workshop, NTUA, April 17-20, 2003 20

  24. Tuning of pull distributions (plot before corrections) N N |h|<0.8 1.6<|h|<2.5 DRf DRf • single muons tracks pT =1 GeV/c • Pixel + SCT using iPatRec N 0.8<|h|<1.6 Pull ~ .87 in barrel ~ .91 in endcap Overestimated X0 in b-layer guessed 3% X0  corrected DRf Nectarios Ch. Benekos EESFYE – HEP 2003 Workshop, NTUA, April 17-20, 2003 21

  25. Tuning (cont.) N N |h|<0.8 1.6<|h|<2.5 DRf DRf 200 GeV muons using Pixel+SCT Rel. 6.0.1. using iPatRec N Pull ~ 1.0 in barrel ~ .91 in endcap Errors slighlty over-estimated at higher h 0.8<|h|<1.6 DRf Nectarios Ch. Benekos EESFYE – HEP 2003 Workshop, NTUA, April 17-20, 2003 22

  26. Momentum resolution vs eta In the absence of multiple scattering: In the presence of multiple scattering: • reducing further the pT, the effect of multiple scattering • is starting to dominate and • at pT=1 GeV/c multiple scattering is dominating at all • |h| with a marked degradation in resolution and • with degrading resolution with increasing |h|. • non-uniform magnetic field correction in forward region • (higher h ) Nectarios Ch. Benekos EESFYE – HEP 2003 Workshop, NTUA, April 17-20, 2003 23

  27. Eta dependency on impact parameter resolution (TDR 11 mm) (TDR 73 mm / √(sinθ)) Nectarios Ch. Benekos EESFYE – HEP 2003 Workshop, NTUA, April 17-20, 2003 24

  28. Conclusions • The single track reconstruction performance of the ATLAS ID has been • investigated using the simulation of single muons. • Material tuning in iPatRec • resolution studied of the impact parameters, over the complete studied |h| and pT-range • Measurement errors understood and correctly accounted • Due to the updated ID layout (more realistic material) the • impact parameter resolution was found to be: • ~ 100 mm (as a function of sinq) for pT=1 GeV/c • (multiple scattering effect is dominated) • and ~14 mm for pT=200 GeV/c Nectarios Ch. Benekos EESFYE – HEP 2003 Workshop, NTUA, April 17-20, 2003 25

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