Space-point Distortions Particle Tracking and Identification at High Rates Friday, 16th December 2016 Seminar - Talk Yannik Vetter

Outline Motivation Motion of charged particles Static Distortions Misalignment: Field Cage Misalignment: E x B Calibration Dynamic Distortions Space-Charge ALICE TPC RUN2 Outlook: RUN3 GEM Update Summary Yannik Vetter Space-Point Distortions 16.12.2016

Motivation reconstruct actual track identify particles (p, dE/dx,…) highest possible resolution several sources that distort path of ionization electrons definition of distorting effects correction necessary ‘cross-check’ with e.g. ITS and TRD tracks Yannik Vetter Space-Point Distortions 16.12.2016

Outline Motivation Motion of charged particles Static Distortions Misalignment: Field Cage Misalignment: E x B Calibration Dynamic Distortions Space-Charge ALICE TPC RUN2 Outlook: RUN3 GEM Update Summary Yannik Vetter Space-Point Distortions 16.12.2016

Charged particle in Fields Langevin-Equation: with static solution: three directional dependencies ions move much slower than electrons:  Electrons cleared from volume much faster than ions  Ions create so called space charges Yannik Vetter Space-Point Distortions 16.12.2016

Outline Motivation Motion of charged particles Static Distortions Misalignment: Field Cage Misalignment: E x B Calibration Dynamic Distortions Space-Charge ALICE TPC RUN2 Outlook: RUN3 GEM Update Summary Yannik Vetter Space-Point Distortions 16.12.2016

Static Distortions Misalignment: Field Cage E-Field pointing towards central electrode simplest analogy: plate capacitor field cage to prevent inhomogeities at boundary resistor rods and strips to correct these last strip left out <<<<<<<< WHY??? >>>>>>>>>>> [http://www.itacanet.org] Sketch: D.Vranic Yannik Vetter Space-Point Distortions 16.12.2016

Static Distortions Misalignment: Field Cage need very precise resistor values for best result deviations result in E-field distortion, especially at very low/high radii distortions up to 6 mm due misalignment E-field main component misalignment up to 2 mm distortions also: misalignment of central electrode, ROCs and resistor rods misalignment of 0.1 mm results in distortion of up to 6 mm What else? Yannik Vetter Space-Point Distortions [Rossegger et al.] 16.12.2016

Static Distortions Misalignment: E x B (always) and components E x B – term in static solution of EoM ≠ 0  Rossegger et al.: 2nd order solution of the Langevin-Equation expanding to only 2nd order terms x- and y- velocities strongly suppressed, but not 0! Rossegger et al. Yannik Vetter Space-Point Distortions 16.12.2016

Static Distortions Misalignment: E x B z – component; with v0 nominal drift velocity at nominal field E0: resulting distortions after integration of ux,y,z: plug in equations above to get dependence on E and B distortion of 1 cm in 2nd order known to 4 µm < resolution TPC Delta_z??? How comes the form? Yannik Vetter Space-Point Distortions 16.12.2016

Static Distortions Misalignment: E x B E x B biggest distortion effects, O(cm) B-field inhom. up to 8 mm distortions B-field main component misalignment up to 2 mm distortions Yannik Vetter Space-Point Distortions 16.12.2016

Static Distortions Misalignment: E x B E x B corrections for B inhom. B = 0.5 T FC misalignment correction Yannik Vetter Space-Point Distortions 16.12.2016

Static Distortions Calibration assume distortion transformations to commute and approximate as linear combination of partial distortions: BUT: cannot observe distortions! Define observables: RUN1: ITS+TPC or TPC+TRD or TPC+primary vertex RUN2+3: already have space charge and distortion maps, can use known calibration Partial distortions, like E x B, misalignment etc. Yannik Vetter Space-Point Distortions 16.12.2016

Static Distortions Calibration Partial distortions, like E x B, misalignment etc. Yannik Vetter Space-Point Distortions 16.12.2016

Static Distortions Calibration Partial distortions, like E x B, misalignment etc. Yannik Vetter Space-Point Distortions 16.12.2016

Outline Motivation Motion of charged particles Static Distortions Misalignment: Field Cage Misalignment: E x B Calibration Dynamic Distortions Space-Charge ALICE TPC RUN2 Outlook: RUN3 GEM Update Summary Yannik Vetter Space-Point Distortions 16.12.2016

Dynamic Distortions Space-Charge accumulation of charges in space TPC: clusters of ions from tracks (low effect), backflowing ions from readout planes (IBF, MWPC ~0, GEM ~1%) varying over time  dynamic distortion depending on interaction rate (track number) parametrized by charge density Laplace equation yields resulting E-field obtain distortions with Langevin-equation Yannik Vetter Space-Point Distortions 16.12.2016

Dynamic Distortions Space-Charge Low z and low r  high track number Yannik Vetter Space-Point Distortions 16.12.2016

Dynamic Distortions Space-Charge hot spots of space charges mostly in between the sector boundaries resulting E-field distortion has focussing effect Better picture!!! Yannik Vetter Space-Point Distortions 16.12.2016

Dynamic Distortions Space-Charge Central Electrode ROCs Yannik Vetter Space-Point Distortions 16.12.2016

Dynamic Distortions Space-Charge Residuals, Yannik Vetter Space-Point Distortions 16.12.2016

Dynamic Distortions Alice tpc Run2 Measurement of space charges: use data from ITS, TRD, (TOF) compare tracks with TPC tracks differences into distortion map Space charge fluctuations observed dominant term for F << 2 and F >> 2 Ntracks number of tracks; NQ charge for one track; F fraction where Q is deposited See other slides! Step by step correction; add fluctuation picture Yannik Vetter Space-Point Distortions 16.12.2016

Dynamic Distortions Alice tpc Run2 track with default dist. map match to ITS, TRD, TOF refit latter part and interpolate to TPC (reference) determine differences between distorted and reference 3D vector of distortion in each voxel smooth parametrization; time dependence, redo every ~40 min See other slides! Step by step correction; add fluctuation picture Yannik Vetter Space-Point Distortions 16.12.2016

Dynamic Distortions Alice tpc Run2 big change in distortions due to change of gas mixture after RUN1 Local distortions factor 10 – 20 higher in RUN2, factor 2 expected dependence on interaction rate: non-linear, seems to saturate distortions depend on the orientation of the B-field (++, --) See other slides! Step by step correction; add fluctuation picture Yannik Vetter Space-Point Distortions 16.12.2016

Dynamic Distortions Alice tpc Run2 1 rel. IR = 5 kHz Yannik Vetter Space-Point Distortions 16.12.2016

Dynamic Distortions Alice tpc Run2 positive B-field negative B-field Yannik Vetter Space-Point Distortions 16.12.2016

Outline Motivation Motion of charged particles Static Distortions Misalignment: Field Cage Misalignment: E x B Calibration Dynamic Distortions Space-Charge ALICE TPC RUN2 Outlook: RUN3 GEM Update Summary Yannik Vetter Space-Point Distortions 16.12.2016

Outlook: RUN3 Gem update ion backflow ~1% (MWPC ~0): GEMs are not opaque for ions like GG much higher IR new calibration necessary: we measure I(x,y,t) density maps become time dependent in RUN3 otherwise calibration is same as in previous run Include calibration with epsilon(x,y,t), see TDR Yannik Vetter Space-Point Distortions 16.12.2016

Outlook: RUN3 Gem update Partial distortions, like E x B, misalignment etc. space charge density maps for different occupancies, normalized to 10 events, z = 10 cm Yannik Vetter Space-Point Distortions 16.12.2016

Outlook: RUN3 Gem update Partial distortions, like E x B, misalignment etc. distortion map for B = 0 (left) and B = 0.5 T (right); symmetry broken due E x B; RUN3 simulation Yannik Vetter Space-Point Distortions 16.12.2016

Outline Motivation Motion of charged particles Static Distortions Misalignment: Field Cage Misalignment: E x B Calibration Dynamic Distortions Space-Charge ALICE TPC RUN2 Outlook: RUN3 GEM Update Summary Yannik Vetter Space-Point Distortions 16.12.2016

Summary Yannik Vetter Space-Point Distortions 16.12.2016